Electron‐Beam‐Evaporated Nickel Oxide Hole Transport Layers for Perovskite‐Based Photovoltaics

High‐quality charge carrier transport materials are of key importance for stable and efficient perovskite‐based photovoltaics. This work reports on electron‐beam‐evaporated nickel oxide (NiO_x) layers, resulting in stable power conversion efficiencies (PCEs) of up to 18.5% when integrated into solar cells employing inkjet‐printed perovskite absorbers. By adding oxygen as a process gas and optimizing the layer thickness, transparent and efficient NiO_x hole transport layers (HTLs) are fabricated, exhibiting an average absorptance of only 1%. The versatility of the material is demonstrated for different absorber compositions and deposition techniques. As another highlight of this work, all‐evaporated perovskite solar cells employing an inorganic NiO_x HTL are presented, achieving stable PCEs of up to 15.4%. Along with good PCEs, devices with electron‐beam‐evaporated NiO_x show improved stability under realistic operating conditions with negligible degradation after 40 h of maximum power point tracking at 75 °C. Additionally, a strong improvement in device stability under ultraviolet radiation is found if compared to conventional perovskite solar cell architectures employing other metal oxide charge transport layers (e.g., titanium dioxide). Finally, an all‐evaporated perovskite solar mini‐module with a NiO_x HTL is presented, reaching a PCE of 12.4% on an active device area of 2.3 cm².     

Record Open‐Circuit Voltage Wide‐Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure

In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (V_OC) of 1.31 V. The V_OC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic?inorganic hybrid perovskite solar cells and the V_OC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, V_OC is enhanced by 80 mV.     

Role of Ionic Functional Groups on Ion Transport at Perovskite Interfaces

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite‐based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial‐adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite‐based memristors. This fundamental study of I–V hysteresis in perovskite‐based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.     

Rear Surface Passivation for Ink-Based,Submicron CuIn(S,Se)2 Solar Cells

A N, N-dimethylformamide and thiourea-based route is developed to fabricate submicron (0.55 and 0.75 µm) thick CuIn(S,Se)₂ (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. However, reducing the absorber film thickness below 1 µm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly-recombinative, and mediocre-reflective Mo/CISSe rear interface. For the first time, to mitigate the rear recombination losses, a novel rear contacting structure involving a surface passivation layer and point contact openings is developed for solution processed CISSe films and demonstrated in tangible devices. An atomic layer deposited Al₂O₃ film is employed to passivate the Mo/CISSe rear surface while precipitates formed via chemical bath deposition of CdS are used to generate nanosized point openings. Consequently, Al₂O₃ passivated CISSe solar cells show an increase in the open-circuit voltage (V_OC) and short-circuit current density when compared to reference cells with equivalent absorber thicknesses. Notably, a V_OC increase of 59 mV contributes to active area efficiencies of 14.2% for rear passivated devices with 0.75 µm thick absorber layers, the highest reported value for submicron-based solution processed, low bandgap CISSe solar cells.     

Graded Bandgap Perovskite with Intrinsic n–p Homojunction Expands Photon Harvesting Range and Enables All Transport Layer‐Free Perovskite Solar Cells

Organic–inorganic halide perovskites are promising materials for next‐generation photovoltaic device due to their attractive photoelectrical properties such as strong light absorption, high carrier mobility, and tunable bandgap. Generally, perovskite solar cells require carrier transport layers (CTL) to provide a built‐in electric field and reduce the recombination rate. However, the construction of suitable electron‐ and hole‐transport layers is not cost effective, impairing the commercial application of the devices. An n–p perovskite homojunction absorber with a graded bandgap is developed by introducing a three‐step dynamic spin‐coating strategy and variable valence Sn elements. The bandgap of the perovskite absorber is gradually manipulated from 1.53 eV (the bottom) to 1.27 eV (the top). The electronic behavior is also transformed from n‐type (excess PbI₂, the bottom) to p‐type (Sn vacancy, the top) in a very short distance (50 nm). This designed perovskite homojunction electronic structure not only expands the light harvesting range from 800 to 970 nm which provides potential to break the PCE limits, but also promotes oriented carrier transportation and weakens the dependence on CTL. The demonstrated asymmetrical active layer shows a brand‐new approach to simplify the device structure and boost the performance of CTL‐free perovskite solar cells.     

