Single Crystal Perovskite Solar Cells: Development and Perspectives

The efficiency of perovskite solar cells has increased to a certified value of 25.2% in the past 10 years, benefiting from the superior properties of metal halide perovskite materials. Compared with the widely investigated polycrystalline thin films, single crystal perovskites without grain boundaries have better optoelectronic properties, showing great potential for photovoltaics with higher efficiency and stability. Additionally, single crystal perovskite solar cells are a fantastic model system for further investigating the working principles related to the surface and grain boundaries of perovskite materials. Unfortunately, only a handful of groups have participated in the development of single crystal perovskite solar cells; thus, the development of this area lags far behind that of its polycrystalline counterpart. Therefore, a review paper that discusses the recent developments and challenges of single crystal perovskite solar cells is urgently required to provide guidelines for this emerging field. In this progress report, the optical and electrical properties of single crystal and polycrystalline perovskite thin films are compared, followed by the recent developments in the growth of single crystal perovskite thin films and the photovoltaic applications of this material. Finally, the challenges and perspectives of single crystal perovskite solar cells are discussed in detail.     

Co-Evaporated MAPbI3 with Graded Fermi Levels Enables Highly Performing,Scalable, and Flexible p-i-n Perovskite Solar Cells

Recent progress of vapor-deposited perovskite solar cells (PSCs) has proved the feasibility of this deposition method in achieving promising photovoltaic devices. For the first time, it is probed the versatility of the co-evaporation process in creating perovskite layers customizable for different device architectures. A gradient of composition is created within the perovskite films by tuning the background chamber pressure during the growth process. This method leads to co-evaporated MAPbI₃ film with graded Fermi levels across the thickness. Here it is proved that this growth process is beneficial for p-i-n PSCs as it can guarantee a favorable energy alignment at the charge selective interfaces. Co-evaporated p-i-n PSCs, with different hole transporting layers, consistently achieve power conversion efficiency (PCE) over 20% with a champion value of 20.6%, one of the highest reported to date. The scaled-up p-i-n PSCs, with active areas of 1 and 1.96 cm², achieved the record PCEs of 19.1% and 17.2%, respectively, while the flexible PSCs reached a PCE of 19.3%. Unencapsulated PSCs demonstrate remarkable long-term stability, retaining ≈90% of their initial PCE when stored in ambient for 1000 h. These PSCs also preserve over 80% of their initial PCE after 500 h of thermal aging at 85 °C.     

Fast Growth of Thin MAPbI3 Crystal Wafers on Aqueous Solution Surface for Efficient Lateral‐Structure Perovskite Solar Cells

Solar‐grade single or multiple crystalline wafers are needed in large quantities in the solar cell industry, and are generally formed by a top‐down process from crystal ingots, which causes a significant waste of materials and energy during slicing, polishing, and other processing. Here, a bottom‐up technique that allows the growth of wafer‐size hybrid perovskite multiple crystals directly from aqueous solution is reported. Single‐crystalline hybrid perovskite wafers with centimeter size are grown at the top surface of a perovskite precursor solution. As well as saving raw materials, this method provides unprecedented advantages such as easily tunable thickness and rapid growth of the crystals. These crystalline wafers show high crystallinity, broader light absorption, and a long carrier recombination lifetime, comparable with those of bulk single crystals. Lateral‐structure perovskite solar cells made of these crystals demonstrate a record power conversion efficiency of 5.9%.     

Enhancing Efficiency and Intrinsic Stability of Large-Area Blade-Coated Wide-Bandgap Perovskite Solar Cells Through Strain Release

The realization of efficient large-area perovskite solar cells stands as a pivotal milestone for propelling their future commercial viability. However, the upscaling fabrication of perovskite solar cells is hampered by efficiency losses, and the underlying growth mechanism remains enigmatic. Here, it is unveiled that a prevalent upscaling technology, namely blade-coating, inherently triggers top-down inhomogeneity strains, predominantly concentrated on the surface of wide-bandgap perovskite films. Through strain mitigation strategies, the perovskite films exhibit reduced halide vacancies, leading to enhanced stability and improved optoelectronic characteristics. Consequently, the blade-coated perovskite solar cells achieve minimal efficiency loss when transitioning from small-area to large-area devices, enabling the realization of 1 cm²-area 1.77 eV-bandgap cells with a remarkable efficiency of 18.71%. Additionally, the strain-relieved device exhibits an exceptional 109% retention of its initial efficiency even after 400 h of continuous operation, in stark contrast to the control device which experiences a decline to 91%. Furthermore, the resulting 4-terminal all-perovskite tandem solar cells crafted utilizing blade-coated 1.77 eV-bandgap subcells achieve a maximum efficiency of 27.64% (stabilized at 27.28%). This study not only sheds light on the intricacies of upscaling preparation techniques but also overcomes potential obstacles that can impede the trajectory toward achieving large-scale perovskite solar cells.     

