Enhancing the optoelectronic properties and performance of lead-based quasi-two-dimensional Ruddlesden–Popper perovskites through partial substitution of lead with cadmium for solar cells

Lead-based quasi-two-dimensional (2DRP) perovskites are known for their tunable optoelectronic properties, high moisture resistance, and environmental stability. However, these perovskites exhibit a wide band gap (Eg), low visible light absorption, and high exciton binding energy, which limits charge mobility and results in low photovoltaic efficiency. To address these issues, we propose a strategy involving butylammonium (BA) and methylammonium (MA) cations in the chemical formula BA₂(MA)(Cd_xPb_(1-x))₂I₇, where partial substitution of lead (Pb) with cadmium (Cd) is used to adjust the band gap and improve the optoelectronic properties. Using first-principles calculations, our results show that Cd incorporation induces a transition from orthorhombic to monoclinic crystal structure. This transition reduces the distances between the inorganic layers, while the hybridisation of the 6s orbitals of lead and the 5s orbitals of cadmium leads to a progressive decrease in Eg, from 2.19 eV (pure Pb) to 1.81 eV (x = 0.25) and 1.55 eV (x = 0.5), thus enhancing visible light absorption. Additionally, reflectivity decreases from 24 % to 17 %, along with a reduction in exciton binding energy and effective mass, thus facilitating charge transport. The optimised composition, BA₂(MA)(Pb $\frac{1}{2}$ Cd$\frac{1}{2}$)₂I₇, with a thickness of less than 10 μm, achieved a short-circuit current density (Jsc) of 27 mA/cm² and a maximum light-to-electricity conversion efficiency (SLME) of 30 %, surpassing the pure lead-based perovskite (SLME 19 %, Jsc 11 mA/cm²). These results suggest that partial substitution of lead with cadmium in 2DRP perovskites is a promising strategy for enhancing photovoltaic performance and developing efficient and stable perovskite solar cells.