Electron Transport Layer Material Research Articles

Electronic and Photochemical Passivation by a Classic Sunscreen Material Leading to Reduced Voc Losses and Enhanced Stability in Organic Solar Cells.

Organic solar cells (OSCs) have been a popular topic of research for a long time. As a well-known electron transport layer (ETL) material for inverted device architecture, sol-gel-derived zinc oxide (ZnO) displays certain defective surfaces that cause excessive charge recombination and lower device performance. While ultraviolet (UV)-light soaking is sometimes necessary for the ZnO layer to function properly, the latter can also cause the photodegradation of conjugated organic semiconductors. The photostability of OSCs has always been a hot research topic, as the radiation of UV light may cause changes in the material's properties, and that, in turn, may cause rapid attenuation of the devices. Herein, ZnO is modified by inserting the commonly used sunscreen ingredient benzophenone-3 (BP-3) between the photoactive layer, consisting of a PM6:Y6 blend, and ZnO to reduce the impact of UV radiation on the photosensitive layer. The addition of BP-3 successfully enhances the photovoltaic parameters, and a remarkable open-circuit voltage (Voc) value of 0.887 V is obtained for PM6:Y6-based inverted solar cells, corresponding to a Voc loss as small as 0.547 V. Finally, the application of this strategy increases the device's power conversion efficiency from 12.44 to 13.71% and provides improved UV stability.

Design of n-i-p and p-i-n Sb2Se3 solar cells: role of band alignment

Investigations into novel device architectures and interfaces that enhance charge transport and collection are necessary to increase the power conversion efficiency (PCE) of antimony selenide (Sb2Se3) solar cells, which have shown great promise as a low-cost and high-efficiency alternative to conventional silicon-based solar cells. The current work uses device simulations to design p-i-n and n-i-p Sb2Se3-based solar cell structures. The n-i-p configuration is investigated by comparing distinct electron transport layer (ETL) materials to get the best performance. While certain ETL materials may yield higher efficiencies, the J–V curve may exhibit S-shaped behavior if there is a misalignment of the bands at the ETL/absorber interface. To address this issue, a proposed double ETL structure is introduced to achieve proper band alignment and conduction band offset for electron transport. A PCE of 20.15% was achieved utilizing (ZnO/ZnSe) as a double ETL and Spiro-OMeTAD as a hole transport layer (HTL). Further, the p-i-n configuration is designed by proposing a double HTL structure to facilitate hole transport and achieve a proper valence band offset. A double HTL consisting of (CuI/CuSCN) is used in conjunction with ETL-free configuration to achieve a PCE of 21.72%. The simulation study is conducted using the SCAPS-1D device simulator and is validated versus a previously fabricated cell based on the configuration FTO/CdS/Sb2Se3/Spiro-OMeTAD/Au.

Recent Advances of Doped SnO2 as Electron Transport Layer for High-Performance Perovskite Solar Cells.

Perovskite solar cells (PSCs) have garnered considerable attention over the past decade owing to their low cost and proven high power conversion efficiency of over 25%. In the planar heterojunction PSC structure, tin oxide was utilized as a substitute material for the TiO2 electron transport layer (ETL) owing to its similar physical properties and high mobility, which is suitable for electron mining. Nevertheless, the defects and morphology significantly changed the performance of SnO2 according to the different deposition techniques, resulting in the poor performance of PSCs. In this review, we provide a comprehensive insight into the factors that specifically influence the ETL in PSC. The properties of the SnO2 materials are briefly introduced. In particular, the general operating principles, as well as the suitability level of doping in SnO2, are elucidated along with the details of the obtained results. Subsequently, the potential for doping is evaluated from the obtained results to achieve better results in PSCs. This review aims to provide a systematic and comprehensive understanding of the effects of different types of doping on the performance of ETL SnO2 and potentially instigate further development of PSCs with an extension to SnO2-based PSCs.

Modeling and simulation of > 19% highly efficient PbS colloidal quantum dot solar cell: A step towards unleashing the prospect of quantum dot absorber

We used the SCAPS-1D simulation tool to maximize the performance of lead sulfide (PbS) solar cells. A PbS solar cell was first modeled and then experimentally verified from past research. The ZnO electron-transport layer (ETL) was then replaced with ZnO:Al ETL material. Additionally, the fluorine-doped tin oxide work function, PbS-TBAI, ZnO:Al layer thicknesses, ZnO:Al/PbS-TBAI, PbS-TBAI/PbS-EDT defect density, PbS-TBAI defect density, ZnO:Al, and PbS-TBAI doping concentration were optimized. Results showed a greater alignment of the absorber-layer valence band with the HOMO and LUMO of ZnO:Al than that of ZnO ETL. The PbS solar cell exhibited the greatest efficiency at the optimum values of 0.4 µm, 0.04 µm, 1019 cm–3, 4.0 eV, 1017 cm–3, 1014 cm–2, 1018 cm–2, and 3.8 eV for absorber thickness, ETL thickness, ETL doping concentration, work function of front contact, PbS-TBAI doping concentration, and electron affinity of ZnO:Al, respectively. The PbS solar cell also performed best at interface defect densities of 1014 and 1018 cm–2 for ZnO:Al/PbS-TBAI and PbS-TBAI/PbS-EDT, respectively. The PCE of ZnO:Al ETL-based device also increased from 15.036% before optimization to 19.169% after. The optimized result was found to be affected by temperature. The PbS-optimized device with inorganic hole-transport layers (HTLs) demonstrated higher performance than the device with organic HTL. This finding was attributed to the low hole mobility in the organic HTL than those of inorganic ones. Therefore, the ZnO:Al ETL-based device was more efficient than the ordinary ZnO ETL-based device.

Rare earth element-doped SnO2 for enhancing the efficiency and stability of Cs2AgBiBr6 lead-free perovskite solar cells

The mesoporous architecture of perovskite solar cells (PSCs) can promote the crystallization of the perovskite layer by providing numerous heterogeneous nucleation sites. The morphology of the electron transport layer (ETL) and its interface with the perovskite layer are two of the key factors in improving the performance of PSCs. While SnO2 is a commonly used ETL material owing to its wide band gap, high transparency and high electron mobility. However, the uncontrollable nature of oxygen vacancies in tin oxide during preparation can lead to severe carrier recombination at certain sites, thus adversely affecting the open-circuit voltage (Voc) of the device. In this work, by constructing a three-dimensional structure of tin oxide and doping it with samarium as the electron transport layer, the Cs2AgBiBr6 perovskite solar cell was assembled to solve the problem of severe carrier recombination, and the prepared Sm-doped mesoporous tin dioxide (Sm@m-SnO2) electron transport layer was flatter, denser and smoother than that of the non-samarium-doped one, and the perovskite film was more uniform with higher crystallinity. The device achieves an optimum photovoltaic conversion efficiency of 2.19% while demonstrating excellent environmental tolerance and long-term stability without encapsulation.

Bidirectional passivation of the SnO2/perovskite interface in perovskite solar cells through organic salt 3-Hydroxytyramine hydrochloride

SnO2 offers compelling advantages as an excellent electron transport layer material in perovskite solar cells. However, the defects caused by oxygen vacancies are unfavorable to the interface contact and would hinder the device performance. A simple and effective interface passivation strategy using 3-Hydroxytyramine hydrochloride (3-Hh) is developed, which realizes the bidirectional passivation of electron transport layer and perovskite. The passivation mechanism of 3-Hh is investigated. Further, the morphology of the subsequent deposited perovskite film is improved. The modified perovskite film exhibits increased grain size with better uniformity, reduced grain boundaries, lowered defect density. A relatively higher power conversion efficiency of 22.63% is achieved for the perovskite solar cells based on the bidirectional passivation of SnO2/perovskite interface. Meanwhile, the modified devices can retain 80% and 82% of the initial efficiency after storage for 1400 ​h in air and 700 ​h in 85 ​°C, respectively. The results show that 3-Hh could be a promising multifunctional interface material to further enhance the efficiency and the stability of perovskite solar cells.

