Perovskite Solar Cell Architecture without Charge Transport Materials

Perovskite Solar Cell Architecture without Charge Transport Materials | 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 Perovskite Solar Cell Architecture without Charge Transport Materials Tsutomu Miyasaka, Zhanhao Hu, Nao Saito, Masashi Ikegami, Naoyuki Shibayama This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6336970/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract State-of-the-art perovskite solar cells (PSCs) employ a multilayer device structure, incorporating a combination of charge transport layers and interfacial modifications to achieve efficient charge extraction. However, simplifying the device structure is highly desirable for cost-effective mass production. One promising approach is to integrate multiple functionalities into one single molecule, which effectively replicates the functions that a multilayer structure provides. To explore this concept, we propose a device architecture where a combination of p-type and n-type self-assembled monolayers are employed to construct the hole-extraction and electron-extraction interfaces in a PSC without charge transport layers or additional modifications. The resulting device successfully establishes charge selectivity, achieving a substantial photovoltaic output and promising stability not far from those of state-of-the-art PSCs. Our findings suggest that replacing the complex multi-layered junction interfaces with functional monolayer interfaces is a promising approach to make efficient solar cells with a minimum demand of materials and a simple fabrication process. The proposed device architecture could be extended to other types of thin-film electronic devices and opens up a new pathway to achieve high efficiency by designing the interfaces at the molecular level. Physical sciences/Materials science/Materials for devices/Electronic devices Physical sciences/Chemistry/Physical chemistry/Electron transfer Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction In thin-film electronic devices, heterojunction interfaces where charge carriers encounter energetic and electronic discontinuities are often sites of significant physical processes, both desirable and undesirable. Designing an interface with optimal efficiency requires careful consideration of multiple factors. In a perovskite solar cell (PSC), an ideal interface should maximize the extraction efficiency of photogenerated carriers while minimizing energy losses 1–3 . To achieve this, the interface must possess high charge mobilities, minimal trap states and appropriate energy levels. However, meeting all of these diverse requirements poses substantial challenges. As a result, complex interfacial structures comprising multiple layers, each tailored to fulfill one or a set of the requirements, are generally adopted in state-of-the-art PSCs. A typical device may incorporate one or more hole transport layers, one or more electron transport layers, and additional passivation treatments at the heterojunction interfaces 4 . However, the multilayer device structure and its complex deposition processes can increase costs and lower yields in large-scale production. Therefore, minimizing the number of interfacial layers and treatments is always desirable. One approach to simplify the interfaces is to reproduce functions of the multilayer interfaces with a single molecule. The molecule is composed of multiple segments, each of which fulfill one or a set of the requirements to form a desirable interface. Such an approach is well-established in molecular electronics where molecules with precisely engineered multifunctionalities are designed to replicate the functions of bulk materials in a conventional device 5 . One type of the molecules is those that readily form an ordered self-assembled monolayer (SAM) through bonding with the adjacent layers 6–8 . A typical SAM molecule consists of three components including an anchoring group, a spacer group and a head group (Fig. 1a). These components can be tailored to control properties such as charge transport, energy level alignment, interfacial passivation and surface energy, offering a rich versatility and provides a template to design interfaces from bottom-up 9,10 . For their applications in PSCs, Henry Snaith reported using SAM to modify the electron transport layer of TiO 2 in 2014 11 . In 2019, Steve Albrecht’ group developed carbazole-based SAMs to replace the hole transport layer of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine. Both the power conversion efficiency (PCE) and device stability are improved 12 . A recent achievement sees 25.12% efficiency with SAM at the anode by optimizing the thin film deposition process 13 . Nonetheless, state-of-the-art PSCs still rely on charge transport layers either on both sides of the electrodes or on one side. Commonly used charge transport materials such as TiO 2 and NiO x require high-temperature annealing which is incompatible with flexible substrates 14 . The organic hole transport layers such as 2,2',7,7'-Tetrakis-( N,N -di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) are sensitive to moisture and high temperature, leading to accelerated device degradation 15 . Therefore, eliminating charge transport layers without compromising device efficiency is highly desirable. In this study, we replace charge transport layers at both the anode and cathode with a single layer of a p-type SAM (p-SAM) and an n-type SAM (n-SAM) respectively (Fig. 1b). Indium tin oxide (ITO) is adopted for both electrodes to ensure that any device performance can be attributed to the sole contribution from the SAMs. The resultant device exhibits a successful establishment of an asymmetric charge injection. More promisingly, a photovoltaic output with an open-circuit voltage (V OC ) and short-circuit current density (J SC ) close to state-of-the-art PSCs are achieved. Results and Discussions Device structure and fabrication Our device has a simple structure of anode/p-SAM/perovskite/n-SAM/cathode (Fig. 1b). Specifically, ITO is used for both electrodes to ensure that establishment of charge selectivity is attributed to the SAMs exclusively instead of the built-in potentials induced by the difference of electrode work functions. Furthermore, a hydroxylated surface can be easily formed by treating ITO with UV-Ozone which provides covalent bonding sites to SAM molecules that include phosphonic acids (-PO(OH) 2 ) or carboxylic acids (-COOH) as the anchoring group. [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) (MeO-2PACz) is employed for the p-SAM, and 4-(1',5'-Dihydro-1'-methyl-2'H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2'-yl)benzoic acid (C60-SAM) is employed for the n-SAM (Fig. 1c). Both molecules were well-studied and commercially available 11,12,16 . To make the device, p-SAM and n-SAM are separately spin-coated on two pieces of ITO substrates (Fig. 2a). The deposited SAMs are thermally