Proposing lead-free Cs2SnI6-nBrn for silicon heterojunction-based tandem solar cell

Abstract

Emerging Lead (Pb)-free perovskite solar cells are environment-friendly and show higher stability. Such materials are considered the future of the photovoltaic industry. However, the detailed understanding of these materials is still very limited experimentally as well as theoretically. In this work, all inorganic Pb-free perovskites Cs2SnI6-nBrn have been studied as top cell materials for silicon heterojunction (SHJ)-based tandem solar cells. The optoelectronics characteristics including bandgap, optical absorption, effective masses and carrier mobility of Cs2SnI6-nBrn have been calculated by the first-principle with varying Br- alloying. Details theoretical study show that the bandgap of Cs2SnI6-nBrn can tuned over a range of 1.05 eV to 1.99 eV. In addition, carrier mobility has been modelled for the studied perovskites by deformation potential theory. Based on the calculated optoelectronics characteristics, a standalone Cs2SnI6-nBrn solar cell is simulated and 20.5 % efficiency is reported. SHJ-based solar cell is used as the bottom cell with 28.5 % standalone efficiency under standard spectrum illuminations. Finally, the Cs2SnI6-nBrn/SHJ tandem solar cell is designed and a simulated PCE of 39.1 % has been achieved with n = 1. The highest 1.46 V Voc is obtained with a 40.5 mA/cm2 matched current between the top and bottom cells. The proposed work gives a detailed study of Cs2SnI6-nBrn perovskites and is expected to facilitate the production of Pb-free perovskites/SHJ tandem solar cells.