Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell

Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell Hambalee Mahamu, Shigeo Asahi, Takashi Kita This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4362355/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Nov, 2024 Read the published version in Scientific Reports → Version 1 posted 11 You are reading this latest preprint version Abstract Two-step photon upconversion solar cells (TPU-SCs) based on III–V semiconductors can achieve enhanced sub-bandgap photon absorption because of intraband transitions at the heterointerface. From a technological aspect, the question arose whether similar intraband transitions can be realized by using perovskite/III–V semiconductor heterointerfaces. In this article, we demonstrate a TPU-SC based on a CsPbBr 3 /GaAs heterointerface. Such a solar cell can ideally achieve an energy conversion efficiency of 48.5% under 1-sun illumination. This is 2.1% higher than the theoretical efficiency of an Al 0.3 Ga 0.7 As/GaAs-based TPU-SC. Experimental results of the CsPbBr 3 /GaAs-based TPU-SC show that both the short-circuit current J SC and the open-circuit voltage V OC increase with additional illumination of sub-bandgap photons. We analyze the excitation power dependence of J SC for different excitation conditions to discuss the mechanisms behind the enhancement. In addition, the observed voltage-boost clarifies that the J SC enhancement is caused by an adiabatic optical process at the CsPbBr 3 /GaAs heterointerface, where sub-bandgap photons efficiently pump the electrons accumulated at the heterointerface to the conduction band of CsPbBr 3 . Besides the exceptional optoelectronic properties of CsPbBr 3 and GaAs, the availability of a CsPbBr 3 /GaAs heterointerface for two-step photon upconversion paves the way for the development of high-efficiency perovskite/III–V semiconductor-based single-junction solar cells. Physical sciences/Energy science and technology/Renewable energy/Solar energy/Photovoltaics/Solar cells Physical sciences/Physics/Condensed matter physics/Surfaces interfaces and thin films Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Introduction Photovoltaics is the field of science concerned with the conversion of photons to electrical energy. To characterize the physics of a considered photovoltaic device, usually, several parameters including the energy conversion efficiency (the ratio of the output electrical power to the power provided by the incident light) are used. The energy conversion efficiency of a solar cell (SC) can be estimated using thermodynamic approaches by considering the energy generation in the SC through absorption and the energy losses during the energy conversion process. The “unavoidable” losses including the transmission of sub-bandgap photons, thermalization of high-energy carriers, the Carnot loss, the Boltzmann loss, and emission losses can be taken into account to accurately estimate the efficiency 1 – 3 . By considering these losses, it is possible to derive an expression for the ideal energy conversion efficiency, known as the Shockley–Queisser (SQ) limit or the detailed balance limit. The SQ limit predicts that the maximum conversion efficiency of a single junction SC is ~ 31 % nder 1-sun illumination 1 – 4 . To overcome the SQ limit, a SC design is needed that makes a part of the “unavoidable” losses “avoidable” (and suppresses them). For example, various SC designs have been proposed to reduce the transmission loss 5 – 7 , which plays an important role in SCs and is usually due to the small absorption coefficient of semiconductors for sub-bandgap photons. In 1997, the concept of the intermediate-band solar cell (IBSC) was proposed to harvest energy in the region below the bandgap of the absorber layer of a single-junction SC 8 . An IBSC contains additional real states between the conduction band (CB) and the valence band (VB) of the absorber layer, which are called intermediate band (IB) states and can be realized by using quantum structures or by incorporating impurities 9 . The enhancement of absorption in IBSCs is caused by a sequential excitation of electrons from the VB to the IB and from the IB to the CB using sub-bandgap photons (in addition to the direct band-to-band transitions). Because the band-to-band transitions are independent of the excitation via the IB, an IBSC is equivalent to a parallel circuit: one branch contains one diode corresponding to band-to-band transitions, and the other branch contains two diodes connected in series corresponding to the sequential absorption of sub-bandgap photons via the IB 9 – 14 . As a result, an IBSC is not affected by current and voltage mismatches, which are problematic in the case of multi-junction solar cells 1 . On the other hand, for IBSCs, a low density of states (DOS) at the IB levels is problematic, because this leads to weak carrier generation via the IB. Moreover, regarding the aspect of fabrication complexity, an IB needs a periodic long-range-ordered structure in order to create an electronic band. This requires complex fabrication processes, especially when the band is realized using quantum structures, e.g. quantum dots (QDs). Such a trade-off relation between fabrication complexity and efficiency hinders the commercialization of IBSCs. On the other hand, we demonstrated a so-called two-step photon upconversion solar cell (TPU-SC) based on Al 0.3 Ga 0.7 As and GaAs including a single layer of InAs QDs at the heterointerface (the QDs were prepared by using the Stranski–Krastanov growth mode) 10 – 13 . The fabrication of such a TPU-SC is relatively simple compared to IBSCs. A TPU-SC consists of a wide bandgap semiconductor (WGS) and a narrow bandgap semiconductor (NGS) to form a heterointerface, where intraband transitions can be induced by sub-bandgap photons. For the WGS and the NGS, we previously used Al 0.3 Ga 0.7 As and GaAs, respectively. The presence of QDs at the heterointerface can enhance the intraband transitions at the heterointerface, because the three-dimensional confinement of the QD relaxes the optical selection rule and allows in-plane electronic transitions 10 , 14 – 16 . Consequently, in addition to the band-to-band (interband) excitation of the WGS and the NGS, intraband excitation can be achieved, which results in a photocurrent enhancement: The process responsible for the photocurrent enhancement is called two-step photon upconversion (TPU), because each additional electron is first excited to the NGS CB by an interband-transition step and then experiences an intraband-transition step to reach the WGS CB. Regarding the photovoltage, optically induced intraband transitions lead to an increase of the quasi-Fermi level of the electrons due to an increase in the electron density of the WGS CB. Thus, the TPU-SC design is able to improve the photocurrent and the photovoltage, and therefore, the SC efficiency. The maximum theoretical efficiencies of a TPU-SC with a CB discontinuity at the heterointerface that is 50% larger than the VB discontinuity are 46.7% and 63.2% under 1-sun and 10,000-sun illumination respectively 1 , 17 . Therefore, the TPU-SC concept is one of the SC concepts that may help to realize high-efficiency single-junction SCs. Although design concepts for SCs with high theoretical conversion efficiencies are very important, the actually used semiconductors are important as well. For example, lead halide perovskite semiconductors have been widely studied due to their excellent optoelectronic properties and advantages in terms of fabrication, since they can be prepared relatively easily using low-cost fabrication techniques 18 – 21 . However, stability is a concern, especially for organo-lead halide perovskites. These latter perovskites contain organic cations, e.g. methylammonium ions (MA + ) or formamidinium ions (FA + ), which are known for their chemical sensitivity to water molecules. This leads to a relatively fast degradation, and hence, the instability of organo-lead halide perovskite SCs under ambient conditions 21 – 27 . Several studies suggested the substitution of MA + and FA + with inorganic cations, i.e., cesium ions (Cs + ), to create perovskites that are more stable (this substitution transforms an organic-cation-based perovskite into an all-inorganic perovskite), and significant stability improvements have been reported for perovskites that include Cs + instead of MA + or FA + 21,28 . There are three main types of Cs-based perovskites: CsPbCl 3 , CsPbBr 3 , and CsPbI 3 . Theoretical studies have predicted that CsPbI 3 can provide the highest conversion efficiency. In addition, CsPbI 3 has a small bandgap energy ( E g ) of 1.73 eV, and CsPbI 3 is currently the most studied Cs-based perovskite. While CsPbI 3 has certain advantages for applications, the perovskite phase of CsPbI 3 is only stable above temperatures of 300°C. As a result, CsPbI 3 tends to form undesired non-perovskite phases at room temperature, especially under ambient conditions, leading to significant difficulties for SCs operating at room temperature 29 , 30 . The main reason for the poor stability of CsPbI 3 under ambient conditions is the large ion radius of iodine, which accelerates chemical reactions. The optimum I/Br ratio in CsPbI x Br 3- x for phase stability was identified by Sanchez et al., but the corresponding photovoltaic performance was rather limited 31 . It has been shown that Cs-based perovskites with a higher iodine concentration contain a larger volume of the unstable α-CsPbI 3 phase 32 . Therefore, CsPbI x Br 3- x compound perovskites degrade faster than CsPbBr 3 . Albeit CsPbBr 3 may be not the best choice for SC applications due to its wide bandgap ( E g = 2.33 eV), it can be more suitable than CsPbCl 3 ( E g = 3.00 eV) and CsPbI 3 . It has been reported that single-crystal CsPbBr 3 films exhibit a high electron mobility of 1000 cm 2 (Vs) – 1 with a lifetime of 2.5 ms 21 , 33 , 34 . The carrier diffusion length, which influences the carrier collection efficiency in a SC, is approximately 1 µm and 12 µm for electrons and holes, respectively 35 , and the open-circuit voltage of CsPbBr 3 -based SCs can exceed 1.6 V 36 , 37 . In addition, CsPbBr 3 has a high melting temperature of 570°C, which means a relatively low degradation rate at room temperature. CsPbBr 3 can form several perovskite crystal phases including the cubic α-phase, the tetragonal β-phase, and the orthorhombic γ-phase, depending on the environmental temperature. The formation of these three crystal phases is reversible; the orthorhombic γ-phase forms at room temperature but can be converted to the tetragonal β-phase and the cubic α-phase by heating the crystal up to 88°C (some studies reported a phase-transition temperature of ~ 107°C) and 130°C, respectively 38 – 40 . Although CsPbBr 3 exhibits three different crystal phases, their differences in terms of optoelectronic properties are not significant, and thus this material can be used in SCs over a wide range of operating temperatures. The extraordinary good optoelectronic properties are promising for the realization of high-efficiency SCs. New technological insights might be gained by fabricating a TPU-SC based on CsPbBr 3 and GaAs to study the potential of the CsPbBr 3 /GaAs heterointerface for TPU-SCs and intraband transitions in general. In this work, we fabricated a TPU-SC with CsPbBr 3 and GaAs as the WGS and the NGS, respectively, and confirmed enhancements in the photocurrent and the photovoltage in the case of additional illumination with sub-bandgap photons. The wide bandgap of CsPbBr 3 (2.33 eV) is suitable for application in a TPU-SC together with GaAs (1.42 eV) due to the achieved ratio of the band discontinuities in the CB and VB 1 , 17 . We used a QD-free structure to study the effect of the interface states on the TPU process. Furthermore, we simplified the fabrication technique; the growth of CsPbBr 3 was performed under ambient conditions. Although our CsPbBr 3 /GaAs-based TPU-SC contains no QDs, we believe that the interface states behave like quantized electronic states, which allow intraband transitions if the SC is irradiated with sub-bandgap photons in addition to the light used for interband excitation of the absorber layers in this device. The excitation power-dependence of the short-circuit current and that of the open-circuit voltage revealed the carrier dynamics at the CsPbBr 3 /GaAs-heterointerface as well as the quasi-Fermi-level splitting as a result of a TPU process. The effect of the temperature on the photocurrent and the photovoltage was investigated to distinguish the TPU process from thermal activation. Results and Discussion Device Structure of the TPU-SC The fabricated TPU-SC consists of an n–i–p CsPbBr 3 /GaAs heterojunction. The device structure is shown in Fig. 1 a. For the WGS, we grew a CsPbBr 3 layer (shown in green), and for the NGS, we used a 1-inch p-doped GaAs substrate (001) (shown in black). Zinc oxide (ZnO) was used as the electron transport layer (ETL). We denote the CsPbBr 3 /GaAs heterointerface as HI-I, and the ZnO/CsPbBr 3 heterointerface as HI-II. The band diagram of the ZnO/CsPbBr 3 /p-GaAs structure was simulated using the COMSOL Multiphysics® software, and the result is shown in Fig. 1 b. A TPU process is expected to occur at HI-I after the photogenerated electrons in the CB of GaAs have accumulated at the interface states. Results of the TPU process occurring at HI-I are discussed in the following sections. The Ideal Energy Conversion Efficiency Figure 2 shows two maps of the theoretical conversion efficiency as functions of the NGS and WGS bandgaps for two different band-offset configurations. Here, the band-offset configuration is expressed by the ratio of the CB and VB discontinuities (Δ E CB :Δ E VB ). For the prediction of the efficiency limit under 1-sun illumination, we used the detailed balance approach 1 , 10 , 17 and the AM 1.5G spectrum. Figure 2 a shows the data for Δ E CB :Δ E VB = 3:2. This value of Δ E CB :Δ E VB is nearly equal to that of an Al 0.3 Ga 0.7 As/GaAs heterojunction 15 . The highest conversion efficiency is 46.4%, which is achieved by using NGS and WGS bandgap energies of E g, NGS ≈ 1.60 eV and E g, WGS ≈ 3.10 eV, respectively. Therefore, GaAs ( E g = 1.42 eV) is a reasonable good choice for the NGS. However, the optimal WGS bandgap is very large, and thus Al x Ga 1- x As is not the best choice for the WGS of such a high-efficiency TPU-SC (the bandgap energy of Al x Ga 1- x As is only about 2.20 eV even for x = 0.9). Note that Al 0.3 Ga 0.7 As ( E g = 1.81 eV) was used in the previously reported TPU-SCs. Furthermore, although a larger Al molar fraction x provides a larger bandgap, Al x Ga 1- x As switches from a direct bandgap semiconductor to an indirect bandgap semiconductor when x ≥ 0.45 41 . The conversion efficiency map for Δ E CB :Δ E VB = 7:1 is shown in Fig. 2 b. Since the electron affinity ( χ ) and the E g values of CsPbBr 3 and GaAs are known, we can estimate the barrier heights for both the CB and the VB: The χ values of CsPbBr 3 and GaAs are 3.30 eV and 4.07 eV, respectively 42 , 43 . Therefore, the ratio of Δ E CB to Δ E VB becomes 7:1 (Δ E CB = 0.77 eV and Δ E VB = 0.11 eV). Figure 2 b indicates that the maximum conversion efficiency is 48.5% for E g, NGS ≈ 1.50 eV and E g, WGS ≈ 2.60 eV. This efficiency maximum is 2.1% higher than that for Δ E CB :Δ E VB = 3:2. Furthermore, we find that CsPbBr 3 is suitable for the WGS of such a TPU-SC, because CsPbBr 3 perovskite has an E g of 2.33 eV. In comparison to the Al 0.3 Ga 0.7 As/GaAs-based TPU-SC, the CsPbBr 3 /GaAs-based TPU-SCs provide a higher maximum conversion efficiency in addition to the exceptional optoelectronic properties of CsPbBr 3 and GaAs. EQE Spectra The room-temperature EQE spectra of our CsPbBr 3 /GaAs-based TPU-SC are shown in Fig. 3 . The blue curve represents the EQE spectrum measured under single-color excitation conditions, which means that carrier generation occurs mainly through interband excitation. The EQE signal gradually increases as the excitation wavelength decreases. At wavelengths longer than ~ 900 nm, there is no EQE signal, because these photons cannot excite any semiconductor in this device. The EQE signal shows a sharp onset when the excitation wavelength is about 900 nm, where GaAs starts to absorb the photons. The signal reaches a plateau at about 870 nm, which corresponds to the GaAs bandgap. As a result of the built-in electric field, the photogenerated electrons in the CB of GaAs (generated by the photons in the wavelength range from approximately 530 to 870 nm) drift toward HI-I, while the photogenerated holes drift to the rear electrode (the holes are not influenced by HI-I). The electrons are trapped at the interface states of HI-I and accumulate there. These accumulated electrons partially recombine with the holes that reach HI-I by diffusion. We described this scenario in previous publications 10 – 13 . A part of the electrons at HI-I can overcome the energy barrier by thermal excitation (evidence for thermionic emission is provided in Influence of the Temperature). At an excitation wavelength of about 530 nm, the EQE signal abruptly increases, because the corresponding photon energy is high enough to induce interband transitions in both CsPbBr 3 and GaAs. As the wavelength becomes shorter, the photon energy becomes higher than the CsPbBr 3 bandgap energy and the photons are stronger absorbed by the CsPbBr 3 layer (the number of photons that reach the GaAs layer becomes smaller). Since the holes generated in the CsPbBr 3 VB have a longer path to the rear electrode, which implies a larger recombination probability, the EQE should decrease if the excitation wavelength is reduced further. The dark red curve in Fig. 3 represents the EQE spectrum measured under two-color excitation conditions. The comparison with the data for single-color excitation reveals that the EQE in the wavelength range between the CsPbBr 3 bandgap and the GaAs bandgap (~ 530–870 nm) decreases when the device is additionally irradiated with the IR photons, and in the wavelength range below 530 nm (the CsPbBr 3 bandgap), the IR photons lead to an enhancement of the EQE signal. In contrast, our previous work on TPU-SCs with a single heterointerface showed an IR-induced EQE enhancement in the range between the WGS and NGS, and no enhancement in the short-wavelength range. In particular, in an Al 0.3 Ga 0.7 As/GaAs-based TPU-SC, there is no IR-induced EQE enhancement when the wavelength of the photons for interband excitation is shorter than 680 nm (the Al 0.3 Ga 0.7 As bandgap). This result is attributed to the fact that high-energy photons are strongly absorbed by the WGS, 10 – 13 which causes a reduction of the number of photons that reach the NGS layer, leading to a reduction of the electron density at the heterointerface. As a result, the IR-induced photocurrent enhancement gradually disappears as the wavelength of the photons for interband excitation becomes shorter; if there are no photogenerated electrons in the NGS CB, the IR laser beam has no effect, because the photogenerated electrons in the WGS CB do not accumulate at the energy barrier 10 – 12 . To clarify the behavior of our CsPbBr 3 /GaAs-based TPU-SC, we consider our previous results on a double-heterointerface TPU-SC with an additional Al 0.7 Ga 0.3 As (~ 2.50 eV) layer grown on the Al 0.3 Ga 0.7 As/GaAs (1.79 eV/1.42 eV) structure: this device exhibited an IR-induced EQE improvement in the short-wavelength range 13 . Hence, the IR-induced EQE enhancement observed in Fig. 3 at wavelengths below 530 nm is likely due to the interface states of HI-II. To support this interpretation, we estimated the fraction of 500-nm light absorbed in each layer of the CsPbBr 3 /GaAs-based TPU-SC (the calculation details are explained in Section S4 of the Supplementary Information). Our estimation reveals that photons with such short wavelengths do not induce electron accumulation at HI-I. Furthermore, in this measurement, non-linear phenomena such as multi-photon absorption in CsPbBr 3 are negligible, because we used a tungsten-halogen lamp and a CW laser for the excitation, i.e., light sources with a relatively low energy density. Therefore, we believe that the IR-induced EQE enhancement in the short-wavelength regime is caused by electron accumulation at HI-II. Conversely, it has been found that the EQE signal in the wavelength region between the bandgaps of CsPbBr 3 and GaAs reduces with the IR irradiation. The change in EQE with the IR irradiation strongly depends on the excitation power density. We discuss the details in Excitation Power-Dependence of J SC . Figure 4 shows the temperature dependence of the EQE spectrum for single-color excitation (the wavelength of the photons for interband excitation is shown on the x -axis, and the temperature is shown on the y -axis). At temperatures below 180 K, the CsPbBr 3 /GaAs-based TPU-SC hardly produces photocurrent due to the CB discontinuity at HI-I: The difference between the electron affinities χ of CsPbBr 3 and GaAs provides a barrier height of 0.77 eV at HI-I. Since the thermal activation energy at 297 K is only 0.026 eV, the electrons accumulated at the heterointerface can hardly overcome the barrier at much lower temperatures. In this measurement, the temperature governs the electronic excitation at HI-I, because 1319-nm (sub-bandgap) photons were not used. Consequently, Fig. 4 confirms the thermal activation of electrons accumulated at HI-I, which plays an important role in the photocurrent generation when sub-bandgap photons are absent 44 . Figure 4 also reveals a clear shift of the absorption edge of GaAs: it shows a blueshift with decreasing temperature due to the well-known effect responsible for the temperature-dependent changes in the bandgaps of III–V semiconductors 45 , 46 . The absorption edge of CsPbBr 3 , on the other hand, is almost insensitive to the temperature within the measured range. It has been reported that the bandgap energy of lead bromide perovskites ( X PbBr 3 , with X = FA + , MA + , or Cs + ) decreases with decreasing temperature, in contrast to the behavior of III–V semiconductors. The increase of the X PbBr 3 bandgap energy with increasing temperature is attributed to changes in the crystal phase. In general, X PbBr 3 perovskites can exist in various crystal phases including tetragonal and cubic phases. In the case of CsPbBr 3 , the orthorhombic phase can be observed easily at room temperature, because of its good stability, and as the temperature increases, phase changes occur. Mannino et al. reported that the E g of CsPbBr 3 has a small dependence on the temperature in the range 200–300 K 38 . This phenomenon is caused by the thermal expansion of the orthorhombic phase. At about 380 K (107°C), the CsPbBr 3 bandgap changes abruptly due to the phase transition from the orthorhombic phase to the tetragonal phase (several investigations confirmed this phase transition already at 361 K 21 , 39 , 40 ). This is in contrast to FAPbBr 3 and MAPbBr 3 , where a tetragonal–cubic phase change occurs in the temperature range 200–300 K, and results in an E g change of CsPbBr 3 of approximately 0.02 eV in the range 200–380 K 38 . Within the temperature range used for Fig. 4 , CsPbBr 3 is in the orthorhombic phase. As a result, the absorption edge is almost constant 2.33 eV (the E g change is within ~ 0.008 eV). JV Characteristics The JV characteristics under different illumination conditions are shown in Fig. 5 ; the black, blue and dark red points represent the JV characteristics in the dark, under single-color excitation conditions ( P Inter = 101 mW/cm 2 ), and under two-color excitation conditions ( P Inter = 101 mW/cm 2 and P Intra = 86 mW/cm 2 ), respectively. Because the ZnO and CsPbBr 3 layers are transparent for 784-nm photons, these photons are absorbed in the p-GaAs substrate and generate electrons and holes in the GaAs CB and VB, respectively. The photogenerated electrons drift toward HI-I, where they can accumulate at the interface states. The photogenerated holes drift to the rear electrode 10 , 11 , 13 . While thermal excitation can in principle excite the electrons at HI-I to the CB of CsPbBr 3 , thermal activation hardly occurs since the energy difference between the CB minima of CsPbBr 3 and GaAs is 0.77 eV, which is much higher than the thermal energy at room temperature (0.026 eV). This is the reason for the small magnitude of the detected photocurrent in the case of single-color excitation. For two-color excitation, a slight photocurrent and photovoltage enhancement due to the additional IR photons can be confirmed. The IR-induced gain in the short-circuit current (Δ J SC ) and the IR-induced gain in the open-circuit voltage (Δ V OC ) are 0.011 µA/cm 2 and 0.025 V respectively. Although this data at first seems inconsistent with the EQE results, both results are actually in agreement. To understand this, we need to consider that a tungsten-halogen lamp was employed for the EQE measurement, which only provided 6.0×10 –4 mW/cm 2 at about 780 nm (a photon flux of 2.4×10 16 m – 2 s – 1 ). As shown later, intraband transitions hardly occur as long as P Inter is below 20 mW/cm 2 (a photon flux of 8.0×10 20 m – 2 s – 1 ). Therefore, the power density of the monochromatic beam for interband excitation used in the EQE measurement was too low. For the JV measurements, we employed a CW solid-state laser operating at 784 nm, which provided a photon flux of 4.0×10 21 m – 2 s – 1 for interband excitation. The behavior of the open-circuit voltage ( V OC ) is interpreted as follows: The V OC enhancement in the case of additional illumination with sub-bandgap photons (i.e., in the case of optically-induced intraband transitions) occurs because the 784-nm photons induce interband transitions only in the GaAs layer. When the electrons accumulated at HI-I are excited by the 1319-nm photons and reach the CB of CsPbBr 3 , the electron quasi-Fermi level shifts to a higher level and thereby increases the V OC . Thus, the observed positive Δ V OC evidences an adiabatic intraband excitation process at HI-I 10 , 44 . In Excitation Power-Dependence of the Open-Circuit Voltage, we show how Δ V OC depends on P Inter and P Intra . Excitation Power-Dependence of J SC Figure 6 shows the dependence of the short-circuit current ( J SC ) on P Inter for single-color excitation. The 784-nm photons excite the GaAs substrate and intraband transitions are not induced, because sub-bandgap photons are absent. The photogenerated electrons in the CB of GaAs drift toward HI-I, where electrons accumulate due to the CB discontinuity. To overcome the barrier, an energy is required that is nearly 30 times the thermal energy at room temperature. Therefore, the temperature is important for the J SC under single-color excitation conditions. The effect of the temperature is discussed in Influence of the Temperature. To discuss the data in Fig. 6 , we fitted the data to a single power law, \({\text{J}}_{\text{SC}}\text{ }\text{∝}\text{ }{\text{P}}_{\text{Inter}}^{\text{ n}}\) . The estimated power exponent is n = 0.67, which significantly deviates from unity. The reason for this sublinear power dependence is the weak built-in electric field and the recombination with holes: The electrons generated in the GaAs CB drift toward HI-I due to a weak electric field. As the electrons accumulate at HI-I, the electric field becomes weaker, which increases the electron–hole recombination rate. Therefore, the rate of thermal activation at HI-I decreases as P Inter increases, leading to the observed sublinear relationship. Figure 7 a shows the photocurrent gain under short-circuit conditions ( \(\text{∆}{\text{J}}_{\text{SC}}\text{ =}{\text{ }\text{J}}_{\text{SC, with infrared}}\text{ – }{\text{J}}_{\text{SC, without infrared}}\) ) as a function of the power density of the 784-nm laser beam in the case of P Intra = 440 mW/cm 2 . The Δ J SC increases with P Inter , because stronger interband excitation provides a higher electron density at HI-I (the 784-nm photons induce interband transitions in GaAs, and the photogenerated electrons are transported to HI-I). The data can be well fitted using a single power law, Δ J SC \(\text{ }\text{∝}\text{ }{\text{P}}_{\text{Inter}}^{\text{ n}}\) , with n = 0.83. Ideally, Δ J SC should be proportional to the electron density at HI-I. Furthermore, the recombination in ZnO and CsPbBr 3 (as well as that at HI-II) should be negligible, because the 784-nm photons induce interband transitions only in the GaAs layer, and thus there are no photogenerated holes in ZnO and CsPbBr 3 . The sublinear characteristic, therefore, originates from a recombination within GaAs or at HI-I. While stronger interband excitation leads to a higher electron density at HI-I, this increased electron density reduces the built-in electric field, facilitating electron–hole recombination within GaAs. Hence, the electron density at HI-I sublinearly increases with P Inter . This behavior was also observed in our previous reports 11 , 47 . Figure 7 b clarifies the dependence of Δ J SC on P Intra in the case of P Inter = 450 mW/cm 2 . This data cannot be fitted to Δ J SC \(\text{ }\text{∝}\text{ }{\text{P}}_{\text{Intra}}^{\text{ n}}\) , because the data shows a gradual change of the power exponent, i.e., the value of n decreases as P Intra is increased. In the low excitation regime, we find n = 0.26, which is the maximum value in the measured range of power densities. The minimum value is n = 0.11, observed at P Intra ~ 450 mW/cm 2 . This reduction of n can be interpreted as a reduction of the carrier separation efficiency at HI-I. Since the 784-nm photons induce interband transitions only in the GaAs layer, recombination in ZnO and CsPbBr 3 (including that at HI-II) can be ignored. Therefore, the observed reduction of the carrier separation efficiency occurs at HI-I. The origin of the reduction is a lower built-in electric field at higher electron densities in the CBs of ZnO and CsPbBr 3 (and at HI-II), because the electrons that are optically excited to the CB of CsPbBr 3 hardly drift to HI-II if the built-in field is weak. This means that the functionality of HI-I is limited by the electron densities in ZnO and CsPbBr 3 13 . Figure 8 shows Δ J SC as functions of P Inter ( x -axis) and P Intra ( y -axis). The data scatters around zero and mostly shows negative values when P Inter is below ~ 20 mW/cm 2 . When P Inter exceeds 20 mW/cm 2 , an increase in P Inter also leads to a remarkable increase in Δ J SC . Note that, in the region below 20 mW/cm 2 , the data is independent of the intensity of the IR light. We interpret this behavior as follows: Since the electron density in the GaAs CB is mainly determined by the intensity of the photons used to induce interband transitions, the electron density remains low as long as P Inter is low. In addition, we used a p-GaAs substrate for the NGS layer (there is no i-GaAs layer), which makes it more difficult to increase the electron density compared to an Al 0.3 Ga 0.7 As/GaAs-based TPU-SC. Because the possibility of upconversion at HI-I is determined by the electron density at the initial energy levels for the intraband transitions, there is almost no Δ J SC in the case of weak interband excitation, even at high values of P Intra . The result of the negative Δ J SC when P Inter is below ~ 20 mW/cm 2 , shown in Fig. 8 , agrees with the EQE results in Fig. 3 . Since the photons used for interband excitation in our EQE measurement were generated by a tungsten-halogen lamp, the photon density was below the threshold. The power density of the monochromatic 784-nm beam generated by the tungsten-halogen lamp and the monochromator was approximately 6.0×10 –4 mW/cm 2 , which is equivalent to 0.005 suns. In other words, the used power density was too small to produce a significant density of electrons at HI-I. According to Fig. 8 , P Inter needs to be higher than ~ 20 mW/cm 2 to let a significant amount of electrons accumulate at HI-I. This result points out that electron accumulation at the heterointerface is a prerequisite for adiabatic intraband transitions caused by sub-bandgap photons. It is well known that non-radiative recombination plays a significant role due to the presence of interface states 48 , 49 . For two-color excitation conditions, the IR light is able to pump out carriers passivating the interface states 50 , which additionally induces non-radiative recombination at the heterointerface, and, thereby, the IR-induced EQE reduction occurs. Excitation Power-Dependence of the Open-Circuit Voltage The excitation power dependence of the V OC for single-color excitation is provided in Fig. 9 . The V OC increases with P Inter , because the electron density in the CB of GaAs increases. Although the accumulated electrons are thermally excited to the CB of CsPbBr 3 as shown in Fig. 6 , the V OC under single-color excitation conditions is determined by the quasi-Fermi-level splitting in GaAs, because in this case, the excitation at HI-I is a thermionic emission process. Figure 10 a shows the IR-induced gain in the open-circuit voltage (Δ V OC ) as a function of P Inter in the case of P Intra = 228 mW/cm 2 , and Fig. 10 b shows the Δ V OC as a function of P Intra in the case of P Inter = 260 mW/cm 2 . The positive Δ V OC values originate from the increase of the electron density in the CB of CsPbBr 3 caused by the additional illumination with the 1319-nm photons (optically induced intraband transitions): Figure 10 a shows an increase in Δ V OC with increasing P Inter . We consider that the carrier extraction at HI-I is an adiabatic optical process 10 , 44 . Therefore, the V OC increases. Compared to the thermionic emission and tunneling processes at HI-I, the intraband transitions induced by the 1319-nm photons provide an increased electron population in the CB of CsPbBr 3 . This induces a split between electron quasi-Fermi levels of CsPbBr 3 and GaAs. The positive Δ V OC values in Fig. 10 evidence the presence of an adiabatic TPU process at HI-I. Since stronger interband excitation causes a higher electron density at HI-I, the Δ V OC increases with P Inter . Similarly, Fig. 10 b shows an increase of Δ V OC with increasing P Intra , in agreement with the Δ J SC data shown in Fig. 7 . As P Intra increases, more electrons accumulated at HI-I are upconverted to the CB of CsPbBr 3 , which induces a widening of the quasi-Fermi-level splitting. Figure 11 shows Δ V OC as functions of P Inter and P Intra . Overall, this map shows an increase in Δ V OC as both P Inter and P Intra increase. Furthermore, a feature identical to that observed in Fig. 8 can be seen: when P Inter is below ~ 20 mW/cm 2 , the Δ V OC values are scattered around zero. The increase with either P Inter or P Intra only appears for sufficiently high P Inter values. This feature indicates that the mechanism responsible for the IR-induced enhancement of V OC is identical to that for the IR-induced enhancement of J SC ; the adiabatic upconversion of electrons contributes to both photocurrent and photovoltage. Furthermore, also the threshold characteristic in Fig. 11 is due to the requirement of a sufficiently high electron density at HI-I. Influence of the Temperature In this section, the effect of the temperature on the photocurrent and photovoltage of the CsPbBr 3 /GaAs-based TPU-SC is discussed. The temperature dependence of J SC for single-color excitation with P Inter = 428 mW/cm 2 is shown in the Arrhenius plot in Fig. 12 . In general, the J SC increases with temperature, in agreement with the temperature dependence of the EQE in Fig. 4 . This increase evidences thermionic emission at least at one of the two heterointerfaces in this device, since a higher temperature implies a higher thermal energy. Hence, at higher temperatures, the photogenerated electrons in the CB of GaAs can more easily overcome the band discontinuity by thermal excitation and then are collected at the front electrode. The dashed line in Fig. 12 represents the result of fitting the data to the Arrhenius equation, $${\text{J}}_{\text{SC}}\text{ = A}\text{exp}\left(\text{–}\frac{{\text{E}}_{\text{A}}}{{\text{k}}_{\text{B}}}\text{∙}\frac{\text{1}}{\text{T}}\right) \text{,} \text{(1)}$$ where A is a fitting parameter, k B is the Boltzmann constant, T is the absolute temperature, and E A is the thermal activation energy. Regarding E A , the observed IR-induced photocurrent gain (Figs. 7 a and b) would suggest an E A that corresponds to the CB discontinuity at HI-I. However, the fit resulted in an E A of 0.108 eV, which is significantly smaller than the estimated CB discontinuity of 0.770 eV. We ascribe the obtained value of E A to an average energy level of occupied interface states at HI-II: As indicated in the band diagram in Fig. 1 b, the CB discontinuity at HI-II has a barrier height of ~ 1.00 eV ( \({\text{χ}}_{\text{ZnO}}\) is much larger than \({\text{χ}}_{\text{CsPbB}{\text{r}}_{\text{3}}}\) ). In this case, the electrons in the CB of CsPbBr 3 should not need thermal energy to reach ZnO. Therefore, we consider