Emerging 2D Layered Materials for Perovskite Solar Cells

Perovskite solar cells (PSCs) are now at the forefront of the state‐of‐the‐art photovoltaic technologies due to their high efficiency and low fabrication costs. To further realize the potential of this fascinating class of solar cells, nanostructured functional materials have been playing important roles. 2D layered materials have attracted a great deal of interest due to their fascinating properties and unique structure. Recently, the exploration of a wide range of novel 2D materials for use in PSCs has seen considerable progress, but still a lot remains to be done in this field. In this progress report, the advancements that have recently been made in the application of these emerging 2D materials, beyond graphene, for PSCs are presented. Both the advantages and challenges of these 2D materials for PSCs are highlighted. Finally, important directions for the future advancements toward efficient, low‐cost, and stable PSCs are outlined.     

Self‐Assembled 2D Perovskite Layers for Efficient Printable Solar Cells

2D organic–inorganic hybrid Ruddlesden–Popper perovskites have emerged recently as candidates for the light‐absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D‐perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spin‐coating step. In this work, a facile, spin‐coating‐free, directly scalable drop‐cast method is reported for depositing precursor solutions that self‐assemble into highly oriented, uniform 2D‐perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm²). This is the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by nonspin‐coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D‐perovskite layer deposition using a slot‐die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot‐die coating on a glass substrate or R2R slot‐die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.     

Spontaneously Self‐Assembly of a 2D/3D Heterostructure Enhances the Efficiency and Stability in Printed Perovskite Solar Cells

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V_OC) of the devices by up to 100 mV. An ultrahigh V_OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.     

Few‐Layer MoS2 Flakes as Active Buffer Layer for Stable Perovskite Solar Cells

Solution‐processed few‐layer MoS₂ flakes are exploited as an active buffer layer in hybrid lead–halide perovskite solar cells (PSCs). Glass/FTO/compact‐TiO₂/mesoporous‐TiO₂/CH₃NH₃PbI₃/MoS₂/Spiro‐OMeTAD/Au solar cells are realized with the MoS₂ flakes having a twofold function, acting both as a protective layer, by preventing the formation of shunt contacts between the perovskite and the Au electrode, and as a hole transport layer from the perovskite to the Spiro‐OMeTAD. As prepared PSC demonstrates a power conversion efficiency (η) of 13.3%, along with a higher lifetime stability over 550 h with respect to reference PSC without MoS₂ (Δη/η = ?7% vs. Δη/η = ?34%). Large‐area PSCs (1.05 cm² active area) are also fabricated to demonstrate the scalability of this approach, achieving η of 11.5%. Our results pave the way toward the implementation of MoS₂ as a material able to boost the shelf life of large‐area perovskite solar cells in view of their commercialization.     

Mixed 3D–2D Passivation Treatment for Mixed‐Cation Lead Mixed‐Halide Perovskite Solar Cells for Higher Efficiency and Better Stability

Layered low‐dimensional perovskite structures employing bulky organic ammonium cations have shown significant improvement on stability but poorer performance generally compared to their 3D counterparts. Here, a mixed passivation (MP) treatment is reported that uses a mixture of bulky organic ammonium iodide (iso‐butylammonium iodide, iBAI) and formammidinium iodide (FAI), enhancing both power conversion efficiency and stability. Through a combination of inactivation of the interfacial trap sites, characterized by photoluminescence measurement, and formation of an interfacial energetic barrier by which ionic transport is reduced, demonstrated by Kelvin probe force microscopy, MP treatment of the perovskite/hole transport layer interface significantly suppresses photocurrent hysteresis. Using this MP treatment, the champion mixed‐halide perovskite cell achieves a reverse scan and stabilized power conversion efficiency of 21.7%. Without encapsulation, the devices show excellent moisture stability, sustaining over 87% of the original performance after 38 d storage in ambient environment under 75 ± 20% relative humidity. This work shows that FAI/ i BAI, is a new and promising material combination for passivating perovskite/selective‐contact interfaces.     

