Halide-substituted electronic properties of organometal halide perovskite films: direct and inverse photoemission studies.

Solution-processed perovskite solar cells are attracting increasing interest due to the potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions in various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of electronic properties with the qualitatively blue shift along the I → Br → Cl series, showing the increase in ionization potentials from 5.96 eV to 7.04 eV and the change of transport band gaps in the range from 1.70 eV to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 nm to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes.