Multi-site Mixing and Entropy Stabilization of CsPbI 3 with Potential Application in Photovoltaics

Metal halide perovskite solar cells have achieved dramatic improvements in their power conversion efficiency in the recent past. Since compositional engineering plays an important role in optimizing material properties, we investigate the effect of alloying at Cs and Pb sites on the energetics and electronic structure of CsPbI 3 using the cluster expansion method in combination with first-principles calculations. For Ge mixing at the Pb site, the α- and β-phases are considered with emphasis on the electronic structure, transition probability, absorption coefficient, efficiency, and carrier mobility of higher-symmetry configurations. CsPb 0.50 Ge 0.50 I 3 (Cs 2 PbGeI 6 ), which takes up a double perovskite (elpasolite) structure, has a direct band gap with no parity-forbidden transitions. Further, we utilize the alloy entropic effect to improve the material stability (energetic) and optoelectronic properties of CsPbI 3 by multi-element mixing. For the proposed mixed compositions, the Fröhlich electron–phonon coupling constant is determined. Scattering rates and electron mobility are obtained from first-principles inputs. These lower Pb-content inorganic perovskites offer great promise as efficient solar cell materials for photovoltaic applications. Our work provides quantitative first-principles benchmarks on how multisite mixing at Cs/Rb and Pb/Ge sites modifies band structure, carrier mobility, and entropy-driven free energies. These insights guide future alloy design strategies for reducing Pb-content and band gap engineering.