Investigation of near-surface material composition of Cu(In,Ga)Se2 film after air exposure and chemical etching for its Cd-free solar cell

Cd-free Cu(In,Ga)Se2 (CIGS) solar cells were fabricated with the structure of soda lime glass/Mo/CIGS/(Zn,Mg)O/In2O3:Sn. The Zn doped into CIGS surface called a Zn-doped CIGS absorber was performed by the deposition of 5-nm-thick Zn layer on the CIGS surface by the sputtering process followed by annealing the sample. It is revealed that conversion efficiency (η) of the Cd-free CIGS solar cell with the Zn-doped CIGS absorber is increased possibly due to the formation of the pn-homojunction. In addition, the material compositions near the CIGS surfaces were systematically examined using X-ray photoelectron spectroscopy (XPS) after the air exposure of the CIGS films with different time periods as well as before and after the KCN etching or dipping in the deionized water of the films. The native oxidation near CIGS surfaces, corresponding to In-oxide, Ga-oxide, and Se-oxide, is enhanced after the air exposure of the CIGS films, thus demonstrating the detrimental impact on cell performances. The native oxidation near CIGS surfaces after the air exposure is obviously reduced through the KCN etching on the CIGS films. As a result, the η (12.6%) of the Cd-free CIGS solar cell is comparable to that (11.9%) of the CdS-buffered CIGS solar cell.