Influence of Cu on Ga diffusion during post-selenizing the electrodeposited Cu/In/Ga metallic precursor process

Gallium accumulation, caused by faster reaction of indium with Se than gallium with Se, is a critical issue for post-selenization process, which limits the optimization of surface energy bandgap and open circuit voltage of CIGSe thin film solar cell. In this study, large-grained and compact CIGSe thin film was successful fabricated by electrodeposition and a three-step vapor Se/N2/vapor Se reaction process. The influence of the Cu content on gallium diffusion and grain growth of CIGSe thin film was studied. The compositions, element distributions, and morphologies of CIGSe thin films were studied. The results revealed that the gallium diffused homogenizely through the film with Cu/(In+Ga) < 0.9, while accumulated at the back contact region with Cu/(In+Ga) > 0.9. Single phase and large-grained CIGSe thin films were obtained with Cu/(In+Ga) ratios between 0.78 and 0.9. Cu(In,Ga)Se2 solar cells made with the CIGSe thin films presented highest efficiency of 11.22%. The solar cell based on the single phase and large-grained CIGSe absorber with Cu/(In+Ga) = 0.85 presents highest conversion efficiency of 11.22%, fill factor of 63.83% and VOC of 514 mV.