Scalable Fabrication of Stable High Efficiency Perovskite Solar Cells and Modules Utilizing Room Temperature Sputtered SnO2 Electron Transport Layer

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost‐effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO₂ are systematically studied. Ar and O₂ are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO₂ films. With the optimized structure, SnO₂ films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO₂ electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T₈₀ operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO₂ film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm² fabricated on 5 × 5 cm² substrates (geometry fill factor = 91%), and a T₈₀ operational lifetime of 515 h.     

A Review on Scaling Up Perovskite Solar Cells

Power conversion efficiency of perovskite solar cells (PSCs) has been boosted to 25.5% among the highest efficiency for single-junction solar cells, making PSCs extremely promising to realize industrial production and commercialization. Scaling up PSCs to fabricate efficient perovskite solar modules (PSMs) is the fundamental for applications. Here, present progresses on scaling up PSCs are reviewed. The structure design for PSMs is discussed. Various scalable methods and related morphology control strategies for large-area uniform perovskite films are summarized. Potential charge transport materials and electrode materials together with their scalable methods for low-cost, efficient, and stable PSMs are also summarized. Besides, current attempts on encapsulation for improving stability and reducing lead leakage are introduced, and the calculated cost and environment influence of PSMs are also outlined.     

Efficient and Stable Inverted Wide-Bandgap Perovskite Solar Cells and Modules Enabled by Hybrid Evaporation-Solution Method

Wide-bandgap perovskite solar cells (WBG-PSCs), when partnered with Si bottom cells in tandem configuration, can provide efficiencies up to 44%; yet, the development of stable, efficient, and scalable WBG-PSCs is required. Here, the utility of the hybrid evaporation-solution method (HESM) is investigated to meet these demanding requirements via its unique advantages including ease of control and reproducibility. A PbI₂/CsBr layer is co-evaporated followed by coating of organic-halide solutions in a green solvent. Bandgaps between 1.55–1.67 eV are systematically screened by varying CsBr and MABr content. Champion efficiencies of 21.06% and 20.35% in cells and 19.83% and 18.73% in mini-modules (16 cm²) for perovskites with 1.64 and 1.67 eV bandgaps are achieved, respectively. Additionally, 18.51%-efficient semi-transparent WBG-PSCs are implemented in 4T perovskite/bifacial silicon configuration, reaching a projected power output of 30.61 mW cm⁻² based on PD IEC TS 60904-1-2 (BiFi200) protocol. Despite similar bandgaps achieved by incorporating Br via MABr solution and/or CsBr evaporation, PSCs having a perovskite layer without MABr addition show significantly higher thermal and moisture stability. This study proves scalable, high-performance, and stable WBG-PSCs are enabled by HESM, hence their use in tandems and in emerging applications such as indoor photovoltaics are now within reach.     

Band Engineering via Gradient Molecular Dopants for CsFA Perovskite Solar Cells

Perovskites with the multi-cation composition of cesium (Cs), methylammonium (MA), and formamidinium (FA) (CsMAFA) are pursued for their high power conversion efficiencies, but they are limited by their thermal stability. To withstand damp-heat accelerated aging MA-free compositions such as CsFA are of interest, but these exhibit lower carrier diffusion lengths and thus lesser performance in photovoltaic devices. A band engineering strategy that overcomes limited carrier diffusion within inverted perovskite solar cells based on CsFA is reported. A joint experimental-computational study shows that treating the perovskite with an n-type molecular dopant increases band bending, shaping the electric field across the active layer to overcome limited diffusive transport. Using this strategy, CsFA solar cell devices with stabilized power conversion efficiencies of 20.3%, a high value for devices using CsFA active layers, are fabricated.     

