Ultrathin Glass-based Perovskite Solar Cells Employing Bilayer Electron Transport Layer

Ultrathin Glass-based Perovskite Solar Cells Employing Bilayer Electron Transport Layer | 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 Research Article Ultrathin Glass-based Perovskite Solar Cells Employing Bilayer Electron Transport Layer Wooyeon Kim, Jian Cheng, Joonwon Choi, Seoyeong Lee, Yongwoo Lee, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4591034/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract In recent studies, flexible perovskite solar cells (PSCs) have exhibited high power conversion efficiency ( PCE ) coupled with remarkable mechanical stability. However, the conventional polymer substrates used in flexible PSCs possess high permeability to moisture and oxygen, leading to the rapid degradation of perovskite materials. In this work, we address these issues by employing ultrathin glass (UTG) substrates, which provide moisture impermeability while retaining flexibility. Additionally, we introduce a strategically designed SnO 2 /TiO 2 bilayer as the electron transport layer (ETL). Our results reveal that PSCs incorporating the bilayer ETL achieve higher PCE than those with a monolayer ETL on conventional glass and UTG substrates. Furthermore, moisture permeability tests demonstrate that PSCs based on UTG substrates sustain their PCE over time, compared to their polymer-based counterparts. These results imply that UTG substrates, combined with a SnO 2 /TiO 2 bilayer ETL, offer a promising solution for developing durable, high-performance, flexible PSCs suitable for long-term applications. electron transport layer ultrathin glass titanium oxide perovskite solar cell transparent conductive oxide Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Organic-inorganic hybrid perovskite solar cells (PSCs) have recently achieved a power conversion efficiency ( PCE ) exceeding 26% [ 1 ]. This achievement is due to the high absorption coefficient, high charge mobility, and low exciton binding energy inherent to perovskite materials [ 2 – 5 ]. A notable advantage of these materials is their exceptional mechanical flexibility, which stems from their hybrid organic-inorganic structure [ 6 , 7 ]. Consequently, flexible PSCs have been extensively investigated, demonstrating high mechanical stability and performance [ 8 – 10 ]. Fabricating flexible PSCs typically involves constructing each layer from a polymer substrate. To provide the flexibility, polymer substrates such as polyethylene naphthalate (PEN), known for their suitable optical and mechanical properties, are commonly used [ 9 , 11 , 12 ]. However, these polymer substrates exhibit higher moisture and oxygen permeability than glass substrates [ 13 – 15 ]. Considering that perovskite materials are easily degraded by moisture and oxygen, polymer substrates are less suitable for long-term applications [ 16 , 17 ]. Ultrathin glass (UTG) substrates present a viable alternative to polymer substrates. With a thickness of less than 100 µm, UTG maintains adequate flexibility while offering the advantageous properties of glass, including a moisture permeability over 100,000 times lower than that of polymer substrates [ 14 , 18 , 19 ]. Consequently, UTG-based flexible PSCs are expected to achieve long-term moisture stability. Another key consideration in fabricating flexible PSCs is the selection of an efficient electron transport layer (ETL). The ETL is a crucial component in PSCs, significantly affecting their performance and stability [ 20 , 21 ]. Positioned between the transparent electrode and the perovskite layer, the ETL efficiently extracts photogenerated electrons from the perovskite layer to the electrode. For effective hole blocking, it is essential for the ETL to completely cover the bottom electrode. Additionally, the ETL should exhibit favorable energy level alignment between the perovskite and the transparent electrode to ensure efficient electron extraction. One strategy to achieve this is the implementation of a bilayer ETL with a favorable energy level design. Recent studies have indicated that bilayer ETLs can provide higher PCE and enhanced stability than monolayer ETLs [ 9 , 11 , 22 ]. In this study, we designed a SnO 2 /TiO 2 bilayer for the ETL to fabricate flexible PSCs. Initially, we synthesized colloidal TiO 2 nanocrystals (NCs), which were capped with oleic acid (OA) to ensure dispersibility in hexane, a non-polar solvent. To prepare the bilayer ETL, TiO 2 dispersion was spin-coated onto a SnO 2 film, and the organic ligands were removed via UV irradiation. We confirmed that this TiO 2 layer possesses favorable energy level alignment between the SnO 2 and perovskite layers, resulting in efficient charge extraction. The bilayer ETL-based PSCs exhibited a PCE of 23.5%, an improvement over the 22.7% PCE of PSCs with a SnO 2 monolayer ETL. Moreover, PSCs using UTG substrates showed significant differences in PCE , with bilayer and monolayer ETLs achieving 15.2% and 8.3%, respectively. We demonstrated that the performance degradation due to UTG roughness is mitigated by employing the bilayer ETL. Furthermore, we highlighted the effectiveness of the UTG substrate in preventing moisture penetration by comparing UTG-based PSCs with PEN-based PSCs. 2. Experimental Chemicals Tin(IV) oxide (SnO 2 NPs, 15% in H 2 O colloidal dispersion), formamidine acetate salt (99%), and lead(II) iodide (PbI 2 , 99.999%) were purchased from Alfa Aesar. Oleic acid (OA, 90%), titanium(IV) isopropoxide (TTIP, 97%), n-hexane (anhydrous, 95%), hydroiodic acid (HI, 57 wt% in water), 2-methoxyethanol (2ME, anhydrous, 99.8%), 2-propanol (IPA, anhydrous, 99.5%), chlorobenzene (CB, anhydrous, 99.8%), N,N -dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, > 99.5%), methylammonium chloride (MACl, 98%), acetonitrile (ACN, anhydrous, 99.8%), and 4- tert -butylpyridine (tBP, 98%) were purchased from Sigma-Aldrich. Phenethylammonium iodide (PEAI) was acquired from Greatcell Solar. Ethyl alcohol (EtOH, pure) and diethyl ether (extra pure grade) were purchased from Duksan. 2,2',7,7'-Tetrakis(N, N -di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209) were obtained from Lumtec. Ultrathin glass (UTG, 50 µm) based tin-doped indium oxide (ITO) substrates were purchased from Quantum Plasma, Korea. Synthesis of colloidal TiO 2 nanocrystals TiO 2 NCs were synthesized through a modified non-hydrolytic sol-gel reaction method, as reported previously [ 23 ]. The synthesis began by combining 88 mmol of OA and 30 mmol of TTIP in a three-neck round-bottom flask under a continuous argon (Ar) gas flow. This mixture was heated to 270°C for 2 h with constant stirring. During the formation of TiO 2 NCs, the reaction mixture's color transitioned from transparent yellow to white. The precipitates were washed with excess ethanol and purified by centrifugation at 3000 rpm for 30 min. The final product, dissolved in n-hexane at 25 mg·ml − 1 , was stored under an N 2 gas within a glove box. Synthesis of the perovskite precursor FAI and FAPbI 3 powders were synthesized using previously established methods [ 21 ]. To synthesize FAI powder, formamidine acetate salt (25 g) was mixed with HI (50 mL) in a 500 mL round-bottom flask and stirred vigorously. The solvent was evaporated under vacuum at 80°C for 1 h, yielding a light-yellow powder. This powder was dissolved in ethanol and precipitated with diethyl ether, repeated three times until a pure white powder was obtained. For further purification, the FAI powder was recrystallized using an ethanol and diethyl ether mixture (1:1 v/v) in a refrigerator. The final white powder was filtrated through a glass filter and dried at 60°C for 24 h. For synthesizing FAPbI 3 powder, the prepared FAI (0.8 M) was mixed with PbI 2 in a 1:1 molar ratio in 2ME with vigorous stirring. The resulting yellow solution was heated and stirred vigorously at 120°C, then subjected to recrystallization using the retrograde method. The final black powder was filtered through a glass filter and baked at 150°C for 1 h to obtain the desired FAPbI 3 powder. Fabrication of perovskite solar cells Patterned ITO substrates were sequentially cleaned by sonication in detergent water, deionized water, and IPA for 10 min each. To produce SnO 2 ETLs, the SnO 2 NCs were diluted with deionized water (SnO 2 NCs:H 2 O = 1:6). This solution was dropped onto UV-ozone-treated transparent ITO substrates and spin-coated at 3000 rpm for 30 s, followed by annealing at 150°C for 30 min. To prepare the SnO 2 /TiO 2 bilayer electron transport layer, a highly dispersed solution of TiO 2 NCs in n-hexane (25 mg·ml − 1 ) was dropped onto a transparent conducting oxide (TCO) substrate and immediately spin-coated at 5000 rpm for 30 s. The TiO 