the following process: First, the photogenerated electrons in the CB of GaAs accumulate at the interface states of HI-I. Then, the electrons are thermally excited to the CB of CsPbBr 3 and transported to the front electrode. A fraction of these electrons relaxes from the CB to the interface states of HI-II and are trapped. These electrons need to be thermally activated to reach the CB of ZnO in order to be collected at the front electrode. This means that these electrons are first thermally excited at HI-I, and then are again thermally excited at HI-II. As a result, the physical meaning of the obtained value of E A may be the energy difference between the CB minimum of ZnO and the average level of occupied interface states at HI-II. As shown in Figs. 8 and 11 , Δ J SC and Δ V OC increase with both P Inter and P Intra (if P Inter > 20 mW/cm 2 ). In Fig. 13 , we provide additional evidence for the presence of optically induced intraband transitions at HI-I. Figure 13 shows two data sets of Δ V OC as a function of Δ J SC for P Inter = 450 mW/cm 2 : The dark red points represent the Δ J SC –Δ V OC data acquired at room temperature as a function of P Intra (the data showing the IR-induced gain in J SC and V OC ). This dataset indicates an increase in both Δ J SC and Δ V OC with increasing P Intra . The blue points represent the Δ J SC –Δ V OC data for P Intra = 0 as a function of the temperature (the data showing the temperature-induced changes in J SC and V OC ). This dataset exhibits a clearly different feature, i.e., the Δ V OC decreases as Δ J SC increases. These two data sets allow us to compare the influence of optically induced intraband transitions (sub-bandgap photons) and thermal activation (thermal energy) on the SC performance. In Fig. 12 , we demonstrated thermal activation, which provides more additional photocurrent (Δ J SC ) as the temperature increases. However, the advantage gained by this process is not identical to the advantage gained by the photocurrent increase through optically induced intraband transitions. To understand the difference, the Δ V OC in the case of thermal activation needs to be considered. It is well-known that the V OC has a strong inverse dependence on the dark saturation current, which increases with temperature 51 , 52 . Therefore, the V OC decreases with increasing temperature and the temperature-induced change in Δ V OC becomes more negative with increasing temperature (Fig. 13 ; blue data), in contrast to the Δ V OC due to the IR-induced intraband transitions (Fig. 13 ; red data). Because IR-induced intraband transitions are optical processes, the Δ V OC is positive (the quasi-Fermi-level splitting increases as discussed in Excitation Power-Dependence of the Open-Circuit Voltage). Consequently, the two Δ J SC –Δ V OC data sets have a different origin for the observed Δ J SC , and the positive Δ V OC values obtained using sub-bandgap photons confirms the presence of adiabatic optical excitation at HI-I 10 , 44 . Conclusions We have presented current and voltage data of a CsPbBr 3 /GaAs-based TPU-SC under different excitation conditions. The CsPbBr 3 perovskite layer was grown using a solution-based method under ambient conditions. The EQE measured with and without additional illumination with sub-bandgap photons revealed clear absorption edges corresponding to the semiconductors in this SC. The enhancement of the photocurrent and photovoltage observed in the case of additional illumination with sub-bandgap photons confirms a TPU process at the CsPbBr 3 /GaAs heterointerface. This distinguishes the effect of optically induced intraband transitions from the effect of elevated temperatures. We elucidated the carrier dynamics in the SC by investigating the influences of the temperature as well as the power densities of the two laser beams used to induce interband and intraband transitions. Despite the observation of a TPU process in this SC, the efficiency of this SC is still too low for applications. The optimization of the CsPbBr 3 quality may lead to better results. Furthermore, the large difference between the electron affinities of CsPbBr 3 and ZnO might have led to an open-circuit voltage reduction 53 . To overcome this problem, it has been proposed to use AlGaN as the ETL 54 . The TPU process observed at the CsPbBr 3 /GaAs heterointerface in this work is a first step in studying perovskite/III–V semiconductor interfaces. We believe that the observation of the TPU process at the CsPbBr 3 /GaAs heterointerface is important for the future development of TPU-SCs, and hence, high-efficiency single junction SCs. Methods Fabrication of CsPbBr 3 /GaAs-Based TPU-SCs We fabricated the TPU-SC as follows: First, a 1-inch p-doped GaAs substrate (001) was washed in acetone, methanol, isopropanol, and deionized water (in each step, sonication was performed for 15 min, and the whole cleaning process is referred to as the AMID process). Then, we deposited Au-Zn/Au electrodes at the rear surface of the GaAs substrate. The deposition of Au-Zn was done using a thermal evaporator. We deposited about 200 nm of Au-Zn, and this was followed by 500 nm of Au. To improve the ohmic characteristics of the p-GaAs/Au-Zn/Au contact, we annealed the substrate at 430°C for 2.5 min under N 2 gas. The substrate was washed again using the AMID process. Then, the substrate was treated with hydrofluoric acid for 20 s to remove the native oxide layer on the surface. After that, the substrate was cleaned in a UV/O 3 cleaner for 10 min. We chose a conventional multi-step spin-coating deposition technique to grow the \(\text{CsPbB}{\text{r}}_{\text{3}}\) layer, since this allows us to easily prepare a perovskite layer with high crystallinity and good surface coverage under ambient conditions 19 , 21 , 28 . We prepared two precursor solutions for the \(\text{CsPbB}{\text{r}}_{\text{3}}\) layer: (A) 1.00 M of lead (II) bromide (PbBr 2 , Tokyo Chemicals Industry) was dissolved in N,N -Dimethylformamide (DMF, Sigma-Aldrich), and (B) 0.07 M of cesium bromide (CsBr, Tokyo Chemicals Industry) was dissolved in a mixture of methanol (FUJIFILM Wako Pure Chemical Corp.) and deionized water with a ratio of 5:1. Both precursor solutions were stirred at 90°C for 30 min until the solution became clear, and then both solutions were filtered using microporous filters to remove microparticles and microcrystals. The precursor solution A was drop-casted on the cleaned p-GaAs substrate until the surface was covered with the solution. Then, the substrate was spun at 2000 rpm for 30 s. Five seconds after the start of the spin-coating process, 100 µL of chlorobenzene (FUJIFILM Wako Pure Chemical Corp.) was dropped on the spinning substrate. Subsequently, the substrate was heated at 90°C for 30 min under ambient conditions. Then, precursor solution B was drop-casted on the substrate. The substrate was again spun at 2000 rpm for 30 s, and then heated at 250°C for 5 min. As a result of the optimization of the number of deposition cycles of precursor B, we repeated this process four times to achieve a high-purity CsPbBr 3 perovskite layer and to suppress the formation of CsPb 2 Br 5 and Cs 4 PbBr 6 (it has been reported that the formation of the latter two phases depends on the concentration of PbBr 2 and CsBr 21 , and we optimized the concentration of CsBr by using several the deposition cycles). The high purity of the CsPbBr 3 perovskite phase can be confirmed in the X-ray diffractograms shown in the Supplementary Information (the optimization of CsBr deposition cycles was done using glass substrates). We also confirmed the formation of the CsPbBr 3 perovskite phase by photoluminescence (PL) spectroscopy. The PL spectrum in the Supplementary Information shows a PL peak at about 535 nm, which coincides with the peak position reported in previous publications 19 , 21 , 37 , 43 . The ZnO ETL was deposited on the CsPbBr 3 /p-GaAs substrate by sputtering. The ZnO target was purchased from Furuuchi Chemical. The background pressure inside the chamber was at 7.3×10 − 4 Pa, and for the sputtering process we first supplied a mixture of Ar and \({\text{O}}_{\text{2}}\) (with a 1:1 ratio) to the chamber until the total pressure reached 4.5 to 5 Pa. Then, an Ar:O 2 plasma was generated by applying an electric power of 50 W and we gradually increased the applied power to 100 W while the Ar:O 2 pressure was gradually reduced to 1.3 Pa. After the pressure reached 1.3 Pa, the deposition was continued for 5 min. With this procedure, an approximately 130-nm-thick ZnO ETL is obtained. To finalize the TPU-SC device, a 450-nm-thick Ag electrode was deposited on the top of the ZnO layer. The electrode was patterned using a metal mask with an aperture size of 3.0 mm × 3.0 mm. External Quantum Efficiency Measurements To measure the external quantum efficiency (EQE), we employed a tungsten-halogen lamp as a light source for interband excitation. This lamp provides a broad spectrum, and the wavelength-integrated power density was 0.5 mW/cm 2 , which is much smaller than the solar irradiance. The lamp was combined with a 140-mm single monochromator to select specific wavelengths for excitation (note that the output beam intensity depends on the wavelength). Furthermore, an optical chopper with a chopping frequency of 250 Hz was inserted into the excitation path. This monochromatic light was used to induce interband transitions (the beam spot diameter on the SC surface was 1.2 mm). The short-circuit current was amplified by a current amplifier and detected by a lock-in amplifier synchronized to the optical chopper. Moreover, a fixed fraction of the excitation light was detected by a Si photodetector to estimate the number of incident photons at a given wavelength. This setup is for the EQE measurements under single-color excitation conditions, which were performed at room temperature without a temperature controller. For the EQE measurements under two-color excitation conditions, we additionally illuminated the device with 1319-nm photons generated by a continuous-wave (CW) solid-state laser. These infrared (IR) photons can induce intraband transitions in addition to the interband transitions induced by the light from the tungsten-halogen lamp. The power density of the IR laser beam was set to P Intra = 86 mW/cm 2 by a variable neutral-density filter (hereafter, P Intra is always used to refer to the power density of the 1319-nm laser beam). The beam spot diameter on the SC surface was 1.2 mm, and the excitation spot coincided with that for interband excitation, since we employed a single optical fiber to guide the excitation light to the sample. Such a two-color excitation condition should provide a larger photocurrent and hence a better EQE due to the following mechanism: First, the photons generated by the tungsten-halogen lamp induce interband transitions in either of the three semiconductor layers in the device or even all, depending on the photon energy. For example, 700-nm photons will not be absorbed in the ZnO and CsPbBr 3 layers and thus induce interband transitions only in the GaAs layer. The photogenerated electrons in the CB of GaAs mainly stay at the CB minimum. A part of these electrons will be trapped at the interface states at HI-I. In the case of single-color excitation conditions, the electrons can overcome the potential barrier of CsPbBr 3 by thermal activation (and are then collected at the Ag electrode). On the other hand, due to the additional IR photons in the case of two-color excitation conditions, the electrons can also be optically excited to the CB of CsPbBr 3 . This process provides an additional photocurrent (it reduces the probability of trap-assisted recombination at HI-I). The details of the carrier dynamics will be discussed in Section 3. To determine the temperature-dependence of the EQE spectrum, we used the same experimental setup as for the single-color excitation, but here the SC was installed in a cryostat to control the temperature. The EQE signals were recorded without the IR light in order to observe the effect of the temperature. The measurements were performed in the temperature range 180–300 K, because the photocurrent generated by the SC at temperatures lower than 180 K can hardly be detected. Current–Voltage ( JV ) Characteristics The JV curves were measured using three different excitation conditions: (i) no illumination, (ii) illumination with 784-nm photons from a CW solid-state laser (single-color excitation), and (iii) simultaneous illumination with 784-nm photons and 1319-nm photons (two-color excitation). The 784-nm laser light had a photon flux of 4.0×10 21 m – 2 s – 1 , which corresponds to P Inter = 101 mW/cm 2 (hereafter, P Inter is always used to refer to the power density of the 784-nm laser beam). The 784-nm photons induce interband transitions only in the GaAs layer, and the photogenerated electrons can accumulate at HI-I and can then be thermally or optically excited to the CB of CsPbBr 3 . The 1319-nm laser light had a photon flux of 5.7×10 21 m – 2 s – 1 ( P Intra = 86 mW/cm 2 ), and these IR photons are used to optically excite the accumulated electrons. The measurement was done at room temperature without controlling the temperature. The JV curves were recorded using a source measure unit (Keithley 2400). Excitation Power-Dependence of Δ J SC and Δ V OC The IR-induced gain in the short-circuit current (Δ J SC ) and the IR-induced gain in the open-circuit voltage (Δ V OC ) are important parameters for TPU-SCs. They are defined as \(\text{∆}{\text{J}}_{\text{SC}}\text{ =}{\text{ }\text{J}}_{\text{SC, with infrared}}\text{ – }{\text{J}}_{\text{SC, without infrared}}\) and \(\text{∆}{\text{V}}_{\text{OC}}\text{ = }{\text{V}}_{\text{OC, with infrared}}\text{ – }{\text{V}}_{\text{OC, without infrared}}\) . The Keithley 2400 source measure unit was employed to record the short-circuit current ( J SC ). First, we measured the J SC for single-color excitation as a function of P Inter . Then, we measured the J SC values under two-color excitation conditions. This allows us to determine Δ J SC as functions of both P Inter and P Intra . The intensities of the laser beams were controlled by two variable neutral-density filters. The values of Δ V OC were obtained by measuring the V OC values under single- and two-color excitation conditions, but the measurement of V OC is slightly different from that of J SC . In the case of the J SC measurement, the data was directly recorded by the source measure unit. On the other hand, the V OC was determined by the following procedure: First, we applied a voltage where the current is approximately zero. Then, we used the JV data to estimate the values of V OC using a linear least-squares method to estimate linear functions defined by the slope and the intercept on the y -axis. By obtaining these two parameters for each measured JV dataset, we can estimate the V OC values, which are equal to the intercepts of the linear functions on the x -axis. As a result, the Δ V OC can be estimated as functions of P Inter and P Intra . Both types of measurements were conducted at room temperature without using a temperature controller. To verify the influence of the temperature on the SC operation, we measured the J SC as a function of the temperature for single-color excitation with P Inter = 428 mW/cm 2 (a photon flux of 1.69×10 22 m – 2 s – 1 ). The beam spot diameter on the SC was 1.2 mm. The SC was installed in the (evacuated) cryostat and we changed the temperature in the range from 280 to 300 K. The analysis was done using the Arrhenius equation, because we consider that the accumulated electrons overcome the heterointerface by thermionic emission. Similarly, we also measured the temperature-induced changes in J SC and V OC under single-color excitation conditions. The same experimental system as for the temperature dependence of J SC was employed, and P Inter was 450 mW/cm 2 . Note that for this experiment (which is discussed in Section 3.6), the parameters Δ J SC and Δ V OC are defined as \(\text{∆}{\text{J}}_{\text{SC}}\text{ =}{\text{ }\text{J}}_{\text{SC, at high temperature}}\text{ – }{\text{J}}_{\text{SC, at reference temperature}}\) and \(\text{∆}{\text{V}}_{\text{OC}}\text{ = }{\text{V}}_{\text{OC, at high temperature}}\text{ – }{\text{V}}_{\text{OC, at reference temperature}}\) (the reference temperature is 282 K). Therefore, in this experiment, Δ J SC and Δ V OC express the changes in the photocurrent and the open-circuit voltage excluding the effect of the IR light (the TPU process). Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Declarations Competing interests The authors declare no competing financial interest. Additional Information The Supplementary Information is available free of charge at https://doi/XXXX . X-ray diffractograms of CsPbBr 3 deposited on GaAs, photoluminescence spectrum of CsPbBr 3 deposited on GaAs for four deposition cycles, cross-sectional scanning electron microscopy image of the CsPbBr 3 /GaAs-based TPU-SC, and the calculation of the total amount of 500-nm photons absorbed in each layer. Author Contribution Hambalee Mahamu (H. M.) conceptualized this project. The manuscript was written by H. M. under the revision provided by Shigeo Asahi (S. A.) and Takashi Kita (T. K.). The main financial support for this project was acquired by H. M. while S. A. and T. K. were responsible for additional financial support. Acknowledgements One of the authors (Hambalee Mahamu) would like to thank the Graduate School of Engineering, Kobe University, for financial support through a scholarship. He gratefully acknowledges the Sasakura Enviro-Science Foundation for research support. 