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

• Published in: Solar energy materials and solar cells Vol. 119; pp. 281 - 286
• Main Authors: Puttnins, S., Levcenco, S., Schwarzburg, K., Benndorf, G., Daume, F., Rahm, A., Braun, A., Grundmann, M., Unold, T
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2013
• Subjects: COPPER INDIUM SELENIDE; COPPER SELENIDE; Cu(In,Ga)Se2; DEPOSITION; Electric potential; Minority carrier lifetime; Optimization; Photovoltaic cells; Sodium; Solar cell; Solar cells; THIN FILMS; Time resolved photoluminescence; VOLTAGE; Solar cell; Time resolved photoluminescence; Sodium; Minority carrier lifetime; Cu(In,Ga)Se2
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2013.08.029

Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage (VOC). 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 VOC 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 50ns to about 5ns 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. •Minority carrier lifetimes of CIGS measured via time-resolved photoluminescence.•Sodium increases efficiency and VOC but decreases minority carrier lifetimes.•Influence of sodium on lifetimes found for three independent deposition processes.•C–V profiling shows increase of net doping concentration with sodium content.•Consolidation of measurements by assumption of deep defect proportional to doping.