Effect of sodium on material and device quality deposited Cu(In,Ga)Se2

Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se-2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage (Vac). For this work CIGSe absorbers were produced in a low temperature deposition process on polyimide substrates with varying sodium content. In agreement with previous studies we find an increase of V-OC and overall solar cell performance with increasing sodium content and a decrease of the overall performance for very high sodium content. At the same time time-resolved photoluminescence measurements indicate that the minority carrier lifetime decreases from 50 ns to about 5 ns for increasing sodium content in the absorber. This correlation was found to be valid for three different types of low-temperature CIGSe deposition processes. The results can be explained by an increase in deep defect density with increasing Na content, proportional to the increased net charge carrier density as measured by capacitance-voltage profiling. For very high sodium content this leads to an actual decrease in the overall performance of the solar cells. (C) 2013 Elsevier B.V. All rights reserved.