Synergistic Effect to High-Performance Perovskite Solar Cells with Reduced Hysteresis and Improved Stability by the Introduction of Na-Treated TiO2 and Spraying-Deposited CuI as Transport Layers

Critical Review of Cu-Based Hole Transport Materials for Perovskite Solar Cells: From Theoretical Insights to Experimental Validation

Despite the remarkable efficiency of perovskite solar cells (PSCs), long-term stability remains the primary barrier to their commercialization. The prospect of enhancing stability by substituting organic transport layers with suitable inorganic compounds, particularly Cu-based inorganic hole-transport materials (HTMs), holds promise due to their high valence band maximum (VBM) aligning with perovskite characteristics. This review assesses the advantages and disadvantages of these five types of Cu-based HTMs. Although Cu-based binary oxides and chalcogenides face narrow bandgap issues, the "chemical modulation of the valence band" (CMVB) strategy has successfully broadened the bandgap for Cu-based ternary oxides and chalcogenides. However, Cu-based ternary oxides encounter challenges with low mobility, and Cu-based ternary chalcogenides face mismatches in VBM alignment with perovskites. Cu-based binary halides, especially CuI, exhibit excellent properties such as wider bandgap, high mobility, and defect tolerance, but their stability remains a concern. These limitations of single anion compounds are insightfully discussed, offering solutions from the perspective of practical application. Future research can focus on Cu-based composite anion compounds, which merge the advantages of single anion compounds. Additionally, mixed-cation chalcogenides such as CuxM1-xS enable the customization of HTM properties by selecting and adjusting the proportions of cation M.

Copper(I) Iodide Thin Films: Deposition Methods and Hole-Transporting Performance

The pursuit of p-type semiconductors has garnered considerable attention in academia and industry. Among the potential candidates, copper iodide (CuI) stands out as a highly promising p-type material due to its conductivity, cost-effectiveness, and low environmental impact. CuI can be employed to create thin films with >80% transparency within the visible range (400-750 nm) and utilizing various low-temperature, scalable deposition techniques. This review summarizes the deposition techniques for CuI as a hole-transport material and their performance in perovskite solar cells, thin-film transistors, and light-emitting diodes using diverse processing methods. The preparation methods of making thin films are divided into two categories: wet and neat methods. The advancements in CuI as a hole-transporting material and interface engineering techniques hold promising implications for the continued development of such devices.

Effects of Chemical Valences of Sulfur on the Performance of CsFAMA Perovskite Solar Cells

The low electrical conductivity and the high surface defect density of the TiO₂ electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO₂ layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO₂/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO₂ ETL layers treated with Na₂S, Na₂S₂O₃, and Na₂SO₄, respectively. Experimental results show that the Na₂S and Na₂S₂O₃ interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO₂/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na₂SO₄ interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO₂/PVK interface and device performance. These results indicate that S^2- can obviously improve the quality of TiO₂ and PVK layers and TiO₂/PVK interfaces, while SO₄^2- has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.

Thermally Evaporated Copper Iodide Hole-Transporter for Stable CdS/CdTe Thin-Film Solar Cells

This study focuses on fabricating efficient CdS/CdTe thin-film solar cells with thermally evaporated cuprous iodide (CuI) as hole-transporting material (HTM) by replacing Cu back contact in conventional CdS/CdTe solar cells to avoid Cu diffusion. In this study, a simple thermal evaporation method was used for the CuI deposition. The current-voltage characteristic of devices with CuI films of thickness 5 nm to 30 nm was examined under illuminations of 100 mW/cm² (1 sun) with an Air Mass (AM) of 1.5 filter. A CdS/CdTe solar cell device with thermally evaporated CuI/Au showed power conversion efficiency (PCE) of 6.92% with J_SC, V_OC, and FF of 21.98 mA/cm², 0.64 V, and 0.49 under optimized fabrication conditions. Moreover, stability studies show that fabricated CdS/CdTe thin-film solar cells with CuI hole-transporters have better stability than CdS/CdTe thin-film solar cells with Cu/Au back contacts. The significant increase in FF and, hence, PCE, and the stability of CdS/CdTe solar cells with CuI, reveals that Cu diffusion could be avoided by replacing Cu with CuI, which provides good band alignment with CdTe, as confirmed by XPS. Such an electronic band structure alignment allows smooth hole transport from CdTe to CuI, which acts as an electron reflector. Hence, CuI is a promising alternative stable hole-transporter for CdS/CdTe thin-film solar cells that increases the PCE and stability.

High-Efficiency Electron Transport Layer-Free Perovskite/GeTe Tandem Solar Cell: Numerical Simulation

The primary purpose of recent research has been to achieve a higher power conversion efficiency (PCE) with stable characteristics, either through experimental studies or through modeling and simulation. In this study, a theoretical analysis of an efficient perovskite solar cell (PSC) with cuprous oxide (Cu2O) as the hole transport material (HTM) and zinc oxysulfide (ZnOS) as the electron transport material (ETM) was proposed to replace the traditional HTMs or ETMs. In addition, the impact of doping the perovskite layer was investigated. The results show that the heterostructure of n-p PSC without an electron transport layer (ETL) could replace the traditional n-i-p structure with better performance metrics and more stability due to reducing the number of layers and interfaces. The impact of HTM doping and thickness was investigated. In addition, the influence of the energy gap of the absorber layer was studied. Furthermore, the proposed PSC without ETL was used as a top sub-cell with germanium-telluride (GeTe) as a bottom sub-cell to produce an efficient tandem cell and boost the PCE. An ETL-free PSC/GeTe tandem cell is proposed for the first time to provide an efficient and stable tandem solar cell with a PCE of 45.99%. Finally, a comparison between the performance metrics of the proposed tandem solar cell and those of other recent studies is provided. All the simulations performed in this study are accomplished by using SCAPS-1D.

High-Performance Planar Perovskite Solar Cells with a Reduced Energy Barrier and Enhanced Charge Extraction via a Na2WO4-Modified SnO2 Electron Transport Layer

Tin oxide (SnO₂) has been commonly used as an electron transport layer (ETL) in planar perovskite solar cells (p-PSCs) because it can be prepared by a low-temperature solution-processed method. However, the device performance has been restricted due to the limited electrical performance of SnO₂ and its mismatched energy level alignment with the perovskite absorber. Considering these problems, sodium tungstate (Na₂WO₄) has been employed to modify the SnO₂ ETL. The conduction band minimum of SnO₂ increases and the defects at the ETL/perovskite interface decrease by the modification of the SnO₂ ETL with Na₂WO₄, thus reducing the energy barrier between the ETL and perovskite. In addition, the electron extraction ability has been enhanced and the interface recombination between the ETL and perovskite has also been inhibited. As a result, the photovoltaic performance of p-PSCs based on the modified ETL has been improved, and a champion power conversion efficiency of 21.16% has been achieved compared with the control device of 17.30% with an open circuit voltage increased from 1.075 to 1.162 V.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

Band Matching Strategy for All-Inorganic Cs2AgBiBr6 Double Perovskite Solar Cells with High Photovoltage

The lead-free double perovskite has been proven to be one of the promising alternatives to solve the stability and toxicity problems of lead-based organic-inorganic hybrid perovskite solar cells. Here, high-quality Cs₂AgBiBr₆ double perovskite films with large grains and smooth surface have been prepared through a sequential-vapor-deposition method, and a low-cost and eco-friendly Cu₂O film with a suitable energy level and good electrical properties was prepared as an efficient hole transport layer by vacuum vapor deposition for the first time. The Cu₂O-based devices achieve a champion power conversion efficiency increasing from 1.03 to 1.52% and an enhancement of photovoltage from 1.083 to 1.198 V compared with their organic counterparts. More importantly, the Cu₂O-based devices have excellent stability; they maintained the initial 96% efficiency under environmental conditions after 33 days of unpackaged storage. These results also point out the direction for the further development of these new promising perovskite solar cells.

Engineering Copper Iodide (CuI) for Multifunctional p-Type Transparent Semiconductors and Conductors

Developing transparent p-type semiconductors and conductors has attracted significant interest in both academia and industry because metal oxides only show efficient n-type characteristics at room temperature. Among the different candidates, copper iodide (CuI) is one of the most promising p-type materials because of its widely adjustable conductivity from transparent electrodes to semiconducting layers in transistors. CuI can form thin films with high transparency in the visible light region using various low-temperature deposition techniques. This progress report aims to provide a basic understanding of CuI-based materials and recent progress in the development of various devices. The first section provides a brief introduction to CuI with respect to electronic structure, defect states, charge transport physics, and overviews the CuI film deposition methods. The material design concepts through doping/alloying approaches to adjust the optoelectrical properties are also discussed in the first section. The following section presents recent advances in state-of-the-art CuI-based devices, including transparent electrodes, thermoelectric devices, p-n diodes, p-channel transistors, light emitting diodes, and solar cells. In conclusion, current challenges and perspective opportunities are highlighted.

