Organic-inorganic halide perovskites efficiently convert sunlight to electricity in solar cells. The choice of halide (Cl, Br or I) can be used to chemically tune the spectral response of the materials and the positions of the valence and conduction bands (i.e. the ionisation potential and electron affinity). Here the band offsets of the methylammonium lead halides are reported, including relativistic corrections and using the Pb 1s core level as a reference state. The binding energy of the valence band decreases monotonically down the series, primarily due to the change from 3p to 4p to 5p valence orbitals of the halide. Type I band alignments are predicted, which implies that Br and Cl secondary phases in CH3NH3PbI3 thin-films would act as barriers to charge transport in photovoltaic devices.
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