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Tin dioxide (SnO 2 ) with high conductivity and low photocatalytic activity has been reported as one of the best candidates for highly efficient electron transport layer (ETL) in perovskite solar cell (PSC). The state-of-the-art SnO 2 layer is achieved by chemical bath deposition with tunable properties, while the commercial SnO 2 nanocrystals (NCs) with low tunability still face the necessity of further improvement. Here, a kind of highly crystallized Cl-doped SnO 2 NCs is reported that can form very stable aqueous dispersion with shelf life up to one year without any stabilizer, which can facilitate the fabrication of PSCs with satisfactory performance. Compared to the commercial SnO 2 NCs regardless of the extrinsic Cl-doping conditions, the intrinsic Cl-doped SnO 2 NCs effectively suppress the energy barrier and reduces the trap state density at the buried interface between perovskite and ETL. Consequently, stable PSCs based on such Cl-doped SnO 2 NCs achieve a champion efficiency up to ≈25% for small cell (0.085 cm 2 ) and ≈20% for mini-module (12.125 cm 2 ), indicating its potential as a promising candidate for ETL in high-performance perovskite photovoltaics., (© 2023 Wiley-VCH GmbH.)

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic-inorganic hybrid perovskite because the functional organic cations' substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI 3 ) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA 4 Pb 3 I 10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI 3 films. Moreover, the BTAI surface terminated BTAI-MAPbI 3 shows better passivation effect than BTA 4 Pb 3 I 10 -MAPbI 3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm 2 ) and mini-module (11.52 cm 2 ) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI 3 based ones., (© 2023 Wiley-VCH GmbH.)

Black-phase formamidinium lead iodide (FAPbI 3 ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI 2 ) residue, most sequential deposition methods of FAPbI 3 -based perovskite will introduce external ions like methylammonium (MA + ), cesium (Cs + ), and bromide (Br - ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO 2 ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI 3 . The SnO 2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI 2 in the perovskite layer, which reduces the side effect of the residual PbI 2 . Besides, the FSA functionalization also modifies SnO 2 ETL to suppress deep-level defects at the perovskite/SnO 2 interface. As a result, the FSA-FAPbI 3 -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI 3 PSCs with improved optoelectronic performance., (© 2022 Wiley-VCH GmbH.)

All-inorganic perovskites have attracted increasing attention for applications in perovskite solar cells (PSCs) and optoelectronics, including light-emitting devices (LEDs). Cesium lead halide perovskites with tunable I/Br ratios and a band gap aligning with the sunlight region are promising candidates for PSCs. Although impressive progress has been made to improve device efficiency from the initial 2.9 % with low phase stability to over 20 % with high stability, there are still questions regarding the perovskite crystal growth mechanism, especially at low temperatures. In this Minireview, we summarize recent developments in using an organic matrix, including the addition and use of organic ions, polymers, and solvent molecules, for the crystallization of black phase inorganic perovskites at temperatures lower than the phase transition point. We also discuss possible mechanisms for this low-temperature crystallization and their effect on the stability of black phase perovskites. We conclude with an outlook and perspective for further fabrication of large-scale inorganic perovskites for optoelectronic applications., (© 2021 Wiley-VCH GmbH.)

Inorganic CsPbI 3 perovskite with high chemical stability is attractive for efficient deep-red perovskite light-emitting diodes (PeLEDs) with high color purity. Compared to PeLEDs based on ex-situ-synthesized CsPbI 3 nanocrystals/quantum dots suffering from low conductivity and efficiency droop under high current densities, in situ deposited 3D CsPbI 3 films from precursor solutions can maintain high conductivity but show high trap density. Here, it is demonstrated that introducing diammonium iodide can increase the size of colloids in the precursor solution, retard the phase-transition rate, and passivate trap states of the in-situ-formed cuboid crystallites. The PeLED based on the one-step-formed 3D CsPbI 3 cuboid crystallite films shows a peak external quantum efficiency (EQE) value up to 15.03% because of the high conductivity and reduced trap states. Furthermore, this one-step method also has a wide processing window, which is attractive for flow-line production of large-area PeLED modules. The fabrication of a 9 cm 2 PeLED that exhibits a peak EQE of 10.30% is successfully demonstrated., (© 2021 Wiley-VCH GmbH.)

Defect-triggered phase degradation is generally considered as the main issue that causes phase instability and limited device performance for CsPbI 3 inorganic perovskites. Here, a defect compensation in CsPbI 3 perovskite through crystal secondary growth of inorganic perovskites is demonstrated, and highly efficient inorganic photovoltaics are realized. This secondary growth is achieved by a solid-state reaction between a bromine salt and defective CsPbI 3 perovskite. Upon solid-state reaction, the Br - ions can diffuse over the entire CsPbI 3 perovskite layer to heal the undercoordinated Pb 2+ and conduct certain solid-state I/Br ion exchange reaction, while the organic cations can potentially heal the Cs + cation vacancies through coupling with [PbI 6 ] 4- octahedra. The carrier dynamics confirm that this crystal secondary growth can realize defect compensation in CsPbI 3 . The as-achieved defect-compensated CsPbI 3 not only improves the charge dynamics but also enhances the photoactive phase stability. Finally, the CsPbI 3 -based solar cell delivers 20.04% efficiency with excellent operational stability. Overall, this work proposes a novel concept of defect compensation in inorganic perovskites through crystal secondary growth induced by solid-state reaction that is promising for various optoelectronic applications., (© 2021 Wiley-VCH GmbH.)

The in situ formation of reduced dimensional perovskite layer via post-synthesis ion exchange has been an effective way of passivating organic-inorganic hybrid perovskites. In contrast, cesium ions in Cs-based inorganic perovskite with strong ionic binding energy cannot exchange with those well-known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA + ) cation can intercalate into CsPbI 3 to effectively substitute the Cs cation and to form one-dimensional (1D) TBAPbI 3 layer in the post-synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI 3 protective layer to heal defects at the surface of inorganic CsPbI 3 perovskite. The TBAPbI 3 -CsPbI 3 perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one., (© 2021 Wiley-VCH GmbH.)

Research on chemically stable inorganic perovskites has achieved rapid progress in terms of high efficiency exceeding 19% and high thermal stabilities, making it one of the most promising candidates for thermodynamically stable and high-efficiency perovskite solar cells. Among those inorganic perovskites, CsPbI 3 with good chemical components stability possesses the suitable bandgap (≈1.7 eV) for single-junction and tandem solar cells. Comparing to the anisotropic organic cations, the isotropic cesium cation without hydrogen bond and cation orientation renders CsPbI 3 exhibit unique optoelectronic properties. However, the unideal tolerance factor of CsPbI 3 induces the challenges of different crystal phase competition and room temperature phase stability. Herein, the latest important developments regarding understanding of the crystal structure and phase of CsPbI 3 perovskite are presented. The development of various solution chemistry approaches for depositing high-quality phase-pure CsPbI 3 perovskite is summarized. Furthermore, some important phase stabilization strategies for black phase CsPbI 3 are discussed. The latest experimental and theoretical studies on the fundamental physical properties of photoactive phase CsPbI 3 have deepened the understanding of inorganic perovskites. The future development and research directions toward achieving highly stable CsPbI 3 materials will further advance inorganic perovskite for highly stable and efficient photovoltaics., (© 2020 Wiley-VCH GmbH.)