2 NC-coated substrate was then treated with UV irradiation using a UV cure machine (JHCI-051B, JECO) equipped with a mercury lamp (peak wavelength: 365 nm). For perovskite layer deposition, the UV-ozone-treated substrate was spin-coated with the perovskite precursor solution at 8000 rpm for 50 s. After 10 s of spin coating, 1 mL of diethyl ether was dropped onto the spinning film. The deposited film was then annealed at 150°C for 15 min and 100°C for 1 h on a hotplate. The perovskite precursor solution was prepared by dissolving 778 mg of synthesized FAPbI 3 powder and 23 mg of MACl in 0.5 mL of DMF/DMSO (4:1 v/v). After cooling, the perovskite films were coated with PEAI (3 mg·ml − 1 in IPA) at 3000 rpm for 30 s. Spiro-OMeTAD, the hole transport layer (HTL) in the PSCs, was spin-coated onto the films at 4000 rpm for 30 s. The Spiro-OMeTAD solution was prepared by doping 90 mg/mL Spiro-OMeTAD in CB with 39 µL of tBP, 23 µL of Li-TFSI (520 mg·ml − 1 in ACN), and 5 µL of FK209 (180 mg·ml − 1 in ACN). Finally, an Au electrode with a thickness of 80 nm was deposited using a thermal evaporator. Characterization The crystal structure of the TiO 2 NCs was characterized using a high-resolution X-ray diffractometer (XRD, D8 ADVANCE, BRUKER) with Cu-Kα radiation (λ = 1.541 Å). The morphology of SnO 2 and TiO 2 NCs was analyzed with transmission electron microscopy (TEM, JEM2100F, JEOL), while their films were examined using scanning electron microscopy (FE-SEM, Verios G4 UC, FEI) and atomic force microscopy (AFM, XE-100, Park Systems). The optical properties of the thin films were investigated using a UV–Vis spectrometer (Shimadzu, UV-2600i). The absorption coefficient (α) for direct bandgap semiconductors near the band edge was calculated using the formula αhν = A(hν–E g ) 2 , where A denotes the proportionality constant, hν denotes the photon energy, and E g denotes the optical bandgap. Specifically, E g was estimated by extrapolating the tangent line to the plotted curve. ATR–FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific) was employed to measure ligand removal after UV treatment. Steady-state PL and TRPL spectra were obtained using a Fluoromax-4 (iHR320, HORIBA Scientific) spectrometer, with an excitation wavelength of 463 nm. TRPL measurements were conducted to determine the PL maximum (~ 800 nm) of the perovskite film. Under dry conditions (20 ± 5°C, below 15% RH), the current density–voltage ( J − V ) curve and MPP tracking characteristics of the devices were measured using a Keithley 2400 source meter under a Xenon-lamp-based solar simulator (Newport 91160s, AAA class). A Si solar cell calibrated by the National Renewable Energy Laboratory (NREL) was used to adjust the light intensity to AM 1.5G 1 sun conditions (100 mW·cm − 2 ). The voltage sweep rate was 20 mV with a delay of 20 ms for the reverse (forward) sweeping direction from 1.3 V (− 0.1 V) to − 0.1 V (1.3 V). The devices were measured with a SUS304 aperture with an area of 0.09 cm 2 . The incident photon-to-current conversion efficiency (IPCE) was characterized using an IPCE measurement system (PV Measurement, Inc.). 3. Results and Discussion SnO 2 NCs are commonly used as the ETL in PSCs [ 24 ]. However, achieving perfect hole-blocking ability with SnO 2 ETL alone is challenging because it is difficult to fully cover the surface while maintaining a thin layer. As the thickness of the SnO 2 ETL increases, the PCE tends to decrease due to heightened resistance [ 9 ]. A facile strategy to address this issue is the sequential deposition of different ETLs to create a bilayer ETL. When forming a bilayer ETL, the lower layer should possess a higher conduction band minimum (CBM) to enhance electron extraction [ 9 , 11 , 22 ]. Anatase TiO 2 , with a slightly higher CBM than SnO 2 , is an appropriate choice [ 25 , 26 ]. Consequently, we developed a bilayer ETL with a TiO 2 layer on top of a SnO 2 layer. To minimize the impact on the SnO 2 layer, which can dissolve in polar solvents, we synthesized TiO 2 NCs dispersed in a non-polar solvent. The transmission electron microscopy (TEM) image of the synthesized TiO 2 NCs is shown in Figure S1 , confirming the rod-shaped nanocrystals as previously reported [ 23 ]. These TiO 2 NCs are capped with a long-chain organic ligand (OA) to facilitate dispersion. However, these surface stabilizers were removed by UV irradiation in the thin-film state, as they would impede charge transport [ 10 , 23 ]. The removal of organic ligands by UV irradiation was confirmed by ATR-FTIR analysis ( Figure S2 ). The UV-sintered TiO 2 film does not exist strong vibration peaks at 2925 cm –1 and 2855 cm –1 , corresponding to the asymmetric and symmetric CH 2 stretching vibrations of OA. Other vibration peaks of the organic ligands, such as COO – (1525 cm –1 , asymmetric) and CH 2 (1440 cm –1 , bending), also disappear after the UV sintering. X-ray diffraction (XRD) patterns of the TiO 2 film showed that all reflection peaks correspond to the anatase TiO 2 crystal structure (PDF #21-1272) ( Figure S3 ). The SnO 2 /TiO 2 bilayer film was prepared using the UV-sintering method for the ETL, as depicted in the scheme of Fig. 1 a. We initially compared the transmittance of both SnO 2 and SnO 2 /TiO 2 films. The UV-Vis transmittance of each layer on glass/ITO substrates is shown in Fig. 1 b. The SnO 2 /TiO 2 film demonstrated a slightly higher transmittance than the SnO 2 film at 350–500 nm, indicating that more near-UV light could reach the light-absorbing layer. Figure 1 c illustrates the band alignment for the designed ETLs, with this band diagram estimated from ultraviolet photoelectron spectroscopy (UPS) and Tauc plots in Figure S4 . The estimated CBM of SnO 2 , TiO 2 , and perovskite are − 4.30 eV, -4.20 eV, and − 3.96 eV, respectively. The results indicate that the CBMs of TiO 2 and SnO 2 are arranged in a cascade, providing favorable energy levels for charge extraction [ 22 ]. The steady-state TRPL decay was measured to investigate the charge transfer ability at the interface of each ETL and the perovskite. Figure 1 d illustrates that devices utilizing the bilayer ETL exhibit more effective PL quenching than those employing only the SnO 2 layer, indicating that charge extraction from perovskite to the bilayer ETL is more efficient than with only a SnO 2 layer [ 27 – 29 ]. Figure 1 e shows the PL decay transients of perovskite coated on each ETL, from which the corresponding PL lifetimes and amplitudes were obtained by fitting with a bi-exponential decay function, as listed in Table S1 . The fast decay component ( \({\tau }_{1}\) ) originates from the quenching of charge carriers at the interface, while the slow decay component ( \({\tau }_{2}\) ) is attributed to the radiative recombination of free charge carriers due to traps in the absorption layer [ 30 ]. Compared to the perovskite-only film, the samples using ETL exhibited a shorter \({\tau }_{1}\) with an increasing amplitude of the fast decay ( \({A}_{1}\) ), indicating that charge transfer from the perovskite film to the ETL became dominant. The average lifetime for the SnO 2 /perovskite sample was 251.88 ns, which reduced to 172.13 ns for the bilayer ETL. Additionally, \({\tau }_{1}\) decreased from 164.17 ns to 71.65 ns in the bilayer ETL sample, with an increasing \({A}_{1}\) . This implies that the bilayer ETL is more efficient in extracting charge from the perovskite. To compare the thickness and uniformity of each SnO 2 film, we used scanning electron microscopy (SEM) to measure each ETL-based PSC (device structure: ITO/ETL/perovskite/Spiro-OMeTAD/Au). These images show thicknesses of 20–30 nm for the SnO 2 layer and 60–80 nm for the bilayer (Figs. 2 a and 2 b). Additionally, they indicate that the bilayer ETL is deposited uniformly. We also compared the morphology of perovskite films deposited on each ETL layer, revealing that the underlying layer does not significantly affect the perovskite crystal grains ( Figure S5 ). Figure 2 c shows the J − V curves of each ETL-based solar cell, with their photovoltaic performances summarized in Table S2 . The bilayer ETL-based device exhibited a higher PCE compared to the SnO 2 -based devices, achieving a PCE of 23.47% (open-circuit voltage ( V OC ) = 1.19 V, short-circuit current density ( J SC ) = 23.43 mA·cm − 2 , fill factor ( FF ) = 84.07%). In contrast, the SnO 2 ETL-based device had a PCE of 22.66% ( V OC = 1.19 V, J SC = 22.87 mA·cm − 2 , FF = 83.04%). We measured the IPCE spectra to assess the charge collection efficiency of each ETL on the photocurrent of PSCs. The J − V characteristics measured in both reverse and forward directions of the devices show negligible differences ( Figure S6a ). Figure S6b presents the IPCE spectra of solar cells with different