annealed and washed by their solvents to remove excessive unbonded molecules. Subsequently, the perovskite layer of methylammonium lead iodide (MAPbI 3 ) is spin-coated onto p-SAM and n-SAM respectively. MAPbI 3 is selected due to its well-studied properties and widespread use as a benchmark perovskite composition . The two samples are then stacked with the perovskite layers in contact, and annealed at 150 ℃ under a pressure of about 1.2×10 7 Pa in a hot-press for 20 min (Fig. S1). After this process, the two perovskite layers fuse to form one single layer with high uniformity (Fig. 2b). The perovskite layer before and after fusion is observed by scanning electron microscopy (SEM). Fig. 1c and d show the cross-section and surface of the perovskite layer before hot-press. After hot-press, we detach one of the ITO glass to reveal the fused perovskite underneath (Fig. 1e and f). The fused perovskite layer is continuous in the vertical direction and the large grains span the full layer thickness of about 1 μm. The fusion of two perovskites under heat and pressure can be explained by Ostwald ripening of ionic crystals 17–19 . Mass transport between grains merges the two unconnected layers and leads to a growth of the grain sizes after hot-press (Fig. 2c-f). In the meantime, the sequential process of thermally induced perovskite decomposition and its reversible formation in the closed system could also have played a role 20,21 . For MAPbI 3 annealed in the open air at 150 ℃ for 20 min, the X-ray diffraction peak of the decomposed species of PbI 2 (12.6°) increased rapidly indicating accelerated degradation of MAPbI 3 at this temperature (Fig. 1g). However, for the hot-pressed perovskite annealed at the same temperature, PbI 2 peak is largely suppressed and the crystal structure of MAPbI 3 is maintained. The closed system created by the intimately contacting, two pieces of ITO substrates prevents the decomposed gaseous species from escaping far from the film and thereby enables the back reaction route to reform the perovskite. Fusion of two perovskites and its intact crystalline quality through the hot-press process have also been confirmed by several other groups 22–25 . Electrical and photovoltaic characteristics We fabricated four types of devices to investigate their electrical and photovoltaic characteristics (Fig. 3a): (1) the device with no SAMs and thereby has a symmetric structure of ITO/perovskite/ITO; (2) the device with the p-SAM, i.e., ITO/p-SAM/perovskite/ITO; (3) the device with the n-SAM, i.e., ITO/perovskite/n-SAM/ITO; and (4) the complete device with both the p-SAM and n-SAM, i.e., ITO/p-SAM/perovskite/n-SAM/ITO. Dark current density-voltage ( J - V ) characteristics from 0 V to 1.50 V, and subsequently from 0 V to −1.50 V are given in Fig. 3b. Unsurprisingly, the symmetric device with no SAMs exhibits symmetric current injection characteristics under the positive and negative bias. The turn-on voltage is observed at 1.02 V and −1.02 V, with the current density reaching approximately 26 mA/cm 2 at both 1.50 V and −1.50 V. By adding p-SAM on one side of the device, charge injection under positive bias is slightly enhanced, reducing the turn-on voltage to about 0.88 V. Conversely, charge injection at negative bias is suppressed, increasing the turn-on voltage to −1.13 V. Similarly, the device with only the n-SAM shows an enhancement in charge injection under positive bias yielding a turn-on voltage of 0.96 V. Injection under negative bias is suppressed with a turn-on voltage of −1.08 V. For the complete device incorporating both p-SAM and n-SAM, the turn-on voltage is 0.92 V under positive bias and −1.13 V under negative bias. The device achieves a highest current density of 36.8 mA/cm 2 at 1.50 V, while exhibiting a relatively suppressed current density of 20.9 mA/cm 2 at −1.50 V. Dark J - V curves obtained by a continuous forward and reverse scan is provided in Fig. S2, where different hysteresis is observed indicating different charge injection barriers among the four types of devices. As a result, despite that the rectification ratio is modest, an asymmetric charge injection is successfully established with the insertion of SAM at the interfaces, and the best performance is obtained in the device incorporating both the p-SAM and n-SAM. Notably, clear electroluminescence is observed at positive bias (Fig. 3b), indicating that our device architecture is also applicable to construct light-emitting diodes. It is then interesting to investigate whether such a simple device structure can give a notable asymmetric charge extraction under light illumination, i.e., a photovoltaic output. Since ITO glass is used for both electrodes, the device can be illuminated from either side. For consistent comparison, AM1.5G 1-Sun light illumination (100 mW/cm 2 ) was directed onto the anode side, while a black sheet was placed on the cathode side to prevent reflected light from re-entering the device. The photocurrent is shown in Fig. 3c. As expected, the symmetric device without SAMs exhibits symmetric J - V characteristics with no photovoltaic output (a wider scan from −1.2 V to 1.2 V is given in Fig. S3). The mismatch of the forward and reverse scans can be attributed to the hysteresis 26,27 . Upon adding p-SAM on one side of the device, a substantial photovoltaic output arises yielding a V OC of 0.62 V, J SC of 16.6 mA/cm 2 and a fill-factor (FF) of 37.6%. This corresponds to a PCE of 3.85 %. Similarly, adding n-SAM on one side of the device achieves a PCE of 3.56 % with a J SC of 12.5 mA/cm 2 , V OC of 0.84 V and FF of 34.1%. When both the p-SAM and n-SAM are incorporated into the device, a promising PCE of 8.40 % is achieved which is higher than both of the devices incorporating only the p-SAM and only the n-SAM (statistical performance data is provided in Fig. S4). All key parameters show improvement. Notably, J SC reaches 19.1 mA/cm 2 which is approximately 81% of the value previously reported for the state-of-the-art PSCs based on MAPbI 3 28 . The integrated current density from the external quantum efficiency (EQE) spectrum matches the J SC as shown in Fig. 3d. Meanwhile, V OC reaches 0.98 V which is approximately 88 % of the value reported for the state-of-the-art MAPbI 3 device 28 . However, FF remains relatively low at 45.2% which will be discussed in the next section. Our results demonstrate that the device architecture with SAMs alone at the interfaces are sufficient to achieve a substantial photovoltaic output. To the best of our knowledge, charge-transport-layer-free perovskite solar cell with SAMs was previously reported by Udo Bach et al., who deposited two different types of SAMs onto horizontally placed gold electrodes in a back-contact device structure 29 . MAPbI 3 was directly deposited onto the SAM-modified electrodes, yielding a V OC of 0.56V, J SC of 11.4 mA/cm 2 , FF of 40.5 % and PCE of 2.59 %. In comparison, our results using a sandwiched device structure show improvements across all key parameters, suggesting more efficient charge extraction in our architecture. It is worth noting that that the use of highly transparent ITO glass as both the electrodes, combined with a fully charge-transport-layer-free structure that minimizes parasitic absorption, enables our device to function as an ideal bifacial solar cell. A very small performance difference is observed when light is incident on the anode compared to the cathode (Fig. 3e). A bifaciality factor of 93% is obtained which is on par with the best bifacial silicon solar cells, and is among the highest achieved in PSCs 30,31 . Furthermore, the device exhibited a promising stability without additional encapsulation. 