Optimized Growth Conditions for Cu(In, Ga)Se2 Layers Grown by Co-Evaporation at High Deposition Rates

Abstract

AbstractWe have investigated the compositional dependency on solar cell device quality for four different Cu(In, Ga)Se2 (CIGS) processes: Homogeneous and Ga-graded CIGS deposited at 60 min, Homogenous and Ga-graded CIGS deposited at 15 min. The Cu/(In+Ga) ratio was for varied between 0.65 and 1. An improved device performance was observed as the Cu/(In+Ga) ratio was increased from 0.65 up to 0.85 - 0.95 for all the different CIGS processes. This improvement was both due to a more efficient carrier collection at long wavelengths, increasing the Jsc, and an enhanced Voc. We explain theses improvements by an increased diffusion length in the CIGS material. The CIGS layers deposited at 15 min resulted in similar device performance as the CIGS layers deposited at 60 min, at low Cu/(In+Ga) ratios. However, for Cu/(In+Ga) ratios above 0.9 the efficiency difference increased to above 1 % unit, mainly due to a lower collection efficiency, especially at long wavelengths, resulting in a lower Jsc. By introducing a Ga-gradient in the CIGS layer the device performance improved for both deposition times, most clearly for the 15 min deposition time at Cu/(In+Ga) ratios below 0.9. At 15 minutes deposition time the reason for the improvement is a combination of small improvements in FF, Voc and Jsc, reflecting reduced recombination in general. For the single device, deposited at 60 minutes, that showed a significant improvement with Ga-grading, an improved carrier collection at long wavelengths was observed.