High quality baseline for high efficiency, Cu(In1−x,Gax)Se2 solar cells

We report on the progress that we have made in the quality of our baseline process for the production of high efficiency soda lime glass/Mo/Cu(In,Ga)Se2 (CIGS)/CdS/i-ZnO/ZnO:Al/MgF2 solar cells. The enhancement of the average performance level has enabled us to reach conversion efficiencies of up to 19·3% (internal measurement). The new quality initiative uses process control, optical and electrical modelling, and the critical revision of all process steps as tools for the attainment of the 19% efficiency level. Our experiments show that the compositional process window for CIGS solar cells that have an efficiency of η ≈ 19% is very wide. Accordingly, we suggest that an efficiency of 19·0–19·5% is achievable in the following compositional process windows: 0·69 ≤ Cu/(Ga + In) ≤ 0·98 and 0·21 ≤ Ga/(Ga + In) ≤ 0·38. In addition, our results show that large CIGS grains are not a necessary requirement for high efficiencies up to 19%. These findings and the partly lacking ability to correlate certain aspects of our progress with experimental parameters lead us to the conclusion that there are still some important process variables undiscovered. From this conclusion and from the evaluation of the available data we infer that there is a potential for the enhancement of CIGS solar cell efficiencies beyond 20%. Copyright © 2007 John Wiley & Sons, Ltd.