Modified PEDOT Layer Makes a 1.52 V Voc for Perovskite/PCBM Solar Cells

The work functions for charge transport layers in perovskite solar cells affect device performance significantly. In this work, the regular poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is modified by adding a polymer electrolyte PSS‐Na to improve its HTL function in perovskite solar cells. The modified PEDOT:PSS films (called m‐PEDOT:PSS) possess higher work function than the regular one. Its energy level matches the valence band of perovskite very well, leading to enhanced V_oc and PCE (power conversion efficiency). When CH₃NH₃PbI₃ is used as the light absorber, the cell with PEDOT:PSS HTL gives a V_oc of 0.96 V and a PCE of 12.35%, while the cell with m‐PEDOT:PSS layer gives a V_oc of 1.11 V and a PCE of 15.56%. Enhanced V_oc and PCE are also achieved when CH₃NH₃PbI₂Br or CH₃NH₃PbBr₃ is used as the light absorber. The m‐PEDOT:PSS/CH₃NH₃PbBr₃/PC₆₁BM solar cells demonstrate an outstanding V_oc of 1.52 V.     

In Situ Polymerization of Cross-Linked Perovskite–Polymer Composites for Highly Stable and Efficient Perovskite Solar Cells

Mixed-halide perovskites have emerged as outstanding light absorbers that enable the fabrication of efficient solar cells; however, their instability hinders the commercialization of such systems. Grain-boundary (GB) defects and lattice tensile strain are critical intrinsic-instability factors in polycrystalline perovskite films. In this study, the light-induced cross-linking of acrylamide (Am) monomers with non-crystalline perovskite films is used to fabricate highly efficient and stable perovskite solar cells (PSCs). The Am monomers induce the preferred crystal orientation in the polycrystalline perovskite films, enlarge the perovskite grain size, and cross-link the perovskite grains. Additionally, the liquid properties of Am effectively releases lattice strain during perovskite-film crystallization. The cross-linked interfacial layer functions as an airtight wall that protects the perovskite film from water corrosion. Devices fabricated using the proposed strategy show an excellent power conversion efficiency (PCE) of 24.45% with an open-circuit voltage (V_OC) of 1.199 V, which, to date, is the highest V_OC reported for hybrid PSCs with electron transport layers (ETLs) comprised of TiO₂. Large-area PSC modules fabricated using the proposed strategy show a power conversion efficiency of 20.31% (with a high fill factor of 77.1%) over an active area of 33 cm², with excellent storage stability.     

Effect of Photogenerated Dipoles in the Hole Transport Layer on Photovoltaic Performance of Organic–Inorganic Perovskite Solar Cells

Judicious choice of transport layer in organic–inorganic halide perovskite solar cells can be one of the essential parameters in photovoltaic design and fabrication techniques. This article reports the effect of optically generated dipoles in transport layer on the photovoltaic actions in active layer in perovskite solar cells with the architecture of indium tin oxide (ITO)/TiO _x /CH₃NH₃PbI_3–x Cl _x /hole transport layer (HTL)/Au. Here, PTB7‐thieno[3,4‐b]thiophene‐alt‐benzodithiophene and P3HT‐poly(3‐hexylthiophene) are separately used as the HTL with significant and negligible photoinduced dipoles, respectively. Electric field‐induced photoluminescence quenching provides the first‐hand evidence to indicate that the photoinduced dipoles are partially aligned in the amorphous PTB7 layer under the influence of device built‐in field. By monitoring the recombination process through magneto‐photocurrent measurements under device operation condition, it is shown that the photoinduced dipoles in PTB7 layer can decrease the recombination of photogenerated carriers in the active layer in perovskite solar cells. Furthermore, the capacitance measurements suggest that the photoinduced dipoles in PTB7 can decrease charge accumulation at the electrode interface. Therefore, the studies indicate the important role of photoinduced dipoles in the HTL on charge recombination dynamics and provide a fundamental insight on how the polarization in transport layer can influence the device performance in perovskite solar cells.     