Comparative Simulation Study of the Performance of Conventional and Inverted Hybrid Tin-Based Perovskite Solar Cells

A hybrid tin-based (GA0.2FA0.78SnI3-1% EDAI2) perovskite solar cell (PSC) with a p-i-n inverted structure has been reported to pass all the rigorous standard tests successfully and achieve a certified power conversion efficiency (PCE) of 8.3%. Our previous numerical study showed that this PCE could be considerably increased to 24.1% by engineering and controlling the interfaces of the cell. The aim of the current study is to compare the performance of a conventional n-i-p structure with its inverted p-i-n analog quantitatively, and demonstrate that, by improving the conventional structure, it can achieve a PCE score approximately equal to the inverted p-i-n structure. To that end, the absorber layer was chosen to be GA0.2FA0.78SnI3-1% EDAI2, while four ETL (electron transport layer) materials (TiO2, WS2, SnO2, and ZnOS), and four HTL (hole transport layer) materials (PEDOT:PSS, Cu2O, CuSCN, and CuI) were considered. Most used ETL/HTL combinations have been rigorously investigated with the aim of finding the ultimate configuration, providing the highest photovoltaic properties. Additionally, the effect of the layers’ thicknesses and their doping concentrations were inspected, and their impact on the photovoltaic properties of the PSC was investigated. The optimized structure with CuI (copper iodide) as the HTL and ZnOS (zinc oxysulphide) as the ETL scored a PCE of 24.1%, which is comparable to the value found with the inverted structure (26%). The current numerical simulation on GA0.2FA0.78SnI3-1% EDAI2 could be considered as a milestone in its chances for commercial development.

Fabrication of perovskite solar cells with PCE of 21.84% in open air by bottom-up defect passivation and stress releasement

The archetypical n-type semiconductor material, tin oxide (SnO2), is common and well-known electron transport layer (ETL) material in regular perovskite solar cells (PSCs) due to the superior electron mobility and suitable energy level alignment with perovskite. However, plentiful detrimental defects concentrated at perovskite/SnO2 ETLs interface and unfavorable residual stress within perovskite films, compromise and dampen the photovoltaic performance and shelf stability of PSCs. Herein, alkali metal salts (i.e., 2, 2′-biquinoline-4, 4′-dicarboxylic acid alkali metal salts (BCA-2H, BCA-2Na, and BCA-2K)) are employed as pre-buried modifiers in SnO2 films to achieve a holistic amelioration of ETLs, perovskite photoactive layer, and their interface heterojunction. The BCA modification facilitates crystal orientation and crystalline of SnO2 nanocrystals, instrumental in decreasing trap state densities within functional layers, releasing interfacial residual tensile strain, and upgrading perovskite film performance. In comparison to BCA-2H- and BCA-2Na-doped ETLs, the BCA-2K decoration corroborate better defect passivation and stress release effect originating from preferable interface enhancement and desirable surface chemical properties. Consequently, the PSCs based on SnO2-BCA-2K ETL fabricated under open-air conditions deliver the champion efficiency of 21.84% accompanied by negligible hysteresis. Moreover, the unpacked devices maintain 81.2% of its original value after 1200 h storage under an ambient condition of 55 ± 5% (relative humidity, RH) and 25 ± 5 ℃ with open-circuit conditions.

Synthesis and Optoelectronic Applications of dπ-pπ Conjugated Polymers with a Di-metallaaromatic Acceptor.

The development of conjugated polymers especially n-type polymer semiconductors is powered by the design and synthesis of electron-deficient building blocks. Herein, a strong acceptor building block with di-metallaaromatic structure was designed and synthesized by connecting two electron-deficient metallaaromatic units through a π-conjugated bridge. Then, a double-monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well-defined model oligomers indicated polymer structures. Kinetic studies based on NMR and UV/Vis spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ-pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28% based on the PM6:EH-HD-4F non-fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Synthesis and Optoelectronic Applications of dπ–pπ Conjugated Polymers with a Di‐metallaaromatic Acceptor

