Theoretical and experimental investigations on the bulk photovoltaic effect in lead-free perovskites MASnI3 and FASnI3

Perovskite solar cells based on the lead free hybrid organic-inorganic CH3NH3SnI3 (MASnI(3)) and CH4N2SnI3 (FASnI(3)) perovskites were fabricated, and the photoelectric conversion efficiency (PCE) was assessed. FASnI(3)'s PCE was higher than MASnI(3)'s efficiency. To study the different photovoltaic properties, we calculated their structural, electronic, and optical properties using density functional theory via the Perdew-Burke-Ernzerhof and spin-orbit coupling (PBE-SOC) methods. The results show that FASnI(3) exhibits an appropriate band gap, substantial stability, marked optical properties, and significant hole and electron conductive behavior compared with MASnI(3). The interaction of organic cations (FA(+)) with the inorganic framework of FASnI(3) was stronger than that with MASnI(3), so they affected the band length and band angle distribution, causing the structure of the FASnI(3) and MASnI(3) to change. The calculations also demonstrated that energy splitting was evident in FASnI(3) due to the spin-orbit coupling effect, however, it was moderate in MASnI(3), which was caused by the H bond effect. This research not only furthers the understanding of these functional materials, but also can assist the development of highly efficient and stable non-lead perovskite solar cells.