80% of its initial PCE is maintained after about 250 hours under a continuous AM1.5G light illumination in ambient air (Fig. 3f). The two ITO glass electrodes serve as self-encapsulation which significantly retards perovskite degradation. Decomposed PbI 2 was observed to proceed from the device edges indicating oxygen and water infiltration primarily through the gap between the two ITO electrodes (Fig. S5). This inherent stability could eliminate the need for high quality encapsulations, thereby reducing the associated costs. The ultra-simple device structure can be extended to other perovskite formulations such as formamidinium (FA)-based perovskites, which have higher thermal stability compared to MAPbI 3 . Devices incorporating FAPbI 3 and Cs 0.05 FA 0.85 MA 0.1 PbI 3 ( CsFAMAPbI ) were fabricated where the perovskites were prepared through a modified recipe (provided in Supplementary Materials) from the previous reports 32,33 . The photovoltaic performance is compared in Fig. 3g and Table S1. Both FAPbI 3 and CsFAMAPbI exhibit promising photovoltaic outputs, reaching a maximum PCE of 7.34% and 8.22% respectively. Notably, V OC as large as 1.01 V is observed with FAPbI 3 , and a negligible hysteresis (forward-to-reverse PCE ratio of 98.8%) is achieved with CsFAMAPbI. Energy level alignment To better understand the establishment of electrical polarity from SAMs, the energy level alignment was investigated by Ultraviolet Photoelectron Spectroscopy (Fig. S6). The bandgaps were calculated from UV-vis spectroscopy (Fig. S7). The valence band maximum (VBM) and conduction band minimum (CBM) of MAPbI 3 are from the previous report 34 . The resulting energy diagram is presented in Fig. 4a. The ITO substrate exhibits a work-function of 4.7 eV. Adding p-SAM and n-SAM slightly modifies the surface work-function to 4.8 eV and 4.6 eV respectively. The modest difference of 0.2 eV indicates a limited built-in electric field within the perovskite layer in contrast to state-of-the-art PSCs that employ electrodes with significantly larger work-function disparity 35–37 . The areal surface potential of the p-SAM- and n-SAM-modified ITO are shown in Fig. 4b measured by Kelvin probe force microscopy, revealing a uniform SAM coverage. Regarding the energy barriers, at the anode side, the highest occupied molecular orbital (HOMO) of p-SAM is located at −5.1 eV forming no barrier to hole extraction from the VBM of MAPbI 3 (−5.9 eV). However, the hole injection barrier of approximately 0.8 eV remains relatively large which could have induced energy loss and thereby a lower V OC in the p-SAM-only device compared to the n-SAM-only device (Fig. 3c). The lowest unoccupied molecular orbital (LUMO) of p-SAM is located at −1.9 eV, which can effectively block electron leakage from the CBM of MAPbI 3 (−4.3 eV). At the cathode side, the HOMO of n-SAM is positioned at −5.9 eV, which may lead to insufficient hole blocking. Additionally, the LUMO of n-SAM is located at −3.8 eV, forming an energy barrier of approximately 0.5 eV from the CBM of MAPbI 3 . This barrier is not insurmountable, but it could impede efficient electron extraction leading to a smaller J SC in the n-SAM-only device compared to the p-SAM-only device (Fig. 3c). As a result, the energy level alignment of the device is far from ideal, which could have contributed to a retarded charge extraction under the positive bias and thereby a relatively small FF (Fig. 3c, Table S1) 38 . Consistent with the interpretation, a relatively large ideality factor ( n ) of 2.43 is extracted from the light-intensity dependent V OC (Fig. 4c), and a slope ( α ) of 0.84 is obtained from the light-intensity dependent J SC (Fig. 4d), both indicating substantial non-radiative recombination losses 39,40 . Nonetheless, the energy diagram still provides an asymmetric energy level alignment to enable favorable extraction of holes to the anode and electrons to the cathode. The selective extraction of holes and electrons is further facilitated by the hole-transporting carbazole moieties in p-SAM and the electron-transporting fullerene moieties in n-SAM respectively in the complete device 6,16,41 . Photoluminescence (PL) measurements show a consistent result revealing an enhanced charge extraction. A significant PL quench is observed for MAPbI 3 deposited on p-SAM and n-SAM, compared to MAPbI 3 coated on the bare ITO substrate (Fig. 4e). It is therefore reasonable to predict that the device performance can be further improved by optimizing the energy level alignment to minimize the hole extraction barrier and maximizing the electron extraction barrier at the anode, and vice versa at the cathode. Conclusion In this study, we replaced the complex multilayered heterojunction interfaces commonly used in state-of-the-art PSCs with a single layer of p-SAM and n-SAM at the anode and cathode respectively. This ultra-simple device structure free of charge transport layers achieves a J SC and V OC close to state-of-the-art PSCs with a promising stability. More importantly, because symmetric ITO electrodes are employed in the sandwiched device, our result indicates that SAMs alone can generate a charge selectivity that is sufficient to produce a substantial photovoltaic output. Our findings introduce a fundamentally new PSC architecture to eliminate the need for bulk charge transport materials in heterojunction devices. We believe that the device efficiency achieved here can be further improved by developing SAMs that form better energy level alignment with the perovskite. Furthermore, the concept of using molecular monolayers to replace multilayer interfaces could be extended to other devices such as light-emitting diodes and memristors, opening up a new pathway to cost-effective electronics. Declarations Data availability: The data that support the findings of this study are available from the corresponding authors upon reasonable request. Acknowledgments: This research was supported by JSPS KAKENHI Grant Number 24H00488 (to T.M.) and JSPS KAKENHI Grant Number 24K08273 (to NY.S.). We thank Z. Liu for preparing some of the perovskite films and T. Tobe for developing the program for device stability measurements. We also appreciate K. Yamagishi for preparing the substrates and performing the SEM imaging. Author contributions: T.M. and NY.S. supervised the research. 