Effect of lead-free (CH3NH3)3Bi2I9 perovskite addition on spectrum absorption and enhanced photovoltaic performance of bismuth triiodide solar cells

Active composite layers of bismuth triiodide (BiI3) and lead-free (CH3NH3)3Bi2I9 (MBI) perovskite were prepared using a simple chemical solution method under ambient conditions for thin-film solar cells. Results of X-ray diffraction and scanning electron microscopy indicated that the crystallization and surface morphologies of the composite films varied with perovskite contents. Multi-absorption was observed in the composite films due to the bandgap difference between BiI3 and MBI perovskite. Moreover, band bending at the BiI3-perovskite interfaces resulted in the realignment of energy levels in the composite films, and this phenomenon was beneficial to the efficient injection of excited electrons from the active layers into the TiO2 layers. Accordingly, due to the optimized crystallization and realigned energy level, when 20% of MBI perovskite was introduced into the active layers, the open-circuit voltage obviously increased from 0.44 V to 0.57 V in the (BiI3)0.8(MBI)0.2 composites solar cells, enhancing their power conversion efficiency by 67% compared with that in pure BiI3 solar cell. This study developed a new route for designing more efficient lead-free solar cells