Lead-free formamidinium bismuth perovskites (FA)(3)Bi2I9 with low bandgap for potential photovoltaic application

Bismuth halide perovskites are potential lead-free absorbing materials for solar cells. However, methylamine and cesium-based bismuth perovskites are insufficient for visible light absorption because of their large bandgaps. In this work, we developed a new formamidinium (FA)-based bismuth perovskite material, (FA)(3)Bi2I9. (FA)(3)Bi2I9 shows a hexagonal phase with an enlarged unit cell than the conventional methylamine-based bismuth perovskite (MA)(3)Bi2I9. It has a bandgap of 2.19 eV and a Wannier-Mott exciton binding energy of 260 meV, which are lower than those of (MA)(3)Bi2I9 (2.25 eV and 320 meV, respectively) in our experiments. Thus, (FA)(3)Bi2I9 is more suitable as an absorbing layer for solar cells than (MA)(3)Bi2I9. In addition, solvents can effectively control the growth of (FA)(3)Bi2I9 films. In specific, using dimethyl sulfoxide as the solvent resulted in enhanced c-axis oriented growth and plate-like microstructure. With these bismuth perovskite films as absorbing layers, mesoporous-structured (FA)(3)Bi2I9 solar cells were fabricated for the first time. The initial devices show the highest open voltage of 0.48 V and a power conversion efficiency of 0.022%, which are higher than those of methylamine-based bismuth perovskite solar cells, offering a potential candidate for lead-free perovskite solar cells.