High-Efficiency, Low-Hysteresis Planar Perovskite Solar Cells by Inserting the NaBr Interlayer

With great research potential, the perovskite solar cells (PSCs) have been well developed in recent years, but there are still some urgent issues like efficiency and hysteresis defects that severely limit their commercialization. Interface modification is a significant measure to reduce defects and promote performance. In the article, an easy and effective strategy of modifying the electron transport layer (ETL) with NaBr is proposed to improve efficiency and reduce hysteresis. The charge carrier dynamics can be greatly optimized by diffusing NaBr on the ETL. The efficiency of the NaBr coated device can achieve 21.16%, which is extremely higher than the control one and shows low hysteresis behavior with a hysteresis index reduced from 0.135 to 0.025. The results indicate that the NaBr modification provides a novel strategy for preparing PSCs with high efficiency and low hysteresis.

Interface Defects Passivation and Conductivity Improvement in Planar Perovskite Solar Cells Using Na2S-Doped Compact TiO2 Electron Transport Layers

Numerous trap states and low conductivity of compact TiO₂ layers are major obstacles for achieving high power conversion efficiency and high-stability perovskite solar cells. Here we report an effective Na₂S-doped TiO₂ layer, which can improve the conductivity of TiO₂ layers, the contact of the TiO₂/perovskite interface, and the crystallinity of perovskite layers. Comprehensive investigations demonstrate that Na cations increase the conductivity of TiO₂ layers while S anions change the wettability of TiO₂ layers, thus improving the crystallinity of perovskite layers and passivate defects at the TiO₂/PVK interface. The synergetic effects of dopants lead to a champion efficiency as high as 21.25% in unencapsulated perovskite solar cells (PSCs), with much-improved stability. Our work provides new insights on anion dopants in TiO₂ layers, which is usually neglected in previous reports, and also proposes a simple approach to produce low-cost and high-performance electron transport layers for high-performance PSCs.

Modifying Mesoporous TiO2 by Ammonium Sulfonate Boosts Performance of Perovskite Solar Cells

Mesoporous-structure perovskite solar cells (meso-PVKSCs) have been widely utilized due to the achieved high efficiency for which the TiO₂ layer usually suffers from sufficient electron trap states, low electron mobility, and inavoidable catalytic activity. Herein, a mesoporous TiO₂ (m-TiO₂) layer is modified by tetraethylammonium p-toluenesulfonate (abbreviated as TEATS) for the first time, leading to a significant photoelectric conversion efficiency enhancement from 19.14 to 20.69% for Cs_0.05MA_0.12FA_0.83PbI_2.55Br_0.45 (abbreviated as CsMAFA) meso-PVKSCs. In particular, the obtained champion open-circuit voltage (V_oc) is 1.18 V, which is a record high value for meso-PVKSCs with CsMAFA triple cation mixed perovskite. A series of measurements were employed to investigate the influences of TEATS modification on the energy band structures of TiO₂ as well as the CsMAFA perovskite layer atop, unveiling that TEATS modification benefits defect passivation of the TiO₂ film along with a decrease in the work function of TiO₂. Besides, TEATS modification helps to improve the wettability of perovskite precursors on the m-TiO₂ substrate, affording improved film quality of perovskite with enhanced crystallinity and grain size. Consequently, the trap states existed in the perovskite film can be passivated, and the interfacial charge recombination is suppressed. This further benefits the improvement of the ambient stability of devices.

