Structural and optoelectronic properties of BA₂(MA)Pb₂I₇ and BDA(MA)Pb₂I₇ perovskites for solar cell applications: A DFT study on the role of organic cations

Abstract

Two-dimensional (2D) perovskites, renowned for their exceptional optical properties and stability, are increasingly explored for photovoltaic applications. However, the influence of interspace cations on the structural, optoelectronic, surface, water interaction, and mechanical properties of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases is still lacking. Using density functional theory (DFT), this study examines the effects of interspace butylammonium (BA) and butanediammonium (BDA) cations, alongside the intercage methylammonium (MA) cation, on the perovskites BA₂(MA)Pb₂I₇ and BDA(MA)Pb₂I₇. Our results show that the BDA cation, with its higher dielectric constant and reduced spatial occupation between inorganic layers, shifts electron absorption in the Pb 6p conduction orbitals to photon energies of 1.69 eV, resulting in improved light absorption, lower exciton binding energy, enhanced charge filtering, and reduced carrier recombination rates compared to the BA-based perovskite (1.86 eV). This feature of BDA lowers the work function, facilitating electron extraction. Additionally, reduced ionisation potentials and electron affinities suggest enhanced charge transport and extraction capabilities. The BDA-based perovskite, with reduced interlayer spacing and increased mass density, minimises voids and strengthens interlayer interactions, providing higher mechanical resistance and improved ductility, as indicated by higher values for Poisson's ratio, Young's modulus, and bulk modulus, as well as better surface resistance to water compared to BA-based perovskites. These attributes make BDA-based perovskites promising candidates for thin-film photovoltaic applications, surpassing BA-based variants in machinability. This study highlights cations' crucial role, spatial occupation, and characteristics in modulating optoelectronic and structural properties, paving the way to optimise 2D perovskites for high-performance photovoltaics.