CuInSe2 and CuGaSe2, the materials that form the alloy Cu(In,Ga)Se2, belong to the semiconducting I-III-VI2 materials family that crystallize in the tetragonal chalcopyrite structure. The chalcopyrite structure of, for example, CuInSe2 is obtained from the cubic zinc blende structure of II-VI materials like ZnSe by occupying the Zn sites alternately with Cu and In atoms. . In CIGS some In atoms are replaced by Ga atoms.
For CIGS based solar cell usually Mo is used for back contact. It gives extra benefit as it form MoSe2 film on Mo surface by reacting with CIGS/ CIS. MoSe2 is p-type semiconductor with direct band gap of 1.3 eV. Because of layer bandgap of MoSe2 compared with that of standard CIGS film, the MoSe2 layer provides a low recombination back surface or the photo generated minority carries (electrons) in the CIGS absorber layer and at the same time provide low resistance contact for the majority carriers Research has shown that Au, Ti, Mo, and Ni all form fairly reproducible low resistance contacts to CIGS/CIS. However Au and Ti show a significant diffusion into the CIS/CIGS with annealing at elevated temperatures and thus, only Ni and Mo have emerged as valid candidates for being ohmic contacts in these devices. But due to extra benefit of MoSe2 formation Mo is used widely.
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