High performance planar perovskite solar cells with a perovskite of mixed organic cations and mixed halides, MA_1-xFA_xPbI_3-yCl_y

Hybrid organic-inorganic perovskite solar cells (PSCs) have attracted great interest owing to their low fabrication costs and high power conversion efficiency. Most studies have focused on the devices with methylammonium lead trihalide perovskites. Here, we explore a new perovskite with mixed organic cations and mixed halides, MA_1-xFA_xPbI_3-yCl_y. MA_1-xFA_xPbI_3-yCl_y films can be fabricated by annealing at a temperature of 80-110 °C. Planar heterojunction PSCs using this perovskite as the active material can exhibit a high power conversion efficiency (PCE) of up to 18.14% with short-circuit photocurrent density (J_sc) of 21.55 ± 0.55 mA cm^-2, open-circuit voltage (V_oc) of 1.100 ± 0.010 V, and fill factor (FF) of 0.75 ± 0.02. The PCE is much higher than those of the control devices with other commonly employed perovskites including MAPbI₃, MAPbI_3-yCl_y, MAPbI_3-yBr_y, and MA_1-xFA_xPbI₃. The superior performance is mainly attributed to the enhancement of J_sc, which is a result of long charge diffusion lengths due to the presence of mixed organic cations and mixed halides. In addition, there is no obvious hysteresis in the J-V curves along the forward and reverse scan directions. The formation of undesirable δ-phase perovskite that has a band gap of 2.8 eV is not observed in the MA_1-xFA_xPbI_3-yCl_y films. These findings pave the way for the design of new hybrid perovskites with stronger light absorption over a wide range, lower charge recombination, and improved charge transport properties through compositional engineering.