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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-6336970","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":476760515,"identity":"1379d603-8358-4475-a9c4-be2895666c17","order_by":0,"name":"Tsutomu Miyasaka","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0klEQVRIiWNgGAWjYDACZjCZwMAPJA9AhA4QqUWyjWgtDFAtBseIdZduO4/hxx81afLG95sPHi5gsJNnYDyL3xqzwzzG0jzHcgy3HWNLODyDIdmwgeFcAiEtBtIMbBWM247xGBzmYWAGKj9jQNCWnz/+VdhvbgNrqSdKi5kEb1tO4gY2sJbDxGhhK7Pm7UtLnnEsLQHoyOOGbQT9cv7w5ps/viXb9jcfPvyZp6Janl+CQIgxMHAgOwPIZpM4Q0AHA/sDNAH+HkJaRsEoGAWjYIQBAH/QRJaaSSizAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-8535-7911","institution":"Toin University of Yokohama","correspondingAuthor":true,"prefix":"","firstName":"Tsutomu","middleName":"","lastName":"Miyasaka","suffix":""},{"id":476760516,"identity":"a9200e64-f355-4099-a196-1cd7dc23de91","order_by":1,"name":"Zhanhao Hu","email":"","orcid":"https://orcid.org/0000-0002-2892-6388","institution":"Toin University of Yokohama","correspondingAuthor":false,"prefix":"","firstName":"Zhanhao","middleName":"","lastName":"Hu","suffix":""},{"id":476760517,"identity":"d8d9d94a-09af-4f9d-aae6-2fe641fffce5","order_by":2,"name":"Nao Saito","email":"","orcid":"https://orcid.org/0009-0000-9460-6561","institution":"Toin University of Yokohama","correspondingAuthor":false,"prefix":"","firstName":"Nao","middleName":"","lastName":"Saito","suffix":""},{"id":476760518,"identity":"02eebfe7-ccd9-4303-a7dd-454452380643","order_by":3,"name":"Masashi Ikegami","email":"","orcid":"","institution":"Toin University of Yokohama","correspondingAuthor":false,"prefix":"","firstName":"Masashi","middleName":"","lastName":"Ikegami","suffix":""},{"id":476760519,"identity":"f437fc04-4749-47ed-8ab7-3b1d5cb657cb","order_by":4,"name":"Naoyuki Shibayama","email":"","orcid":"https://orcid.org/0000-0003-2182-049X","institution":"Toin University of Yokohama","correspondingAuthor":false,"prefix":"","firstName":"Naoyuki","middleName":"","lastName":"Shibayama","suffix":""}],"badges":[],"createdAt":"2025-03-30 06:00:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6336970/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6336970/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85484000,"identity":"ec710003-a777-40c5-ae66-18e72d19af22","added_by":"auto","created_at":"2025-06-26 11:37:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":306780,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMolecular structures of SAMs and the device architecture.\u003c/strong\u003e \u003cstrong\u003ea, \u003c/strong\u003eThe components of a SAM molecule and its self-assembly on the substrate. \u003cstrong\u003eb,\u003c/strong\u003e The charge-transport-layer free device architecture incorporating SAMs at the interfaces. \u003cstrong\u003ec,\u003c/strong\u003e The molecular structures of the p-SAM (MeO-2PACz) and n-SAM (C60-SAM) used in this experiment.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/f0c2cf6ffcfd3a9495f2e467.png"},{"id":85484847,"identity":"99c2d866-09ae-475e-a7f3-a7e9d0ebdbf1","added_by":"auto","created_at":"2025-06-26 11:45:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":445768,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDevice fabrication. a,\u003c/strong\u003eThe device is fabricated by depositing the perovskite on p-SAM-modified ITO and n-SAM-modified ITO separately, followed by thermal annealing under a pressure in a hot-press. \u003cstrong\u003eb,\u003c/strong\u003e The two perovskite films fuse into one single uniform layer after the hot-press process. \u003cstrong\u003ec,\u003c/strong\u003e The cross-section of the perovskite before hot-press. \u003cstrong\u003ed,\u003c/strong\u003e The surface of the perovskite before hot-press. \u003cstrong\u003ee,\u003c/strong\u003e The cross-section of the perovskite after hot-press. \u003cstrong\u003ef,\u003c/strong\u003eThe surface of the perovskite after hot-press. \u003cstrong\u003eg,\u003c/strong\u003e XRD patterns of the as-deposited perovskite, the fused perovskite after the hot-press process and the perovskite thermally annealed in the open air.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/d2332baff3f4c4dc5c3f5d5e.png"},{"id":85484001,"identity":"71252690-c27a-4696-b5e0-bcaa20d3f43d","added_by":"auto","created_at":"2025-06-26 11:37:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":626158,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDark \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eJ\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e and photovoltaic characteristics.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e The four device structures under investigation. \u003cstrong\u003eb,\u003c/strong\u003e Dark \u003cem\u003eJ\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e of the four types of devices with MAPbI\u003csub\u003e3 \u003c/sub\u003eas the perovskite layer. \u003cu\u003eThe picture shows the electroluminescence of the device with both p-SAM and n-SAM operated at about 15 V.\u003c/u\u003e \u003cstrong\u003ec,\u003c/strong\u003e Photovoltaic characteristics of the four types of devices with MAPbI\u003csub\u003e3 \u003c/sub\u003eas the perovskite layer. \u003cstrong\u003ed,\u003c/strong\u003e The EQE spectrum (left y-axis) of the device ITO/p-SAM/MAPbI\u003csub\u003e3\u003c/sub\u003e/n-SAM/ITO. The integrated current density is plotted to the right y-axis. \u003cstrong\u003ee,\u003c/strong\u003e Photovoltaic characteristics of the device ITO/p-SAM/MAPbI\u003csub\u003e3\u003c/sub\u003e/n-SAM/ITO with light incident on the anode and cathode respectively. \u003cstrong\u003ef,\u003c/strong\u003e Device (unencapsulated) stability of ITO/p-SAM/MAPbI\u003csub\u003e3\u003c/sub\u003e/n-SAM/ITO under maximum power point tracking. \u003cstrong\u003eg,\u003c/strong\u003e Photovoltaic characteristics of the devices (ITO/p-SAM/perovskite/n-SAM/ITO) with MAPbI\u003csub\u003e3\u003c/sub\u003e, FAPbI\u003csub\u003e3\u003c/sub\u003e and CsFAMAPbI as the perovskite layer respectively.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/46bb2a81770f0dbf663baa7d.png"},{"id":85484003,"identity":"789d9838-e2b5-40f3-99f8-ab62aad4eb4a","added_by":"auto","created_at":"2025-06-26 11:37:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":337394,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEnergetic properties.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e The energy diagram of the complete device. \u003cstrong\u003eb, \u003c/strong\u003eSurface potential of the p-SAM-coated ITO and the n-SAM-coated ITO. \u003cstrong\u003ec\u003c/strong\u003e, \u003cu\u003eLight-intensity dependent V\u003c/u\u003e\u003csub\u003e\u003cu\u003eOC\u003c/u\u003e\u003c/sub\u003e\u003cu\u003e \u003c/u\u003e\u003cu\u003e\u003cstrong\u003ed,\u003c/strong\u003e\u003c/u\u003e\u003cu\u003e Light-intensity dependent\u003c/u\u003e J\u003csub\u003eSC\u003c/sub\u003e \u003cstrong\u003ee,\u003c/strong\u003e Photoluminescence of MAPbI\u003csub\u003e3\u003c/sub\u003e deposited on the bare ITO, p-SAM-coated ITO and n-SAM-coated ITO.