A Highly Tolerant Printing for Scalable and Flexible Perovskite Solar Cells

Homogeneity and stability of flexible perovskite solar cells (PSCs) are significant for the commercial feasibility in upscaling fabrication. Concretely, the mismatching between bottom interface and perovskite precursor ink can cause uncontrollable crystallization and undesired dangling bonds during the printing process. Herein, methylammonium acetate, serving as ink assistant (IAS) can effectively avoid the micron-scale defects of perovskite film. The in situ optical microscope is applied to prove the IAS can inhibit the colloidal aggregation and induce more adequate crystallization growth, thus avoiding the micron-scale defects of pinholes and intergranular cracking. Concurrently, 4-chlorobenzenesulfonic acid is introduced into the electrode surface as a passivation layer to restore the deep traps at perovskite interface in nano-scale. Finally, the target flexible devices (1.01 cm²) deliver a superior efficiency of 18.12% with improved air atmosphere stability. This multi-scale defect repair strategy provides an integrated design concept of homogeneity and stability for scalable and flexible PSCs.     

Moisture Induced Secondary Crystal Growth Boosting the Efficiency of Hole Transport Layer-Free Carbon-Based Perovskite Solar Cells beyond 19.5%

Hole transport layer (HTL)-free carbon-based perovskite solar cells (C-PSCs) show promising commercial application potential due to their attractive advantages of low cost and high stability. However, the power conversion efficiency of C-PSCs is relatively low, mainly due to the poor crystalline quality of the C-PSC applicable perovskite films and the energy level mismatch between the perovskite and carbon electrode. Herein, a moisture-induced secondary crystal growth strategy to simultaneously improve the crystalline quality and optimize the energy level of perovskite film is proposed. The presence of moisture renders the surface of perovskite grains reactive by forming metastable intermediates. It is demonstrated that the commonly considered harmful intermediates can trigger secondary crystal growth. This secondary growth strategy results in improved crystallinity, larger grain size, and better morphology of the perovskite films, which reduce the density of defect states and also benefit the interface contact between the perovskite film and carbon electrode. Furthermore, the secondary growth modulates the surface composition of the film to achieve an optimized energy level alignment. As a result, this secondary growth strategy reduces the charge recombination loss and accelerates the charge transport process in C-PSCs. Consequently, a new record efficiency of 19.52% is achieved for HTL-free C-PSCs.     

Designing Heterovalent Substitution with Antioxidant Attribute for High-Performance Sn-Pb Alloyed Perovskite Solar Cells

All-perovskite tandem solar cells are promising for breaking through the single-junction Shockley–Queisser limit, and that potentially raises interest in configuring efficient Sn-Pb alloyed narrow-bandgap perovskite solar cells (PSCs). However, the Sn-Pb alloyed perovskites are commonly plagued by uncontrollable crystallization dynamics and severe p-doping levels. Herein, an effective additive molecule is designed with heterovalent substitution and antioxidant functions, whereby an organic metal coordination compound of tris(2,4-pentanedionato)gallium (TPGa) is employed to upgrade the quality of perovskite films. Ga³⁺ substitution obviously boosts the formation energy of Sn vacancies and heals the trap states. Meanwhile, the crystal structure evolution process is improved by the anchoring effect of 2,4-pentanedionato. The PSCs incorporating these improvements deliver not only a power conversion efficiency of 21.5% but also outstanding stability, as demonstrated by retaining 80% of the initial efficiency for over 1500 h. In addition, 23.14%-efficient all-perovskite tandem solar cells are further obtained by pairing this PSC with a wide-bandgap (1.74 eV) top cell. This study supports the feasibility of doping trivalent ions into the Sn-Pb alloyed perovskites to compromise the self-p-doping effect and highlights the importance of acetylacetone for passivating defects and hindering oxidation.     

