Development of ultra-thin CIGS absorber material with alkali treatment for solar cell production

Abstract

As a thin-film photovoltaic technology, Cu(In,Ga)Se2 (CIGS) is gaining popularity in recent years due to increasing efficiencies and fast-growing market prospects. The incorporation of alkali elements such as Na, K, Rb, and Cs into CIGS absorber layers is one of the techniques used for enhancing photovoltaic efficiencies. However, easily reproducible methodologies for this addition are still under investigation. To date, it has been reported that with alkali atom post-deposition treatment (PDT), solar cell characteristics can be improved in terms of Voc, Fill Factor, and thus overall efficiency. In this work, selenization parameters are discussed for the creation of a baseline and co-evaporated CIGS samples are enhanced using post-deposition treatments with fluoride and chloride alkali salt. The PDT is designed as an easily reproducible enhancement using a spin coater and heat treatment. Results of baseline experiments are evaluated using SEM/EDS, PL, and Current-Voltage measurements. For the selenization baseline, higher annealing temperatures resulted in increased solar cell performance and the added pre-annealing step with N2 caused a homogenization of the CIGS surface. PL-data from the alkali PDT experiments suggest improvements in the CIGS absorber quality, with increased PL intensity measured for larger alkali atoms. SEM images reveal alkali crystal structures for Li, Na, and K, which could be useful for the creation of openings in oxide layers when developing CIGS/CdS front passivation.