Remarkable Stability and Optoelectronic Properties of an All-Inorganic CsSnGeI Perovskite Solar Cell.

Sn-Ge mixed perovskites have been proposed as promising lead-free candidates in the photovoltaics (PV) field. In this work, we discovered a stable 1 phase Sn-Ge mixed structure (CsSnGeI) with an appropriate band gap value of 1.19 eV, which manifests its unique structural stability and physics properties. The thermodynamic stability of this mixed structure under different growth conditions and all possible native defects are depicted in detail. The formation energies and dominant native point defects indicate that 1 phase CsSnGeI exhibits unipolar self-doping behavior (p-type conductivity) and good defect tolerance while the growth condition changes. In addition, the calculation of light absorption confirmed that the 1 phase has a higher light absorption coefficient than that of MAPbI in the visible light range, showing excellent light absorption. Our work not only provides theoretical guidance for unraveling the unusual structural stability of Sn-Ge mixed perovskites, but also offers a useful scheme to modulate the stability and optoelectronic properties of Ge-based perovskites through alloy engineering.