Sodium incorporation strategies for CIGS growth at different temperatures

Incorporation of sodium into Cu(In,Ga)Se2 (CIGS) absorber layers and low-temperature absorber growth processes are important issues for the processing of flexible CIGS solar cells, particularly when polymer substrates are used. Na present during growth of CIGS is known to influence the growth kinetics and to lead to structurally and electronically modified absorber material. With post-deposition Na incorporation, predominantly electronic properties are altered. We compared the performances of solar cells processed with Na diffusing from the glass substrate during growth with those of cells with post-deposition Na incorporation and thus obtained an indication of the significance of Na-induced modifications of CIGS growth for cell efficiency. When the absorbers were grown at low substrate temperatures (below 500 °C), we found that absorbers with post-deposition-incorporated Na performed better. This is attributed to an impeding influence of Na on CIGS phase formation, which became most apparent when the growth temperature was 370 °C. With higher substrate temperatures (around or above 500 °C), the performance of cells grown in the presence of Na became similar or superior, which might indicate a beneficial influence of Na on CIGS growth once a certain growth temperature is exceeded. With post-deposition Na incorporation, a cell efficiency of 13.8% was achieved at 400 °C substrate temperature.