Low‐Temperature Solution‐Processed CuCrO2 Hole‐Transporting Layer for Efficient and Photostable Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PVSCs) have become the front‐running photovoltaic technology nowadays and are expected to profoundly impact society in the near future. However, their practical applications are currently hampered by the challenges of realizing high performance and long‐term stability simultaneously. Herein, the development of inverted PVSCs is reported based on low temperature solution‐processed CuCrO₂ nanocrystals as a hole‐transporting layer (HTL), to replace the extensively studied NiO_x counterpart due to its suitable electronic structure and charge carrier transporting properties. A ≈45 nm thick compact CuCrO₂ layer is incorporated into an inverted planar configuration of indium tin oxides (ITO)/c‐CuCrO₂/perovskite/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/bathocuproine (BCP)/Ag, to result in the high steady‐state power conversion efficiency of 19.0% versus 17.1% for the typical low temperature solution‐processed NiO_x‐based devices. More importantly, the optimized CuCrO₂‐based device exhibits a much enhanced photostability than the reference device due to the greater UV light‐harvesting of the CuCrO₂ layer, which can efficiently prevent the perovskite film from intense UV light exposure to avoid associated degradation. The results demonstrate the promising potential of CuCrO₂ nanocrystals as an efficient HTL for realizing high‐performance and photostable inverted PVSCs.     

Metal Halide Perovskite Materials for Solar Cells with Long‐Term Stability

Metal halide perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaic technology with their power conversion efficiencies over 23%. For prototypical organic–inorganic metal halide perovskites, their intrinsic instability poses significant challenges to the commercialization of PSCs. Recently, the scientific community has done tremendous work in composition engineering to develop more robust light‐absorbing layers, including mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites. This review provides an overview of the impact of these perovskites on the efficiency and long‐term stability of PSCs.     

Stable Inverted Planar Perovskite Solar Cells with Low‐Temperature‐Processed Hole‐Transport Bilayer

Low‐temperature‐processed perovskite solar cells (PSCs), which can be fabricated on rigid or flexible substrates, are attracting increasing attention because they have a wide range of potential applications. In this study, the stability of reduced graphene oxide and the ability of a poly(triarylamine) underlayer to improve the quality of overlying perovskite films to construct hole‐transport bilayer by means of a low‐temperature method are taken advantage of. The bilayer is used in both flexible and rigid inverted planar PSCs with the following configuration: substrate/indium tin oxide/reduced graphene oxide/polytriarylamine/CH₃NH₃PbI₃/PCBM/bathocuproine/Ag (PCBM = [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester). The flexible and rigid PSCs show power conversion efficiencies of 15.7 and 17.2%, respectively, for the aperture area of 1.02 cm². Moreover, the PSC based the bilayer shows outstanding light‐soaking stability, retaining ≈90% of its original efficiency after continuous illumination for 500 h at 100 mW cm^?2.     

High‐Performance Flexible Perovskite Solar Cells via Precise Control of Electron Transport Layer

Flexible perovskite solar cells (f‐PSCs) have attracted great attention due to their promising commercial prospects. However, the performance of f‐PSCs is generally worse than that of their rigid counterparts. Herein, it is found that the unsatisfactory performance of planar heterojunction (PHJ) f‐PSCs can be attributed to the undesirable morphology of electron transport layer (ETL), which results from the rough surface of the flexible substrate. Precise control over the thickness and morphology of ETL tin dioxide (SnO₂) not only reduces the reflectance of the indium tin oxide (ITO) on polyethylene 2,6‐naphthalate (PEN) substrate and enhances photon collection, but also decreases the trap‐state densities of perovskite films and the charge transfer resistance, leading to a great enhancement of device performance. Consequently, the f‐PSCs, with a structure of PEN/ITO/SnO₂/perovskite/Spiro‐OMeTAD/Ag, exhibit a power conversion efficiency (PCE) up to 19.51% and a steady output of 19.01%. Furthermore, the f‐PSCs show a robust bending resistance and maintain about 95% of initial PCE after 6000 bending cycles at a bending radius of 8 mm, and they present an outstanding long‐term stability and retain about 90% of the initial performance after >1000 h storage in air (10% relative humidity) without encapsulation.     