ETLs, which align well with the values obtained from the J − V curves in Fig. 2 c. The solar cells based on the bilayer ETL exhibited higher IPCE values with a strong spectral response from 350 to 500 nm. Photons with shorter wavelengths have a shallower penetration depth and are typically absorbed near the front region of the perovskite layer [ 10 , 26 ]. Therefore, the observed increase in IPCE values for shorter wavelengths implies a more efficient electron extraction at the interface between the SnO 2 /TiO 2 bilayer and the perovskite, as well as improved optical transparency of the bilayer ETL. For flexibility, we fabricated a device with the same structure on an ITO-deposited ultrathin glass (UTG) substrate. The thickness of conventional glass/ITO is 1100 µm, whereas the UTG/ITO used is 50 µm. It is noteworthy that performance differences between each ETL-based PSC are more pronounced when using UTG substrates. In Fig. 2 d, we compared the J − V curves of each ETL-based solar cell employing UTG substrates. The bilayer ETL demonstrated a PCE of 15.2%, while the SnO 2 ETL-based device exhibited a significantly lower PCE of 8.3%. To understand this performance difference, we compared the properties of conventional ITO substrates and UTG substrates. The UV-Vis transmittance of each substrate revealed slightly lower transmittance for UTG substrates ( Figure S7 ). This might explain the lower J SC value of UTG-based devices. Furthermore, we compared the roughness of each substrate using atomic force microscopy (AFM). As shown in Fig. 3 a, the roughness of the UTG substrate (RMS of 81.0 nm) is significantly higher than that of the conventional substrate (RMS of 1.4 nm). Additionally, we compared films deposited with a SnO 2 layer and a SnO 2 /TiO 2 bilayer on the UTG substrate (Fig. 3 b). The SnO 2 film (RMS of 2.6 nm) exhibited higher roughness than the bilayer film (RMS of 1.0 nm). Moreover, images of the SnO 2 film revealed that SnO 2 alone could not sufficiently cover the high roughness of the UTG surface, exposing the ITO layer (Fig. 3 c). These results indicate that the bilayer ETL not only improves the performance of PSCs but also provides sufficient coverage even for films with significant roughness. We fabricated devices using UTG/ITO and PEN/ITO substrates to assess their moisture permeability. Water was applied to the back side of the device in direct contact with the substrate, and the PCE was monitored for over 1000 min. The initial J − V curves of the devices used for comparison are depicted in Figure S8 . The PEN-based device achieved a PCE of 18.0%, slightly higher performance than the UTG-based device, which had a PCE of 15.2%. After 1000 min, the PEN-based device showed a 71% reduction in its initial PCE (Fig. 4 a). Conversely, the UTG-based device exhibited a relatively low moisture permeability, with only a 5% reduction in initial PCE . The optical images in Fig. 4 b visually illustrate the state of the devices, revealing that the perovskite layer in the PEN-based device dissolved, whereas the UTG-based device did not. Given that water was applied only to the backsides of the devices, this indicates that moisture penetration from the substrate influenced the outcome. Thus, UTG substrates are recommended to fabricate flexible perovskite solar cells that are moisture-resistant. 4. Conclusions In this study, we demonstrated that a strategically designed SnO 2 /TiO 2 bilayer is an efficient ETL for PSCs, enhancing charge carrier extraction from the perovskite to the transparent electrode by modifying the ETL's energy levels. This bilayer ETL proved more effective on substrates with high roughness, such as UTG substrates, due to its superior coverage compared to a monolayer ETL, resulting in better hole-blocking properties. Consequently, the PCE of bilayer ETL-based PSCs improved to 15.2%, compared to 8.3% for monolayer ETL-based PSCs. This demonstrates the bilayer ETL's efficiency even on substrates with significant roughness. Furthermore, we emphasized the low moisture permeability of UTG substrates compared to conventional polymer substrates. Moisture permeability tests indicated that devices based on polymer substrates experience a sharp decline in PCE, while those based on UTG substrates maintain their PCE. This suggests the potential of UTG substrates for the long-term use of flexible PSCs. Declarations Data availability The data are available upon request from the authors, and access to the data requires approval from the researchers. Acknowledgements This study is the result of a research project conducted with the funds of the Open R&D program of Korea Electric Power Corporation. (R23XH02) This work was also supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (Sector coupling energy industry advancement manpower training program, 20224000000440). Author information These authors contributed equally: Wooyeon Kim, Jian Cheng, and Joonwon Choi Authors and Affiliations Department of Chemical Engineering, Hanyang University, Seoul, 04763, Republic of Korea Wooyeon Kim, Jian Cheng, Joonwon Choi, Seoyeong Lee, Yongwoo Lee, Doyeon Lee, Min Jae Ko Corresponding author Correspondence to Min Jae Ko Ethics declarations Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References H. Chen, C. Liu, J. Xu, A. Maxwell, W. Zhou, Y. Yang, Q. Zhou, A.S.R. Bati, H. Wan, Z. Wang, L. Zeng, J. Wang, P. Serles, Y. Liu, S. Teale, Y. Liu, M.I. Saidaminov, M. Li, N. Rolston, S. Hoogland, T. Filleter, M.G. Kanatzidis, B. Chen, Z. Ning, E.H. Sargent, Science 384, 189–193 (2024) E.M. Hutter, M.C. Gélvez-Rueda, A. Osherov, V. Bulović, F.C. Grozema, S.D. Stranks, T.J. Savenije, Nature Materials 16, 115–120 (2017) G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy & Environmental Science 7, 982–988 (2014) Z. Zhang, W. Kim, M.J. Ko, Y. Li, Nano Convergence 10, 23 (2023) S. Bae, J.W. Jo, P. Lee, M.J. Ko, ACS Applied Materials & Interfaces 11, 17452–17458 (2019) F. Song, D. Zheng, J. Feng, J. Liu, T. Ye, Z. Li, K. Wang, S. Liu, D. Yang, Advanced Materials 36, 2312041 (2024) J. Feng, APL Materials 2, (2014) H.S. Jung, G.S. Han, N.-G. Park, M.J. Ko, Joule 3, 1850–1880 (2019) D.S. Lee, K.W. Kim, Y.-H. Seo, M.H. Ann, W. Lee, J. Nam, J. Chung, G. Seo, S. Nam, B.S. Ma, T.-S. Kim, Y. Kang, N.J. Jeon, J. Seo, S.S. Shin, Joule 8, 1380–1393 (2024) W. Kim, J. Kim, D. Kim, B. Koo, S. Yu, Y. Li, Y. Kim, M.J. Ko, npj Flexible Electronics 8, 20 (2024) J. Chung, S.S. Shin, K. Hwang, G. Kim, K.W. Kim, D.S. Lee, W. Kim, B.S. Ma, Y.-K. Kim, T.-S. Kim, J. Seo, Energy & Environmental Science 13, 4854–4861 (2020) M. Park, J.-Y. Kim, H.J. Son, C.-H. Lee, S.S. Jang, M.J. Ko, Nano Energy 26, 208–215 (2016) S. Castro-Hermosa, M. Top, J. Dagar, J. Fahlteich, T.M. Brown, Advanced Electronic Materials 5, 1800978 (2019) S.B.M. Jaime, R.M.V. Alves, P.F.J. Bócoli, Journal of Drug Delivery Science and Technology 71, 103330 (2022) J.W. Jo, Y. Yoo, T. Jeong, S. Ahn, M.J. Ko, Electronic Materials Letters 14, 657–668 (2018) J. Yang, B.D. Siempelkamp, D. Liu, T.L. Kelly, ACS Nano 9, 1955–1963 (2015) J.A. Christians, P.A. Miranda Herrera, P.V. Kamat, Journal of the American Chemical Society 137, 1530–1538 (2015) S. Castro-Hermosa, G. Lucarelli, M. Top, M. Fahland, J. Fahlteich, T.M. Brown, Cell Reports Physical Science 1, 100045 (2020) B. Dou, E.M. Miller, J.A. Christians, E.M. Sanehira, T.R. Klein, F.S. Barnes, S.E. Shaheen, S.M. Garner, S. Ghosh, A. Mallick, D. Basak, M.F.A.M. van Hest, The Journal of Physical Chemistry Letters 8, 4960–4966 (2017) J.J. Yoo, G. Seo, M.R. Chua, T.G. Park, Y. Lu, F. Rotermund, Y.-K. Kim, C.S. Moon, N.J. Jeon, J.-P. Correa-Baena, V. Bulović, S.S. Shin, M.G. Bawendi, J. Seo, Nature 590, 587–593 (2021) M. Kim, J. Jeong, H. Lu, T.K. Lee, F.T. Eickemeyer, Y. Liu, I.W. Choi, S.J. Choi, Y. Jo, H.-B. Kim, S.-I. Mo, Y.-K. Kim, H. Lee, N.G. An, S. Cho, W.R. Tress, S.M. Zakeeruddin, A. Hagfeldt, J.Y. Kim, M. Grätzel, D.S. Kim, Science 375, 302–306 (2022) H.G. Lemos, J.H.H. Rossato, R.A. Ramos, J.V.M. Lima, L.J. Affonço, S. Trofimov, J.J.I. Michel, S.L. Fernandes, B. Naydenov, C.F.O. Graeff, Journal of Materials Chemistry C 11, 3571–3580 (2023) I. Jeong, H. Jung, M. Park, J.S. Park, H.J. Son, J. Joo, J. Lee, M.J. Ko, Nano Energy 28, 380–389 (2016) G.S. Han, J. Kim, S. Bae, S. Han, Y.J. Kim, O.Y. Gong, P. Lee, M.J. Ko, H.S. Jung, ACS Energy Letters 4, 1845–1851 (2019) K. Wang, S. Olthof, W.S. Subhani, X. Jiang, Y. Cao, L. Duan, H. Wang, M. Du, S. Liu, Nano Energy 68, 104289 (2020) I. Jeong, Y.H. Park, S. Bae, M. Park, H. Jeong, P. Lee, M.J. Ko, ACS Applied Materials & Interfaces 9, 36865–36874 (2017) W. Xiang, S. Liu, W. Tress, Angewandte Chemie International Edition 60, 26440–26453 (2021) Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 5, 5784 (2014) Z. Li, Z. Wang, C. Jia, Z. Wan, C. Zhi, C. Li, M. Zhang, C. Zhang, Z. Li, Nano Energy 94, 106919 (2022) D. Yang, R.X. Yang, K. Wang, C.C. Wu, X.J. Zhu, J.S. Feng, X.D. Ren, G.J. Fang, S. Priya, S.Z. Liu, Nat. Commun. 