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/25c4350e29745c13c61724fc.png"},{"id":86503793,"identity":"f6bc968e-bb03-4e0d-94c6-ea321600c769","added_by":"auto","created_at":"2025-07-11 11:38:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2325673,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/4502f10c-e39a-4a04-b550-b3332fe51837.pdf"},{"id":85485621,"identity":"242d5710-c2a0-41cd-946b-293cd4679354","added_by":"auto","created_at":"2025-06-26 11:53:44","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1039695,"visible":true,"origin":"","legend":"Suplementary information","description":"","filename":"SupplementaryMaterialsMiyasakaetal.docx","url":"https://assets-eu.researchsquare.com/files/rs-6336970/v1/1f29bc3258a6ccdfa807e1ed.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Perovskite Solar Cell Architecture without Charge Transport Materials","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIn thin-film electronic devices, heterojunction interfaces where charge carriers encounter energetic and electronic discontinuities are often sites of significant physical processes, both desirable and undesirable. Designing an interface with optimal efficiency requires careful consideration of multiple factors. In a perovskite solar cell (PSC), an ideal interface should maximize the extraction efficiency of photogenerated carriers while minimizing energy losses\u003csup\u003e1–3\u003c/sup\u003e. To achieve this, the interface must possess high charge mobilities, minimal trap states and appropriate energy levels. However, meeting all of these diverse requirements poses substantial challenges. As a result, complex interfacial structures comprising multiple layers, each tailored to fulfill one or a set of the requirements, are generally adopted in state-of-the-art PSCs. A typical device may incorporate one or more hole transport layers, one or more electron transport layers, and additional passivation treatments at the heterojunction interfaces\u003csup\u003e4\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHowever, the multilayer device structure and its complex deposition processes can increase costs and lower yields in large-scale production. Therefore, minimizing the number of interfacial layers and treatments is always desirable. One approach to simplify the interfaces is to reproduce functions of the multilayer interfaces with a single molecule. The molecule is composed of multiple segments, each of which fulfill one or a set of the requirements to form a desirable interface. Such an approach is well-established in molecular electronics where molecules with precisely engineered multifunctionalities are designed to replicate the functions of bulk materials in a conventional device\u003csup\u003e5\u003c/sup\u003e. One type of the molecules is those that readily form an ordered self-assembled monolayer (SAM) through bonding with the adjacent layers\u003csup\u003e6–8\u003c/sup\u003e. A typical SAM molecule consists of three components including an anchoring group, a spacer group and a head group (Fig. 1a). These components can be tailored to control properties such as charge transport, energy level alignment, interfacial passivation and surface energy, offering a rich versatility and provides a template to design interfaces from bottom-up\u003csup\u003e9,10\u003c/sup\u003e. For their applications in PSCs, Henry Snaith reported using SAM to modify the electron transport layer of TiO\u003csub\u003e2\u003c/sub\u003e in 2014\u003csup\u003e11\u003c/sup\u003e. In 2019, Steve Albrecht’ group developed carbazole-based SAMs to replace the hole transport layer of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine. Both the power conversion efficiency (PCE) and device stability are improved\u003csup\u003e12\u003c/sup\u003e. A recent achievement sees 25.12% efficiency with SAM at the anode by optimizing the thin film deposition process\u003csup\u003e13\u003c/sup\u003e. Nonetheless, state-of-the-art PSCs still rely on charge transport layers either on both sides of the electrodes or on one side. Commonly used charge transport materials such as TiO\u003csub\u003e2\u003c/sub\u003e and NiO\u003csub\u003ex\u003c/sub\u003e require high-temperature annealing which is incompatible with flexible substrates\u003csup\u003e14\u003c/sup\u003e. The organic hole transport layers such as 2,2',7,7'-Tetrakis-(\u003cem\u003eN,N\u003c/em\u003e-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) are sensitive to moisture and high temperature, leading to accelerated device degradation\u003csup\u003e15\u003c/sup\u003e. Therefore, eliminating charge transport layers without compromising device efficiency is highly desirable.\u003c/p\u003e\n\u003cp\u003eIn this study, we replace charge transport layers at both the anode and cathode with a single layer of a p-type SAM (p-SAM) and an n-type SAM (n-SAM) respectively (Fig. 1b). Indium tin oxide (ITO) is adopted for both electrodes to ensure that any device performance can be attributed to the sole contribution from the SAMs. The resultant device exhibits a successful establishment of an asymmetric charge injection. More promisingly, a photovoltaic output with an open-circuit voltage (V\u003csub\u003eOC\u003c/sub\u003e) and short-circuit current density (J\u003csub\u003eSC\u003c/sub\u003e) close to state-of-the-art PSCs are achieved.\u0026nbsp;\u003c/p\u003e"},{"header":"Results and Discussions","content":"\u003cp\u003e\u003cstrong\u003eDevice structure and fabrication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOur device has a simple structure of anode/p-SAM/perovskite/n-SAM/cathode (Fig. 1b). Specifically, ITO is used for both electrodes to ensure that establishment of charge selectivity is attributed to the SAMs exclusively instead of the built-in potentials induced by the difference of electrode work functions. Furthermore, a hydroxylated surface can be easily formed by treating ITO with UV-Ozone which provides covalent bonding sites to SAM molecules that include phosphonic acids (-PO(OH)\u003csub\u003e2\u003c/sub\u003e) or carboxylic acids (-COOH) as the anchoring group. [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) (MeO-2PACz) is employed for the p-SAM, and 4-(1\u0026apos;,5\u0026apos;-Dihydro-1\u0026apos;-methyl-2\u0026apos;H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2\u0026apos;-yl)benzoic acid (C60-SAM) is employed for the n-SAM (Fig. 1c). Both molecules were well-studied and commercially available\u003csup\u003e11,12,16\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo make the device, p-SAM and n-SAM are separately spin-coated on two pieces of ITO substrates (Fig. 2a). The deposited SAMs are thermally annealed and washed by their solvents to remove excessive unbonded molecules. Subsequently, the perovskite layer of methylammonium lead iodide (MAPbI\u003csub\u003e3\u003c/sub\u003e) is spin-coated onto p-SAM and n-SAM respectively.