Tailoring In-Gap States for Optimizing the Band Gap in Mixed-Valent Bromometallates, Towards Lead-Free Photovoltaics

Abstract

Motivated by the task to replace toxic lead halide perovskites with lead-free environmentally benign light-harvesting materials for photovoltaics, we have synthesized two new hybrid bromometallates, (AH2)4[Sb2Br10(SbBr6)2(Br)2] (1) and (AH2)2[(BiBr6)(SbBr6)]·2H2O (2), where A = 1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonan. Their crystal structures show combinations of Sb3+/Sb5+ (1) or Bi3+/Sb5+ (2) centers each surrounded by six Br− anions forming distorted (Sb3+ or Bi3+) or almost regular (Sb5+) octahedra. These octahedra are arranged in the crystal structure in a perfectly ordered way owing to weak Br…Br interactions within the inorganic anionic part and to hydrogen bonds linking inorganic anions and organic cations. The electronic structures of both compounds are alike in that they have M3+/Br− states (M = Sb or Bi) dominating the top of the valence band and acceptor in-gap states formed by the Sb5+/Br− interactions. Such an electronic structure leads to the reduced band gap of 1.27 (1) and 1.58 eV (2), favorable for creating light-harvesting materials for photovoltaics. We support our data by powder and single-crystal X-ray diffraction study, 121Sb Mossbauer spectroscopy, and optical spectroscopy. We infer that tailoring the band gap is a very important step on the way to lead-free light-harvesting materials for photovoltaic applications.