First-principles study of thermoelectric properties of mixed iodide perovskite Cs(B,B′)I₃ (B, B′ = Ge, Sn, and Pb)

The thermoelectric properties of the mixed iodide perovskites Cs(B,B′)I₃ (B, B′ = Ge, Sn, and Pb), which are related to the well-known organic–inorganic hybrid perovskite used in solar cells, were studied using first-principles calculation. We evaluated the thermoelectric properties of stable partially substituted structures, which were explored by applying a cluster expansion method in our previous work based on the Boltzmann transport equation. The calculated figure of merit, ZT, is larger for N-type systems than for P-type systems. The gradient of the density of states (DOS) at the conduction band edge is steep for all the compositions. This steep conduction edge results in large Seebeck coefficients in N-type systems. Excellent thermoelectric performance can be expected in N-type systems. Among the lead-free compounds, a Cs(Ge,Sn)I₃ solid solution with a high germanium content is a possible candidate for a high-ZT material. It was observed that ZT and the Seebeck coefficient increase with increasing gradient of the DOS near the Fermi energy. In addition, ZT and the Seebeck coefficient increase as the band gap increases, which is remarkable in a P-type system. This result indicates a close relationship among the ZT, Seebeck coefficient, and band gap in P-type systems.