ALKALI POST-DEPOSITION TREATMENT OF Cu(In,Ga)(S,Se)_2 SOLAR CELL ABSORBERS GROWN UNDER ATMOSPHERIC PRESSURE

摘要

This work presents the adaptation of post-deposition treatments with sodium fluoride and/or rubidium fluoride, as previously developed for our multi-source co-evaporated absorbers, to sequentially prepared Cu(In,Ga)(S,Se)_2 (CIGSSe) thin-films using fast atmospheric pressure chalcogenization of sputtered precursors. The incorporation of the alkali metals into the films with different Cu/(Ga+In) (CGI) ratios and their effect on material and device properties is analyzed and compared to findings obtained for multi-source co-evaporated absorbers. It was found that a Mo(S,Se)x layer at the back contact of the sequentially prepared films possibly acts as a sink for alkali ions, leading to in-depth distributions of alkali metals that differ from those in absorbers prepared by co-evaporation. The best device results were obtained with a sequentially prepared CIGSSe sample with CGI=0.87, using a NaF followed by a RbF-PDT, and resulted in a maximum Voc ≈ 600 mV and a Voc deficit of around 400mV. Additionally, the FF was enhanced. Ultimately, by using photolithographic cell definition and a MgF_2 anti-reflective coating, the NaF+RbF PDT resulted in solar cells with independently certified efficiencies of up to 18.4%, a significant improvement of the previous value without PDT. At this early development stage, these results demonstrate that the PDT is useful to exploit the potential of absorbers prepared by fast, vacuum-free processing.