First-principles study of lead-free Ge-based 2D Ruddlesden-Popper hybrid perovskites for solar cell applications

Recently, 2D halide perovskites have attracted attention because they are excellent photo absorbing materials for perovskite solar cells. To date, the majority of 2D perovskite-based devices have been made of Pb, a material with toxic properties and environmental concerns. Thus, lead-free alternatives are essential to enable the expansion of photovoltaic systems based on perovskites. Herein, we examine the structural, electronic, optical and stability properties of Pb-free 2D Ruddlesden-Popper (RP) perovskites (BA)₂(MA)GeI₃ (BA = CH₃(CH₂)₃NH₃⁺; MA = CH₃NH₃⁺; n = 1-5, and ∝) by using DFT calculations and comparing the results to their Pb-based counterparts (BA)₂(MA)PbI₃ (n = 1-5, and ∝). Theoretical analysis indicates that Pb and Ge-based 2D perovskites are significantly more thermodynamically stable than their corresponding 3D materials. A more accurate bandgap is achieved using the HSE06 + SOC scheme and compared to the findings of the PBE and PBE + SOC. These materials are direct bandgap semiconductors. Due to spin-orbit coupling, Pb-based perovskite displays higher Rashba energy splitting than Ge-based ones. The bandgap changes from 2.37 eV (n = 1) to 1.79 eV (n = 5), and from 1.92 eV (n = 1) to 1.56 eV (n = 5) for Pb and Ge-based perovskites, respectively. The bandgap of all Ge-based perovskites is lower than their corresponding Pb-based ones. We show that the 2D perovskites could serve as hole-transporting materials when they are alongside 3D perovskites. The trade-off between thermodynamic stability and absorption coefficient of the considered compounds indicates that 2D RP perovskites BA₂MA₄Ge₅I₁₆ are promising Pb-free halide semiconductors for solar cell applications.