Electron-transport-layer free perovskite solar cells (ETL-free PSCs) have attracted great attention due to their low cost and simple manufacturing process. However, an additional interface layer has to be introduced, and the currently achieved efficiency remains far from full-structure PSCs. Here, we report an in situ interface engineering strategy by the methylammonium acetate (MAAc) ionic liquid perovskite precursor. We found that a dipole layer was in situ constructed through the physical adsorption of the residual MAAc polar molecules on the indium tin oxide electrode, which is significantly different from the treatment by the interface layer in previous reports. This allows a decrease of the effective work function and enables in situ band bending in the perovskite semiconductor. The in situ band bending facilitates charge collection and hinders interfacial charge recombination, leading to ETL-free PSCs with a maximum power conversion efficiency of 21.08%, which is the highest report to date.
Pb-based metal halide perovskites (MHPs) have emerged as efficient light absorbers in third-generation photovoltaic devices, and the latest certified power conversion efficiency (PCE) of Pb-based perovskite solar cells (PSCs) has reached 25.2%. Despite great progress, Pb-based MHPs are affected by toxicity, which hinders their market entry in a potential future large-scale commercialization effort. Therefore, the exploration of Pb-free MHPs has become one of the alternative solutions sought in the community. Among all the Pb-free MHPs, Sn-based MHPs show great promise owing to their similar or even superior theoretical optoelectronic characteristics. After several years of development, the PCE of Sn-based PSCs has recently been approaching 10%, with the breakthroughs mainly coming from A-site cation engineering of Sn-based MHPs. In this review, the crucial status of A-site cation engineering strategies in the research of Sn-based PSCs is highlighted. First, the way the features of A-site cation influence the structure and characteristics of MHPs is systematically demonstrated. Then, the state-of-the-art developments, focusing on A-site cation engineering of Sn-based MHPs, are comprehensively reviewed. Subsequently, the current challenges and opportunities for further boosting the performance of Sn-based PSCs are discussed. Finally, conclusions and perspectives on the promising Sn-based optoelectronic devices are discussed.
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