AbstractThe development of conjugated polymers especially n‐type polymer semiconductors is powered by the design and synthesis of electron‐deficient building blocks. Herein, a strong acceptor building block with di‐metallaaromatic structure was designed and synthesized by connecting two electron‐deficient metallaaromatic units through a π‐conjugated bridge. Then, a double‐monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well‐defined model oligomers indicated polymer structures. Kinetic studies based on nuclear magnetic resonance and ultraviolet–visible spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ–pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28 % based on the PM6 : EH‐HD‐4F non‐fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Solution-processed pristine metal oxides as electron-transporting materials for perovskite solar cells

In recent years, perovskite material-based photovoltaic devices have attracted great attention of researchers because of an expeditious improvement in their efficiency from 3.8% to over 25%. The electron transport layer (ETL), which functions for the extraction and transportation of photogenerated electrons from active perovskite material to the electrodes, is a vital part of these perovskite solar cells (PSCs). The optoelectronic properties of these electron transport layer materials also have an impact on the performance of these perovskite solar cells, and for commercialized flexible perovskite solar cells, low-temperature and solution-processable electron transport layers having high stability and suitable optoelectronic properties are needed. In this regard, the solution-processable films of different metal oxides have been largely investigated by many research groups. So, this review summarizes the optoelectronic properties of the different metal oxide-based electron transport layers and the development in the performance of the perovskite solar cells, which have solution-processable metal oxides as electron transport layers.

Study on effect of different HTL and ETL materials on the perovskite solar cell performance with TCAD simulator

Perovskite solar cell (PSC) has gained lots of attention in the scientific community because of achieving high efficiency in a short period of time. The basic structure of PSC is the active layer sandwiched between the electron transport layer (ETL) and hole transport layer (HTL). Overall device performance depends on the materials of ETL and HTL. This research studied the effect of different ETL and HTL materials and the variation of active layer thickness on the PSC performance with the Sentaurus TCAD simulator. For HTL and ETL, two types of materials were used: organic and inorganic. Organic HTL material was spiro-OMeTAD, inorganic HTL material was NiOx, Organic ETL was PCBM, and inorganic ETL was SnO2. It was observed that inorganic ETL and organic HTL showed the best result for the specific active layer thickness, and the best result was PCE of 21.61%, Voc of 1.23 V, Jsc of 19.31 mA/cm2 and FF of 90%.

Boosting the Performance of Perovskite Solar Cells through Systematic Investigation of the Annealing Effect of E-Beam Evaporated TiO2.

Electron transport layer (ETL) plays an undeniable role in improving the performance of n-i-p planar perovskite solar cells (PSCs). Titanium dioxide (TiO2) is known as a promising ETL material for perovskite solar cell. In this work, the effect of annealing temperature on optical, electrical, and surface morphology of the electron-beam (EB)-evaporated TiO2 ETL, and consequently on the performance of perovskite solar cell, was investigated. It was found that annealing treatment at an optimized temperature of 480 °C considerably improved the surface smoothness, density of grain boundaries, and carrier mobility of TiO2 film, which resulted in nearly 10-fold improvement in power conversion efficiency (11.16%) in comparison with the unannealed device (1.08%). The improvement in performance of the optimized PSC is attributed to the acceleration of charge carrier extraction, as well as suppression of the recombination at the ETL/Perovskite interface.

Device Simulation of Highly Stable and 29% Efficient FA0.75MA0.25Sn0.95Ge0.05I3-Based Perovskite Solar Cell.

A new type of perovskite solar cell based on mixed tin and germanium has the potential to achieve good power conversion efficiency and extreme air stability. However, improving its efficiency is crucial for practical application in solar cells. This paper presents a quantitative analysis of lead-free FA0.75MA0.25Sn0.95Ge0.05I3 using a solar cell capacitance simulator to optimize its structure. Various electron transport layer materials were thoroughly investigated to enhance efficiency. The study considered the impact of energy level alignment between the absorber and electron transport layer interface, thickness and doping concentration of the electron transport layer, thickness and defect density of the absorber, and the rear metal work function. The optimized structures included poly (3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) as the hole transport layer and either zinc oxide (ZnO) or zinc magnesium oxide (Zn0.7Mg0.3O) as the electron transport layer. The power conversion efficiency obtained was 29%, which was over three times higher than the initial structure. Performing numerical simulations on FA0.75MA0.25Sn0.95Ge0.05I3 can significantly enhance the likelihood of its commercialization. The optimized values resulting from the conducted parametric study are as follows: a short-circuit current density of 30.13 mA·cm-2), an open-circuit voltage of 1.08 V, a fill factor of 86.56%, and a power conversion efficiency of 28.31% for the intended solar cell.

