Control of valence band offset at CdS/Cu(In,Ga)Se2 interface by inserting wide-bandgap materials for suppression of interfacial recombination in Cu(In,Ga)Se2 solar cells

Abstract

We inserted Cu(In,Ga)3Se5 into the CdS/Cu(In,Ga)Se2 interface of Cu(In,Ga)Se2 solar cells with a flat band profile and energy bandgaps (Eg) of 1.2 and 1.4 eV in order to investigate the repelling of holes by the effect of valence band offset (ΔEv). We found that open circuit voltage (VOC) was clearly improved from 0.66 to 0.75 V with Eg of 1.4 eV, although VOC was only increased from 0.63 to 0.64 V with Eg of 1.2 eV. For high efficiency, we fabricated Cu(In,Ga)Se2 solar cells with a single-graded band profile and an average Eg of 1.4 eV. Eventually, a conversion efficiency of 14.4% was obtained when Cu(In,Ga)3Se5 with a thickness of 30 nm was inserted, although the conversion efficiency was 10.5% without Cu(In,Ga)3Se5. These results suggest the importance of ΔEv in the suppression of interfacial recombination by repelling holes and possibility that the highest efficiency of Cu(In,Ga)Se2 solar cells with an average Eg of 1.4 eV could be achieved.