Double-graded bandgap in Cu(In,Ga)Se2 thin film solar cells by low toxicity selenization process

A low-toxic selenization with post gallium diffusion (PGD) treatment has been demonstrated to increase the bandgap in the surface Cu(In,Ga)Se-2 (CIGSe) absorbers and to form double-graded bandgap profiles to improve the cell efficiency. The CIGSe absorber with PGD for 5 min increased open-circuit voltage from 0.49 to 0.66V and efficiency from 9.2% to 13.2%, contributed by the enhancement of carrier recombination in the space-charge region. The reduction in short-circuit current from 30.8 to 29.9 mA/cm(2), attributed to the absorption loss in long-wavelength regions, can be potentially improved by further optimization of the minimum bandgap value in gradient valley. (C) 2014 AIP Publishing LLC.