Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted widespread attention due to their high power conversion efficiency (PCE) and low manufacturing cost. Although the certified PCE has reached 25.8%, the stability of PSCs under high temperature, high humidity, and continuous light exposure is still significantly inferior to that of traditional cells, which hinders their commercialization. Developing and applying highly stable inorganic hole transport materials (HTMs) is currently one of the effective methods to solve the photo-thermal stability of devices, which can effectively shield water and oxygen from corroding the perovskite absorption layer, thereby avoiding the formation of ion migration channels. This paper outlines the approximate classification and photoelectric properties of inorganic HTMs, introduces relevant research progress, summarizes performance optimization strategies for inorganic HTMs devices, including element doping, additive engineering, and interface engineering, and finally prospects the future development directions. It is necessary to further study the microstructure of inorganic HTMs and their relationship with the performance of PSCs to achieve more efficient and stable PSCs.

Power conversion efficiency of single-junction solar cells is fundamentally limited by the Shockley- Queisser (S-Q) limit. The most promising practical technology to break through the S-Q limit is to use two-terminal tandem structure which can simultaneously solve the problems, spectral mismatch and thermal relaxation energy loss, in single-junction devices. As one of the important components of the interconnecting layer, the recombination layer in the two-terminal tandem solar cells can provide recombination sites for electrons and holes extracted from the electron transporting layer and the hole transporting layer, avoiding the open-circuit voltage loss caused by charge accumulation and promoting the current flow of tandem solar cells. The recombination layer is considered as one of the key factors of achieving high-performance tandem devices. The ideal recombination layer should possess high conductivity to improve the charge recombination rate, high optical transmittance to ensure effective light absorption of the rear subcells, good chemical stability to reduce the damage caused by the solvent, and low preparation cost to promote the commercial production process. At present, a variety of materials have been used in two-terminal tandem solar cells, such as thin metals, transparent conductive oxides, conductive polymers, graphene oxide, etc., which play an important role in perovskite-perovskite, perovskite-organic, and perovskite-silicon two terminal tandem devices. In this review, the research progress of recombination layers in different types of tandem solar cells is summarized, together with types, design principles, preparation processes, and their advantages and disadvantages. Meanwhile, problems and challenges of the current recombination layers are proposed, which provides a useful reference for the design of high-performance tandem cells.

In recent years, organic-inorganic hybrid perovskite solar cells have received a lot of attention for their excellent performance and low manufacturing cost. However, the toxicity of lead in organic-inorganic hybrid perovskite solar cells and instability inhibits its further commercialization. Double perovskite Cs₂AgBiBr₆ possess excellent stability, low toxicity, long carrier lifetime, and small effective carrier mass, and is considered as a promising photovoltaic material. It has been applied in solar cells and displayed superior performance. However, the power conversion efficiency of Cs₂AgBiBr₆ perovskite solar cell still lags behind organic-inorganic hybrid perovskite solar cells, and its development faces various challenges. This review firstly introduces the crystal structure and the structural parameters such as tolerance factor of Cs₂AgBiBr₆. And then, the progress of thin film preparation technologies such as solution processing method, anti-solvent assisted film forming method, vapor deposition processing method, vacuum-assisted film forming method, spray-coating method are summarized, and the advantages and disadvantages of various preparation technologies are discussed. The performance optimization strategies of Cs₂AgBiBr₆ perovskite solar cells are analyzed from three aspects: element doping, additive engineering, and interface engineering (interface energy level matching and interface defect passivation), and the research progress in recent years is reviewed. Finally, the challenges faced by Cs₂AgBiBr₆ perovskite solar cells are pointed out, and future research directions are prospected from three aspects: precursor solvent engineering, bandgap engineering, and device degradation mechanism.

Hybrid organic-inorganic perovskite solar cells (PSCs) have attracted global attention as one of the most promising photovoltaic materials due to their high efficiency, low energy consumption and low cost. However, non-radiative recombination caused by interface defects severely inhibits the performance of PSCs. To solve this critical issue, the particle size of nickel oxide (NiO_x) hole transport layer was reduced to improve the particle size uniformity and achieve efficient hole transport. Furthermore, the antisolvent acting time of the perovskite film was optimized to reduce the interfacial non-radiative recombination and interfacial defect. As a result, the crystalline quality is improved and power conversion efficiency (PCE) of the perovskite solar cells increase from 10.11% to 18.37%. Kelvin probe atomic force microscopy (KPFM) study shows that the contact potential difference (CPD) of the optimized perovskite film in the illumination condition increases by 120.39 mV compared with that under the dark condition. Analysis by piezoelectric atomic force microscopy (PFM) reveals that the ferroelectric polarization of the optimized interfacial perovskite films hardly changes from illumination to dark states, indicating that reducing interfacial defects can decrease the hysteresis effect of the PSCs. It is concluded that optimizing the NiO_x hole transport layer and improving the quality of perovskite film can reduce the interface defects, the non-radiative recombination and the hysteresis effect, and improve PCE of perovskite solar cells.