9 (2018) Supplementary Files SI240614.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 18 Jun, 2024 Reviewers invited by journal 18 Jun, 2024 Editor assigned by journal 18 Jun, 2024 First submitted to journal 16 Jun, 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-4591034","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":315786174,"identity":"f6795cc1-663f-4a33-a31b-1fb7a9531de5","order_by":0,"name":"Wooyeon Kim","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Wooyeon","middleName":"","lastName":"Kim","suffix":""},{"id":315786175,"identity":"3ea7ae26-3b21-4293-974a-54758a62185f","order_by":1,"name":"Jian Cheng","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Jian","middleName":"","lastName":"Cheng","suffix":""},{"id":315786176,"identity":"2260ac27-df02-4247-aaf7-5cd188a3e3fb","order_by":2,"name":"Joonwon Choi","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Joonwon","middleName":"","lastName":"Choi","suffix":""},{"id":315786177,"identity":"7271459d-4cef-4307-aef2-d05f38eb07ac","order_by":3,"name":"Seoyeong Lee","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Seoyeong","middleName":"","lastName":"Lee","suffix":""},{"id":315786178,"identity":"19adba7d-3e75-46e1-a9a2-5970811f21ae","order_by":4,"name":"Yongwoo Lee","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Yongwoo","middleName":"","lastName":"Lee","suffix":""},{"id":315786179,"identity":"6cdea129-7374-4bd4-a3de-d0ff084c99bf","order_by":5,"name":"Doyeon Lee","email":"","orcid":"","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":false,"prefix":"","firstName":"Doyeon","middleName":"","lastName":"Lee","suffix":""},{"id":315786180,"identity":"2178d7e4-025b-49ae-9820-daa6cc80939f","order_by":6,"name":"Min Jae Ko","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3ElEQVRIiWNgGAWjYFCCYwwMHxuQ+BLEaGGcSaIWNgZmXpK06DYeS3xsu8Muz+D42cMvGGrsGCRnH8CvxezAscPGuWeSiw3O5KVZMBxLZpDmSyCk5XibdG4bc+KGAzlmBgxsBxjkeAg4DKzFsq0+ccP5N0At/4jScuyYNGPb4cQNN3KMHzC2HWCQJkJLsmFv2/HEmTfemDEk9iXzSPYQ0nLjmOGDn23ViX3nc4w/fPhmJydxhoAWBokDEFrhAAObRAIDAyFnAQF/A4SWb2Bg/kBY+SgYBaNgFIxEAABitkiXyJRJXwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-4842-3235","institution":"Hanyang University - Seoul Campus: Hanyang University","correspondingAuthor":true,"prefix":"","firstName":"Min","middleName":"Jae","lastName":"Ko","suffix":""}],"badges":[],"createdAt":"2024-06-16 23:43:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4591034/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4591034/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":59629842,"identity":"07388317-94e7-4fa5-9eeb-795e838d5429","added_by":"auto","created_at":"2024-07-04 04:57:11","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":88495,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Schematic illustration of the UV sintering process for SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer formation. \u003cstrong\u003eb\u003c/strong\u003e Transmittance spectra of SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e films on glass/ITO. \u003cstrong\u003ec\u003c/strong\u003e Energy level diagrams of SnO\u003csub\u003e2\u003c/sub\u003e, TiO\u003csub\u003e2\u003c/sub\u003e, and perovskite calculated from the Tauc plot and UPS spectrum. \u003cstrong\u003ed\u003c/strong\u003e Steady-state PL spectra of the perovskite films deposited on SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2 \u003c/sub\u003efilms. \u003cstrong\u003ee\u003c/strong\u003e TRPL spectra with an excitation wavelength of 463 nm for perovskite films deposited on SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2 \u003c/sub\u003efilms.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/a11688f5519838dbd1dddb65.jpg"},{"id":59630069,"identity":"173f25d2-2ead-4296-88a8-582e5868ad6e","added_by":"auto","created_at":"2024-07-04 05:05:11","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":90857,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional SEM image of \u003cstrong\u003ea\u003c/strong\u003e SnO\u003csub\u003e2\u003c/sub\u003e and \u003cstrong\u003eb\u003c/strong\u003e SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer ETL-based PSCs. \u003cstrong\u003ec\u003c/strong\u003e \u003cem\u003eJ\u003c/em\u003e−\u003cem\u003eV\u003c/em\u003e characteristics of conventional glass/ITO substrate-based PSCs using SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer ETLs. \u003cstrong\u003ed \u003c/strong\u003e\u003cem\u003eJ\u003c/em\u003e−\u003cem\u003eV\u003c/em\u003e curves of UTG/ITO substrate-based PSCs with various ETLs.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/364c78d338b69d6fe39422b5.jpg"},{"id":59629843,"identity":"9565b52b-f8a3-4575-8127-e96e4f130a6a","added_by":"auto","created_at":"2024-07-04 04:57:11","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":82194,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea \u003c/strong\u003eAFM images of conventional glass/ITO and UTG/ITO substrates. \u003cstrong\u003eb \u003c/strong\u003eAFM images of SnO\u003csub\u003e2\u003c/sub\u003e layer and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer coated on a glass/ITO substrate. \u003cstrong\u003ec \u003c/strong\u003eSchematic representations of PSCs using SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e ETLs.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/fecbc2dbc767e75224ec5142.jpg"},{"id":59629844,"identity":"3939f24d-8c77-44da-8a8a-59ce6e82cc54","added_by":"auto","created_at":"2024-07-04 04:57:11","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":58695,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea \u003c/strong\u003eNormalized \u003cem\u003ePCE\u003c/em\u003e obtained from moisture permeability tests of PEN/ITO, and UTG/ITO substrate-based PSCs. \u003cstrong\u003eb \u003c/strong\u003eOptical images of\u003cstrong\u003e \u003c/strong\u003eeach PSC before and after the moisture permeability test.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/2ad968596be14ee4eae27da2.jpg"},{"id":59630987,"identity":"a50ba68c-178b-4abf-8122-84303014eb5d","added_by":"auto","created_at":"2024-07-04 05:21:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":811464,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/4fed8e6b-5a0d-48f0-8973-d2e53d2a1c02.pdf"},{"id":59629846,"identity":"33e747bb-fcd3-4a64-98fa-3ff38831b535","added_by":"auto","created_at":"2024-07-04 04:57:11","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":1119140,"visible":true,"origin":"","legend":"","description":"","filename":"SI240614.docx","url":"https://assets-eu.researchsquare.com/files/rs-4591034/v1/03fe53753802a187effec5ac.docx"}],"financialInterests":"","formattedTitle":"Ultrathin Glass-based Perovskite Solar Cells Employing Bilayer Electron Transport Layer","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOrganic-inorganic hybrid perovskite solar cells (PSCs) have recently achieved a power conversion efficiency (\u003cem\u003ePCE\u003c/em\u003e) exceeding 26% [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. This achievement is due to the high absorption coefficient, high charge mobility, and low exciton binding energy inherent to perovskite materials [\u003cspan additionalcitationids=\"CR3 CR4\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. A notable advantage of these materials is their exceptional mechanical flexibility, which stems from their hybrid organic-inorganic structure [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Consequently, flexible PSCs have been extensively investigated, demonstrating high mechanical stability and performance [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFabricating flexible PSCs typically involves constructing each layer from a polymer substrate. To provide the flexibility, polymer substrates such as polyethylene naphthalate (PEN), known for their suitable optical and mechanical properties, are commonly used [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. However, these polymer substrates exhibit higher moisture and oxygen permeability than glass substrates [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Considering that perovskite materials are easily degraded by moisture and oxygen, polymer substrates are less suitable for long-term applications [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Ultrathin glass (UTG) substrates present a viable alternative to polymer substrates. With a thickness of less than 100 \u0026micro;m, UTG maintains adequate flexibility while offering the advantageous properties of glass, including a moisture permeability over 100,000 times lower than that of polymer substrates [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Consequently, UTG-based flexible PSCs are expected to achieve long-term moisture stability.