Electronic structures of lead-free Sn- or Ge-hybrid perovskite halides studied by first-principles calculation

Although CH3NH3PbI3-based perovskite compounds have excellent photovoltaic properties, the lead (Pb) is harmful to the environment, and Pb-free perovskite compounds are needed for next generation solar cells. Theoretical calculations could be one of the good methods to design the Pb-free compounds, and the first-principles calculation was used to predict the properties of the present proposed materials. The instability of organic molecules such as CH3NH3 is also a problem, and alkali elements such as cesium and rubidium could improve the stability. From these point of view effects of tin (Sn) or germanium (Ge) on the lead-free Cs- or Rb- -based perovskites including double perovskite structures were investigated by first-principles calculation in the present work. For the standard single perovskite structures, perovskites with chlorine have suitable band gap energies, higher electron mobilities, and photovoltaic performance compared with other iodine or bromine-based perovskites. For the double perovskite structures, bromine was found to be the suitable halogen, and Ge provided a higher electron density than Sn. The double perovskite structures also provided wider band gap energies and improved stability compared with the standard single perovskites, and the photovoltaic properties could be affected by hybridization of Sn/Ge or halogen ions.