Physical Properties and Photovoltaic Performance of Perovskite Solar Cells Based on Lead-Free A3Bi2I9 (A = CH3NH3, Cs) Active Layers

Perovskite solar cells based on spin-coated (CH3NH3)3Bi2I9 and Cs3Bi2I9 thin films were fabricated, and the thickness-dependent structural, morphological, optical, and photovoltaic properties were studied. The x-ray diffraction studies revealed the formation of a hexagonal structure in the studied perovskite layers, and an improvement in crystallinity and morphology was also observed with increasing thickness. The absorption coefficient was of the order of 104 cm−1, and depending on film thickness, the band gap values varied from 1.70 eV to 2.02 eV and from 1.95 eV to 2.25 eV for (CH3NH3)3Bi2I9 and Cs3Bi2I9 active layers, respectively. We also investigated the effect of the thickness of these films on device performance. Results showed that with increasing thickness of the active layers, improvements of 21% in open-circuit voltage (VOC) and 36% in efficiency were observed for (CH3NH3)3Bi2I9 perovskite devices, compared with improvements of 28% and 57% in VOC and cell efficiency, respectively, for Cs3Bi2I9 solar cells.