Two-Dimensional Lead-Free Double Perovskite with Superior Stability and Optoelectronic Properties for Solar Cell Application

2D Dion–Jacobson (DJ) perovskites have recently drawn much attention due to their superior charge transport and high environmental stability. Unfortunately, the issue of toxicity regarding the Pb element in perovskites has yet to be solved. Herein, we propose a new BDA2CuBiI8 (1,4-butanediamine) 2D perovskite by substituting two Pb2+ with the pair of Cu+ and Bi3+ to achieve Pb-free halide perovskites. The first-principles calculations demonstrated the thermodynamic and dynamic stabilities of 2D BDA2CuBiI8. Furthermore, the results indicate that this new material has a direct band gap of 1.58 eV suitable for solar cells with a theoretical maximal efficiency over 30% and allows direct transitions between band edges contrary to its bulk counterpart. The interlayer interaction of apical I–I is found to play a crucial role in determining the band edges. The findings indicate DJ structure type and double substitution together provide a new method for designing lead-free perovskites for efficient solar cells.