Advances in CIGS thin film solar cells with emphasis on the alkali element post-deposition treatment

In the past tens of years, the power conversion efficiency of Cu(In,Ga)Se2 (CIGS) has continuously improved and been one of the fastest growing photovoltaic technologies that can also help us achieve the goal of carbon emissions reduction. Among several key advances, the alkali element post-deposition treatment (AlK PDT) is regarded as the most important finding in the last 10 years, which has led to the improvement of CIGS solar cell efficiency from 20.4% to 23.35%. A profound understanding of the influence of alkali element on the chemical and electrical properties of the CIGS absorber along with the underlying mechanisms is of great importance. In this review, we summarize the strategies of the alkali element doping in CIGS solar cell, the problems to be noted in the PDT process, the effects on the CdS buffer layer, the effects of different alkali elements on the structure and morphology of the CIGS absorber layer, and retrospect the progress in the CIGS solar cell with emphasis on the alkali element post deposition treatment.