Perovskite solar cells (PSCs) are developing rapidly and their power conversion efficiency (PCE) has been repeatedly refreshed, but their long-term stability still needs to be improved. At present, most of the preparation of high-efficiency PSCs is completed in the inert gas, with high cost and limited operating space, which is not conducive to its industrial application. Here, perovskite solar cells with mixed cation, displaying ultra-long stability, were successfully prepared in the air. Effects of A-site cation doping on the microstructure, optoelectronic properties and stability of the perovskite were systematically investigated. The experimental results show that FA⁺ and Cs⁺ co-doping improves the quality of perovskite films, modulates the energy level arrangement of perovskite/SnO₂, suppresses carrier complexation, and significantly improves the PCE, long-term, wet and thermal stability of the cell. The optimal PCE of Cs_0.05MA_0.35FA_0.6PbI₃ cells is 19.34%, maintaining 85% of the initial efficiency after reserving for 242 d in dark environment at (20±5) ℃ and <5% relative humidity. In contrast, the PCE of the MAPbI₃ cell decreased to 30% of the initial value after reserving for 112 d under the same test conditions. FA⁺ and Cs⁺ co-doping also significantly improved the thermal and moisture resistance of the cells. Cs_0.05MA_0.35FA_0.6PbI₃ PSCs remain 99% and 84% of initial PCE after aging for 96 h at (85±5) ℃ and 20%-30% relative humidity, (20±5) ℃ and 80%-90% relative humidity in the dark, respectively. In contrast, PCEs of MAPbI₃ PSCs under the same conditions remain only 70% and 56%. This study provides a reference for the preparation of highly efficient and ultra-long stable mixed cation solar cells in the air.

Organic-inorganic hybrid perovskite is an ideal light absorption material due to its high light absorption coefficient, adjustable band gap and bipolar charge conduction characteristics. However, perovskite thin films prepared by solution method possess various defects in the surface and interface, which inhibit carrier transport and trigger recombination. In this study, a multifunctional amino acid derivative, 9-fluorenylmethoxycarbonyl- L-phenylalanine-L-phenylalanine (Fmoc-FF-OH), was selected as an additive to reduce defects of perovskite film and to inhibit carrier recombination at grain boundaries. When the concentration of Fmoc-FF-OH is 0.6 g·L^-1, the particle size of the perovskite thin film increases from 138 to 210 nm, and the defect state density decreases from 2.46×10¹⁵ to 2.17×10¹⁵ cm^-3. Perovskite solar cells also exhibit optimal performance with open circuit voltage increasing from 1.05 to 1.10 V, and photoelectric conversion efficiency (PCE) improved from 15.50% to 17.44%. After stability test for 220 h, the photoelectric conversion efficiency of the device can still maintain 71% of the initial.

With a bandgap of 1.1-1.4 eV, Sn/Pb mixed halide perovskites are ideal materials for single-junction solar cells to reach the power conversion efficiencies (PCEs) limit of Shockley-Queisser (S-Q) theory. Their chemical composition gradient in the vertical direction of the perovskite films affect the transport and separation of carriers by changing the energy band structures. Therefore, it is very important to control the crystallization process of tin-lead mixed perovskite thin films. In this work, it was found that different vertical composition gradients were formed when tin-lead mixed perovskites were prepared with different amounts of the anti-solvent. Larger amounts of anti-solvent was contributed to higher lead content on the film surface. The vertical composition gradient of tin-lead mixed perovskite could be regulated by adjusting the solvent composition, among which increasing V(DMSO):V(DMF) in the solvent could form a vertical composition gradient with a lead-rich bottom and a tin-rich surface. When V(DMSO):V(DMF) in lead-based precursor solutions was optimized to 1 : 2, compared with the control group of 1 : 4, open circuit voltage of the device under standard light conditions increased from 0.725 to 0.769 V, short circuit current density from 30.95 to 31.65 mA·cm^-2, and PCE from 16.22% to nearly 18%. Numerical simulations using SCAPS further proved the necessity of forming a vertical composition gradient. When the bottom of the perovskite film is rich in lead and the top is rich in tin, the recombination of carriers in the hole transport layer interface region is reduced, which can improve the device’s performance.