\u003cu\u003e\u0026nbsp;MAPbI\u003csub\u003e3\u003c/sub\u003e is selected due to its well-studied properties and widespread use as a benchmark perovskite composition\u003c/u\u003e. The two samples are then stacked with the perovskite layers in contact, and annealed at 150 ℃ under a pressure of about 1.2\u0026times;10\u003csup\u003e7\u003c/sup\u003e Pa in a hot-press for 20 min (Fig. S1). After this process, the two perovskite layers fuse to form one single layer with high uniformity (Fig. 2b). The perovskite layer before and after fusion is observed by scanning electron microscopy (SEM). Fig. 1c and d show the cross-section and surface of the perovskite layer before hot-press. After hot-press, we detach one of the ITO glass to reveal the fused perovskite underneath (Fig. 1e and f). The fused perovskite layer is continuous in the vertical direction and the large grains span the full layer thickness of about 1 \u0026mu;m. The fusion of two perovskites under heat and pressure can be explained by Ostwald ripening of ionic crystals\u003csup\u003e17\u0026ndash;19\u003c/sup\u003e. Mass transport between grains merges the two unconnected layers and leads to a growth of the grain sizes after hot-press (Fig. 2c-f). In the meantime, the sequential process of thermally induced perovskite decomposition and its reversible formation in the closed system could also have played a role\u003csup\u003e20,21\u003c/sup\u003e. For MAPbI\u003csub\u003e3\u003c/sub\u003e annealed in the open air at 150 ℃ for 20 min, the X-ray diffraction peak of the decomposed species of PbI\u003csub\u003e2\u003c/sub\u003e (12.6\u0026deg;) increased rapidly indicating accelerated degradation of MAPbI\u003csub\u003e3\u003c/sub\u003e at this temperature (Fig. 1g). However, for the hot-pressed perovskite annealed at the same temperature, PbI\u003csub\u003e2\u003c/sub\u003e peak is largely suppressed and the crystal structure of MAPbI\u003csub\u003e3\u003c/sub\u003e is maintained. The closed system created by the intimately contacting, two pieces of ITO substrates prevents the decomposed gaseous species from escaping far from the film and thereby enables the back reaction route to reform the perovskite. Fusion of two perovskites and its intact crystalline quality through the hot-press process have also been confirmed by several other groups\u003csup\u003e22\u0026ndash;25\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrical and photovoltaic characteristics\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe fabricated four types of devices to investigate their electrical and photovoltaic characteristics (Fig. 3a): (1) the device with no SAMs and thereby has a symmetric structure of ITO/perovskite/ITO; (2) the device with the p-SAM, i.e., ITO/p-SAM/perovskite/ITO; (3) the device with the n-SAM, i.e., ITO/perovskite/n-SAM/ITO; and (4) the complete device with both the p-SAM and n-SAM, i.e., ITO/p-SAM/perovskite/n-SAM/ITO. Dark current density-voltage (\u003cem\u003eJ\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e) characteristics from 0 V to 1.50 V, and subsequently from 0 V to \u0026minus;1.50 V are given in Fig. 3b. Unsurprisingly, the symmetric device with no SAMs exhibits symmetric current injection characteristics under the positive and negative bias. The turn-on voltage is observed at 1.02 V and \u0026minus;1.02 V, with the current density reaching approximately 26 mA/cm\u003csup\u003e2\u003c/sup\u003e at both 1.50 V and \u0026minus;1.50 V. By adding p-SAM on one side of the device, charge injection under positive bias is slightly enhanced, reducing the turn-on voltage to about 0.88 V. Conversely, charge injection at negative bias is suppressed, increasing the turn-on voltage to \u0026minus;1.13 V. Similarly, the device with only the n-SAM shows an enhancement in charge injection under positive bias yielding a turn-on voltage of 0.96 V. Injection under negative bias is suppressed with a turn-on voltage of \u0026minus;1.08 V. For the complete device incorporating both p-SAM and n-SAM, the turn-on voltage is 0.92 V under positive bias and \u0026minus;1.13 V under negative bias. The device achieves a highest current density of 36.8 mA/cm\u003csup\u003e2\u003c/sup\u003e at 1.50 V, while exhibiting a relatively suppressed current density of 20.9 mA/cm\u003csup\u003e2\u003c/sup\u003e at \u0026minus;1.50 V. Dark \u003cem\u003eJ\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e curves obtained by a continuous forward and reverse scan is provided in Fig. S2, where different hysteresis is observed indicating different charge injection barriers among the four types of devices. As a result, despite that the rectification ratio is modest, an asymmetric charge injection is successfully established with the insertion of SAM at the interfaces, and the best performance is obtained in the device incorporating both the p-SAM and n-SAM.\u003cu\u003e\u0026nbsp;Notably, clear electroluminescence is observed at positive bias (Fig. 3b), indicating that our device architecture is also applicable to construct light-emitting diodes.\u003c/u\u003e\u003c/p\u003e\n\u003cp\u003eIt is then interesting to investigate whether such a simple device structure can give a notable asymmetric charge extraction under light illumination, i.e., a photovoltaic output. Since ITO glass is used for both electrodes, the device can be illuminated from either side. For consistent comparison, AM1.5G 1-Sun light illumination (100 mW/cm\u003csup\u003e2\u003c/sup\u003e) was directed onto the anode side, while a black sheet was placed on the cathode side to prevent reflected light from re-entering the device. The photocurrent is shown in Fig. 3c. As expected, the symmetric device without SAMs exhibits symmetric \u003cem\u003eJ\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e characteristics with no photovoltaic output (a wider scan from \u0026minus;1.2 V to 1.2 V is given in Fig. S3). The mismatch of the forward and reverse scans can be attributed to the hysteresis\u003csup\u003e26,27\u003c/sup\u003e. Upon adding p-SAM on one side of the device, a substantial photovoltaic output arises yielding a V\u003csub\u003eOC\u003c/sub\u003e of 0.62 V, J\u003csub\u003eSC\u003c/sub\u003e of 16.6 mA/cm\u003csup\u003e2\u003c/sup\u003e and a fill-factor (FF) of 37.6%. This corresponds to a PCE of 3.85 %. Similarly, adding n-SAM on one side of the device achieves a PCE of 3.56 % with a J\u003csub\u003eSC\u003c/sub\u003e of 12.5 mA/cm\u003csup\u003e2\u003c/sup\u003e, V\u003csub\u003eOC\u003c/sub\u003e of 0.84 V and FF of 34.1%. When both the p-SAM and n-SAM are incorporated into the device, a promising PCE of 8.40 % is achieved which is higher than both of the devices incorporating only the p-SAM and only the n-SAM (statistical performance data is provided in Fig. S4). All key parameters show improvement. Notably, J\u003csub\u003eSC\u003c/sub\u003e reaches 19.1 mA/cm\u003csup\u003e2\u003c/sup\u003e which is approximately 81% of the value previously reported for the state-of-the-art PSCs based on MAPbI\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e28\u003c/sup\u003e. The integrated current density from the external quantum efficiency (EQE) spectrum matches the J\u003csub\u003eSC\u003c/sub\u003e as shown in Fig. 3d. Meanwhile, V\u003csub\u003eOC\u003c/sub\u003e reaches 0.98 V which is approximately 88 % of the value reported for the state-of-the-art MAPbI\u003csub\u003e3\u003c/sub\u003e device\u003csup\u003e28\u003c/sup\u003e. However, FF remains relatively low at 45.2% which will be discussed in the next section.