  The electronic properties of ABX3 (A = Cs, CH3NH3, NH2CHNH2; B = Sn, Pb; X =
Cl, Br, I) type compounds in the cubic phase are systematically studied using
the first-principles calculations. We find that these compounds have direct
band gaps at R point where the valance band maximum is an anti-bonding state of
B s-X p coupling, while the conduction band minimum is a non-bonding state with
B p characters. The chemical trend of their properties as A or B or X varies is
fully investigated, which is of great importance to understand and optimize
this kind of solar cell materials. We find that: (i) as the size of A
increases, the band gap of ABX3 will increase; (ii) as B varies from Sn to Pb,
the band gap of ABX3 will increase; and (iii) as X ranges from Cl to Br to I,
the band gap will decrease. We explained these trends by analyzing their band
structures. Furthermore, optical properties of the ABX3 compounds are
investigated. Our calculations show that taking into account the spin-orbit
coupling effect is crucial for predicting the accurate band gap of these halide
perovskites. We predict that CH3NH3SnBr3 is a promising material for solar
cells absorber with a perfect band gap and good optical absorption.