Carbon-based perovskite solar cells (C-PSCs) play an important role in industrialization research due to their stability and low cost. In this work, high-quality NiO_x mesoporous layer was selected as a hole transport layer (HTL) based on MAPbI₃ material to enhance the performance of C-PSCs. The effect of preparation methods of the NiO_x mesoporous layer on the solar cell performance and the optimum thickness of the NiO_x mesoporous layer were investigated. It was found that mesoporous layers prepared by screen-printing process with well-distributed pores facilitated the filling of perovskite (PVK) precursor solution in the underlayer mesoporous scaffold. Finally, an HTL-contained perovskite solar cell with high efficiency and almost negligible hysteresis was achieved, possessing an open-circuit voltage (V_OC) of 910 mV, a power conversion efficiency (PCE) of 14.63%, and certified efficiency reached 14.88%. Moreover, PCE of the solar cell displayed outstanding stability after being stored in air for nearly 900 h.

Solution-processed perovskite films usually contain a large quantity of grain boundaries, which decrease the film crystalline quality and introduce severe defect recombination, hindering performance of the devices based on them. Therefore, preparation of high-quality films to achieve high power conversion efficiencies remains a great challenge for perovskite solar cells. Due to high abilities of self-assembly and morphology-tuning for liquid crystal molecules, a single-molecule liquid crystal 4-cyano-4′-pentyl biphenyl (5CB) was employed as additive in CH(NH₂)₂PbI₃ (FAPbI₃) precursor solution to increase the perovskite grain size and decrease the grain boundaries. In addition, the cyano group in 5CB passivates the uncoordinated Pb²⁺ in the perovskite films, which reduces the trap density concentration and inhibits the nonradiative recombination. The resulting perovskite solar cells with 0.2 mg/mL 5CB in the precursor achieve an efficient power conversion efficiency of 21.27% with an open circuit voltage of 1.086 V, a current density of 24.17 mA/cm², and a fill factor of 80.96%. In conclusion, introducing single-molecule liquid crystal as additive is a facile and efficient strategy for improving the performance of FAPbI₃ solar cells.

Perovskite solar cells (PSCs) with structure of TiO₂/ZrO₂/carbon triple-layer are attractive recently because of their inexpensive raw materials, scalable fabrication process, and outstanding stability. But little progress has been made in the low temperature fabrication of TiO₂/ZrO₂/carbon triple-layer structured PSCs. A major reason is that it is rather difficult to construct the ZrO₂ spacer layer at low temperature. Herein, we report a facile low-temperature spray-coating method to prepare effective ZrO₂ spacer layer in TiO₂/ZrO₂/carbon triple-layer PSCs using urea to tune the porosity. After optimizing the amount of urea and the thickness of zirconia to 1100 nm, power conversion efficiencies (PCE) of 14.7% for a single cell and 10.8% for a module with 5 cells connected in series (5×0.9 cm× 2.5 cm) were achieved. Furthermore, the PSCs could be stable for 200 d at constant temperature (25 ℃) and humidity (40%). With this spray coating method, the zirconia layer on flexible substrate can endure 50 times of bending without any cracking. Compared to the conventional screen-printing method of ZrO₂ spacer layer, the spray-coating alternative developed in this work shows advantages of more convenient to process, preparation under lower temperature, and compatibility to flexible substrate.

Density functional theory calculations play an important role in the study of defects in halide perovskites. Although the traditional semi-local functionals (such as PBE) can obtain the band gaps close to the experiments, they fail to accurately describe the positions of the band edges. Utilizing more accurate hybrid functionals combined with the spin-orbit coupling (SOC) effect with full structure relaxation is considered to be necessary for the prediction of defect properties. There are two types of hybrid functionals in the literature, namely the screened HSE and the unscreened PBE0. In this study, taking the orthorhombic phase CsPbI₃ as an example, these methods were compared for the calculation of defect properties. The results show that there is no obvious difference between two methods for bulk properties, but qualitative differences appear for the defect properties. Most of the shallow-level defects predicted in the HSE calculations become deep-level defects in the PBE0 calculations. Meanwhile, there are qualitative differences between the defect transition levels and the Kohn-Sham levels. The origin of above differences lies in the fact that the Hartree-Fock exchange potential has long-range interaction. Therefore, in unscreened hybrid functionals, such as PBE0, it is more difficult to obtain convergent results with a manageable supercell size. In contrast, HSE exhibits a screening effect on the Hartree-Fock exchange potential and can obtain accurate defect energy levels using relatively small supercell sizes. Therefore, all results here demonstrate that the HSE hybrid functional owns a significant advantage in dealing with this problem even though a large Hartree-Fock mixing parameter (about 0.43) is needed.