\u003c/p\u003e\n\u003cp\u003eOur results demonstrate that the device architecture with SAMs alone at the interfaces are sufficient to achieve a substantial photovoltaic output. To the best of our knowledge, charge-transport-layer-free perovskite solar cell with SAMs was previously reported by Udo Bach et al., who deposited two different types of SAMs onto horizontally placed gold electrodes in a back-contact device structure\u003csup\u003e29\u003c/sup\u003e. MAPbI\u003csub\u003e3\u003c/sub\u003e was directly deposited onto the SAM-modified electrodes, yielding a V\u003csub\u003eOC\u003c/sub\u003e of 0.56V, J\u003csub\u003eSC\u003c/sub\u003e of 11.4 mA/cm\u003csup\u003e2\u003c/sup\u003e, FF of 40.5 % and PCE of 2.59 %. In comparison, our results using a sandwiched device structure show improvements across all key parameters, suggesting more efficient charge extraction in our architecture.\u003c/p\u003e\n\u003cp\u003eIt is worth noting that that the use of highly transparent ITO glass as both the electrodes, combined with a fully charge-transport-layer-free structure that minimizes parasitic absorption, enables our device to function as an ideal bifacial solar cell. A very small performance difference is observed when light is incident on the anode compared to the cathode (Fig. 3e). A bifaciality factor of 93% is obtained which is on par with the best bifacial silicon solar cells, and is among the highest achieved in PSCs\u003csup\u003e30,31\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFurthermore,\u0026nbsp;the device exhibited a promising stability without additional encapsulation. 80% of its initial PCE is maintained after about 250 hours under a continuous AM1.5G light illumination in ambient air (Fig. 3f). The two ITO glass electrodes serve as self-encapsulation which significantly retards perovskite degradation. Decomposed PbI\u003csub\u003e2\u003c/sub\u003e was observed to proceed from the device edges indicating oxygen and water infiltration primarily through the gap between the two ITO electrodes (Fig. S5). This inherent stability could eliminate the need for high quality encapsulations, thereby reducing the associated costs.\u003c/p\u003e\n\u003cp\u003eThe ultra-simple device structure can be extended to other perovskite formulations such as formamidinium (FA)-based perovskites,\u0026nbsp;which have higher thermal stability compared to MAPbI\u003csub\u003e3\u003c/sub\u003e. Devices incorporating FAPbI\u003csub\u003e3\u003c/sub\u003e and Cs\u003csub\u003e0.05\u003c/sub\u003eFA\u003csub\u003e0.85\u003c/sub\u003eMA\u003csub\u003e0.1\u003c/sub\u003ePbI\u003csub\u003e3\u003c/sub\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003e(\u003cu\u003eCsFAMAPbI\u003c/u\u003e) were fabricated where the perovskites were prepared through a modified recipe (provided in Supplementary Materials) from the previous reports\u003csup\u003e32,33\u003c/sup\u003e. The photovoltaic performance is compared in Fig. 3g and Table S1. Both FAPbI\u003csub\u003e3\u003c/sub\u003e and CsFAMAPbI exhibit promising photovoltaic outputs, reaching a maximum PCE of 7.34% and 8.22% respectively. Notably, V\u003csub\u003eOC\u003c/sub\u003e as large as 1.01 V is observed with FAPbI\u003csub\u003e3\u003c/sub\u003e, and a negligible hysteresis (forward-to-reverse PCE ratio of 98.8%) is achieved with CsFAMAPbI.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEnergy level alignment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo better understand the establishment of electrical polarity from SAMs, the energy level alignment was investigated by Ultraviolet Photoelectron Spectroscopy (Fig. S6). The bandgaps were calculated from UV-vis spectroscopy (Fig. S7). The valence band maximum (VBM) and conduction band minimum (CBM) of MAPbI\u003csub\u003e3\u003c/sub\u003e are from the previous report\u003csup\u003e34\u003c/sup\u003e. The resulting energy diagram is presented in Fig. 4a. The ITO substrate exhibits a work-function of 4.7 eV. Adding p-SAM and n-SAM slightly modifies the surface work-function to 4.8 eV and 4.6 eV respectively. The modest difference of 0.2 eV indicates a limited built-in electric field within the perovskite layer in contrast to state-of-the-art PSCs that employ electrodes with significantly larger work-function disparity\u003csup\u003e35\u0026ndash;37\u003c/sup\u003e. The areal surface potential\u0026nbsp;of the p-SAM- and n-SAM-modified ITO are shown in Fig. 4b measured by Kelvin probe force microscopy, revealing a uniform SAM coverage. Regarding the energy barriers, at the anode side, the highest occupied molecular orbital (HOMO) of p-SAM is located at \u0026minus;5.1 eV forming no barrier to hole extraction from the VBM of MAPbI\u003csub\u003e3\u003c/sub\u003e (\u0026minus;5.9 eV). However, the hole injection barrier of approximately 0.8 eV remains relatively large which could have induced energy loss and thereby a lower V\u003csub\u003eOC\u003c/sub\u003e in the p-SAM-only device compared to the n-SAM-only device (Fig. 3c). The lowest unoccupied molecular orbital (LUMO) of p-SAM is located at \u0026minus;1.9 eV, which can effectively block electron leakage from the CBM of MAPbI\u003csub\u003e3\u003c/sub\u003e (\u0026minus;4.3 eV). At the cathode side, the HOMO of n-SAM is positioned at \u0026minus;5.9 eV, which may lead to insufficient hole blocking. Additionally, the LUMO of n-SAM is located at \u0026minus;3.8 eV, forming an energy barrier of approximately 0.5 eV from the CBM of MAPbI\u003csub\u003e3\u003c/sub\u003e. This barrier is not insurmountable, but it could impede efficient electron extraction leading to a smaller J\u003csub\u003eSC\u003c/sub\u003e in the n-SAM-only device compared to the p-SAM-only device (Fig. 3c). As a result, the energy level alignment of the device is far from ideal, which could have contributed to a retarded charge extraction under the positive bias and thereby a relatively small FF (Fig. 3c, Table S1)\u003csup\u003e38\u003c/sup\u003e.