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ZnO electron transporting layer engineering realized over 20% efficiency and over 1.28 V open-circuit voltage in all-inorganic perovskite solar cells. EcoMat 4, 1–11 (2022). Hombe, A., Saiki, S., Mori, T., Saito, Y. & Tanimoto, T. AlGaN as an electron transport layer for wide-bandgap perovskite solar cells. Jpn. J. Appl. Phys. 62, (2023). Additional Declarations No competing interests reported. Supplementary Files SupplementaryInformationFinalised.docx Cite Share Download PDF Status: Published Journal Publication published 06 Nov, 2024 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 30 Sep, 2024 Reviews received at journal 09 Sep, 2024 Reviewers agreed at journal 05 Sep, 2024 Reviews received at journal 30 Jun, 2024 Reviewers agreed at journal 24 Jun, 2024 Reviewers agreed at journal 20 Jun, 2024 Reviewers invited by journal 20 May, 2024 Editor assigned by journal 20 May, 2024 Editor invited by journal 08 May, 2024 Submission checks completed at journal 08 May, 2024 First submitted to journal 03 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4362355","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":301397099,"identity":"304748bd-dfc6-4d2e-9707-928271612345","order_by":0,"name":"Hambalee Mahamu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABC0lEQVRIie3QMUvEMBjG8bcE2qW9rm/pl0gI1KXgV2mWuogOwtFBJEeltxy4VugHcUwp1KXc3LHgesOBow4GlQORlHOTI/8lQ/gRngDYbP+wkHgK9kUKIRAJ4AIF/LxwjSRab8SqHnKMpHMkoYNiZXDfIVU/yUxjdjEFMr/jz+3qxV8CD+NSER8WVybh1KKV0VOKySBK7m8hiZo+08S9MRGCmZBMb0lGp4qDClI6Xp69aSKkgbiYMSmqDnl9INd7Mkd8XzHZakLxiyT6FZgl6G3EoxzyqNZbWLNFrrdQp6HmLeedp17fizQM11077ZYpe4jLCXZFb/yxX69+n7TPjiWHbv9ObDab7VT7AKTAVQiz5jPLAAAAAElFTkSuQmCC","orcid":"","institution":"Graduate School of Engineering, Kobe University","correspondingAuthor":true,"prefix":"","firstName":"Hambalee","middleName":"","lastName":"Mahamu","suffix":""},{"id":301397100,"identity":"bddb1089-e22a-4151-912e-9beb07891042","order_by":1,"name":"Shigeo Asahi","email":"","orcid":"","institution":"Graduate School of Engineering, Kobe University","correspondingAuthor":false,"prefix":"","firstName":"Shigeo","middleName":"","lastName":"Asahi","suffix":""},{"id":301397101,"identity":"669554c8-78bb-4efb-a281-b2bc50d2d091","order_by":2,"name":"Takashi Kita","email":"","orcid":"","institution":"Graduate School of Engineering, Kobe University","correspondingAuthor":false,"prefix":"","firstName":"Takashi","middleName":"","lastName":"Kita","suffix":""}],"badges":[],"createdAt":"2024-05-03 06:53:02","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4362355/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4362355/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-78257-x","type":"published","date":"2024-11-06T15:56:50+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":56501607,"identity":"18b151ef-ee30-4272-8a52-1b311157ec74","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":173235,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic illustration of the CsPbBr\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/GaAs-based TPU-SC and its band diagram.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) The structure of the TPU-SC fabricated in this work. The p-GaAs substrate serves as the p-layer (the hole transport layer), and ZnO serves as the n-layer (the electron transport layer) in this device. (\u003cstrong\u003eb\u003c/strong\u003e) The energy band diagram of this device at short-circuit condition obtained using the COMSOL Multiphysics® software. The CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs and ZnO/CsPbBr\u003csub\u003e3\u003c/sub\u003e heterointerfaces are indicated by the dashed lines with the labels HI-I and HI-II, respectively.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/c307815fb008e13d19f5403d.png"},{"id":56501617,"identity":"50123688-a270-4f0e-8c79-bd7dec170259","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":221026,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTwo-dimensional maps of the ideal conversion efficiency. \u003c/strong\u003eThe maps show the ideal conversion efficiency\u003cstrong\u003e \u003c/strong\u003eunder 1-sun illumination with the AM 1.5G spectrum for different ratios of the CB and VB discontinuities. (\u003cstrong\u003ea\u003c/strong\u003e) Results for Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e = 3:2. (\u003cstrong\u003eb\u003c/strong\u003e) Results for Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB \u003c/sub\u003e= 7:1. \u0026nbsp;\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/3e8d71f87f090701de6d57da.png"},{"id":56503856,"identity":"4b5a7648-c021-442e-8b4a-fb9c515a43dc","added_by":"auto","created_at":"2024-05-15 04:40:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":18382,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe EQE spectra\u003c/strong\u003e \u003cstrong\u003eof the CsPbBr\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/GaAs-based TPU-SC.\u003c/strong\u003e The spectra were obtained with and without illumination with IR photons (\u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra \u003c/sub\u003e= 86 mW/cm\u003csup\u003e2\u003c/sup\u003e) shown by the dark red and blue curves, respectively. The band edges of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs are indicated by the vertical dashed lines.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/a4accc4f5d2afcee12ec6637.png"},{"id":56501990,"identity":"8ce49648-4d2e-4185-89a4-7312fba43ed0","added_by":"auto","created_at":"2024-05-15 04:16:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":15054,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTemperature-dependence of the EQE of the CsPbBr\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/GaAs-based TPU-SC for single-color excitation.\u003c/strong\u003e The data reveals a clear bandgap shift of GaAs. The white dashed lines at 530 nm and 870 nm indicate the band edges of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs at room temperature, respectively.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/4ec9efdc49cdc731cbaa5da1.png"},{"id":56501987,"identity":"9ee9aaa2-e214-48fa-8cb9-87bdfe595427","added_by":"auto","created_at":"2024-05-15 04:16:59","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":17751,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCurrent–voltage (\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJV\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e) characteristics the CsPbBr\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/GaAs-based TPU-SC. \u003c/strong\u003eThe \u003cem\u003eJV\u003c/em\u003e curves were measured under different excitation conditions at room temperature. The black points indicate the \u003cem\u003eJV\u003c/em\u003e characteristics in the dark. The blue and dark red points show the data for single-color excitation with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter \u003c/sub\u003e= 101 mW/cm\u003csup\u003e2\u003c/sup\u003e and two-color excitation with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter \u003c/sub\u003e= 101 mW/cm\u003csup\u003e2\u003c/sup\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 86 mW/cm\u003csup\u003e2\u003c/sup\u003e, respectively.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/f7bbdc5b603430acd76679f0.png"},{"id":56501618,"identity":"9d19775c-37da-4505-8098-b8be4cedcc2f","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":11461,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eSC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e for single-color excitation as a function of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/bca52c4b3613959b960f6c57.png"},{"id":56501606,"identity":"80815b96-d8bf-4989-b736-14f6276edc69","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":27634,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe IR-induced gain in the short-circuit current (Δ\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eSC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) for two-color excitation as a function of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eIntra\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e for a constant 1319-nm photon flux. (\u003cstrong\u003eb\u003c/strong\u003e) Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e for a constant 784-nm photon flux.\u003c/p\u003e","description":"","filename":"Fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/18ae55d20eabd8125100bd0c.png"},{"id":56502355,"identity":"f3ea7bf8-1ad3-4257-8adf-be7fe42ff00f","added_by":"auto","created_at":"2024-05-15 04:24:59","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":17515,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe IR-induced gain in \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eSC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e as functions of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eIntra\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Fig8.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/cfc591e1be34a7f1a381d2c3.png"},{"id":56503308,"identity":"42824301-a350-48d6-beca-b7139e078d71","added_by":"auto","created_at":"2024-05-15 04:32:59","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":8624,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe open-circuit voltage (\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eOC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) for single-color excitation as a function of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Fig9.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/ba123b80323b715156c22114.png"},{"id":56501611,"identity":"a44301a1-8f26-4fa9-ab2a-2de0318bd1c1","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":19757,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe IR-induced gain in the open-circuit voltage (Δ\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eOC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) for single-color excitation as a function of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eand \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eIntra\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e for a constant 1319-nm photon flux. (\u003cstrong\u003eb\u003c/strong\u003e) Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e\u003cem\u003e \u003c/em\u003eas a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e for a constant 784-nm photon flux.\u003c/p\u003e","description":"","filename":"Fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/e82386ad6e1ea36afeaa8848.png"},{"id":56501992,"identity":"8721e557-2f81-44b8-bec8-05fa36e1e550","added_by":"auto","created_at":"2024-05-15 04:16:59","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":17046,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe IR-induced gain in the open-circuit voltage (Δ\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eOC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) as functions of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eIntra\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/e490bdfd30f528670af0cf66.png"},{"id":56501615,"identity":"d46a7119-1be8-4ef1-a6a2-eee54bfcf34b","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":16293,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTemperature dependence of the \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eSC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e for single-color excitation with \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eP\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eInter \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e= 428 mW/cm\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e.\u003c/strong\u003e \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e and \u003cem\u003eλ\u003c/em\u003e\u003csub\u003eInter \u003c/sub\u003eare the estimated thermal activation energy and the excitation wavelength, respectively.\u003c/p\u003e","description":"","filename":"Fig12.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/f16c117a452d8b3469435bbc.png"},{"id":56503310,"identity":"605d3748-1251-4191-90b5-90d1a378cd9f","added_by":"auto","created_at":"2024-05-15 04:32:59","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":12952,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of Δ\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eSC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e–Δ\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003eOC\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e data sets.\u003c/strong\u003e The data was obtained either at room temperature as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e (the dark red points) or as a function of the temperature for \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 0 (the blue points).\u003c/p\u003e","description":"","filename":"Fig13.png","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/b405213a90ae9a6a7f687cbd.png"},{"id":68749650,"identity":"ba008eea-4c8c-46a1-9e4e-221ab1de110c","added_by":"auto","created_at":"2024-11-11 15:59:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1773651,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/43193af1-3c88-428c-a5cc-fce2adf5c239.pdf"},{"id":56501619,"identity":"e544db20-8ba7-4328-852c-600f53152a65","added_by":"auto","created_at":"2024-05-15 04:08:59","extension":"docx","order_by":15,"title":"","display":"","copyAsset":false,"role":"supplement","size":1114560,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformationFinalised.docx","url":"https://assets-eu.researchsquare.com/files/rs-4362355/v1/8ccd216a1a9534426d0e41ed.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePhotovoltaics is the field of science concerned with the conversion of photons to electrical energy. To characterize the physics of a considered photovoltaic device, usually, several parameters including the energy conversion efficiency (the ratio of the output electrical power to the power provided by the incident light) are used. The energy conversion efficiency of a solar cell (SC) can be estimated using thermodynamic approaches by considering the energy generation in the SC through absorption and the energy losses during the energy conversion process. The \u0026ldquo;unavoidable\u0026rdquo; losses including the transmission of sub-bandgap photons, thermalization of high-energy carriers, the Carnot loss, the Boltzmann loss, and emission losses can be taken into account to accurately estimate the efficiency\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. By considering these losses, it is possible to derive an expression for the ideal energy conversion efficiency, known as the Shockley\u0026ndash;Queisser (SQ) limit or the detailed balance limit. The SQ limit predicts that the maximum conversion efficiency of a single junction SC is ~\u0026thinsp;31 % nder 1-sun illumination\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. To overcome the SQ limit, a SC design is needed that makes a part of the \u0026ldquo;unavoidable\u0026rdquo; losses \u0026ldquo;avoidable\u0026rdquo; (and suppresses them). For example, various SC designs have been proposed to reduce the transmission loss\u003csup\u003e\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, which plays an important role in SCs and is usually due to the small absorption coefficient of semiconductors for sub-bandgap photons.\u003c/p\u003e \u003cp\u003eIn 1997, the concept of the intermediate-band solar cell (IBSC) was proposed to harvest energy in the region below the bandgap of the absorber layer of a single-junction SC\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. An IBSC contains additional real states between the conduction band (CB) and the valence band (VB) of the absorber layer, which are called intermediate band (IB) states and can be realized by using quantum structures or by incorporating impurities\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. The enhancement of absorption in IBSCs is caused by a sequential excitation of electrons from the VB to the IB and from the IB to the CB using sub-bandgap photons (in addition to the direct band-to-band transitions). Because the band-to-band transitions are independent of the excitation via the IB, an IBSC is equivalent to a parallel circuit: one branch contains one diode corresponding to band-to-band transitions, and the other branch contains two diodes connected in series corresponding to the sequential absorption of sub-bandgap photons via the IB\u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11 CR12 CR13\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. As a result, an IBSC is not affected by current and voltage mismatches, which are problematic in the case of multi-junction solar cells\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. On the other hand, for IBSCs, a low density of states (DOS) at the IB levels is problematic, because this leads to weak carrier generation via the IB. Moreover, regarding the aspect of fabrication complexity, an IB needs a periodic long-range-ordered structure in order to create an electronic band. This requires complex fabrication processes, especially when the band is realized using quantum structures, e.g. quantum dots (QDs). Such a trade-off relation between fabrication complexity and efficiency hinders the commercialization of IBSCs. On the other hand, we demonstrated a so-called two-step photon upconversion solar cell (TPU-SC) based on Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs and GaAs including a single layer of InAs QDs at the heterointerface (the QDs were prepared by using the Stranski\u0026ndash;Krastanov growth mode)\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. The fabrication of such a TPU-SC is relatively simple compared to IBSCs.\u003c/p\u003e \u003cp\u003eA TPU-SC consists of a wide bandgap semiconductor (WGS) and a narrow bandgap semiconductor (NGS) to form a heterointerface, where intraband transitions can be induced by sub-bandgap photons. For the WGS and the NGS, we previously used Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs and GaAs, respectively. The presence of QDs at the heterointerface can enhance the intraband transitions at the heterointerface, because the three-dimensional confinement of the QD relaxes the optical selection rule and allows in-plane electronic transitions\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Consequently, in addition to the band-to-band (interband) excitation of the WGS and the NGS, intraband excitation can be achieved, which results in a photocurrent enhancement: The process responsible for the photocurrent enhancement is called two-step photon upconversion (TPU), because each additional electron is first excited to the NGS CB by an interband-transition step and then experiences an intraband-transition step to reach the WGS CB. Regarding the photovoltage, optically induced intraband transitions lead to an increase of the quasi-Fermi level of the electrons due to an increase in the electron density of the WGS CB. Thus, the TPU-SC design is able to improve the photocurrent and the photovoltage, and therefore, the SC efficiency. The maximum theoretical efficiencies of a TPU-SC with a CB discontinuity at the heterointerface that is 50% larger than the VB discontinuity are 46.7% and 63.2% under 1-sun and 10,000-sun illumination respectively\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Therefore, the TPU-SC concept is one of the SC concepts that may help to realize high-efficiency single-junction SCs.