Modulation on Electrostatic Potential of Passivator for Highly Efficient and Stable Perovskite Solar Cells

The perovskite layer contains a large number of charged defects that seriously impair the efficiency and stability of perovskite solar cells (PSCs), thus it is essential to develop an effective passivation strategy to heal them. Based on theoretical calculations, it is found that enhancing the electrostatic potential of passivators can improve passivation effect and adsorption energy between charged defects and passivators. Herein, an electrostatic potential modulation (EPM) strategy is developed to design passivators for highly efficient and stable PSCs. With the EPM strategy, 1-phenylethylbiguanide (PEBG) and 1-phenylbiguanide (PBG) are designed. It is found that the charge distribution and electrostatic potential of phenyl- and phenylethyl- substituent on the biguanide are significantly enhanced. The N atom directly bonding to the phenyl group shows larger positive charge than that bonding to the phenylethyl group. The modulated electrostatic potential makes PBG bind stronger with the defects on perovskite surface. Based on the effective passivation of EPM, a champion efficiency of 24.67% is realized and the device retain 91.5% of its initial PCE after ≈1300 h. The promising EPM strategy, which provides a principle of passivator design and allows passivation to be controllable, may advance further optimization and application of perovskite solar cells toward commercialization.     

An Effective Method for Recovering Nonradiative Recombination Loss in Scalable Organic Solar Cells

Regarded as a critical step in commercial applications, scalable printing technology has become a research frontier in the field of organic solar cells. However, inevitable efficiency loss always occurs in the lab‐to‐manufacturing translation due to the different fabrication processes. In fact, the decline of photovoltaic performance is mainly related to voltage loss, which is mainly affected by the diversity of phase separation morphology and the chemical structures of photoactive materials. Fullerene derivative indene‐C₆₀ bisadduct (ICBA) is introduced into a PBDB‐T‐2F:IT‐4F system to control the active layer morphology during blade‐coating process. Accordingly, as a symmetrical fullerene derivative, ICBA can regulate the crystallization tendency and molecular packing orientation and suppress charge carrier recombination. This ternary strategy overcomes the morphology issues caused by weaker shear impulse in blade‐coating process. Benefiting from the reduced nonradiative recombination loss, 1.05 cm² devices are fabricated by blade coating with a power conversion efficiency of 13.70%. This approach provides an effective support for recovering the voltage loss during scalable printing approaches.     

A Universal Strategy of Perovskite Ink - Substrate Interaction to Overcome the Poor Wettability of a Self-Assembled Monolayer for Reproducible Perovskite Solar Cells

Perovskite solar cells employing [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) self-assembled monolayer as the hole transport layer have been reported to demonstrate a high device efficiency. However, the poor perovskite wetting on Me-4PACz caused by poor perovskite ink interaction with the underlying Me-4PACz presents significant challenges for fabricating efficient perovskite devices. A triple co-solvent system comprising dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite ink - Me-4PACz coated substrate interaction and obtain a uniform perovskite layer. In comparison to DMF- and DMSO-based inks, the inclusion of NMP shows considerably higher binding energies of the perovskite ink with Me-4PACz as revealed by density-functional theory calculations. With the optimized triple co-solvent ratio, the perovskite devices deliver high power conversion efficiencies of >20%, 19.5%, and ≈18.5% for active areas of 0.16, 0.72, and 1.08 cm², respectively. Importantly, this perovskite ink–substrate interaction approach is universal and helps in obtaining a uniform layer and high photovoltaic device performance for other perovskite compositions such as MAPbI₃, FA_1−xMA_xPbI_3–yBr_y, and MA-free FA_1−xCs_xPbI_3–yBr_y.     

Interface Energy-Level Management toward Efficient Tin Perovskite Solar Cells with Hole-Transport-Layer-Free Structure

Lead-free tin perovskite solar cells (PSCs) have emerged as a promising candidate toward high-performance and eco-friendly photovoltaic technology with great potential for future application. However, tin PSCs with over 10% efficiency usually feature an organic hole transport layer (HTL) at the illumination side that may induce device degradation during long-term operation. Removing the unstable organic HTL is an important way to solve these stability issues, but the efficiency of HTL-free tin PSCs is still much lower than that of the completed cells. Herein, it is demonstrated that formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band-bending structure to selectively extract the hole in the HTL-free devices. By using this band-bending structure, a promising efficiency of over 10% is first achieved for the lead-free PSCs with a HTL-free structure. More importantly, the optimized cell is highly stable, keeping 95% and 90% of the initial efficiency after continuous light soaking for 40 days and 80 °C annealing for 300 h, respectively. This work paves a route toward the development of efficient, eco-friendly, and highly stable perovskite photovoltaics.     

