Hybrid AMX3 perovskites (A=Cs, CH3NH3; M=Sn, Pb; X=halide) have revolutionized the scenario of emerging photovoltaic technologies. Introduced in 2009 by Kojima et al., a rapid evolution very recently led to 15% efficient solar cells. CH3NH3PbI3 has so far dominated the field, while the similar CH3NH3SnI3 has not been explored for photovoltaic applications, despite the reduced band-gap. Replacement of Pb by the more environment-friendly Sn would facilitate the large uptake of perovskite-based photovoltaics. Despite the extremely fast progress, the materials electronic properties which are key to the photovoltaic performance are relatively little understood. Here we develop an effective GW method incorporating spin-orbit coupling which allows us to accurately model the electronic, optical and transport properties of CH3NH3SnI3 and CH3NH3PbI3, opening the way to new materials design. The different CH3NH3SnI3 and CH3NH3PbI3 properties are discussed in light of their exploitation for solar cells, and found to be entirely due to relativistic effects.
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