Numerical analysis of Al/Gr/ETL/MoS2/Sb2S3/Ni solar cell using non-toxic In2S3/SnS2/ZnSe electron transport layer

In this study, Molybdenum disulfide (MoS2) based thin film solar cells (TFSCs) of the structure Al/Gr/ETL/MoS2/Sb2S3/Ni have been simulated using SCAPS-1D software. Due to their remarkable properties, three Cd-free and non-toxic (In2S3, SnS2, and ZnSe) electron transport layer (ETL) materials were suggested for MoS2-based TFSCs. The effect of various layer parameters on the performance of the cells was investigated for various ETL materials. The considered parameters are the MoS2 absorber layer thickness, the carrier density and thickness of the ETL, and the ETL/MoS2 interface defect density, as well as the impact of the working temperature, series resistance, and shunt resistance. The simulation results show that the highest power conversion efficiency is achieved for the MoS2 absorber layer thickness of 1.0 µm, and the thickness and carrier density of different ETLs of 0.05 µm and 1016 cm-3, respectively. We found that the defect density of the ETL/MoS2 interface should be controlled under 1012 cm-2. The optimized power conversion efficiencies of MoS2-based TFSCs are found by simulation 27.88%, 28.22%, and 28.29% for the cell with In2S3, SnS2, and ZnSe ETL, respectively. Among the three ETL materials used, zinc selenide (ZnSe) shows the best results due to its wider band gap. We have found that the cells with the FTO window layer show slightly better performance than those with the graphene. Maximum photovoltaic efficiency of 28.54% has been achieved for the solar cell of the structure Al/FTO/ZnSe/MoS2/Sb2S3/Ni. The results of simulations reveal that high-efficiency MoS2 solar cells with non-toxic (In2S3, SnS2, and ZnSe) ETLs can be obtained. Overall, the numerical simulation conducted in this study could contribute to the manufacture of Cd-free non-toxic and highly efficient MoS2-based solar cells.

AlGaN as an electron transport layer for wide-bandgap perovskite solar cells

Perovskite solar cells are expected to be applied as photoreceivers for high-efficiency optical wireless power transfer for electric vehicles. The use of aluminum gallium nitride (AlGaN) as an electron transport layer (ETL) for wide-gap perovskite solar cells is hereby proposed in this paper. The electrical properties and energy-band alignment of AlGaN deposited by either hydride vapor phase epitaxy or metal-organic CVD are investigated. AlGaN shows a higher conduction band level than conventional ETL materials. Simulation of the performance of a perovskite solar cell with CH3NH3PbBr3 as the absorbing layer and AlGaN as the ETL was performed using a solar-cell capacitance simulator. The results suggest that AlGaN increases the power conversion efficiency of the solar cell by improving the conduction band offset between the perovskite layer and the ETL.

Rational Design of Fullerene Derivatives for Improved Stability of p-i-n Perovskite Solar Cells

Perovskite solar cells (PSCs) with p-i-n architecture attracted particular attention from the research community due to their simple and scalable fabrication at low temperatures. However, the operational stability of p-i-n PSCs has to be improved, which requires the development of advanced charge transport interlayers. Fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PC61BM) are commonly used as electron transport layer (ETL) materials in PSCs, though they strongly compromise the device stability. Indeed, it has been shown that PC61BM films actively absorb volatile products resulting from photodegradation of lead halide perovskites and transport them towards top metal electrode. Thus, there is an urgent need for development of new fullerene-based electron transport materials with improved properties, in particular the ability to heal defects on the perovskite films surface and block the diffusion of volatile perovskite photodegradation products. To address this challenge, a systematic variation of organic addends structure should be performed in order to tailor the properties of fullerene derivatives. Herein, we rationally designed a series of fullerene derivatives with different side chains and explored their performance as ETL materials in perovskite solar cells. It has been shown that among all studied compounds, a methanofullerene with thiophene pendant group enables both high efficiency and improved device operational stability. The obtained results suggest that further engineering of fullerene-based materials could pave a way for the development of advanced ETL materials enabling long lifetimes of p-i-n perovskite solar cells.

A dπ‐pπ Conjugated System with High Mobility and Strong Emission Simultaneously

AbstractStrong intermolecular interactions usually facilitate charge transport, but impede photoluminescence, therefore, the development of organic π‐conjugated materials that exhibit both semiconducting and light‐emissive properties remains challenging. Herein, a series of perylene diimide (PDI)‐based carbolong complexes with dπ‐pπ conjugation is synthesized. The resulting alcohol‐soluble products exhibit broad and strong absorption combined with outstanding electronic and optical properties, and are applied as electron transport layer materials in organic solar cells, achieving power conversion efficiencies up to 17.36%. In addition, these complexes exhibit an uncommon aggregation‐induced emission phenomenon. These results will aid in future design of π‐conjugated materials with increased functionality.

Unique Properties and Emerging Applications of Carbolong Metallaaromatics.

ConspectusAromatic compounds are important in synthetic chemistry, biomedicines, and materials science. As a special type of aromatic complex, transition-metal-based metallaaromatics contain at least one transition metal in an aromatic framework. The chemistry of metallaaromatics has seen much progress in computational studies and synthetic methods, but their properties and applications are still emerging. In recent years, we have disclosed a series of metal-centered conjugated polycyclic metallacycles in which a carbon chain is chelated to a metal center through at least three metal-carbon bonds. These are termed carbolong complexes and exhibit good stability to water, oxygen, light, and heat on account of their polydentate chelation and aromaticity, making them easy to handle. Carbolong complexes are not only special π-conjugated aromatics but also organometallics; therefore, they have the properties of both species. In this Account, we showcase the recent advances in their applications based on their different properties.First, carbolong complexes are a special kind of π-conjugated aromatic, with the ability to transmit electrons, allowing them to function as single-molecule conductors and candidates for electron transporting layer materials (ETLs) in solar cells. A series of carbolong complexes have been proved to be useful as achievable ETLs which enhance device performance in both organic solar cells and perovskite solar cells.Second, due to the involvement of d orbitals in the conjugation, carbolong complexes normally exhibit strong and broad absorption, even in some cases extending to the near-infrared region (NIR). The absorbed optical energy can be converted into light, heat, and ultrasound; consequently, carbolong compounds can be used as core moieties in smart materials. For example, 7C carbolong complexes were found to exhibit aggregation-enhanced near-infrared emission (AIEE). Some 12C carbolong complexes have been designed into the core moieties of NIR-responsive polymers, such as cylindrical NIR-responsive materials, self-healing materials, and shape memory materials. In contrast to the stereotypically toxic osmium compounds such as the highly toxic OsO4, some osmium carbolong complexes exhibit low cell cytotoxicity and good biocompatibility; consequently, they also have potential applications in the biomedical area. For example, benefiting from broad absorption in the NIR, 9C and 12C carbolong complexes have been used in photoacoustic imaging and photothermal therapy, respectively. In addition, photodynamic therapeutic applications which take advantage of a carbolong peroxo complex are discussed.Third, as special transition-metal complexes chelated by carbon-based ligands, a carbolong peroxo complex has displayed catalytic activity in the dehydrogenation of alcohols and a bimetallic carbolong complex has been used to catalyze difunctionalization reactions of unactivated alkenes.Overall, aromatic carbolong complexes have been applied to photovoltaics, smart materials, phototherapy, and catalytic reactions. Moving forward, we hope that this Account will shed light on future studies and theoretical research and encourage more discoveries of the properties of other metallaaromatics.

What defines the perovskite solar cell efficiency and stability: fullerene-based ETL structure or film morphology?

A study of fullerene derivatives as electron-transport materials for perovskite solar cells revealed that their crystal structures affect charge transport and device efficiency, while the operational stability is governed by the film uniformity.