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It was found that Cu(In,Ga)Se 2 (CIGS) solar cells with single graded (SG) band profile and average bandgap E g(avg) of 1.15 eV has higher efficiency than double graded (DG) band profile with the same E g(avg) , despite no intentional bandgapwidening at a CdS/CIGS interface in SG-CIGS. To explain this contradiction, we focused on the band structure at the CdS/CIGS interface and proposed that the contradiction can be explained by the formation of an ordered vacancy chalcopyrite compound (OVC) layer on the surface of CIGS. Device simulation using wxAMPS revealed that if an OVC layer exists at the interface, conversion efficiency of SG-CIGS solar cells overcomes that of DG-CIGS solar cells due to suppression of surface recombination at the interface, leading to open-circuit voltage (V OC ) improvement. We applied the SG structure to CIGS solar cells with high Ga contents. As a result, the conversion efficiency of 15.1% for the SG-CIGS solar cell with E g(avg) of 1.4 eV was achieved, although in contrast, that of the DG-CIGS solar cells was 12.9%. This efficiency is comparable to the highest efficiency in CIGS solar cells with high Ga contents. Therefore, SG band profile is promising structure and it is expected to achieve high efficiency predicted essentially in CIGS solar cells with high Ga contents. Although CIGS solar cells with the highest conversion efficiency have been expected due to good matching the sunlight spectrum in the case of E g ¼ 1.4 eV, practically, the conversion efficiency was deteriorated with increasing Ga content [5,6] because of several reasons [7][8][9].The conduction band profile of typical CIGS solar cells fabricated by the three stage method has double graded (DG) structure which is useful for the improvement of carrier collection efficiency by an internal electric field, and the improvement of open circuit voltage (V OC ) by bandgapwidening in a depletion region. On the other hand, single graded (SG) structure has been reported lower V OC than DG structure due to no bandgap-widening in a depletion region [10]. However, in this study from experimental results we found that SG structure has higher efficiency than DG structure. To explain this contradiction, we focused on the band structure at the CdS/CIGS interface. Moreover, from the experiment and the calculation, we found the policy to improve the efficiency of high-Ga CIGS solar cells.

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To increase the performance of thin-film solar cells, solar spectrum splitting technique has been considered and studied. It was found from the simulation that the total efficiency of nearly 25% can be obtained at the splitting wavelength of 600 nm with top cell using higher band gap material. The experiment has been carried out to verify the simulation results. Up to now the total efficiency of about 18.7 % has been obtained using a-Si:H and CU(Inl."Ga,)Se2 as the top and bottom cells, respectively, at the splitting wavelength of 614 nm which is similar to the simulation results.