\u003c/p\u003e \u003cp\u003eAnother key consideration in fabricating flexible PSCs is the selection of an efficient electron transport layer (ETL). The ETL is a crucial component in PSCs, significantly affecting their performance and stability [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Positioned between the transparent electrode and the perovskite layer, the ETL efficiently extracts photogenerated electrons from the perovskite layer to the electrode. For effective hole blocking, it is essential for the ETL to completely cover the bottom electrode. Additionally, the ETL should exhibit favorable energy level alignment between the perovskite and the transparent electrode to ensure efficient electron extraction. One strategy to achieve this is the implementation of a bilayer ETL with a favorable energy level design. Recent studies have indicated that bilayer ETLs can provide higher \u003cem\u003ePCE\u003c/em\u003e and enhanced stability than monolayer ETLs [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, we designed a SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer for the ETL to fabricate flexible PSCs. Initially, we synthesized colloidal TiO\u003csub\u003e2\u003c/sub\u003e nanocrystals (NCs), which were capped with oleic acid (OA) to ensure dispersibility in hexane, a non-polar solvent. To prepare the bilayer ETL, TiO\u003csub\u003e2\u003c/sub\u003e dispersion was spin-coated onto a SnO\u003csub\u003e2\u003c/sub\u003e film, and the organic ligands were removed via UV irradiation. We confirmed that this TiO\u003csub\u003e2\u003c/sub\u003e layer possesses favorable energy level alignment between the SnO\u003csub\u003e2\u003c/sub\u003e and perovskite layers, resulting in efficient charge extraction. The bilayer ETL-based PSCs exhibited a \u003cem\u003ePCE\u003c/em\u003e of 23.5%, an improvement over the 22.7% \u003cem\u003ePCE\u003c/em\u003e of PSCs with a SnO\u003csub\u003e2\u003c/sub\u003e monolayer ETL. Moreover, PSCs using UTG substrates showed significant differences in \u003cem\u003ePCE\u003c/em\u003e, with bilayer and monolayer ETLs achieving 15.2% and 8.3%, respectively. We demonstrated that the performance degradation due to UTG roughness is mitigated by employing the bilayer ETL. Furthermore, we highlighted the effectiveness of the UTG substrate in preventing moisture penetration by comparing UTG-based PSCs with PEN-based PSCs.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cp\u003e \u003cb\u003eChemicals\u003c/b\u003e \u003c/p\u003e \u003cp\u003eTin(IV) oxide (SnO\u003csub\u003e2\u003c/sub\u003e NPs, 15% in H\u003csub\u003e2\u003c/sub\u003eO colloidal dispersion), formamidine acetate salt (99%), and lead(II) iodide (PbI\u003csub\u003e2\u003c/sub\u003e, 99.999%) were purchased from Alfa Aesar. Oleic acid (OA, 90%), titanium(IV) isopropoxide (TTIP, 97%), n-hexane (anhydrous, 95%), hydroiodic acid (HI, 57 wt% in water), 2-methoxyethanol (2ME, anhydrous, 99.8%), 2-propanol (IPA, anhydrous, 99.5%), chlorobenzene (CB, anhydrous, 99.8%), \u003cem\u003eN,N\u003c/em\u003e-dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, \u0026gt;\u0026thinsp;99.5%), methylammonium chloride (MACl, 98%), acetonitrile (ACN, anhydrous, 99.8%), and 4-\u003cem\u003etert\u003c/em\u003e-butylpyridine (tBP, 98%) were purchased from Sigma-Aldrich. Phenethylammonium iodide (PEAI) was acquired from Greatcell Solar. Ethyl alcohol (EtOH, pure) and diethyl ether (extra pure grade) were purchased from Duksan. 2,2',7,7'-Tetrakis(N,\u003cem\u003eN\u003c/em\u003e-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209) were obtained from Lumtec. Ultrathin glass (UTG, 50 \u0026micro;m) based tin-doped indium oxide (ITO) substrates were purchased from Quantum Plasma, Korea.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of colloidal TiO\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003enanocrystals\u003c/b\u003e\u003c/p\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e NCs were synthesized through a modified non-hydrolytic sol-gel reaction method, as reported previously [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The synthesis began by combining 88 mmol of OA and 30 mmol of TTIP in a three-neck round-bottom flask under a continuous argon (Ar) gas flow. This mixture was heated to 270\u0026deg;C for 2 h with constant stirring. During the formation of TiO\u003csub\u003e2\u003c/sub\u003e NCs, the reaction mixture's color transitioned from transparent yellow to white. The precipitates were washed with excess ethanol and purified by centrifugation at 3000 rpm for 30 min. The final product, dissolved in n-hexane at 25 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, was stored under an N\u003csub\u003e2\u003c/sub\u003e gas within a glove box.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of the perovskite precursor\u003c/b\u003e \u003c/p\u003e \u003cp\u003eFAI and FAPbI\u003csub\u003e3\u003c/sub\u003e powders were synthesized using previously established methods [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. To synthesize FAI powder, formamidine acetate salt (25 g) was mixed with HI (50 mL) in a 500 mL round-bottom flask and stirred vigorously. The solvent was evaporated under vacuum at 80\u0026deg;C for 1 h, yielding a light-yellow powder. This powder was dissolved in ethanol and precipitated with diethyl ether, repeated three times until a pure white powder was obtained. For further purification, the FAI powder was recrystallized using an ethanol and diethyl ether mixture (1:1 v/v) in a refrigerator. The final white powder was filtrated through a glass filter and dried at 60\u0026deg;C for 24 h. For synthesizing FAPbI\u003csub\u003e3\u003c/sub\u003e powder, the prepared FAI (0.8 M) was mixed with PbI\u003csub\u003e2\u003c/sub\u003e in a 1:1 molar ratio in 2ME with vigorous stirring. The resulting yellow solution was heated and stirred vigorously at 120\u0026deg;C, then subjected to recrystallization using the retrograde method. The final black powder was filtered through a glass filter and baked at 150\u0026deg;C for 1 h to obtain the desired FAPbI\u003csub\u003e3\u003c/sub\u003e powder.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFabrication of perovskite solar cells\u003c/b\u003e \u003c/p\u003e \u003cp\u003ePatterned ITO substrates were sequentially cleaned by sonication in detergent water, deionized water, and IPA for 10 min each. To produce SnO\u003csub\u003e2\u003c/sub\u003e ETLs, the SnO\u003csub\u003e2\u003c/sub\u003e NCs were diluted with deionized water (SnO\u003csub\u003e2\u003c/sub\u003e NCs:H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;=\u0026thinsp;1:6). This solution was dropped onto UV-ozone-treated transparent ITO substrates and spin-coated at 3000 rpm for 30 s, followed by annealing at 150\u0026deg;C for 30 min. To prepare the SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer electron transport layer, a highly dispersed solution of TiO\u003csub\u003e2\u003c/sub\u003e NCs in n-hexane (25 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was dropped onto a transparent conducting oxide (TCO) substrate and immediately spin-coated at 5000 rpm for 30 s. The TiO\u003csub\u003e2\u003c/sub\u003e NC-coated substrate was then treated with UV irradiation using a UV cure machine (JHCI-051B, JECO) equipped with a mercury lamp (peak wavelength: 365 nm). For perovskite layer deposition, the UV-ozone-treated substrate was spin-coated with the perovskite precursor solution at 8000 rpm for 50 s. After 10 s of spin coating, 1 mL of diethyl ether was dropped onto the spinning film. The deposited film was then annealed at 150\u0026deg;C for 15 min and 100\u0026deg;C for 1 h on a hotplate. The perovskite precursor solution was prepared by dissolving 778 mg of synthesized FAPbI\u003csub\u003e3\u003c/sub\u003e powder and 23 mg of MACl in 0.5 mL of DMF/DMSO (4:1 v/v). After cooling, the perovskite films were coated with PEAI (3 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in IPA) at 3000 rpm for 30 s. Spiro-OMeTAD, the hole transport layer (HTL) in the PSCs, was spin-coated onto the films at 4000 rpm for 30 s. The Spiro-OMeTAD solution was prepared by doping 90 mg/mL Spiro-OMeTAD in CB with 39 \u0026micro;L of tBP, 23 \u0026micro;L of Li-TFSI (520 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in ACN), and 5 \u0026micro;L of FK209 (180 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in ACN). Finally, an Au electrode with a thickness of 80 nm was deposited using a thermal evaporator.