Optimized Growth Conditions for Cu(In, Ga)Se 2 Layers Grown by Co-Evaporation at High Deposition Rates
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OPTIMIZED GROWTH CONDITIONS FOR Cu(In,Ga)Se2 LAYERS GROWN BY COEVAPORATION AT HIGH DEPOSITION RATES O. Lundberg, M. Edoff, and L. Stolt Uppsala University, Ångström Solar Center, P.O. Box 534, SE-751 21 Uppsala, Sweden
Abstract: We 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. 1 Introduction Cu(In,Ga)Se2 (CIGS) based thin film solar cells have the potential to become one of the leading PV technologies the coming years. Its success will be strongly dependent on the production cost of the CIGS layer, since the deposition of this layer will carry a significant part of the complete module cost. In order to explore the potential for reduced production costs by reducing the deposition time of the CIGS layer it is important to know: - How and why is the device performance affected by the deposition time? - How and why is the process window affected by the deposition time? - How can the device performance and the process window be improved for short deposition times? Previously we investigated how the device performance changed with the deposition time of the CIGS layer [1]. As shown in Ref [1] and a number of earlier studies [2-5] an improved device performance can be obtained by introducing an increased Ga/(In+Ga) ratio towards the back contact of the CIGS layer, so called Ga-grading. In this work we have studied how the compositional process window is affected by the deposition time. How Ga-grading can improve device performance under these different conditions is also investigated. 2 Experimental
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