All-Inorganic CsPbBr3 Perovskite Solar Cells with 10.45% Efficiency by Evaporation-Assisted Deposition and Setting Intermediate Energy Levels

Nowadays, inorganic CsPbBr₃ perovskite is emerging as a promising candidate as a light-absorbing layer in photovoltaic devices due to its excellent photoelectric property and superior stability under humidity and thermal attacks in comparison with organic cation-based hybrid perovskites. However, the impure perovskite phase and severe interfacial charge recombination have limited the further improvement of device performance. In this work, a vapor-assisted solution technique was introduced to prepare a high-purity CsPbBr₃ film in a perovskite solar cell (PSC). To further reduce the electron-hole recombination and enhance charge extraction, we introduced the novel intermediate energy level of manganese sulfide (MnS) as a hole transport layer in CsPbBr₃ PSC. The as-optimized CsPbBr₃ PSC based on all-inorganic transport layers delivers a power conversion efficiency (PCE) of 10.45% in comparison with 8.16% for the device free of an intermediate layer, which is one of the highest PCEs achieved among the CsPbBr₃-based PSCs to date. Moreover, the optimized device retained 80% PCE of its initial efficiency over 90 days under 80% relative humidity at 85 °C, indicating an excellent environmental tolerance to boost the commercial application of low-cost, efficient, and stable all-inorganic PSCs.

On the Current-Voltage Hysteresis in Perovskite Solar Cells: Dependence on Perovskite Composition and Methods to Remove Hysteresis

Current-density-voltage (J-V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan-direction-dependent J-V curves depending on perovskite composition. This review investigates J-V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis-free PSCs are proposed. There is a specific trend in hysteresis in terms of J-V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis-free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis-free PSCs and extremely high efficiency.

Tuning the Excited-State Dynamics of CuI Films with Electrochemical Bias

Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to -0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible "memory" effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr3 film provides synergy between both CuI and CsPbBr3 in dictating the charge separation and recombination.

Low-Temperature Solution-Processed ZnSe Electron Transport Layer for Efficient Planar Perovskite Solar Cells with Negligible Hysteresis and Improved Photostability

For a typical perovskite solar cell (PKSC), the electron transport layer (ETL) has a great effect on device performance and stability. Herein, we manifest that low-temperature solution-processed ZnSe can be used as a potential ETL for PKSCs. Our optimized device with ZnSe ETL has achieved a high power conversion efficiency (PCE) of 17.78% with negligible hysteresis, compared with the TiO₂ based cell (13.76%). This enhanced photovoltaic performance is attributed to the suitable band alignment, high electron mobility, and reduced charge accumulation at the interface of ETL/perovskite. Encouraging results were obtained when the thin layer of ZnSe cooperated with TiO₂. It shows that the device based on the TiO₂/ZnSe ETL with cascade conduction band level can effectively reduce the interfacial charge recombination and promote carrier transfer with the champion PCE of 18.57%. In addition, the ZnSe-based device exhibits a better photostability than the control device due to the greater ultraviolet (UV) light harvesting of the ZnSe layer, which can efficiently prevent the perovskite film from intense UV-light exposure to avoid associated degradation. Consequently, our results present that a promising ETL can be a potential candidate of the n-type ETL for commercialization of efficient and photostable PKSCs.

Highly crystalline Nb-doped TiO2 nanospindles as superior electron transporting materials for high-performance planar structured perovskite solar cells

Planar-structured perovskite solar cells (PSCs) have received tremendous attention due to their high power conversion efficiency (PCE), simple process and low-cost fabrication. A compact thin film of electron transport materials (ETMs) plays a key role in these PSCs. However, the traditional ETMs of PSCs, TiO₂ nanoparticulate films, suffer from low conductivity and high trap state density. Herein, we exploited TiO₂ nanospindles as a compact ETM in planar PSCs for the first time, and achieved an efficient device with a PCE of 19.1%. By optimization with Nb doping into the TiO₂ nanospindles, the PCE of the PSC was further improved up to 20.8%. The carrier transfer and collection efficiency were significantly improved after Nb⁵⁺ doping, revealed by Mott–Schottky (MS) analysis, space charge limited current (SCLC), photoluminence (PL), time-resolved photoluminence (TRPL) spectra, electrochemical impedance spectra (EIS) and so forth. Moreover, the hysteresis behavior was effectively inhibited and the stability was significantly enhanced. This work may provide a new avenue towards the rational design of efficient ETMs for perovskite solar cells.

Highly crystalline Nb:TiO₂ nanospindles have been successfully exploited as efficient ETMs in planar perovskite solar cells, achieving a power conversion efficiency of 20.8%, superior to that of the undoped one (19.1%).