  The energetics and the electronic structure of methylammonium lead bromine
(CH3NH3PbBr3) perovskite (001) surfaces are studied based on density functional
theory. By examining the surface grand potential, we predict that the
CH3NH3Br-terminated (001) surface is energetically more favorable than the
PbBr2-terminated (001) surface, under thermodynamic equilibrium conditions of
bulk CH3NH3PbBr3. The electronic structure of each of these two different
surface terminations retains some of the characteristics of the bulk, while new
surface states are found near band edges which may affect the photovoltaic
performance in the solar cells based on CH3NH3PbBr3. The calculated electron
affinity of CH3NH3PbBr3 reveals a sizable difference for the two surface
terminations, indicating a possibility of tuning the band offset between the
halide perovskite and adjacent electrode with proper interface engineering.