Polymer Doping for High‐Efficiency Perovskite Solar Cells with Improved Moisture Stability

Each component layer in a perovskite solar cell plays an important role in the cell performance. Here, a few types of polymers including representative p‐type and n‐type semiconductors, and a classical insulator, are chosen to dope into a perovskite film. The long‐chain polymer helps to form a network among the perovskite crystalline grains, as witnessed by the improved film morphology and device stability. The dewetting process is greatly suppressed by the cross‐linking effect of the polymer chains, thereby resulting in uniform perovskite films with large grain sizes. Moreover, it is found that the polymer‐doped perovskite shows a reduced trap‐state density, likely due to the polymer effectively passivating the perovskite grain surface. Meanwhile the doped polymer formed a bridge between grains for efficient charge transport. Using this approach, the solar cell efficiency is improved from 17.43% to as high as 19.19%, with a much improved stability. As it is not required for the polymer to have a strict energy level matching with the perovskite, in principle, one may use a variety of polymers for this type of device design.     

A Printable Organic Electron Transport Layer for Low‐Temperature‐Processed,Hysteresis‐Free,and Stable Planar Perovskite Solar Cells

Despite recent breakthroughs in power conversion efficiencies (PCEs), which have resulted in PCEs exceeding 22%, perovskite solar cells (PSCs) still face serious drawbacks in terms of their printability, reliability, and stability. The most efficient PSC architecture, which is based on titanium dioxide as an electron transport layer, requires an extremely high‐temperature sintering process (≈500 °C), reveals hysterical discrepancies in the device measurement, and suffers from performance degradation under light illumination. These drawbacks hamper the practical development of PSCs fabricated via a printing process on flexible plastic substrates. Herein, an innovative method to fabricate low‐temperature‐processed, hysteresis‐free, and stable PSCs with a large area up to 1 cm² is demonstrated using a versatile organic nanocomposite that combines an electron acceptor and a surface modifier. This nanocomposite forms an ideal, self‐organized electron transport layer (ETL) via a spontaneous vertical phase separation, which leads to hysteresis‐free, planar heterojunction PSCs with stabilized PCEs of over 18%. In addition, the organic nanocomposite concept is successfully applied to the printing process, resulting in a PCE of over 17% in PSCs with printed ETLs.     

Simultaneous Improvement in Efficiency and Stability of Low‐Temperature‐Processed Perovskite Solar Cells by Interfacial Control

In most current state‐of‐the‐art perovskite solar cells (PSCs), high‐temperature (≈500 °C)‐sintered metal oxides are employed as electron‐transporting layers (ETLs). To lower the device processing temperature, the development of low‐temperature‐processable ETL materials (such as solution‐processed ZnO) has received growing attention. However, thus far, the use of solution‐processed ZnO is limited because the reverse decomposition reaction that occurs at ZnO/perovskite interfaces significantly degrades the charge collection and stability of PSCs. In this work, the reverse decomposition reaction is successfully retarded by sulfur passivation of solution‐processed ZnO. The sulfur passivation of ZnO by a simple chemical means, efficiently reduces the oxygen‐deficient defects and surface oxygen‐containing groups, thus effectively preventing reverse decomposition reactions during and after formation of the perovskite active layers. Using the low‐temperature‐processed sulfur‐passivated ZnO (ZnO–S), perovskite layers with higher crystallinity and larger grain size are obtained, while the charge extraction at the ZnO/perovskite interface is significantly improved. As a result, the ZnO–S‐based PSCs achieve substantially improved power‐conversion‐efficiency (PCE) (19.65%) and long‐term air‐storage stability (90% retention after 40 d) compared with pristine ZnO‐based PSCs (16.51% and 1% retention after 40 d). Notably, the PCE achieved is the highest recorded (19.65%) for low‐temperature ZnO‐based PSCs.     

Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.     

Enhanced Efficiency and Stability of Perovskite Solar Cells Through Nd‐Doping of Mesostructured TiO2

Block‐copolymer templated chemical solution deposition is used to prepare mesoporous Nd‐doped TiO₂ electrodes for perovskite‐based solar cells. X‐ray diffraction and photothermal deflection spectroscopy show substitutional incorporation into the TiO₂ crystal lattice for low Nd concentration, and increasing interstitial doping for higher concentrations. Substitutional Nd‐doping leads to an increase in stability and performance of perovskite solar cells by eliminating defects and thus increasing electron transport and reducing charge recombination in the mesoporous TiO₂. The optimized doping concentration of 0.3% Nd enables the preparation of perovskite solar cells with stabilized power conversion efficiency of >18%.     

Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO₃) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO₃/spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO₃/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO₃ layer particularly improves the open‐circuit voltage (V_OC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V_OC (1.17 V in the champion cell). In addition, the WO₃ layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO₃/spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.     

Improving the Performance and Stability of Inverted Planar Flexible Perovskite Solar Cells Employing a Novel NDI‐Based Polymer as the Electron Transport Layer

A new naphthalene diimide (NDI)‐based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT‐TTCN)) is developed as the electron transport layer (ETL) in place of the fullerene‐based ETL in inverted perovskite solar cells (Pero‐SCs). A combination of characterization techniques, including atomic force microscopy, scanning electron microscopy, grazing‐incidence wide‐angle X‐ray scattering, near‐edge X‐ray absorption fine‐structure spectroscopy, space‐charge‐limited current, electrochemical impedance spectroscopy, photoluminescence (PL), and time‐resolved PL decay, is used to demonstrate the interface phenomena between perovskite and P(NDI2DT‐TTCN) or [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). It is found that P(NDI2DT‐TTCN) not only improves the electron extraction ability but also prevents ambient condition interference by forming a hydrophobic ETL surface. In addition, P(NDI2DT‐TTCN) has excellent mechanical stability compared to PCBM in flexible Pero‐SCs. With these improved functionalities, the performance of devices based on P(NDI2DT‐TTCN) significantly outperform those based on PCBM from 14.3 to 17.0%, which is the highest photovoltaic performance with negligible hysteresis in the field of polymeric ETLs.     

Barrier Designs in Perovskite Solar Cells for Long‐Term Stability

Perovskite solar cells (PSCs) have attracted much attention in the past decade and their power conversion efficiency has been rapidly increasing to 25.2%, which is comparable with commercialized solar cells. Currently, the long‐term stability of PSCs remains as a major bottleneck impeding their future commercial applications. Beyond strengthening the perovskite layer itself and developing robust external device encapsulation/packaging technology, integration of effective barriers into PSCs has been recognized to be of equal importance to improve the whole device’s long‐term stability. These barriers can not only shield the critical perovskite layer and other functional layers from external detrimental factors such as heat, light, and H₂O/O₂, but also prevent the undesired ion/molecular diffusion/volatilization from perovskite. In addition, some delicate barrier designs can simultaneously improve the efficiency and stability. In this review article, the research progress on barrier designs in PSCs for improving their long‐term stability is reviewed in terms of the barrier functions, locations in PSCs, and material characteristics. Regarding specific barriers, their preparation methods, chemical/photoelectronic/mechanical properties, and their role in device stability, are further discussed. On the basis of these accumulative efforts, predictions for the further development of effective barriers in PSCs are provided at the end of this review.     

High Electron Affinity Enables Fast Hole Extraction for Efficient Flexible Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) with low‐temperature processed hole transporting materials (HTMs) suffer from poor performance due to the inferior hole‐extraction capability at the HTM/perovskite interfaces. Here, molecules with controlled electron affinity enable a HTM with conductivity improved by more than ten times and a decreased energy gap between the Fermi level and the valence band from 0.60 to 0.24 eV, leading to the enhancement of hole‐extraction capacity by five times. As a result, the 3,6‐difluoro‐2,5,7,7,8,8‐hexacyanoquinodimethane molecules are used for the first time enhancing open‐circuit voltage (V_oc) and fill factor (FF) of the PSCs, which enable rigid‐and flexible‐based inverted perovskite devices achieving highest power conversion efficiencies of 22.13% and 20.01%, respectively. This new method significantly enhances the V_oc and FF of the PSCs, which can be widely combined with HTMs based on not only NiO_x but also PTAA, PEDOTT:PSS, and CuSCN, providing a new way of realizing efficient inverted PSCs.