Optoelectronic and Energy Level Exploration of Bismuth and Antimony-Based Materials for Lead-Free Solar Cells

Bismuth- and antimony-based materials, such as A₃M₂X₉ and AMSX₂ (A = cation, M = Bi, Sb, S = sulfur, X = halogen), are promising candidates as the counterpart to lead halide perovskite. However, the large number of different compositions and crystal structures (dimer, perovskite, etc.) has made these materials largely overlooked; thus, an intuitive evaluation strategy is required. Here, we present a comprehensive study of the energy levels (bandgap, valence band maximum, etc.) and optoelectronics (photoconductivity and charge transfer to charge transport material) of the Bi- and Sb-based materials, which include 6 crystal categories with 44 compositions, by using time-resolved microwave conductivity (TRMC). Importantly, we found an efficient hole transfer from the Sb-based materials to the hole transport materials with the inclusion of the thiophene component, leading to an improved power conversion efficiency of 2.91% for Sb₂S₃-containing SbSI, prepared by a novel one-step method. Our study establishes a key rule for exploring active layer compositions and designing device structures, which would accelerate the evolution of Bi- and Sb-based lead-free solar cells.