photovoltaic devices (23.3%), showing great potential toward practical applications. [1] However, thermal instability of organic components (e.g., methylammonium (MA) and formamidinium (FA)) and high toxicity of heavy metal lead (Pb) pose challenges for practical applications. [2][3][4][5] To address these issues, a new perovskite family of organic-free and lead-free cesium tin tri-iodide (CsSnI 3 ) has been proposed and attracted considerable attention. [6][7][8] One of the potential candidates within the new perovskite family is the inorganic CsSnI 3 . This kind of perovskite possesses favorable direct bandgap (around 1.3 eV), high optical absorption coefficient (about 10 4 cm −1 in the visible range), and ultralow exciton binding energy (18 meV), making it very promising as a photoactive layer for PSCs' construction. [9][10][11] Since the first report in 2014 by Kumar et al., the efficiency of CsSnI 3based perovskite solar cells has witnessed steady improvement from 2.02% to 10.1% in 2021. [12,13] Such development has mainly been attributed to addressing the factors that affect the performance of the devices, such as annealing temperature, [14] film fabrication method, [15] and interface regulation. [16,17] However, little efforts have been made to Organic-free and lead-free CsSnI 3 perovskite solar cells (PSCs) have recently gained growing attention as a promising template to mitigate the thermal instability and lead toxicity of hybrid lead-based PSCs. However, the relatively low device efficiency due to the high content of Sn(II)-related defects hinders its further development. Herein, highly performed CsSnI 3−x Br x compositional perovskite-based PSCs are achieved by using dimethyl ketoxime (C 3 H 7 NO, DMKO) as a multifunctional additive. As a commercially used deoxidant, DMKO can effectively neutralize the oxygen molecule and reduce Sn 4+ back to Sn 2+ , enhancing the oxidation resistance of the film. Besides, the electron-rich oxime group (NOH) in DMKO tends to interact with Sn 2+ ions with extremely low adsorption energy less than −15 eV and inhibits defect formation, resulting in films with low defect density. The corresponding PSCs deliver a considerable open-circuit voltage (V oc ) of 0.75 V with a record efficiency as high as 11.2%, which represents the highest reported efficiency for lead-free all-inorganic PSCs thus far. More importantly, the grain surface distributed DMKO provides an in situ encapsulation of the perovskite, which results in greatly enhanced ambient stability of the un-encapsulated devices.

Perovskite Solar Cells In article number 2202491, Tingting Shi, Hsing‐Lin Wang, and co‐workers report highly performing organic‐free and lead‐free CsSnI2.6Br0.4 perovskite based solar cells, achieved via introducing dimethyl ketoxime (DMKO) as a multifunctional additive. The chemically reducing DMKO located on the grain surface, not only mitigates Sn2+ oxidation but also passivates Sn(II) related defects, forming an in situ encapsulation of the perovskite, which results in greatly enhanced ambient stability of the corresponding device.

Sn–Ge mixed perovskites have been proposed as promising lead-free candidates in the photovoltaics (PV) field. In this work, we discovered a stable P1 phase Sn–Ge mixed structure (CsSn0.5Ge0.5I3) with an appropriate band gap value of 1.19 eV, which manifests its unique structural stability and physics properties. The thermodynamic stability of this mixed structure under different growth conditions and all possible native defects are depicted in detail. The formation energies and dominant native point defects indicate that P1 phase CsSn0.5Ge0.5I3 exhibits unipolar self-doping behavior (p-type conductivity) and good defect tolerance while the growth condition changes. In addition, the calculation of light absorption confirmed that the P1 phase has a higher light absorption coefficient than that of MAPbI3 in the visible light range, showing excellent light absorption. Our work not only provides theoretical guidance for unraveling the unusual structural stability of Sn–Ge mixed perovskites, but also offers a useful scheme to modulate the stability and optoelectronic properties of Ge-based perovskites through alloy engineering.

Monolithic perovskite/organic tandem solar cells (POTSCs) have significant advantages in next‐generation flexible photovoltaics, owing to their capability to overcome the Shockley–Queisser limit and facile device integration. However, the compromised sub‐cells performance challenges the fabrication of high‐efficiency POTSCs. Especially for all‐inorganic wide‐bandgap perovskite front sub‐cells (AIWPSCs) based n‐i‐p structured POTSCs (AIPOTSCs), for which the power conversion efficiency (PCE) is much lower than organic–inorganic mixed‐halide wide‐bandgap perovskite based POTSCs. Herein, an ionic liquid, methylammonium formate (MAFm), based dual‐interface engineering approach is developed to modify the bottom and top interfaces of wide‐bandgap CsPbI2Br films. In particular, the Fm− group of MAFm can effectively passivate the interface defects, and the top interface modification can facilitate the formation of uniform perovskite films with enlarged grain size, thereby mitigating the defects and perovskite grain boundaries induced carrier recombination. As a result, CsPbI2Br‐based AIWPSCs with a high PCE of 17.0% and open‐circuit voltage (VOC) of 1.347 V are achieved. By integrating these dual‐interface engineered CsPbI2Br‐based front sub‐cells with the narrow‐bandgap PM6:CH1007‐based rear sub‐cells, a record PCE of 23.21% is obtained for AIPOTSCs, illustrating the potential of AIPOTSCs for achieving high‐efficiency tandem solar cells.

Vacancy defects are universally regarded to be the main defect that limits the photoelectric conversion efficiency of perovskite solar cells. In perovskite, iodine vacancy dominates the defect proportion due to its low formation energy. However, the defect property of iodine vacancy (VI) is still in dispute. Ideally, the VI defect is considered to be a shallow level defect near conduction band minimum, meaning that it does not act as a Shockley–Read–Hall (SRH) nonradiative recombination center. Herein, we find a direct correlation between compressive strain and VI defect behavior. The compressive strain along the lattice vector b/c direction will drive the VI defect from shallow level to deep level defect, which is related to the formation of Pb-dimer. In addition, the influence of extra electrons is also considered during the structural evolution of VI, which is often observed in the experiments. Therefore, we find that the elimination of compressive strain and extra electrons can be of great significance for improving the photoelectric performance of perovskite solar cells. Our work reveals the defect properties of VI from shallow level one to the SRH recombination center and the inherent physics mechanism of defect evolution under external factors, which provides a strategy to control device defects and eliminate recombination losses.