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCharacterization\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe crystal structure of the TiO\u003csub\u003e2\u003c/sub\u003e NCs was characterized using a high-resolution X-ray diffractometer (XRD, D8 ADVANCE, BRUKER) with Cu-Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.541 \u0026Aring;). The morphology of SnO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e NCs was analyzed with transmission electron microscopy (TEM, JEM2100F, JEOL), while their films were examined using scanning electron microscopy (FE-SEM, Verios G4 UC, FEI) and atomic force microscopy (AFM, XE-100, Park Systems). The optical properties of the thin films were investigated using a UV\u0026ndash;Vis spectrometer (Shimadzu, UV-2600i). The absorption coefficient (α) for direct bandgap semiconductors near the band edge was calculated using the formula αhν\u0026thinsp;=\u0026thinsp;A(hν\u0026ndash;E\u003csub\u003eg\u003c/sub\u003e)\u003csup\u003e2\u003c/sup\u003e, where A denotes the proportionality constant, hν denotes the photon energy, and E\u003csub\u003eg\u003c/sub\u003e denotes the optical bandgap. Specifically, E\u003csub\u003eg\u003c/sub\u003e was estimated by extrapolating the tangent line to the plotted curve. ATR\u0026ndash;FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific) was employed to measure ligand removal after UV treatment. Steady-state PL and TRPL spectra were obtained using a Fluoromax-4 (iHR320, HORIBA Scientific) spectrometer, with an excitation wavelength of 463 nm. TRPL measurements were conducted to determine the PL maximum (~\u0026thinsp;800 nm) of the perovskite film. Under dry conditions (20\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C, below 15% RH), the current density\u0026ndash;voltage (\u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e) curve and MPP tracking characteristics of the devices were measured using a Keithley 2400 source meter under a Xenon-lamp-based solar simulator (Newport 91160s, AAA class). A Si solar cell calibrated by the National Renewable Energy Laboratory (NREL) was used to adjust the light intensity to AM 1.5G 1 sun conditions (100 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e). The voltage sweep rate was 20 mV with a delay of 20 ms for the reverse (forward) sweeping direction from 1.3 V (\u0026minus;\u0026thinsp;0.1 V) to \u0026minus;\u0026thinsp;0.1 V (1.3 V). The devices were measured with a SUS304 aperture with an area of 0.09 cm\u003csup\u003e2\u003c/sup\u003e. The incident photon-to-current conversion efficiency (IPCE) was characterized using an IPCE measurement system (PV Measurement, Inc.).\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eSnO\u003csub\u003e2\u003c/sub\u003e NCs are commonly used as the ETL in PSCs [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. However, achieving perfect hole-blocking ability with SnO\u003csub\u003e2\u003c/sub\u003e ETL alone is challenging because it is difficult to fully cover the surface while maintaining a thin layer. As the thickness of the SnO\u003csub\u003e2\u003c/sub\u003e ETL increases, the \u003cem\u003ePCE\u003c/em\u003e tends to decrease due to heightened resistance [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e]. A facile strategy to address this issue is the sequential deposition of different ETLs to create a bilayer ETL. When forming a bilayer ETL, the lower layer should possess a higher conduction band minimum (CBM) to enhance electron extraction [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]. Anatase TiO\u003csub\u003e2\u003c/sub\u003e, with a slightly higher CBM than SnO\u003csub\u003e2\u003c/sub\u003e, is an appropriate choice [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]. Consequently, we developed a bilayer ETL with a TiO\u003csub\u003e2\u003c/sub\u003e layer on top of a SnO\u003csub\u003e2\u003c/sub\u003e layer.\u003c/p\u003e\n\u003cp\u003eTo minimize the impact on the SnO\u003csub\u003e2\u003c/sub\u003e layer, which can dissolve in polar solvents, we synthesized TiO\u003csub\u003e2\u003c/sub\u003e NCs dispersed in a non-polar solvent. The transmission electron microscopy (TEM) image of the synthesized TiO\u003csub\u003e2\u003c/sub\u003e NCs is shown in \u003cstrong\u003eFigure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/strong\u003e, confirming the rod-shaped nanocrystals as previously reported [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. These TiO\u003csub\u003e2\u003c/sub\u003e NCs are capped with a long-chain organic ligand (OA) to facilitate dispersion. However, these surface stabilizers were removed by UV irradiation in the thin-film state, as they would impede charge transport [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. The removal of organic ligands by UV irradiation was confirmed by ATR-FTIR analysis (\u003cstrong\u003eFigure S2\u003c/strong\u003e). The UV-sintered TiO\u003csub\u003e2\u003c/sub\u003e film does not exist strong vibration peaks at 2925 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and 2855 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, corresponding to the asymmetric and symmetric CH\u003csub\u003e2\u003c/sub\u003e stretching vibrations of OA. Other vibration peaks of the organic ligands, such as COO\u003csup\u003e\u0026ndash;\u003c/sup\u003e (1525 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, asymmetric) and CH\u003csub\u003e2\u003c/sub\u003e (1440 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, bending), also disappear after the UV sintering.\u003c/p\u003e\n\u003cp\u003eX-ray diffraction (XRD) patterns of the TiO\u003csub\u003e2\u003c/sub\u003e film showed that all reflection peaks correspond to the anatase TiO\u003csub\u003e2\u003c/sub\u003e crystal structure (PDF #21-1272) (\u003cstrong\u003eFigure S3\u003c/strong\u003e). The SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer film was prepared using the UV-sintering method for the ETL, as depicted in the scheme of Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea. We initially compared the transmittance of both SnO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e films. The UV-Vis transmittance of each layer on glass/ITO substrates is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb. The SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e film demonstrated a slightly higher transmittance than the SnO\u003csub\u003e2\u003c/sub\u003e film at 350\u0026ndash;500 nm, indicating that more near-UV light could reach the light-absorbing layer. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec illustrates the band alignment for the designed ETLs, with this band diagram estimated from ultraviolet photoelectron spectroscopy (UPS) and Tauc plots in \u003cstrong\u003eFigure S4\u003c/strong\u003e. The estimated CBM of SnO\u003csub\u003e2\u003c/sub\u003e, TiO\u003csub\u003e2\u003c/sub\u003e, and perovskite are \u0026minus;\u0026thinsp;4.30 eV, -4.20 eV, and \u0026minus;\u0026thinsp;3.96 eV, respectively. The results indicate that the CBMs of TiO\u003csub\u003e2\u003c/sub\u003e and SnO\u003csub\u003e2\u003c/sub\u003e are arranged in a cascade, providing favorable energy levels for charge extraction [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eThe steady-state TRPL decay was measured to investigate the charge transfer ability at the interface of each ETL and the perovskite. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed illustrates that devices utilizing the bilayer ETL exhibit more effective PL quenching than those employing only the SnO\u003csub\u003e2\u003c/sub\u003e layer, indicating that charge extraction from perovskite to the bilayer ETL is more efficient than with only a SnO\u003csub\u003e2\u003c/sub\u003e layer [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e]. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the PL decay transients of perovskite coated on each ETL, from which the corresponding PL lifetimes and amplitudes were obtained by fitting with a bi-exponential decay function, as listed in \u003cstrong\u003eTable \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/strong\u003e. The fast decay component (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{1}\\)\u003c/span\u003e\u003c/span\u003e) originates from the quenching of charge carriers at the interface, while the slow decay component (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{2}\\)\u003c/span\u003e\u003c/span\u003e) is attributed to the radiative recombination of free charge carriers due to traps in the absorption layer [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]. Compared to the perovskite-only film, the samples using ETL exhibited a shorter \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{1}\\)\u003c/span\u003e\u003c/span\u003e with an increasing amplitude of the fast decay (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({A}_{1}\\)\u003c/span\u003e\u003c/span\u003e), indicating that charge transfer from the perovskite film to the ETL became dominant. The average lifetime for the SnO\u003csub\u003e2\u003c/sub\u003e/perovskite sample was 251.88 ns, which reduced to 172.13 ns for the bilayer ETL. Additionally,\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{1}\\)\u003c/span\u003e\u003c/span\u003e decreased from 164.17 ns to 71.65 ns in the bilayer ETL sample, with an increasing \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({A}_{1}\\)\u003c/span\u003e\u003c/span\u003e. This implies that the bilayer ETL is more efficient in extracting charge from the perovskite.