Solvent‐Mediated Dimension Tuning of Semiconducting Oxide Nanostructures as Efficient Charge Extraction Thin Films for Perovskite Solar Cells with Efficiency Exceeding 16%

The recent surge in efficiency and progress of organohalide perovskite solar cells (PSCs) has been significant. The PSC performance is significantly influenced by nanostructuring as this varies the intrinsic optical, electrical, and electrochemical properties. Diverse TiO₂ electron transport layers (ETLs) are solvothermally grown on the transparent conducting oxide substrate with different dimensionalities, 0D nanoparticles (TNP), 1D nanowires (TNW) to 2D nanosheets (TNS), by varying the organic solvent used. These layers feature enhanced optical transparency (≈2%–5% transmittance improvement compared to pristine fluorine doped tin oxide, FTO, glass) minimizing light absorption losses. PSCs constructed using 1D TNW or 2D TNS yield enhanced photovoltaic performance compared to the 0D TNP counterparts. This is a result of i) improved infiltration of the perovskite in the porous TNW or TNS network and ii) facilitated electron transport and charge extraction at the TNW/perovskite or TNS/perovskite interfaces, thus reduced interfacial recombination loss. Employing a bilayered ETL film consisting of a self‐assembled TiO₂ blocking layer and a subsequent TNW active layer, produces PSC devices with an efficiency exceeding 16%. This bilayered ETL film can simultaneously block the photogenerated holes and enhance electron ­extraction, therefore improving PSC performance.     

Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction

Perovskite/silicon tandem solar cells are increasingly recognized as promi­sing candidates for next‐generation photovoltaics with performance beyond the single‐junction limit at potentially low production costs. Current designs for monolithic tandems rely on transparent conductive oxides as an intermediate recombination layer, which lead to optical losses and reduced shunt resistance. An improved recombination junction based on nanocrystalline silicon layers to mitigate these losses is demonstrated. When employed in monolithic perovskite/silicon heterojunction tandem cells with a planar front side, this junction is found to increase the bottom cell photocurrent by more than 1 mA cm^?2. In combination with a cesium‐based perovskite top cell, this leads to tandem cell power‐conversion efficiencies of up to 22.7% obtained from J–V measurements and steady‐state efficiencies of up to 22.0% during maximum power point tracking. Thanks to its low lateral conductivity, the nanocrystalline silicon recombination junction enables upscaling of monolithic perovskite/silicon heterojunction tandem cells, resulting in a 12.96 cm² monolithic tandem cell with a steady‐state efficiency of 18%.     

Cesium Doped NiOx as an Efficient Hole Extraction Layer for Inverted Planar Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells have resulted in tremendous interest in developing next generation photovoltaics due to high record efficiency exceeding 22%. For inverted structure perovskite solar cells, the hole extraction layers play a significant role in achieving efficient and stable perovskite solar cell by modifying charge extraction, interfacial recombination losses, and band alignment. Here, cesium doped NiO_x is selected as a hole extraction layer to study the impact of Cs dopant on the optoelectronic properties of NiO_x and the photovoltaic performance. Cs doped NiO_x films are prepared by a simple solution‐based method. Both doped and undoped NiO_x films are smooth and highly transparent, while the Cs doped NiO_x exhibits better electron conductivity and higher work function. Therefore, Cs doping results in a significant improvement in the performance of NiO_x‐based inverted planar perovskite solar cells. The best efficiency of Cs doped NiO_x devices is 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiO_x is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiO_x. This work reveals that Cs doped NiO_x is very promising hole extraction material for high and stable inverted perovskite solar cells.     

Efficient Indium‐Doped TiOx Electron Transport Layers for High‐Performance Perovskite Solar Cells and Perovskite‐Silicon Tandems

In addition to a good perovskite light absorbing layer, the hole and electron transport layers play a crucial role in achieving high‐efficiency perovskite solar cells. Here, a simple, one‐step, solution‐based method is introduced for fabricating high quality indium‐doped titanium oxide electron transport layers. It is shown that indium‐doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO₂, boosting the fill‐factor and voltage of perovskite cells. Using the optimized transport layers, a high steady‐state efficiency of 17.9% for CH₃NH₃PbI₃‐based cells and 19.3% for Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃‐based cells is demonstrated, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively compared to TiO₂‐based control cells. In addition, a steady‐state efficiency of 16.6% for a semi‐transparent cell is reported and it is used to achieve a four‐terminal perovskite‐silicon tandem cell with a steady‐state efficiency of 24.5%.     

