Viability of Lead-Free Perovskites with Mixed Chalcogen and Halogen Anions for Photovoltaic Applications

We assess the viability for photovoltaic applications of proposed Pb-free perovskites with mixed chalcogen and halogen anions, AB(Ch,X)3 (A = Cs or Ba; B = Sb or Bi; Ch = chalcogen; X = halogen), by examining critical issues such as the structural, electronic/optical properties, and stability through the combination of density-functional theory calculations and solid-state reactions. The calculations show that these quaternary Pb-free perovskites are thermodynamically unstable—they are prone to decompose into ternary and/or binary secondary phases or form phases with nonperovskite structures. Solid-state synthesis efforts confirm the theoretically predicted difficulty for preparing these compounds; all attempted reactions do not form the desired perovskite phases with mixed chalcogen and halogen anions under conditions examined. Instead, they form separate binary and ternary compounds. Despite earlier predictions of promising characteristics for these prospective perovskites for photovoltaics, our results suggest that, due to their instability, the Pb-free perovskites with mixed chalcogen and halogen anions may be challenging to form under equilibrium synthetic conditions.