Solar thermal radiant energy is abundant in storage and pollution-free, and is one of the most competitive clean energies in the future. In recent years, halide perovskite quantum dots (PQDs) are widely used in solar cells and luminescent concentrator solar cells due to their excellent photoelectric properties and unique advantages such as quantum confinement effect and solution processing, and possess vast application prospects, but they are still facing many challenges in future commercial applications. In this review, optimization strategies for improving cell performance are emphatically summarized combined with the domestic and foreign research progress in the field of PQD solar cells. The application of PQDs in luminescent concentrator cells is introduced. Finally, the current challenges in this field are elaborated, and its development trends are prospected. This review provides some ideas for the design and development of the photovoltaic technology in the future.

Defects at the surface and grain boundary of the three-dimensional (3D) organic-inorganic metal halide perovskite film incline to cause non-radiative recombination of charge carriers and accelerate decomposition of 3D perovskite, in turn deteriorating the power conversion efficiency (PCE) and stability of the perovskite solar cells (PSCs). In this study, the organic 4-chlorobenzylamine cation was applied to react with 3D perovskite and the residual PbI₂ to in-situ form a two-dimensional (2D) perovskite top layer, which can passivate the surface and grain boundary defects of the 3D perovskite film, and improve the surface hydrophobicity. Based on this strategy, 2D/3D-PSCs with higher PCE and better stability were successfully obtained. Their structure, morphology photoelectric propery and stability of PSCs were systematically studied. All results show that 2D/3D-PSCs achieve PCEs up to 20.88%, much higher than that of 18.70% for the 3D-PSCs. In addition, 2D/3D-PSCs can maintain 82% of the initial PCE after 200 h continuous operation under 1-sun illumination in N₂ atmosphere, exhibiting excellent stability.

Printable perovskite solar cells employ a device structure for which the organic-inorganic perovskite absorber is hosted by an inorganic mesoporous scaffold. Its advantages include simple fabrication process, low material cost and relatively high stability. However, it is challenging to homogeneously deposit high-quality perovskite crystals on the mesoporous scaffold. Herein, by incorporating lead acetate (Pb(Ac)₂) in the typical perovskite precursor, methylamine lead iodine (MAPbI₃), the crystal growth and pore-filling of the perovskite crystals in the mesoporous scaffold is improved by facilitating the crystal nucleation. Meanwhile, Ac^- and MA⁺ form MAAc which releases during thermal annealing process, resulting in excess PbI₂ in the perovskite layer which passivates the grain boundaries. By incorporating 1% molar ratio of Pb(Ac)₂ in the perovskite precursor, a power conversion efficiency of 15.42% is obtained for printable perovskite solar cells. This indicates that it is feasible to enhance the performance of printable perovskite solar cells by employing additives to tune the crystallization of the perovskite absorbers.

The investigation of defects in perovskite crystals is essential to promote the development of perovskite solar cells. The defects result in non-radiative recombination as well as the deterioration of device stability. To reduce the impact of material defects on photovoltaic performance, it is necessary to deeply understand the types of perovskite film defects and the routes of suppression methods. According to electronic characteristics, defects can be divided into electron-rich and electron-deficient defects. Based on Lewis acid-base theory, electron-rich defects can be passivated by Lewis acid, and electron-deficient defects can be passivated by Lewis base or ionic liquids. These passivation additives can be added during the formation of perovskite film, or used to post-treat the surface of the films. This review summarized the defect passivation cases reported in recent years to visually present design strategies of additives and the effects of defect passivation on photovoltaic performance. Finally, developing multi-functional passivators, large-area passivation strategy and advanced charge transport layer were proposed for future perspective. This review is expected to promote the development of perovskite solar cells in the future.

Semitransparent photovoltaics (STPV) is a promising solar energy harvesting technology because it can be integrated to harness huge sun-facing areas of modern buildings for electricity generation. Its thermodynamic efficiency limits are of fundamental interest. This work extended the analyses from neutral STPV for building windows to those installed on the surface of colored building envelops according to the principle of detailed balance. Results show that the efficiency limit of STPV for blue building envelops is as high as 28.3%, which represents a 10% absolute enhancement compared to the neutral STPV for building windows (18.1%). These results demonstrate that STPV can be integrated into both windows and envelops of modern buildings, which has the potential to offset the low density of solar energy. This work provides guidelines on the selection and the development of active materials for STPV.