Improved Air Stability of Tin Halide Perovskite Solar Cells by an N-Type Active Moisture Barrier

Tin halide perovskite solar cells are promising for the next generation of highly efficient photovoltaics. Their commercialization can be accelerated by increasing their stability in moisture and oxygen. Herein, an n-type organic molecule (IO-4Cl) is applied as an interlayer between the perovskite films and electron transport layers in p-i-n structured devices. The electron-rich indacenodithieno-[3,2-b]thiophene enhances electron transport, while the hydrocarbon side chains and rigid conjugated backbone isolate air. It is also shown that the C═O in IO-4Cl can coordinate with Sn²⁺ on perovskite films' surface and grain boundaries to enhance perovskite crystal stability. In addition, IO-4Cl slows down crystallization dynamics, resulting in lower non-radiation recombination. The moisture ingress in the perovskite films is tracked under high relative humidity (RH) and it is found that IO-4Cl can mitigate moisture infiltration. Finally, the devices with IO-4Cl maintain 95% of the initial power conversion efficiency after 1200 h of storage in a nitrogen-filled glovebox, and their stability in ambient air (60–80% RH) is significantly improved against pristine devices, thus demonstrating the beneficial effects of IO-4Cl interlayer on device stability.     

Undoped Hole Transport Layer Toward Efficient and Stable Inorganic Perovskite Solar Cells

Numerous strategies have been practiced to improve the power conversion efficiency of CsPbI₂Br-based perovskite solar cells (PSCs), which definitely makes efficiency gradually approach the theoretical efficiency limit. However, sufficient device stability is still in urgent demand for commercialization, pushing to overcome some instability sources induced by hygroscopicity of spiro-OMeTAD and residual strain of perovskite layer. To address these issues, p-type semiconductor of PCPDTBT is used to replace spiro-OMeTAD, enabling dual functions of hole transport and strain regulation. On the one hand, undoped PCPDTBT performs excellent hole extraction and transport, while avoiding the perovskite degradation caused by the hygroscopicity of common additives. On the other hand, PCPDTBT assisted by a thermally spin-coating method compensates for the thermally-induced residual strain in perovskite layer owing to its high thermal expansion coefficient. Consequently, CsPbI₂Br-based PSCs with PCPDTBT layer achieve improved efficiency of 16.5% as well as enhanced stability. This study provides a simple and facile strategy to achieve efficient and stable CsPbI₂Br-based PSCs.     

Self‐Crystallized Multifunctional 2D Perovskite for Efficient and Stable Perovskite Solar Cells

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.     

Efficient and Thermally Stable Wide Bandgap Perovskite Solar Cells by Dual-Source Vacuum Deposition

Wide bandgap perovskites are being widely studied in view of their potential applications in tandem devices and other semitransparent photovoltaics. Vacuum deposition of perovskite thin films is advantageous as it allows the fabrication of multilayer devices, fine control over thickness and purity, and it can be upscaled to meet production needs. However, the vacuum processing of multicomponent perovskites (typically used to achieve wide bandgaps) is not straightforward, because one needs to simultaneously control several thermal sources during the deposition. Here a simplified dual-source vacuum deposition method to obtain wide bandgap perovskite films is shown. The solar cells obtained with these materials have similar or even larger efficiency as those including multiple A-cations, but are much more thermally stable, up to 3500 h at 85 °C for a perovskite with a bandgap of 1.64 eV. With optimized thickness, record efficiency of >19% and semitransparent devices with stabilized power output in excess of 17% are achieved.     

A Simple Way to Simultaneously Release the Interface Stress and Realize the Inner Encapsulation for Highly Efficient and Stable Perovskite Solar Cells

The mixed halide perovskites have become famous for their outstanding photoelectric conversion efficiency among new‐generation solar cells. Unfortunately, for perovskites, little effort is focused on stress engineering, which should be emphasized for highly efficient solar cells like GaAs. Herein, polystyrene (PS) is introduced into the perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the residual stress in the perovskite during annealing because of its low glass transition temperature. The stress‐free perovskite has less recombination, larger lattices, and a lower ion migration tendency, which significantly improves the cell's efficiency and device stability. Furthermore, the so‐called inner‐encapsulated perovskite solar cells are fabricated with another PS capping layer on the top of perovskite. As high as a 21.89% photoelectric conversion efficiency (PCE) with a steady‐state PCE of 21.5% is achieved, suggesting that the stress‐free cell can retain almost 97% of its initial efficiency after 5 days of “day cycle” stability testing.