<i>A</i>-Site Cation in Inorganic <i>A</i>₃Sb₂I₉ Perovskite Influences Structural Dimensionality, Exciton Binding Energy, and Solar Cell Performance

Abstract

Inspired by the rapid rise in efficiencies of lead halide perovskite (LHP) solar cells, lead-free alternatives are attracting increasing attention. In this work, we study the photovoltaic potential of solution-processed antimony (Sb)-based compounds with the formula A3Sb2I9 (A = Cs, Rb, and K). We experimentally determine bandgap magnitude and type, structure, carrier lifetime, exciton binding energy, film morphology, and photovoltaic device performance. We use density functional theory to compute the equilibrium structures, band structures, carrier effective masses, and phase stability diagrams. We find the A-site cation governs the structural and optoelectronic properties of these compounds. Cs3Sb2I9 has a 0D structure, the largest exciton binding energy (175 ± 9 meV), an indirect bandgap, and, in a solar cell, low photocurrent (0.13 mA cm–2). Rb3Sb2I9 has a 2D structure, a direct bandgap, and, among the materials investigated, the lowest exciton binding energy (101 ± 6 meV) and highest photocurrent (1.67 mA cm–2). K3Sb2I9 has a 2D structure, intermediate exciton binding energies (129 ± 9 meV), and intermediate photocurrents (0.41 mA cm–2). Despite remarkably long lifetimes in all compounds (54, 9, and 30 ns for Cs-, Rb-, and K-based materials, respectively), low photocurrents limit performance of all devices. We conclude that carrier collection is limited by large exciton binding energies (experimentally observed) and large carrier effective masses (calculated from density functional theory). The highest photocurrent and efficiency (0.76%) were observed in the Rb-based compound with a direct bandgap, relatively lower exciton binding energy, and lower calculated electron effective mass. To reliably screen for candidate lead-free photovoltaic absorbers, we advise that faster and more accurate computational tools are needed to calculate exciton binding energies and effective masses.