\u003c/p\u003e\n\u003cp\u003eTo compare the thickness and uniformity of each SnO\u003csub\u003e2\u003c/sub\u003e film, we used scanning electron microscopy (SEM) to measure each ETL-based PSC (device structure: ITO/ETL/perovskite/Spiro-OMeTAD/Au). These images show thicknesses of 20\u0026ndash;30 nm for the SnO\u003csub\u003e2\u003c/sub\u003e layer and 60\u0026ndash;80 nm for the bilayer (Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). Additionally, they indicate that the bilayer ETL is deposited uniformly. We also compared the morphology of perovskite films deposited on each ETL layer, revealing that the underlying layer does not significantly affect the perovskite crystal grains (\u003cstrong\u003eFigure S5\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the \u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e curves of each ETL-based solar cell, with their photovoltaic performances summarized in \u003cstrong\u003eTable S2\u003c/strong\u003e. The bilayer ETL-based device exhibited a higher \u003cem\u003ePCE\u003c/em\u003e compared to the SnO\u003csub\u003e2\u003c/sub\u003e-based devices, achieving a \u003cem\u003ePCE\u003c/em\u003e of 23.47% (open-circuit voltage (\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;1.19 V, short-circuit current density (\u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e)\u0026thinsp;=\u0026thinsp;23.43 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, fill factor (\u003cem\u003eFF\u003c/em\u003e)\u0026thinsp;=\u0026thinsp;84.07%). In contrast, the SnO\u003csub\u003e2\u003c/sub\u003e ETL-based device had a PCE of 22.66% (\u003cem\u003eV\u003c/em\u003e\u003csub\u003eOC\u003c/sub\u003e = 1.19 V, \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e = 22.87 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, \u003cem\u003eFF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;83.04%). We measured the IPCE spectra to assess the charge collection efficiency of each ETL on the photocurrent of PSCs. The \u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e characteristics measured in both reverse and forward directions of the devices show negligible differences (\u003cstrong\u003eFigure S6a\u003c/strong\u003e). \u003cstrong\u003eFigure S6b\u003c/strong\u003e presents the IPCE spectra of solar cells with different ETLs, which align well with the values obtained from the \u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e curves in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec. The solar cells based on the bilayer ETL exhibited higher IPCE values with a strong spectral response from 350 to 500 nm. Photons with shorter wavelengths have a shallower penetration depth and are typically absorbed near the front region of the perovskite layer [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]. Therefore, the observed increase in IPCE values for shorter wavelengths implies a more efficient electron extraction at the interface between the SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer and the perovskite, as well as improved optical transparency of the bilayer ETL.\u003c/p\u003e\n\u003cp\u003eFor flexibility, we fabricated a device with the same structure on an ITO-deposited ultrathin glass (UTG) substrate. The thickness of conventional glass/ITO is 1100 \u0026micro;m, whereas the UTG/ITO used is 50 \u0026micro;m. It is noteworthy that performance differences between each ETL-based PSC are more pronounced when using UTG substrates. In Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed, we compared the \u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e curves of each ETL-based solar cell employing UTG substrates. The bilayer ETL demonstrated a \u003cem\u003ePCE\u003c/em\u003e of 15.2%, while the SnO\u003csub\u003e2\u003c/sub\u003e ETL-based device exhibited a significantly lower \u003cem\u003ePCE\u003c/em\u003e of 8.3%.\u003c/p\u003e\n\u003cp\u003eTo understand this performance difference, we compared the properties of conventional ITO substrates and UTG substrates. The UV-Vis transmittance of each substrate revealed slightly lower transmittance for UTG substrates (\u003cstrong\u003eFigure S7\u003c/strong\u003e). This might explain the lower \u003cem\u003eJ\u003c/em\u003e\u003csub\u003eSC\u003c/sub\u003e value of UTG-based devices. Furthermore, we compared the roughness of each substrate using atomic force microscopy (AFM). As shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, the roughness of the UTG substrate (RMS of 81.0 nm) is significantly higher than that of the conventional substrate (RMS of 1.4 nm). Additionally, we compared films deposited with a SnO\u003csub\u003e2\u003c/sub\u003e layer and a SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer on the UTG substrate (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb). The SnO\u003csub\u003e2\u003c/sub\u003e film (RMS of 2.6 nm) exhibited higher roughness than the bilayer film (RMS of 1.0 nm). Moreover, images of the SnO\u003csub\u003e2\u003c/sub\u003e film revealed that SnO\u003csub\u003e2\u003c/sub\u003e alone could not sufficiently cover the high roughness of the UTG surface, exposing the ITO layer (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). These results indicate that the bilayer ETL not only improves the performance of PSCs but also provides sufficient coverage even for films with significant roughness.\u003c/p\u003e\n\u003cp\u003eWe fabricated devices using UTG/ITO and PEN/ITO substrates to assess their moisture permeability. Water was applied to the back side of the device in direct contact with the substrate, and the \u003cem\u003ePCE\u003c/em\u003e was monitored for over 1000 min. The initial \u003cem\u003eJ\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003eV\u003c/em\u003e curves of the devices used for comparison are depicted in \u003cstrong\u003eFigure S8\u003c/strong\u003e. The PEN-based device achieved a PCE of 18.0%, slightly higher performance than the UTG-based device, which had a PCE of 15.2%. After 1000 min, the PEN-based device showed a 71% reduction in its initial \u003cem\u003ePCE\u003c/em\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea). Conversely, the UTG-based device exhibited a relatively low moisture permeability, with only a 5% reduction in initial \u003cem\u003ePCE\u003c/em\u003e. The optical images in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb visually illustrate the state of the devices, revealing that the perovskite layer in the PEN-based device dissolved, whereas the UTG-based device did not. Given that water was applied only to the backsides of the devices, this indicates that moisture penetration from the substrate influenced the outcome. Thus, UTG substrates are recommended to fabricate flexible perovskite solar cells that are moisture-resistant.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, we demonstrated that a strategically designed SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer is an efficient ETL for PSCs, enhancing charge carrier extraction from the perovskite to the transparent electrode by modifying the ETL's energy levels. This bilayer ETL proved more effective on substrates with high roughness, such as UTG substrates, due to its superior coverage compared to a monolayer ETL, resulting in better hole-blocking properties. Consequently, the \u003cem\u003ePCE\u003c/em\u003e of bilayer ETL-based PSCs improved to 15.2%, compared to 8.3% for monolayer ETL-based PSCs. This demonstrates the bilayer ETL's efficiency even on substrates with significant roughness.\u003c/p\u003e \u003cp\u003eFurthermore, we emphasized the low moisture permeability of UTG substrates compared to conventional polymer substrates. Moisture permeability tests indicated that devices based on polymer substrates experience a sharp decline in PCE, while those based on UTG substrates maintain their PCE. This suggests the potential of UTG substrates for the long-term use of flexible PSCs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data are available upon request from the authors, and access to the data requires approval from the researchers.