In this work, a fully tin‐based, mixed‐organic‐cation perovskite absorber (FA)_x(MA)_1−xSnI₃ (FA = NH₂CH = NH₂⁺, MA = CH₃NH₃⁺) for lead‐free perovskite solar cells (PSCs) with inverted structure is presented. By optimizing the ratio of FA and MA cations, a maximum power conversion efficiency of 8.12% is achieved for the (FA)_0.75(MA)_0.25SnI₃‐based device along with a high open‐circuit voltage of 0.61 V, which originates from improved perovskite film morphology and inhibits recombination process in the device. The cation‐mixing approach proves to be a facile method for the efficiency enhancement of tin‐based PSCs.

During the past several years, methylammonium lead halide perovskites have been widely investigated as light absorbers for thin-film photovoltaic cells. Among the various device architectures, the inverted planar heterojunction perovskite solar cells have attracted special attention for their relatively simple fabrication and high efficiencies. Although promising efficiencies have been obtained in the inverted planar geometry based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) sulfonic acid (PEDOT:PSS) as the hole transport material (HTM), the hydrophilicity of the PEDOT:PSS is a critical factor for long-term stability. In this paper, a CuO_x hole transport layer from a facile solution-processed method was introduced into the inverted planar heterojunction perovskite solar cells. After the optimization of the devices, a champion PCE of 17.1% was obtained with an open circuit voltage (V_oc) of 0.99 V, a short-circuit current (J_sc) of 23.2 mA cm⁻² and a fill factor (FF) of 74.4%. Furthermore, the unencapsulated device cooperating with the CuO_x film exhibited superior performance in the stability test, compared to the device involving the PEDOT:PSS layer, indicating that CuO_x could be a promising HTM for replacing PEDOT:PSS in inverted planar heterojunction perovskite solar cells.