Light Absorption and Recycling in Hybrid Metal Halide Perovskite Photovoltaic Devices

The production of highly efficient single‐ and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH₃NH₃PbI₃ perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ≈67 nm to ≈1.4 µm thickness and transfer‐matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady‐state power conversion efficiency is achieved through incorporation of a perovskite film with near‐optimum predicted thickness (≈709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (≈709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a “dead‐volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.     

Effect of Electron‐Transport Material on Light‐Induced Degradation of Inverted Planar Junction Perovskite Solar Cells

This paper presents a systematic study of the influence of electron‐transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C₆₁ Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time‐of‐flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI₃) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron‐transport material based on the perylendiimide derivative provides good isolation for the MAPbI₃ films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene‐based electron‐transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.     

Spontaneous Interface Ion Exchange: Passivating Surface Defects of Perovskite Solar Cells with Enhanced Photovoltage

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnO_x‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnO_x‐Cl/MAPbI₃. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.     

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.     

Synergic Interface Optimization with Green Solvent Engineering in Mixed Perovskite Solar Cells

Organic–inorganic hybrid halide perovskite solar cells (PSCs) have recently drawn enormous attentions due to their impressive performance (>22%) and low temperature solution processability (<150 °C). Current solution process involves application of a large amount of toxic solvents, such as chlorobenzene, which is heavily employed in both the perovskite layer and the hole transport layer (HTL) deposition. Herein, this study employs green solvent of ethyl acetate for engineering efficient perovskite and HTL layers, which enables a synergic interface (perovskite/HTL) optimization. A champion efficiency of 19.43% is obtained for small cells (0.16 cm² with mask) and over 14% for large size modules (5 × 5 cm²). The PSCs prepared from the green solvent engineering demonstrate superior performance on both efficiency and stability over their chlorobenzene counterparts. These enhancements are ascribed to the in situ inhibition on carrier recombination induced by interfacial defects during the solution processing, which enables about 2/3 reduction of calculated recombination rate. Thus, the green solvent route shows the great potential toward environmental‐friendly manufacturing.     

Overcoming Interfacial Losses in Solution‐Processed Organic Multi‐Junction Solar Cells

Organic solar cells are promising in terms of full‐solution‐processing which enables low‐cost and large‐scale fabrication. While single‐junction solar cells have seen a boost in power conversion efficiency (PCE), multi‐junction solar cells are promising to further enhance the PCE. In all‐solution‐processed multi‐junction solar cells, interfacial losses are often encountered between hole‐transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high‐performance donor and acceptor materials in multi‐junction solar cells. Here, the authors report on a systematic study of interface losses in both single‐junction and multi‐junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO _x nanoparticles successfully overcomes the interfacial losses in both single‐ and multi‐junction solar cells based on various active layers by reducing interface protonation, promoting better energy‐level alignment, and forming a dense and smooth layer. Solution‐processed single‐junction solar cells are demonstrated to reach the same performance as with evaporated MoO _x (over 7%). Multi‐junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO _x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO _x nanoparticles show FF smaller than 50% and PCE less than 5%.     

Scalable Triple Cation Mixed Halide Perovskite–BiVO4 Tandems for Bias‐Free Water Splitting

Strong interest exists in the development of organic–inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, it is revealed how both fields can be seamlessly merged together, to obtain scalable, bias‐free solar water splitting tandem devices. For this purpose, state‐of‐the‐art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiO_x) hole transport layer are employed to produce Field's metal‐epoxy encapsulated photocathodes. Their stability (up to 7 h), photocurrent density (–12.1 ± 0.3 mA cm^?2 at 0 V versus reversible hydrogen electrode, RHE), and reproducibility enable a matching combination with robust BiVO₄ photoanodes, resulting in 0.25 cm² PEC tandems with an excellent stability of up to 20 h and a bias‐free solar‐to‐hydrogen efficiency of 0.35 ± 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10 cm², with a slight decrease in bias‐free photocurrent density from 0.39 ± 0.15 to 0.23 ± 0.10 mA cm^?2 due to an increasing series resistance. To characterize these devices, a versatile 3D‐printed PEC cell is also developed.     

Infrared Light Management Using a Nanocrystalline Silicon Oxide Interlayer in Monolithic Perovskite/Silicon Heterojunction Tandem Solar Cells with Efficiency above 25%

Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm^?² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm² perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm^?2 and a certified stabilized power conversion efficiency of 25.2%.