Abstract
It was found that Cu(In,Ga)Se2 (CIGS) solar cells with single graded (SG) band profile and average bandgap Eg(avg) of 1.15 eV has higher efficiency than double graded (DG) band profile with the same Eg(avg), despite no intentional bandgap‐widening 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 (VOC) 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 Eg(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.
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