Investigations of potassium-induced surface treatment of Cu(In,Ga)Se<inf>2</inf> (CIGSe) thin film solar cells prepared by two-stage process using elemental selenium

Abstract

The influences of alkaline doping of CIGS absorbers by a potassium fluoride post-deposition treatment (KF-PDT) were investigated systematically. Three factors that influence the CIGS solar cell performances were investigated: (i) deposition temperature, (ii) post-deposition annealing, and (iii) the thickness of the potassium fluoride (KF) layer. It is deduced that Cu vacancies were more effectively occupied by K instead of In, which prevents the formation of InCu donor defects and facilitates the subsequent Cd doping into the CIGS region. We confirmed large densities of deep defects induced by KF-PDT, leading to significant FF losses of CIGS solar cells. The potassium diffusion and redistribution during post-annealing is critical to micro-structure and crystallinity of CIGS absorber, as well as the defect properties. We pointed out the effects of surface treatment extend to the bulk of CIGS, dependent on the chemical compositions. With modified post-annealing process towards homogeneous potassium distribution and high-quality CGIS crystals, room temperature KF-PDT remains an effective strategy to enhance the performances of CIGS solar cells prepared by two-stage process.