Modeling Ga-profiles for Cu(In,Ga)Se2 thin film solar cells with varying defect density

The very best Cu(In,Ga)(S,Se)2 solar cells have a double graded band gap in the absorber, i.e. a notch profile, formed by varying the ratio of Ga to In. If this is a prerequisite or a consequence of high quality deposition methods is something yet of discussion. In this work we have constructed a high efficiency model (HE-T61) based on in-house state-of-the art Cu(In,Ga)Se2 solar cells, with efficiencies above 19 %, and investigated the role of the Ga-profile. Notch-type Ga-profiles have been compared with single graded profiles, and the influence of Ga-dependent defect distribution and metastable defects have been investigated showing that the optimum Ga-profile is dependent on such defect variations. It is also shown that within the HE-T61 model the optimized Ga-profile yields up to 3 % absolute efficiency gain, indicating that there is potential in band gap engineering