Stress-Induced Charge Redistribution at Grain Boundaries in Various Alkali Treated Flexible Cu(In,Ga)Se₂ Solar Cells

In recent development, efficiency of flexible Cu(In,Ga)Se₂ (CIGS) solar cells has exceeded 22%, marking a significant step towards commercial viability. [1] In particular, the deliberate alkali treatment boosted their efficiency, yielding a comparable effect to diffused alkali from soda-lime glass substrate. Besides alkali treatment, understanding stress-induced properties is crucial to maximize their performance. In this study, effects of alkalis on charge distribution under stress was investigated. Four flexible CIGS solar cells using a polyimide substrate were prepared with various alkali post deposition treatment including heavy alkalis (Rb and Cs). Among the CIGS solar cells, Na only treated sample showed the highest performance with an efficiency of 16.9% due to the highest carrier density and fill factor. Segregation of alkalis towards grain boundaries (GBs) was observed through transmission electron microscopy. Especially, Rb and Cs added after Na treatment occupied the GBs repelling the existed Na. Furthermore, surface charge transport under stress were probed via conductive-atomic force microscopy with curved holder. [2] Alkali treated CIGS thin film showed enhanced surface current along the GBs compared to CIGS without alkalis. Notably, surface current was degraded under convex bending state and the degradation was lower in the alkali treated CIGS. To explain the current distribution, defect energy levels were calculated using density functional theory and alkali related defects induced additional acceptors contributing to an increase in surface current. Additionally, energy level of electrically active defects changed with existence of compressive or tensile stress. As part of the future work, degradation of surface current will be quantitatively analyzed focusing on the redistribution of charges under stress.<br/><br/>[1] https://www.pv-magazine.com/2022/10/11/swiss-scientists-achieve-22-2-efficiency-for-flexible-cigs-solar-cell/ (access: October 16th, 2023)<br/>[2] H. K. Park et al., npj Flexible Electronics 6 (2022) 91.