\u003c/p\u003e \u003cp\u003eAlthough design concepts for SCs with high theoretical conversion efficiencies are very important, the actually used semiconductors are important as well. For example, lead halide perovskite semiconductors have been widely studied due to their excellent optoelectronic properties and advantages in terms of fabrication, since they can be prepared relatively easily using low-cost fabrication techniques\u003csup\u003e\u003cspan additionalcitationids=\"CR19 CR20\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. However, stability is a concern, especially for organo-lead halide perovskites. These latter perovskites contain organic cations, e.g. methylammonium ions (MA\u003csup\u003e+\u003c/sup\u003e) or formamidinium ions (FA\u003csup\u003e+\u003c/sup\u003e), which are known for their chemical sensitivity to water molecules. This leads to a relatively fast degradation, and hence, the instability of organo-lead halide perovskite SCs under ambient conditions\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Several studies suggested the substitution of MA\u003csup\u003e+\u003c/sup\u003e and FA\u003csup\u003e+\u003c/sup\u003e with inorganic cations, i.e., cesium ions (Cs\u003csup\u003e+\u003c/sup\u003e), to create perovskites that are more stable (this substitution transforms an organic-cation-based perovskite into an all-inorganic perovskite), and significant stability improvements have been reported for perovskites that include Cs\u003csup\u003e+\u003c/sup\u003e instead of MA\u003csup\u003e+\u003c/sup\u003e or FA\u003csup\u003e+ 21,28\u003c/sup\u003e. There are three main types of Cs-based perovskites: CsPbCl\u003csub\u003e3\u003c/sub\u003e, CsPbBr\u003csub\u003e3\u003c/sub\u003e, and CsPbI\u003csub\u003e3\u003c/sub\u003e. Theoretical studies have predicted that CsPbI\u003csub\u003e3\u003c/sub\u003e can provide the highest conversion efficiency. In addition, CsPbI\u003csub\u003e3\u003c/sub\u003e has a small bandgap energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e) of 1.73 eV, and CsPbI\u003csub\u003e3\u003c/sub\u003e is currently the most studied Cs-based perovskite. While CsPbI\u003csub\u003e3\u003c/sub\u003e has certain advantages for applications, the perovskite phase of CsPbI\u003csub\u003e3\u003c/sub\u003e is only stable above temperatures of 300\u0026deg;C. As a result, CsPbI\u003csub\u003e3\u003c/sub\u003e tends to form undesired non-perovskite phases at room temperature, especially under ambient conditions, leading to significant difficulties for SCs operating at room temperature\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe main reason for the poor stability of CsPbI\u003csub\u003e3\u003c/sub\u003e under ambient conditions is the large ion radius of iodine, which accelerates chemical reactions. The optimum I/Br ratio in CsPbI\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eBr\u003csub\u003e3-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e for phase stability was identified by Sanchez et al., but the corresponding photovoltaic performance was rather limited\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. It has been shown that Cs-based perovskites with a higher iodine concentration contain a larger volume of the unstable α-CsPbI\u003csub\u003e3\u003c/sub\u003e phase\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Therefore, CsPbI\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eBr\u003csub\u003e3-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e compound perovskites degrade faster than CsPbBr\u003csub\u003e3\u003c/sub\u003e. Albeit CsPbBr\u003csub\u003e3\u003c/sub\u003e may be not the best choice for SC applications due to its wide bandgap (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 2.33 eV), it can be more suitable than CsPbCl\u003csub\u003e3\u003c/sub\u003e (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 3.00 eV) and CsPbI\u003csub\u003e3\u003c/sub\u003e. It has been reported that single-crystal CsPbBr\u003csub\u003e3\u003c/sub\u003e films exhibit a high electron mobility of 1000 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e (Vs)\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e with a lifetime of 2.5 ms\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The carrier diffusion length, which influences the carrier collection efficiency in a SC, is approximately 1 \u0026micro;m and 12 \u0026micro;m for electrons and holes, respectively\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, and the open-circuit voltage of CsPbBr\u003csub\u003e3\u003c/sub\u003e-based SCs can exceed 1.6 V\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. In addition, CsPbBr\u003csub\u003e3\u003c/sub\u003e has a high melting temperature of 570\u0026deg;C, which means a relatively low degradation rate at room temperature. CsPbBr\u003csub\u003e3\u003c/sub\u003e can form several perovskite crystal phases including the cubic α-phase, the tetragonal β-phase, and the orthorhombic γ-phase, depending on the environmental temperature. The formation of these three crystal phases is reversible; the orthorhombic γ-phase forms at room temperature but can be converted to the tetragonal β-phase and the cubic α-phase by heating the crystal up to 88\u0026deg;C (some studies reported a phase-transition temperature of ~\u0026thinsp;107\u0026deg;C) and 130\u0026deg;C, respectively\u003csup\u003e\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Although CsPbBr\u003csub\u003e3\u003c/sub\u003e exhibits three different crystal phases, their differences in terms of optoelectronic properties are not significant, and thus this material can be used in SCs over a wide range of operating temperatures. The extraordinary good optoelectronic properties are promising for the realization of high-efficiency SCs. New technological insights might be gained by fabricating a TPU-SC based on CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs to study the potential of the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface for TPU-SCs and intraband transitions in general.\u003c/p\u003e \u003cp\u003eIn this work, we fabricated a TPU-SC with CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs as the WGS and the NGS, respectively, and confirmed enhancements in the photocurrent and the photovoltage in the case of additional illumination with sub-bandgap photons. The wide bandgap of CsPbBr\u003csub\u003e3\u003c/sub\u003e (2.33 eV) is suitable for application in a TPU-SC together with GaAs (1.42 eV) due to the achieved ratio of the band discontinuities in the CB and VB\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. We used a QD-free structure to study the effect of the interface states on the TPU process. Furthermore, we simplified the fabrication technique; the growth of CsPbBr\u003csub\u003e3\u003c/sub\u003e was performed under ambient conditions. Although our CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC contains no QDs, we believe that the interface states behave like quantized electronic states, which allow intraband transitions if the SC is irradiated with sub-bandgap photons in addition to the light used for interband excitation of the absorber layers in this device. The excitation power-dependence of the short-circuit current and that of the open-circuit voltage revealed the carrier dynamics at the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-heterointerface as well as the quasi-Fermi-level splitting as a result of a TPU process. The effect of the temperature on the photocurrent and the photovoltage was investigated to distinguish the TPU process from thermal activation.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eDevice Structure of the TPU-SC\u003c/h2\u003e \u003cp\u003eThe fabricated TPU-SC consists of an n\u0026ndash;i\u0026ndash;p CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterojunction. The device structure is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. For the WGS, we grew a CsPbBr\u003csub\u003e3\u003c/sub\u003e layer (shown in green), and for the NGS, we used a 1-inch p-doped GaAs substrate (001) (shown in black). Zinc oxide (ZnO) was used as the electron transport layer (ETL). We denote the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface as HI-I, and the ZnO/CsPbBr\u003csub\u003e3\u003c/sub\u003e heterointerface as HI-II. The band diagram of the ZnO/CsPbBr\u003csub\u003e3\u003c/sub\u003e/p-GaAs structure was simulated using the COMSOL Multiphysics\u0026reg; software, and the result is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. A TPU process is expected to occur at HI-I after the photogenerated electrons in the CB of GaAs have accumulated at the interface states. Results of the TPU process occurring at HI-I are discussed in the following sections.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eThe Ideal Energy Conversion Efficiency\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows two maps of the theoretical conversion efficiency as functions of the NGS and WGS bandgaps for two different band-offset configurations. Here, the band-offset configuration is expressed by the ratio of the CB and VB discontinuities (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e). For the prediction of the efficiency limit under 1-sun illumination, we used the detailed balance approach\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e and the AM 1.5G spectrum. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the data for Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e = 3:2. This value of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e is nearly equal to that of an Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs heterojunction\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The highest conversion efficiency is 46.4%, which is achieved by using NGS and WGS bandgap energies of \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg, NGS\u003c/sub\u003e \u0026asymp; 1.60 eV and \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg, WGS\u003c/sub\u003e \u0026asymp; 3.10 eV, respectively. Therefore, GaAs (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 1.42 eV) is a reasonable good choice for the NGS. However, the optimal WGS bandgap is very large, and thus Al\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eGa\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eAs is not the best choice for the WGS of such a high-efficiency TPU-SC (the bandgap energy of Al\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eGa\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eAs is only about 2.20 eV even for \u003cem\u003ex\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.9). Note that Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 1.81 eV) was used in the previously reported TPU-SCs. Furthermore, although a larger Al molar fraction \u003cem\u003ex\u003c/em\u003e provides a larger bandgap, Al\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eGa\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eAs switches from a direct bandgap semiconductor to an indirect bandgap semiconductor when \u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;0.45\u003csup\u003e41\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe conversion efficiency map for Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e = 7:1 is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. Since the electron affinity (\u003cem\u003eχ\u003c/em\u003e) and the \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e values of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs are known, we can estimate the barrier heights for both the CB and the VB: The \u003cem\u003eχ\u003c/em\u003e values of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs are 3.30 eV and 4.07 eV, respectively\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Therefore, the ratio of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e to Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e becomes 7:1 (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e = 0.77 eV and Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e = 0.11 eV). Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb indicates that the maximum conversion efficiency is 48.5% for \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg, NGS\u003c/sub\u003e \u0026asymp; 1.50 eV and \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg, WGS\u003c/sub\u003e \u0026asymp; 2.60 eV. This efficiency maximum is 2.1% higher than that for Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eCB\u003c/sub\u003e:Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVB\u003c/sub\u003e = 3:2. Furthermore, we find that CsPbBr\u003csub\u003e3\u003c/sub\u003e is suitable for the WGS of such a TPU-SC, because CsPbBr\u003csub\u003e3\u003c/sub\u003e perovskite has an \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e of 2.33 eV. In comparison to the Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs-based TPU-SC, the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SCs provide a higher maximum conversion efficiency in addition to the exceptional optoelectronic properties of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eEQE Spectra\u003c/h3\u003e\n\u003cp\u003eThe room-temperature EQE spectra of our CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The blue curve represents the EQE spectrum measured under single-color excitation conditions, which means that carrier generation occurs mainly through interband excitation. The EQE signal gradually increases as the excitation wavelength decreases. At wavelengths longer than ~\u0026thinsp;900 nm, there is no EQE signal, because these photons cannot excite any semiconductor in this device. The EQE signal shows a sharp onset when the excitation wavelength is about 900 nm, where GaAs starts to absorb the photons. The signal reaches a plateau at about 870 nm, which corresponds to the GaAs bandgap. As a result of the built-in electric field, the photogenerated electrons in the CB of GaAs (generated by the photons in the wavelength range from approximately 530 to 870 nm) drift toward HI-I, while the photogenerated holes drift to the rear electrode (the holes are not influenced by HI-I). The electrons are trapped at the interface states of HI-I and accumulate there. These accumulated electrons partially recombine with the holes that reach HI-I by diffusion. We described this scenario in previous publications\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. A part of the electrons at HI-I can overcome the energy barrier by thermal excitation (evidence for thermionic emission is provided in Influence of the Temperature). At an excitation wavelength of about 530 nm, the EQE signal abruptly increases, because the corresponding photon energy is high enough to induce interband transitions in both CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs. As the wavelength becomes shorter, the photon energy becomes higher than the CsPbBr\u003csub\u003e3\u003c/sub\u003e bandgap energy and the photons are stronger absorbed by the CsPbBr\u003csub\u003e3\u003c/sub\u003e layer (the number of photons that reach the GaAs layer becomes smaller). Since the holes generated in the CsPbBr\u003csub\u003e3\u003c/sub\u003e VB have a longer path to the rear electrode, which implies a larger recombination probability, the EQE should decrease if the excitation wavelength is reduced further.\u003c/p\u003e \u003cp\u003eThe dark red curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e represents the EQE spectrum measured under two-color excitation conditions. The comparison with the data for single-color excitation reveals that the EQE in the wavelength range between the CsPbBr\u003csub\u003e3\u003c/sub\u003e bandgap and the GaAs bandgap (~\u0026thinsp;530\u0026ndash;870 nm) decreases when the device is additionally irradiated with the IR photons, and in the wavelength range below 530 nm (the CsPbBr\u003csub\u003e3\u003c/sub\u003e bandgap), the IR photons lead to an enhancement of the EQE signal. In contrast, our previous work on TPU-SCs with a single heterointerface showed an IR-induced EQE enhancement in the range between the WGS and NGS, and no enhancement in the short-wavelength range. In particular, in an Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs-based TPU-SC, there is no IR-induced EQE enhancement when the wavelength of the photons for interband excitation is shorter than 680 nm (the Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs bandgap). This result is attributed to the fact that high-energy photons are strongly absorbed by the WGS,\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e which causes a reduction of the number of photons that reach the NGS layer, leading to a reduction of the electron density at the heterointerface. As a result, the IR-induced photocurrent enhancement gradually disappears as the wavelength of the photons for interband excitation becomes shorter; if there are no photogenerated electrons in the NGS CB, the IR laser beam has no effect, because the photogenerated electrons in the WGS CB do not accumulate at the energy barrier\u003csup\u003e\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo clarify the behavior of our CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC, we consider our previous results on a double-heterointerface TPU-SC with an additional Al\u003csub\u003e0.7\u003c/sub\u003eGa\u003csub\u003e0.3\u003c/sub\u003eAs (~\u0026thinsp;2.50 eV) layer grown on the Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs (1.79 eV/1.42 eV) structure: this device exhibited an IR-induced EQE improvement in the short-wavelength range\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Hence, the IR-induced EQE enhancement observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e at wavelengths below 530 nm is likely due to the interface states of HI-II. To support this interpretation, we estimated the fraction of 500-nm light absorbed in each layer of the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC (the calculation details are explained in Section S4 of the Supplementary Information). Our estimation reveals that photons with such short wavelengths do not induce electron accumulation at HI-I. Furthermore, in this measurement, non-linear phenomena such as multi-photon absorption in CsPbBr\u003csub\u003e3\u003c/sub\u003e are negligible, because we used a tungsten-halogen lamp and a CW laser for the excitation, i.e., light sources with a relatively low energy density. Therefore, we believe that the IR-induced EQE enhancement in the short-wavelength regime is caused by electron accumulation at HI-II.