Superalkali halide perovskites with suitable direct band gaps for photovoltaic applications

The construction of superalkali halide perovskites has attracted attention for the development of new photovoltaic materials, but stable superalkalis have not been found until now. Herein, to construct new three-dimensional superalkali halide perovskites with a MI₃ frame (M = Sn and Pb), a new Li(H₂O)₃⁺ superalkali cation is designed and selected based on low vertical ionization potential, suitable tolerance factor, small ionic radius and large dissociation energy. High-throughput first-principles calculations show that superalkalis with lower vertical ionization potentials exhibit stronger interactions with the MI₃ frame. The normal and cubic Li(H₂O)₃MI₃ perovskites and cubic Li(H₂O)₄PbI₃ perovskites have direct band gaps, s-p and p-p electron transitions, effective carrier masses of less than 0.45m_e and exciton binding energies of less than 291 meV. Moreover, the cubic Li(H₂O)₃PbI₃ perovskite with a direct band gap of 1.40 eV can in theory show a power conversion efficiency of 33.49%. These results strongly suggest that superalkali cations with large dissociation energy can be used to develop stable superalkali perovskites for photovoltaic applications.