Epitaxial inorganic metal-halide perovskite films with controlled surface terminations

Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet-chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of ${\mathrm{CsPbBr}}_{3}$ and ${\mathrm{CsSnBr}}_{3}$ MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and ${\mathrm{PbBr}}_{2}$ surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the ${\mathrm{PbBr}}_{2}$-terminated surface is increased by $0.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, which has drastic implications for the level alignment at MHP interfaces.