\u003c/p\u003e \u003cp\u003eConversely, it has been found that the EQE signal in the wavelength region between the bandgaps of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs reduces with the IR irradiation. The change in EQE with the IR irradiation strongly depends on the excitation power density. We discuss the details in Excitation Power-Dependence of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the temperature dependence of the EQE spectrum for single-color excitation (the wavelength of the photons for interband excitation is shown on the \u003cem\u003ex\u003c/em\u003e-axis, and the temperature is shown on the \u003cem\u003ey\u003c/em\u003e-axis). At temperatures below 180 K, the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC hardly produces photocurrent due to the CB discontinuity at HI-I: The difference between the electron affinities \u003cem\u003eχ\u003c/em\u003e of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs provides a barrier height of 0.77 eV at HI-I. Since the thermal activation energy at 297 K is only 0.026 eV, the electrons accumulated at the heterointerface can hardly overcome the barrier at much lower temperatures. In this measurement, the temperature governs the electronic excitation at HI-I, because 1319-nm (sub-bandgap) photons were not used. Consequently, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e confirms the thermal activation of electrons accumulated at HI-I, which plays an important role in the photocurrent generation when sub-bandgap photons are absent\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e also reveals a clear shift of the absorption edge of GaAs: it shows a blueshift with decreasing temperature due to the well-known effect responsible for the temperature-dependent changes in the bandgaps of III\u0026ndash;V semiconductors\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The absorption edge of CsPbBr\u003csub\u003e3\u003c/sub\u003e, on the other hand, is almost insensitive to the temperature within the measured range.\u003c/p\u003e \u003cp\u003eIt has been reported that the bandgap energy of lead bromide perovskites (\u003cem\u003eX\u003c/em\u003ePbBr\u003csub\u003e3\u003c/sub\u003e, with \u003cem\u003eX\u003c/em\u003e\u0026thinsp;=\u0026thinsp;FA\u003csup\u003e+\u003c/sup\u003e, MA\u003csup\u003e+\u003c/sup\u003e, or Cs\u003csup\u003e+\u003c/sup\u003e) decreases with decreasing temperature, in contrast to the behavior of III\u0026ndash;V semiconductors. The increase of the \u003cem\u003eX\u003c/em\u003ePbBr\u003csub\u003e3\u003c/sub\u003e bandgap energy with increasing temperature is attributed to changes in the crystal phase. In general, \u003cem\u003eX\u003c/em\u003ePbBr\u003csub\u003e3\u003c/sub\u003e perovskites can exist in various crystal phases including tetragonal and cubic phases. In the case of CsPbBr\u003csub\u003e3\u003c/sub\u003e, the orthorhombic phase can be observed easily at room temperature, because of its good stability, and as the temperature increases, phase changes occur. Mannino et al. reported that the \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e of CsPbBr\u003csub\u003e3\u003c/sub\u003e has a small dependence on the temperature in the range 200\u0026ndash;300 K\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. This phenomenon is caused by the thermal expansion of the orthorhombic phase. At about 380 K (107\u0026deg;C), the CsPbBr\u003csub\u003e3\u003c/sub\u003e bandgap changes abruptly due to the phase transition from the orthorhombic phase to the tetragonal phase (several investigations confirmed this phase transition already at 361 K\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e). This is in contrast to FAPbBr\u003csub\u003e3\u003c/sub\u003e and MAPbBr\u003csub\u003e3\u003c/sub\u003e, where a tetragonal\u0026ndash;cubic phase change occurs in the temperature range 200\u0026ndash;300 K, and results in an \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e change of CsPbBr\u003csub\u003e3\u003c/sub\u003e of approximately 0.02 eV in the range 200\u0026ndash;380 K\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Within the temperature range used for Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, CsPbBr\u003csub\u003e3\u003c/sub\u003e is in the orthorhombic phase. As a result, the absorption edge is almost constant 2.33 eV (the \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e change is within ~\u0026thinsp;0.008 eV).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eJV\u003c/b\u003e \u003cb\u003eCharacteristics\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThe \u003cem\u003eJV\u003c/em\u003e characteristics under different illumination conditions are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e; the black, blue and dark red points represent the \u003cem\u003eJV\u003c/em\u003e characteristics in the dark, under single-color excitation conditions (\u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 101 mW/cm\u003csup\u003e2\u003c/sup\u003e), and under two-color excitation conditions (\u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 101 mW/cm\u003csup\u003e2\u003c/sup\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 86 mW/cm\u003csup\u003e2\u003c/sup\u003e), respectively. Because the ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e layers are transparent for 784-nm photons, these photons are absorbed in the p-GaAs substrate and generate electrons and holes in the GaAs CB and VB, respectively. The photogenerated electrons drift toward HI-I, where they can accumulate at the interface states. The photogenerated holes drift to the rear electrode\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. While thermal excitation can in principle excite the electrons at HI-I to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e, thermal activation hardly occurs since the energy difference between the CB minima of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs is 0.77 eV, which is much higher than the thermal energy at room temperature (0.026 eV). This is the reason for the small magnitude of the detected photocurrent in the case of single-color excitation.\u003c/p\u003e \u003cp\u003eFor two-color excitation, a slight photocurrent and photovoltage enhancement due to the additional IR photons can be confirmed. The IR-induced gain in the short-circuit current (Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e) and the IR-induced gain in the open-circuit voltage (Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e) are 0.011 \u0026micro;A/cm\u003csup\u003e2\u003c/sup\u003e and 0.025 V respectively. Although this data at first seems inconsistent with the EQE results, both results are actually in agreement. To understand this, we need to consider that a tungsten-halogen lamp was employed for the EQE measurement, which only provided 6.0\u0026times;10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e mW/cm\u003csup\u003e2\u003c/sup\u003e at about 780 nm (a photon flux of 2.4\u0026times;10\u003csup\u003e16\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e). As shown later, intraband transitions hardly occur as long as \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is below 20 mW/cm\u003csup\u003e2\u003c/sup\u003e (a photon flux of 8.0\u0026times;10\u003csup\u003e20\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e). Therefore, the power density of the monochromatic beam for interband excitation used in the EQE measurement was too low. For the \u003cem\u003eJV\u003c/em\u003e measurements, we employed a CW solid-state laser operating at 784 nm, which provided a photon flux of 4.0\u0026times;10\u003csup\u003e21\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e for interband excitation.\u003c/p\u003e \u003cp\u003eThe behavior of the open-circuit voltage (\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e) is interpreted as follows: The \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e enhancement in the case of additional illumination with sub-bandgap photons (i.e., in the case of optically-induced intraband transitions) occurs because the 784-nm photons induce interband transitions only in the GaAs layer. When the electrons accumulated at HI-I are excited by the 1319-nm photons and reach the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e, the electron quasi-Fermi level shifts to a higher level and thereby increases the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e. Thus, the observed positive Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e evidences an adiabatic intraband excitation process at HI-I\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. In Excitation Power-Dependence of the Open-Circuit Voltage, we show how Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e depends on \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eExcitation Power-Dependence of\u003c/b\u003e \u003cb\u003eJ\u003c/b\u003e\u003csub\u003e\u003cb\u003eSC\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the dependence of the short-circuit current (\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e) on \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e for single-color excitation. The 784-nm photons excite the GaAs substrate and intraband transitions are not induced, because sub-bandgap photons are absent. The photogenerated electrons in the CB of GaAs drift toward HI-I, where electrons accumulate due to the CB discontinuity. To overcome the barrier, an energy is required that is nearly 30 times the thermal energy at room temperature. Therefore, the temperature is important for the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e under single-color excitation conditions. The effect of the temperature is discussed in Influence of the Temperature.\u003c/p\u003e \u003cp\u003eTo discuss the data in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, we fitted the data to a single power law, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{J}}_{\\text{SC}}\\text{ }\\text{\u0026prop;}\\text{ }{\\text{P}}_{\\text{Inter}}^{\\text{ n}}\\)\u003c/span\u003e\u003c/span\u003e. The estimated power exponent is \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.67, which significantly deviates from unity. The reason for this sublinear power dependence is the weak built-in electric field and the recombination with holes: The electrons generated in the GaAs CB drift toward HI-I due to a weak electric field. As the electrons accumulate at HI-I, the electric field becomes weaker, which increases the electron\u0026ndash;hole recombination rate. Therefore, the rate of thermal activation at HI-I decreases as \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e increases, leading to the observed sublinear relationship.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea shows the photocurrent gain under short-circuit conditions (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{∆}{\\text{J}}_{\\text{SC}}\\text{ =}{\\text{ }\\text{J}}_{\\text{SC, with infrared}}\\text{ \u0026ndash; }{\\text{J}}_{\\text{SC, without infrared}}\\)\u003c/span\u003e\u003c/span\u003e) as a function of the power density of the 784-nm laser beam in the case of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 440 mW/cm\u003csup\u003e2\u003c/sup\u003e. The Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e increases with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e, because stronger interband excitation provides a higher electron density at HI-I (the 784-nm photons induce interband transitions in GaAs, and the photogenerated electrons are transported to HI-I). The data can be well fitted using a single power law, Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{ }\\text{\u0026prop;}\\text{ }{\\text{P}}_{\\text{Inter}}^{\\text{ n}}\\)\u003c/span\u003e\u003c/span\u003e, with \u003cem\u003en\u003c/em\u003e = 0.83. Ideally, Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e should be proportional to the electron density at HI-I. Furthermore, the recombination in ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e (as well as that at HI-II) should be negligible, because the 784-nm photons induce interband transitions only in the GaAs layer, and thus there are no photogenerated holes in ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e. The sublinear characteristic, therefore, originates from a recombination within GaAs or at HI-I. While stronger interband excitation leads to a higher electron density at HI-I, this increased electron density reduces the built-in electric field, facilitating electron\u0026ndash;hole recombination within GaAs. Hence, the electron density at HI-I sublinearly increases with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e. This behavior was also observed in our previous reports\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb clarifies the dependence of Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e on \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e in the case of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 450 mW/cm\u003csup\u003e2\u003c/sup\u003e. This data cannot be fitted to Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{ }\\text{\u0026prop;}\\text{ }{\\text{P}}_{\\text{Intra}}^{\\text{ n}}\\)\u003c/span\u003e\u003c/span\u003e, because the data shows a gradual change of the power exponent, i.e., the value of \u003cem\u003en\u003c/em\u003e decreases as \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e is increased. In the low excitation regime, we find \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.26, which is the maximum value in the measured range of power densities. The minimum value is \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.11, observed at \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e ~ 450 mW/cm\u003csup\u003e2\u003c/sup\u003e. This reduction of \u003cem\u003en\u003c/em\u003e can be interpreted as a reduction of the carrier separation efficiency at HI-I. Since the 784-nm photons induce interband transitions only in the GaAs layer, recombination in ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e (including that at HI-II) can be ignored. Therefore, the observed reduction of the carrier separation efficiency occurs at HI-I. The origin of the reduction is a lower built-in electric field at higher electron densities in the CBs of ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e (and at HI-II), because the electrons that are optically excited to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e hardly drift to HI-II if the built-in field is weak. This means that the functionality of HI-I is limited by the electron densities in ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e13\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e as functions of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e-axis) and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e (\u003cem\u003ey\u003c/em\u003e-axis). The data scatters around zero and mostly shows negative values when \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is below ~\u0026thinsp;20 mW/cm\u003csup\u003e2\u003c/sup\u003e. When \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e exceeds 20 mW/cm\u003csup\u003e2\u003c/sup\u003e, an increase in \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e also leads to a remarkable increase in Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e. Note that, in the region below 20 mW/cm\u003csup\u003e2\u003c/sup\u003e, the data is independent of the intensity of the IR light. We interpret this behavior as follows: Since the electron density in the GaAs CB is mainly determined by the intensity of the photons used to induce interband transitions, the electron density remains low as long as \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is low. In addition, we used a p-GaAs substrate for the NGS layer (there is no i-GaAs layer), which makes it more difficult to increase the electron density compared to an Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs-based TPU-SC. Because the possibility of upconversion at HI-I is determined by the electron density at the initial energy levels for the intraband transitions, there is almost no Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e in the case of weak interband excitation, even at high values of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThe result of the negative Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e when \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is below ~\u0026thinsp;20 mW/cm\u003csup\u003e2\u003c/sup\u003e, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, agrees with the EQE results in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Since the photons used for interband excitation in our EQE measurement were generated by a tungsten-halogen lamp, the photon density was below the threshold. The power density of the monochromatic 784-nm beam generated by the tungsten-halogen lamp and the monochromator was approximately 6.0\u0026times;10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e mW/cm\u003csup\u003e2\u003c/sup\u003e, which is equivalent to 0.005 suns. In other words, the used power density was too small to produce a significant density of electrons at HI-I. