Control of valence band offset at CdS/Cu(In,Ga)Se2interface by inserting wide-bandgap materials for suppression of interfacial recombination in Cu(In,Ga)Se2solar cells

We inserted Cu(In,Ga) 3 Se 5 into the CdS/Cu(In,Ga)Se 2 interface of Cu(In,Ga)Se 2 solar cells with a flat band profile and energy bandgaps ( E g ) of 1.2 and 1.4 eV in order to investigate the repelling of holes by the effect of valence band offset (Δ E v ). We found that open circuit voltage ( V OC ) was clearly improved from 0.66 to 0.75 V with E g of 1.4 eV, although V OC was only increased from 0.63 to 0.64 V with E g of 1.2 eV. For high efficiency, we fabricated Cu(In,Ga)Se 2 solar cells with a single-graded band profile and an average E g of 1.4 eV. Eventually, a conversion efficiency of 14.4% was obtained when Cu(In,Ga) 3 Se 5 with a thickness of 30 nm was inserted, although the conversion efficiency was 10.5% without Cu(In,Ga) 3 Se 5 . These results suggest the importance of Δ E v in the suppression of interfacial recombination by repelling holes and possibility that the highest efficiency of Cu(In,Ga)Se 2 solar cells with an average E g of 1.4 eV could be achieved.