\u0026nbsp;Consistent with the interpretation, a relatively large ideality factor (\u003cem\u003en\u003c/em\u003e) of 2.43 is extracted from the light-intensity dependent V\u003csub\u003eOC\u003c/sub\u003e (Fig. 4c), and a slope (\u003cem\u003e\u0026alpha;\u003c/em\u003e) of 0.84 is obtained from the light-intensity dependent J\u003csub\u003eSC\u003c/sub\u003e (Fig. 4d), both indicating substantial non-radiative recombination losses\u003csup\u003e39,40\u003c/sup\u003e. Nonetheless, the energy diagram still provides an asymmetric energy level alignment to enable favorable extraction of holes to the anode and electrons to the cathode. The selective extraction of holes and electrons is further facilitated by the hole-transporting carbazole moieties in p-SAM and the electron-transporting fullerene moieties in n-SAM respectively in the complete device\u003csup\u003e6,16,41\u003c/sup\u003e. Photoluminescence (PL) measurements show a consistent result revealing an enhanced charge extraction. A significant PL quench is observed for MAPbI\u003csub\u003e3\u003c/sub\u003e deposited on p-SAM and n-SAM, compared to MAPbI\u003csub\u003e3\u003c/sub\u003e coated on the bare ITO substrate (Fig. 4e). It is therefore reasonable to predict that the device performance can be further improved by optimizing the energy level alignment to minimize the hole extraction barrier and maximizing the electron extraction barrier at the anode, and vice versa at the cathode.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, we replaced the complex multilayered heterojunction interfaces commonly used in state-of-the-art PSCs with a single layer of p-SAM and n-SAM at the anode and cathode respectively. This ultra-simple device structure free of charge transport layers achieves a J\u003csub\u003eSC\u003c/sub\u003e and V\u003csub\u003eOC\u003c/sub\u003e close to state-of-the-art PSCs with a promising stability. More importantly, because symmetric ITO electrodes are employed in the sandwiched device, our result indicates that SAMs alone can generate a charge selectivity that is sufficient to produce a substantial photovoltaic output. Our findings introduce a fundamentally new PSC architecture to eliminate the need for bulk charge transport materials in heterojunction devices. We believe that the device efficiency achieved here can be further improved by developing SAMs that form better energy level alignment with the perovskite. Furthermore, the concept of using molecular monolayers to replace multilayer interfaces could be extended to other devices such as light-emitting diodes and memristors, opening up a new pathway to cost-effective electronics.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability:\u003c/strong\u003e The data that support the findings of this study are available from the corresponding authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments:\u003c/strong\u003e This research was supported by JSPS KAKENHI Grant Number 24H00488 (to T.M.) and JSPS KAKENHI Grant Number 24K08273 (to NY.S.). We thank Z. Liu for preparing some of the perovskite films and T. Tobe for developing the program for device stability measurements. We also appreciate K. Yamagishi for preparing the substrates and performing the SEM imaging.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e T.M. and NY.S. supervised the research. Z.H. conceived the ideas and designed the experiments. Z.H. conducted the device fabrication and basic characterizations. M.I. helped with \u003cem\u003eJ\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e and photovoltaic measurements. Z.H., N.S. and M.I. conducted device stability characterizations. Z.H., N.S. and NY.S. participated in data analysis. Z.H. wrote the manuscript, which was revised by all the authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e: Supplementary Materials is available\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWang, S., Sakurai, T., Wen, W. \u0026amp; Qi, Y. B. Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells. \u003cem\u003eAdv Mater Interfaces\u003c/em\u003e\u003cstrong\u003e0\u003c/strong\u003e, 1800260 (2018).\u003c/li\u003e\n\u003cli\u003eNayak, P. K., Mahesh, S., Snaith, H. J. \u0026amp; Cahen, D. Photovoltaic solar cell technologies: analysing the state of the art. \u003cem\u003eNat Rev Mater\u003c/em\u003e\u003cstrong\u003e4\u003c/strong\u003e, 269\u0026ndash;285 (2019).\u003c/li\u003e\n\u003cli\u003eShao, S. \u0026amp; Loi, M. A. 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Stabilizing the Efficiency Beyond 20% with a Mixed Cation Perovskite Solar Cell Fabricated in Ambient Air under Controlled Humidity. \u003cem\u003eAdv Energy Mater\u003c/em\u003e\u003cstrong\u003e8\u003c/strong\u003e, 1700677 (2018).\u003c/li\u003e\n\u003cli\u003eNakamura, T. \u003cem\u003eet al.\u003c/em\u003e Single-isomer bis(pyrrolidino)fullerenes as electron-transporting materials for tin halide perovskite solar cells. \u003cem\u003eChem Sci\u003c/em\u003e (2025) doi:10.1039/D4SC07031C.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6336970/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6336970/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"State-of-the-art perovskite solar cells (PSCs) employ a multilayer device structure, incorporating a combination of charge transport layers and interfacial modifications to achieve efficient charge extraction. However, simplifying the device structure is highly desirable for cost-effective mass production. One promising approach is to integrate multiple functionalities into one single molecule, which effectively replicates the functions that a multilayer structure provides. To explore this concept, we propose a device architecture where a combination of p-type and n-type self-assembled monolayers are employed to construct the hole-extraction and electron-extraction interfaces in a PSC without charge transport layers or additional modifications. The resulting device successfully establishes charge selectivity, achieving a substantial photovoltaic output and promising stability not far from those of state-of-the-art PSCs. Our findings suggest that replacing the complex multi-layered junction interfaces with functional monolayer interfaces is a promising approach to make efficient solar cells with a minimum demand of materials and a simple fabrication process. The proposed device architecture could be extended to other types of thin-film electronic devices and opens up a new pathway to achieve high efficiency by designing the interfaces at the molecular level.","manuscriptTitle":"Perovskite Solar Cell Architecture without Charge Transport Materials","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-26 11:37:39","doi":"10.21203/rs.3.rs-6336970/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3c3d89be-e6e4-40a0-989c-1fd2ab84a71a","owner":[],"postedDate":"June 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":50626634,"name":"Physical sciences/Materials science/Materials for devices/Electronic devices"},{"id":50626635,"name":"Physical sciences/Chemistry/Physical chemistry/Electron transfer"}],"tags":[],"updatedAt":"2025-07-11T11:30:17+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-26 11:37:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6336970","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6336970","identity":"rs-6336970","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}