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is the result of a research project conducted with the funds of the Open R\u0026amp;D program of Korea Electric Power Corporation. (R23XH02) This work was also supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (Sector coupling energy industry advancement manpower training program, 20224000000440).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThese authors contributed equally: Wooyeon Kim,\u0026nbsp;Jian Cheng, and Joonwon Choi\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors and Affiliations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDepartment of Chemical Engineering, Hanyang University, Seoul, 04763, Republic of Korea\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWooyeon Kim, Jian Cheng, Joonwon Choi, Seoyeong Lee, Yongwoo Lee, Doyeon Lee, Min Jae Ko\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorrespondence to Min Jae Ko\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eH. Chen, C. Liu, J. Xu, A. Maxwell, W. Zhou, Y. Yang, Q. Zhou, A.S.R. Bati, H. Wan, Z. Wang, L. Zeng, J. Wang, P. Serles, Y. Liu, S. Teale, Y. Liu, M.I. Saidaminov, M. Li, N. Rolston, S. Hoogland, T. Filleter, M.G. Kanatzidis, B. Chen, Z. Ning, E.H. Sargent, Science 384, 189\u0026ndash;193 (2024)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eE.M. Hutter, M.C. G\u0026eacute;lvez-Rueda, A. Osherov, V. Bulović, F.C. Grozema, S.D. Stranks, T.J. Savenije, Nature Materials 16, 115\u0026ndash;120 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy \u0026amp; Environmental Science 7, 982\u0026ndash;988 (2014)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ. Zhang, W. Kim, M.J. Ko, Y. Li, Nano Convergence 10, 23 (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Bae, J.W. Jo, P. Lee, M.J. Ko, ACS Applied Materials \u0026amp; Interfaces 11, 17452\u0026ndash;17458 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eF. Song, D. Zheng, J. Feng, J. Liu, T. Ye, Z. Li, K. Wang, S. Liu, D. Yang, Advanced Materials 36, 2312041 (2024)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Feng, APL Materials 2, (2014)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH.S. Jung, G.S. Han, N.-G. Park, M.J. Ko, Joule 3, 1850\u0026ndash;1880 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD.S. Lee, K.W. Kim, Y.-H. Seo, M.H. Ann, W. Lee, J. Nam, J. Chung, G. Seo, S. Nam, B.S. Ma, T.-S. Kim, Y. Kang, N.J. Jeon, J. Seo, S.S. Shin, Joule 8, 1380\u0026ndash;1393 (2024)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. Kim, J. Kim, D. Kim, B. Koo, S. Yu, Y. Li, Y. Kim, M.J. Ko, npj Flexible Electronics 8, 20 (2024)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Chung, S.S. Shin, K. Hwang, G. Kim, K.W. Kim, D.S. Lee, W. Kim, B.S. Ma, Y.-K. Kim, T.-S. Kim, J. Seo, Energy \u0026amp; Environmental Science 13, 4854\u0026ndash;4861 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Park, J.-Y. Kim, H.J. Son, C.-H. Lee, S.S. Jang, M.J. Ko, Nano Energy 26, 208\u0026ndash;215 (2016)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Castro-Hermosa, M. Top, J. Dagar, J. Fahlteich, T.M. Brown, Advanced Electronic Materials 5, 1800978 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.B.M. Jaime, R.M.V. Alves, P.F.J. B\u0026oacute;coli, Journal of Drug Delivery Science and Technology 71, 103330 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ.W. Jo, Y. Yoo, T. Jeong, S. Ahn, M.J. Ko, Electronic Materials Letters 14, 657\u0026ndash;668 (2018)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Yang, B.D. Siempelkamp, D. Liu, T.L. Kelly, ACS Nano 9, 1955\u0026ndash;1963 (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ.A. Christians, P.A. Miranda Herrera, P.V. Kamat, Journal of the American Chemical Society 137, 1530\u0026ndash;1538 (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Castro-Hermosa, G. Lucarelli, M. Top, M. Fahland, J. Fahlteich, T.M. Brown, Cell Reports Physical Science 1, 100045 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. Dou, E.M. Miller, J.A. Christians, E.M. Sanehira, T.R. Klein, F.S. Barnes, S.E. Shaheen, S.M. Garner, S. Ghosh, A. Mallick, D. Basak, M.F.A.M. van Hest, The Journal of Physical Chemistry Letters 8, 4960\u0026ndash;4966 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ.J. Yoo, G. Seo, M.R. Chua, T.G. Park, Y. Lu, F. Rotermund, Y.-K. Kim, C.S. Moon, N.J. Jeon, J.-P. Correa-Baena, V. Bulović, S.S. Shin, M.G. Bawendi, J. Seo, Nature 590, 587\u0026ndash;593 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Kim, J. Jeong, H. Lu, T.K. Lee, F.T. Eickemeyer, Y. Liu, I.W. Choi, S.J. Choi, Y. Jo, H.-B. Kim, S.-I. Mo, Y.-K. Kim, H. Lee, N.G. An, S. Cho, W.R. Tress, S.M. Zakeeruddin, A. Hagfeldt, J.Y. Kim, M. Gr\u0026auml;tzel, D.S. Kim, Science 375, 302\u0026ndash;306 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH.G. Lemos, J.H.H. Rossato, R.A. Ramos, J.V.M. Lima, L.J. Affon\u0026ccedil;o, S. Trofimov, J.J.I. Michel, S.L. Fernandes, B. Naydenov, C.F.O. Graeff, Journal of Materials Chemistry C 11, 3571\u0026ndash;3580 (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI. Jeong, H. Jung, M. Park, J.S. Park, H.J. Son, J. Joo, J. Lee, M.J. Ko, Nano Energy 28, 380\u0026ndash;389 (2016)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG.S. Han, J. Kim, S. Bae, S. Han, Y.J. Kim, O.Y. Gong, P. Lee, M.J. Ko, H.S. Jung, ACS Energy Letters 4, 1845\u0026ndash;1851 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK. Wang, S. Olthof, W.S. Subhani, X. Jiang, Y. Cao, L. Duan, H. Wang, M. Du, S. Liu, Nano Energy 68, 104289 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI. Jeong, Y.H. Park, S. Bae, M. Park, H. Jeong, P. Lee, M.J. Ko, ACS Applied Materials \u0026amp; Interfaces 9, 36865\u0026ndash;36874 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. Xiang, S. Liu, W. Tress, Angewandte Chemie International Edition 60, 26440\u0026ndash;26453 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 5, 5784 (2014)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ. Li, Z. Wang, C. Jia, Z. Wan, C. Zhi, C. Li, M. Zhang, C. Zhang, Z. Li, Nano Energy 94, 106919 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Yang, R.X. Yang, K. Wang, C.C. Wu, X.J. Zhu, J.S. Feng, X.D. Ren, G.J. Fang, S. Priya, S.Z. Liu, Nat. Commun. 9 (2018)\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":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"electron transport layer, ultrathin glass, titanium oxide, perovskite solar cell, transparent conductive oxide","lastPublishedDoi":"10.21203/rs.3.rs-4591034/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4591034/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn recent studies, flexible perovskite solar cells (PSCs) have exhibited high power conversion efficiency (\u003cem\u003ePCE\u003c/em\u003e) coupled with remarkable mechanical stability. However, the conventional polymer substrates used in flexible PSCs possess high permeability to moisture and oxygen, leading to the rapid degradation of perovskite materials. In this work, we address these issues by employing ultrathin glass (UTG) substrates, which provide moisture impermeability while retaining flexibility. Additionally, we introduce a strategically designed SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer as the electron transport layer (ETL). Our results reveal that PSCs incorporating the bilayer ETL achieve higher \u003cem\u003ePCE\u003c/em\u003e than those with a monolayer ETL on conventional glass and UTG substrates. Furthermore, moisture permeability tests demonstrate that PSCs based on UTG substrates sustain their \u003cem\u003ePCE\u003c/em\u003e over time, compared to their polymer-based counterparts. These results imply that UTG substrates, combined with a SnO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e bilayer ETL, offer a promising solution for developing durable, high-performance, flexible PSCs suitable for long-term applications.\u003c/p\u003e","manuscriptTitle":"Ultrathin Glass-based Perovskite Solar Cells Employing Bilayer Electron Transport Layer","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-04 04:57:07","doi":"10.21203/rs.3.rs-4591034/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-06-18T08:35:13+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-18T08:06:01+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-18T06:41:55+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2024-06-16T19:42:55+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"93b7640f-d428-4247-ab69-6dfc02f3876e","owner":[],"postedDate":"July 4th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-07-04T04:57:07+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-04 04:57:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4591034","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4591034","identity":"rs-4591034","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}