Electronic structure and optical properties of Cs2AX2′X4 (A=Ge,Sn,Pb; X′,X=Cl,Br,I)

We studied the crystal structures, electronic structures and optical properties of Cs2AX2′X4 (A=Ge,Sn,Pb; X′, X=Cl, Br, I) compounds using the first-principles calculation. Our optimized structures agree well with experimental and theoretical results. Band structure calculations, using the modified Becke-Johnson (mBJ) potential method, indicate that these compounds (with the exception of Cs2PbX2′I4) are semiconductors with the direct band gap ranging from 0.36 to 4.09 eV. We found the compounds Cs2GeBr2I4, Cs2GeCl2I4, Cs2GeI2Br4, Cs2SnI6, and Cs2SnBr2I4 may be good candidates for lead-free solar energy absorber materials.