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e needs to be higher than ~\u0026thinsp;20 mW/cm\u003csup\u003e2\u003c/sup\u003e to let a significant amount of electrons accumulate at HI-I. This result points out that electron accumulation at the heterointerface is a prerequisite for adiabatic intraband transitions caused by sub-bandgap photons. It is well known that non-radiative recombination plays a significant role due to the presence of interface states\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. For two-color excitation conditions, the IR light is able to pump out carriers passivating the interface states\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e, which additionally induces non-radiative recombination at the heterointerface, and, thereby, the IR-induced EQE reduction occurs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eExcitation Power-Dependence of the Open-Circuit Voltage\u003c/h2\u003e \u003cp\u003eThe excitation power dependence of the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e for single-color excitation is provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e increases with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e, because the electron density in the CB of GaAs increases. Although the accumulated electrons are thermally excited to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e under single-color excitation conditions is determined by the quasi-Fermi-level splitting in GaAs, because in this case, the excitation at HI-I is a thermionic emission process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea shows the IR-induced gain in the open-circuit voltage (Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e) as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e in the case of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 228 mW/cm\u003csup\u003e2\u003c/sup\u003e, and Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb shows the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e in the case of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 260 mW/cm\u003csup\u003e2\u003c/sup\u003e. The positive Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values originate from the increase of the electron density in the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e caused by the additional illumination with the 1319-nm photons (optically induced intraband transitions):\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea shows an increase in Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e with increasing \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e. We consider that the carrier extraction at HI-I is an adiabatic optical process\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Therefore, the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e increases. Compared to the thermionic emission and tunneling processes at HI-I, the intraband transitions induced by the 1319-nm photons provide an increased electron population in the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e. This induces a split between electron quasi-Fermi levels of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs. The positive Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e evidence the presence of an adiabatic TPU process at HI-I. Since stronger interband excitation causes a higher electron density at HI-I, the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e increases with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eSimilarly, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb shows an increase of Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e with increasing \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e, in agreement with the Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e data shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. As \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e increases, more electrons accumulated at HI-I are upconverted to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e, which induces a widening of the quasi-Fermi-level splitting.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e as functions of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e. Overall, this map shows an increase in Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e as both \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e increase. Furthermore, a feature identical to that observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e can be seen: when \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is below ~\u0026thinsp;20 mW/cm\u003csup\u003e2\u003c/sup\u003e, the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values are scattered around zero. The increase with either \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e or \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e only appears for sufficiently high \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e values. This feature indicates that the mechanism responsible for the IR-induced enhancement of \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e is identical to that for the IR-induced enhancement of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e; the adiabatic upconversion of electrons contributes to both photocurrent and photovoltage. Furthermore, also the threshold characteristic in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e is due to the requirement of a sufficiently high electron density at HI-I.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eInfluence of the Temperature\u003c/h2\u003e \u003cp\u003eIn this section, the effect of the temperature on the photocurrent and photovoltage of the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC is discussed. The temperature dependence of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e for single-color excitation with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 428 mW/cm\u003csup\u003e2\u003c/sup\u003e is shown in the Arrhenius plot in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. In general, the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e increases with temperature, in agreement with the temperature dependence of the EQE in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. This increase evidences thermionic emission at least at one of the two heterointerfaces in this device, since a higher temperature implies a higher thermal energy. Hence, at higher temperatures, the photogenerated electrons in the CB of GaAs can more easily overcome the band discontinuity by thermal excitation and then are collected at the front electrode.\u003c/p\u003e \u003cp\u003eThe dashed line in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e represents the result of fitting the data to the Arrhenius equation,\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${\\text{J}}_{\\text{SC}}\\text{ = A}\\text{exp}\\left(\\text{\u0026ndash;}\\frac{{\\text{E}}_{\\text{A}}}{{\\text{k}}_{\\text{B}}}\\text{∙}\\frac{\\text{1}}{\\text{T}}\\right) \\text{,} \\text{(1)}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e is a fitting parameter, \u003cem\u003ek\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e is the Boltzmann constant, \u003cem\u003eT\u003c/em\u003e is the absolute temperature, and \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e is the thermal activation energy. Regarding \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e, the observed IR-induced photocurrent gain (Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea and b) would suggest an \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e that corresponds to the CB discontinuity at HI-I. However, the fit resulted in an \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e of 0.108 eV, which is significantly smaller than the estimated CB discontinuity of 0.770 eV. We ascribe the obtained value of \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e to an average energy level of occupied interface states at HI-II:\u003c/p\u003e \u003cp\u003eAs indicated in the band diagram in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, the CB discontinuity at HI-II has a barrier height of ~\u0026thinsp;1.00 eV (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{\u0026chi;}}_{\\text{ZnO}}\\)\u003c/span\u003e\u003c/span\u003e is much larger than \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{\u0026chi;}}_{\\text{CsPbB}{\\text{r}}_{\\text{3}}}\\)\u003c/span\u003e\u003c/span\u003e). In this case, the electrons in the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e should not need thermal energy to reach ZnO. Therefore, we consider the following process: First, the photogenerated electrons in the CB of GaAs accumulate at the interface states of HI-I. Then, the electrons are thermally excited to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e and transported to the front electrode. A fraction of these electrons relaxes from the CB to the interface states of HI-II and are trapped. These electrons need to be thermally activated to reach the CB of ZnO in order to be collected at the front electrode. This means that these electrons are first thermally excited at HI-I, and then are again thermally excited at HI-II. As a result, the physical meaning of the obtained value of \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e may be the energy difference between the CB minimum of ZnO and the average level of occupied interface states at HI-II.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e increase with both \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e (if \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e \u0026gt; 20 mW/cm\u003csup\u003e2\u003c/sup\u003e). In Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, we provide additional evidence for the presence of optically induced intraband transitions at HI-I. Figure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e shows two data sets of Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e as a function of Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e for \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 450 mW/cm\u003csup\u003e2\u003c/sup\u003e: The dark red points represent the Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e\u0026ndash;Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e data acquired at room temperature as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e (the data showing the IR-induced gain in \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e). This dataset indicates an increase in both Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e with increasing \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e. The blue points represent the Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e\u0026ndash;Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e data for \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 0 as a function of the temperature (the data showing the temperature-induced changes in \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e). This dataset exhibits a clearly different feature, i.e., the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e decreases as Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e increases. These two data sets allow us to compare the influence of optically induced intraband transitions (sub-bandgap photons) and thermal activation (thermal energy) on the SC performance.\u003c/p\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, we demonstrated thermal activation, which provides more additional photocurrent (Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e) as the temperature increases. However, the advantage gained by this process is not identical to the advantage gained by the photocurrent increase through optically induced intraband transitions. To understand the difference, the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e in the case of thermal activation needs to be considered. It is well-known that the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e has a strong inverse dependence on the dark saturation current, which increases with temperature\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e,\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. Therefore, the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e decreases with increasing temperature and the temperature-induced change in Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e becomes more negative with increasing temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e; blue data), in contrast to the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e due to the IR-induced intraband transitions (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e; red data). Because IR-induced intraband transitions are optical processes, the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e is positive (the quasi-Fermi-level splitting increases as discussed in Excitation Power-Dependence of the Open-Circuit Voltage). Consequently, the two Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e\u0026ndash;Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e data sets have a different origin for the observed Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e, and the positive Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values obtained using sub-bandgap photons confirms the presence of adiabatic optical excitation at HI-I\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWe have presented current and voltage data of a CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC under different excitation conditions. The CsPbBr\u003csub\u003e3\u003c/sub\u003e perovskite layer was grown using a solution-based method under ambient conditions. The EQE measured with and without additional illumination with sub-bandgap photons revealed clear absorption edges corresponding to the semiconductors in this SC. The enhancement of the photocurrent and photovoltage observed in the case of additional illumination with sub-bandgap photons confirms a TPU process at the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface. This distinguishes the effect of optically induced intraband transitions from the effect of elevated temperatures. We elucidated the carrier dynamics in the SC by investigating the influences of the temperature as well as the power densities of the two laser beams used to induce interband and intraband transitions. Despite the observation of a TPU process in this SC, the efficiency of this SC is still too low for applications. The optimization of the CsPbBr\u003csub\u003e3\u003c/sub\u003e quality may lead to better results. Furthermore, the large difference between the electron affinities of CsPbBr\u003csub\u003e3\u003c/sub\u003e and ZnO might have led to an open-circuit voltage reduction\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. To overcome this problem, it has been proposed to use AlGaN as the ETL\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. The TPU process observed at the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface in this work is a first step in studying perovskite/III\u0026ndash;V semiconductor interfaces. We believe that the observation of the TPU process at the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface is important for the future development of TPU-SCs, and hence, high-efficiency single junction SCs.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eFabrication of CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-Based TPU-SCs\u003c/h2\u003e \u003cp\u003eWe fabricated the TPU-SC as follows: First, a 1-inch p-doped GaAs substrate (001) was washed in acetone, methanol, isopropanol, and deionized water (in each step, sonication was performed for 15 min, and the whole cleaning process is referred to as the AMID process). Then, we deposited Au-Zn/Au electrodes at the rear surface of the GaAs substrate. The deposition of Au-Zn was done using a thermal evaporator. We deposited about 200 nm of Au-Zn, and this was followed by 500 nm of Au. To improve the ohmic characteristics of the p-GaAs/Au-Zn/Au contact, we annealed the substrate at 430\u0026deg;C for 2.5 min under N\u003csub\u003e2\u003c/sub\u003e gas. The substrate was washed again using the AMID process. Then, the substrate was treated with hydrofluoric acid for 20 s to remove the native oxide layer on the surface. After that, the substrate was cleaned in a UV/O\u003csub\u003e3\u003c/sub\u003e cleaner for 10 min.\u003c/p\u003e \u003cp\u003eWe chose a conventional multi-step spin-coating deposition technique to grow the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{CsPbB}{\\text{r}}_{\\text{3}}\\)\u003c/span\u003e\u003c/span\u003e layer, since this allows us to easily prepare a perovskite layer with high crystallinity and good surface coverage under ambient conditions\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. We prepared two precursor solutions for the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{CsPbB}{\\text{r}}_{\\text{3}}\\)\u003c/span\u003e\u003c/span\u003e layer: (A) 1.00 M of lead (II) bromide (PbBr\u003csub\u003e2\u003c/sub\u003e, Tokyo Chemicals Industry) was dissolved in \u003cem\u003eN,N\u003c/em\u003e-Dimethylformamide (DMF, Sigma-Aldrich), and (B) 0.07 M of cesium bromide (CsBr, Tokyo Chemicals Industry) was dissolved in a mixture of methanol (FUJIFILM Wako Pure Chemical Corp.) and deionized water with a ratio of 5:1. Both precursor solutions were stirred at 90\u0026deg;C for 30 min until the solution became clear, and then both solutions were filtered using microporous filters to remove microparticles and microcrystals. The precursor solution A was drop-casted on the cleaned p-GaAs substrate until the surface was covered with the solution. Then, the substrate was spun at 2000 rpm for 30 s. Five seconds after the start of the spin-coating process, 100 \u0026micro;L of chlorobenzene (FUJIFILM Wako Pure Chemical Corp.) was dropped on the spinning substrate. Subsequently, the substrate was heated at 90\u0026deg;C for 30 min under ambient conditions. Then, precursor solution B was drop-casted on the substrate. The substrate was again spun at 2000 rpm for 30 s, and then heated at 250\u0026deg;C for 5 min. As a result of the optimization of the number of deposition cycles of precursor B, we repeated this process four times to achieve a high-purity CsPbBr\u003csub\u003e3\u003c/sub\u003e perovskite layer and to suppress the formation of CsPb\u003csub\u003e2\u003c/sub\u003eBr\u003csub\u003e5\u003c/sub\u003e and Cs\u003csub\u003e4\u003c/sub\u003ePbBr\u003csub\u003e6\u003c/sub\u003e (it has been reported that the formation of the latter two phases depends on the concentration of PbBr\u003csub\u003e2\u003c/sub\u003e and CsBr\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, and we optimized the concentration of CsBr by using several the deposition cycles). The high purity of the CsPbBr\u003csub\u003e3\u003c/sub\u003e perovskite phase can be confirmed in the X-ray diffractograms shown in the Supplementary Information (the optimization of CsBr deposition cycles was done using glass substrates). We also confirmed the formation of the CsPbBr\u003csub\u003e3\u003c/sub\u003e perovskite phase by photoluminescence (PL) spectroscopy. The PL spectrum in the Supplementary Information shows a PL peak at about 535 nm, which coincides with the peak position reported in previous publications\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe ZnO ETL was deposited on the CsPbBr\u003csub\u003e3\u003c/sub\u003e/p-GaAs substrate by sputtering. The ZnO target was purchased from Furuuchi Chemical. The background pressure inside the chamber was at 7.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e Pa, and for the sputtering process we first supplied a mixture of Ar and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{O}}_{\\text{2}}\\)\u003c/span\u003e\u003c/span\u003e (with a 1:1 ratio) to the chamber until the total pressure reached 4.5 to 5 Pa. Then, an Ar:O\u003csub\u003e2\u003c/sub\u003e plasma was generated by applying an electric power of 50 W and we gradually increased the applied power to 100 W while the Ar:O\u003csub\u003e2\u003c/sub\u003e pressure was gradually reduced to 1.3 Pa. After the pressure reached 1.3 Pa, the deposition was continued for 5 min. With this procedure, an approximately 130-nm-thick ZnO ETL is obtained.\u003c/p\u003e \u003cp\u003eTo finalize the TPU-SC device, a 450-nm-thick Ag electrode was deposited on the top of the ZnO layer. The electrode was patterned using a metal mask with an aperture size of 3.0 mm \u0026times; 3.0 mm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eExternal Quantum Efficiency Measurements\u003c/h2\u003e \u003cp\u003eTo measure the external quantum efficiency (EQE), we employed a tungsten-halogen lamp as a light source for interband excitation. This lamp provides a broad spectrum, and the wavelength-integrated power density was 0.5 mW/cm\u003csup\u003e2\u003c/sup\u003e, which is much smaller than the solar irradiance. The lamp was combined with a 140-mm single monochromator to select specific wavelengths for excitation (note that the output beam intensity depends on the wavelength). Furthermore, an optical chopper with a chopping frequency of 250 Hz was inserted into the excitation path. This monochromatic light was used to induce interband transitions (the beam spot diameter on the SC surface was 1.2 mm). The short-circuit current was amplified by a current amplifier and detected by a lock-in amplifier synchronized to the optical chopper. Moreover, a fixed fraction of the excitation light was detected by a Si photodetector to estimate the number of incident photons at a given wavelength. This setup is for the EQE measurements under single-color excitation conditions, which were performed at room temperature without a temperature controller.\u003c/p\u003e \u003cp\u003eFor the EQE measurements under two-color excitation conditions, we additionally illuminated the device with 1319-nm photons generated by a continuous-wave (CW) solid-state laser. These infrared (IR) photons can induce intraband transitions in addition to the interband transitions induced by the light from the tungsten-halogen lamp. The power density of the IR laser beam was set to \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 86 mW/cm\u003csup\u003e2\u003c/sup\u003e by a variable neutral-density filter (hereafter, \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e is always used to refer to the power density of the 1319-nm laser beam). The beam spot diameter on the SC surface was 1.2 mm, and the excitation spot coincided with that for interband excitation, since we employed a single optical fiber to guide the excitation light to the sample. Such a two-color excitation condition should provide a larger photocurrent and hence a better EQE due to the following mechanism: First, the photons generated by the tungsten-halogen lamp induce interband transitions in either of the three semiconductor layers in the device or even all, depending on the photon energy. For example, 700-nm photons will not be absorbed in the ZnO and CsPbBr\u003csub\u003e3\u003c/sub\u003e layers and thus induce interband transitions only in the GaAs layer. The photogenerated electrons in the CB of GaAs mainly stay at the CB minimum. A part of these electrons will be trapped at the interface states at HI-I. In the case of single-color excitation conditions, the electrons can overcome the potential barrier of CsPbBr\u003csub\u003e3\u003c/sub\u003e by thermal activation (and are then collected at the Ag electrode). On the other hand, due to the additional IR photons in the case of two-color excitation conditions, the electrons can also be optically excited to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e. This process provides an additional photocurrent (it reduces the probability of trap-assisted recombination at HI-I). The details of the carrier dynamics will be discussed in Section 3.\u003c/p\u003e \u003cp\u003eTo determine the temperature-dependence of the EQE spectrum, we used the same experimental setup as for the single-color excitation, but here the SC was installed in a cryostat to control the temperature. The EQE signals were recorded without the IR light in order to observe the effect of the temperature. The measurements were performed in the temperature range 180\u0026ndash;300 K, because the photocurrent generated by the SC at temperatures lower than 180 K can hardly be detected.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCurrent\u0026ndash;Voltage (\u003c/b\u003e \u003cb\u003eJV\u003c/b\u003e \u003cb\u003e) Characteristics\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe \u003cem\u003eJV\u003c/em\u003e curves were measured using three different excitation conditions: (i) no illumination, (ii) illumination with 784-nm photons from a CW solid-state laser (single-color excitation), and (iii) simultaneous illumination with 784-nm photons and 1319-nm photons (two-color excitation). The 784-nm laser light had a photon flux of 4.0\u0026times;10\u003csup\u003e21\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, which corresponds to \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 101 mW/cm\u003csup\u003e2\u003c/sup\u003e (hereafter, \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e is always used to refer to the power density of the 784-nm laser beam). The 784-nm photons induce interband transitions only in the GaAs layer, and the photogenerated electrons can accumulate at HI-I and can then be thermally or optically excited to the CB of CsPbBr\u003csub\u003e3\u003c/sub\u003e. The 1319-nm laser light had a photon flux of 5.7\u0026times;10\u003csup\u003e21\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e (\u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e = 86 mW/cm\u003csup\u003e2\u003c/sup\u003e), and these IR photons are used to optically excite the accumulated electrons. The measurement was done at room temperature without controlling the temperature. The \u003cem\u003eJV\u003c/em\u003e curves were recorded using a source measure unit (Keithley 2400).\u003c/p\u003e \u003cp\u003e \u003cb\u003eExcitation Power-Dependence of Δ\u003c/b\u003e \u003cb\u003eJ\u003c/b\u003e \u003csub\u003e \u003cb\u003eSC\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eand Δ\u003c/b\u003e\u003cb\u003eV\u003c/b\u003e\u003csub\u003e\u003cb\u003eOC\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eThe IR-induced gain in the short-circuit current (Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e) and the IR-induced gain in the open-circuit voltage (Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e) are important parameters for TPU-SCs. They are defined as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{∆}{\\text{J}}_{\\text{SC}}\\text{ =}{\\text{ }\\text{J}}_{\\text{SC, with infrared}}\\text{ \u0026ndash; }{\\text{J}}_{\\text{SC, without infrared}}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{∆}{\\text{V}}_{\\text{OC}}\\text{ = }{\\text{V}}_{\\text{OC, with infrared}}\\text{ \u0026ndash; }{\\text{V}}_{\\text{OC, without infrared}}\\)\u003c/span\u003e\u003c/span\u003e. The Keithley 2400 source measure unit was employed to record the short-circuit current (\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e). First, we measured the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e for single-color excitation as a function of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e. Then, we measured the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e values under two-color excitation conditions. This allows us to determine Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e as functions of both \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e. The intensities of the laser beams were controlled by two variable neutral-density filters. The values of Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e were obtained by measuring the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values under single- and two-color excitation conditions, but the measurement of \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e is slightly different from that of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e. In the case of the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e measurement, the data was directly recorded by the source measure unit. On the other hand, the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e was determined by the following procedure: First, we applied a voltage where the current is approximately zero. Then, we used the \u003cem\u003eJV\u003c/em\u003e data to estimate the values of \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e using a linear least-squares method to estimate linear functions defined by the slope and the intercept on the \u003cem\u003ey\u003c/em\u003e-axis. By obtaining these two parameters for each measured \u003cem\u003eJV\u003c/em\u003e dataset, we can estimate the \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e values, which are equal to the intercepts of the linear functions on the \u003cem\u003ex\u003c/em\u003e-axis. As a result, the Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e can be estimated as functions of \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eIntra\u003c/sub\u003e. Both types of measurements were conducted at room temperature without using a temperature controller.\u003c/p\u003e \u003cp\u003eTo verify the influence of the temperature on the SC operation, we measured the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e as a function of the temperature for single-color excitation with \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e = 428 mW/cm\u003csup\u003e2\u003c/sup\u003e (a photon flux of 1.69\u0026times;10\u003csup\u003e22\u003c/sup\u003e m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003es\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e). The beam spot diameter on the SC was 1.2 mm. The SC was installed in the (evacuated) cryostat and we changed the temperature in the range from 280 to 300 K. The analysis was done using the Arrhenius equation, because we consider that the accumulated electrons overcome the heterointerface by thermionic emission.\u003c/p\u003e \u003cp\u003eSimilarly, we also measured the temperature-induced changes in \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e under single-color excitation conditions. The same experimental system as for the temperature dependence of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e was employed, and \u003cem\u003eP\u003c/em\u003e\u003csub\u003eInter\u003c/sub\u003e was 450 mW/cm\u003csup\u003e2\u003c/sup\u003e. Note that for this experiment (which is discussed in Section 3.6), the parameters Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e are defined as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{∆}{\\text{J}}_{\\text{SC}}\\text{ =}{\\text{ }\\text{J}}_{\\text{SC, at high temperature}}\\text{ \u0026ndash; }{\\text{J}}_{\\text{SC, at reference temperature}}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{∆}{\\text{V}}_{\\text{OC}}\\text{ = }{\\text{V}}_{\\text{OC, at high temperature}}\\text{ \u0026ndash; }{\\text{V}}_{\\text{OC, at reference temperature}}\\)\u003c/span\u003e\u003c/span\u003e (the reference temperature is 282 K). Therefore, in this experiment, Δ\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and Δ\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e express the changes in the photocurrent and the open-circuit voltage excluding the effect of the IR light (the TPU process).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eData Availability Statement\u003c/h2\u003e \u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eAdditional Information\u003c/h2\u003e \u003cp\u003eThe Supplementary Information is available free of charge at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi/XXXX\u003c/span\u003e\u003cspan address=\"https://doi/XXXX\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eX-ray diffractograms of CsPbBr\u003csub\u003e3\u003c/sub\u003e deposited on GaAs, photoluminescence spectrum of CsPbBr\u003csub\u003e3\u003c/sub\u003e deposited on GaAs for four deposition cycles, cross-sectional scanning electron microscopy image of the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC, and the calculation of the total amount of 500-nm photons absorbed in each layer.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eHambalee Mahamu (H. M.) conceptualized this project. The manuscript was written by H. M. under the revision provided by Shigeo Asahi (S. A.) and Takashi Kita (T. K.). The main financial support for this project was acquired by H. M. while S. A. and T. K. were responsible for additional financial support.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eOne of the authors (Hambalee Mahamu) would like to thank the Graduate School of Engineering, Kobe University, for financial support through a scholarship. He gratefully acknowledges the Sasakura Enviro-Science Foundation for research support. Furthermore, he would like to thank Mr. Mizuto Kawakami of Kobe University for his help with the calculation of the energy conversion efficiency. This work was supported by JSPS KAKENHI Grant Numbers JP23K26142, JP23K03958.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKita, T., Harada, Y. \u0026amp; Asahi, S. Energy Conversion Efficiency of Solar Cells. (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShockley, W. \u0026amp; Queisser, H. J. Detailed Balance Limit of Efficiency of \u003cem\u003ep-n\u003c/em\u003e Junction Solar Cells. J. Appl. Phys. 32, 510\u0026ndash;519 (1961).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHirst, L. C. \u0026amp; Ekins-Daukes, N. J. Fundamental losses in solar cells. Progress in Photovoltaics: Research and Applications 19, 286\u0026ndash;293 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHenry, C. H. 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Energy Procedia 27, 135\u0026ndash;142 (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWu, X. \u003cem\u003eet al.\u003c/em\u003e ZnO electron transporting layer engineering realized over 20% efficiency and over 1.28 V open-circuit voltage in all-inorganic perovskite solar cells. EcoMat 4, 1\u0026ndash;11 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHombe, A., Saiki, S., Mori, T., Saito, Y. \u0026amp; Tanimoto, T. AlGaN as an electron transport layer for wide-bandgap perovskite solar cells. Jpn. J. Appl. Phys. 62, (2023).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4362355/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4362355/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTwo-step photon upconversion solar cells (TPU-SCs) based on III\u0026ndash;V semiconductors can achieve enhanced sub-bandgap photon absorption because of intraband transitions at the heterointerface. From a technological aspect, the question arose whether similar intraband transitions can be realized by using perovskite/III\u0026ndash;V semiconductor heterointerfaces. In this article, we demonstrate a TPU-SC based on a CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface. Such a solar cell can ideally achieve an energy conversion efficiency of 48.5% under 1-sun illumination. This is 2.1% higher than the theoretical efficiency of an Al\u003csub\u003e0.3\u003c/sub\u003eGa\u003csub\u003e0.7\u003c/sub\u003eAs/GaAs-based TPU-SC. Experimental results of the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs-based TPU-SC show that both the short-circuit current \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e and the open-circuit voltage \u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e increase with additional illumination of sub-bandgap photons. We analyze the excitation power dependence of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e for different excitation conditions to discuss the mechanisms behind the enhancement. In addition, the observed voltage-boost clarifies that the \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e enhancement is caused by an adiabatic optical process at the CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface, where sub-bandgap photons efficiently pump the electrons accumulated at the heterointerface to the conduction band of CsPbBr\u003csub\u003e3\u003c/sub\u003e. Besides the exceptional optoelectronic properties of CsPbBr\u003csub\u003e3\u003c/sub\u003e and GaAs, the availability of a CsPbBr\u003csub\u003e3\u003c/sub\u003e/GaAs heterointerface for two-step photon upconversion paves the way for the development of high-efficiency perovskite/III\u0026ndash;V semiconductor-based single-junction solar cells.\u003c/p\u003e","manuscriptTitle":"Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-15 04:08:54","doi":"10.21203/rs.3.rs-4362355/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-09-30T04:26:46+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-09T05:23:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"22957526986702193471425825478744452662","date":"2024-09-05T08:58:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-30T11:41:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"225529498387987889819153611403229519648","date":"2024-06-24T07:40:36+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"77700490411453598242662333821729874461","date":"2024-06-20T08:08:50+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-20T12:53:21+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-20T12:49:45+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-05-08T17:43:01+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-08T08:53:15+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-05-03T06:51:46+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"814b43d9-1118-42b5-861c-fe1867cd6dd3","owner":[],"postedDate":"May 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":31802596,"name":"Physical sciences/Energy science and technology/Renewable energy/Solar energy/Photovoltaics/Solar cells"},{"id":31802597,"name":"Physical sciences/Physics/Condensed matter physics/Surfaces interfaces and thin films"}],"tags":[],"updatedAt":"2024-11-11T15:58:35+00:00","versionOfRecord":{"articleIdentity":"rs-4362355","link":"https://doi.org/10.1038/s41598-024-78257-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-11-06 15:56:50","publishedOnDateReadable":"November 6th, 2024"},"versionCreatedAt":"2024-05-15 04:08:54","video":"","vorDoi":"10.1038/s41598-024-78257-x","vorDoiUrl":"https://doi.org/10.1038/s41598-024-78257-x","workflowStages":[]},"version":"v1","identity":"rs-4362355","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4362355","identity":"rs-4362355","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}