Electronic structure and optical properties of Cs 2 AX 2 ′ X 4 (A=Ge,Sn,Pb; X′,X=Cl,Br,I)

We studied the crystal structures, electronic structures and optical properties of Cs 2 AX 2 ′ X 4 (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 Cs 2 PbX 2 ′ I 4 ) 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