|In-situ Microscopy Characterization of Cu(In,Ga)Se2 Potential-Induced Degradation|
|Chuanxiao Xiao, Chun-Sheng Jiang, Steve Harvey, Lorelle Mansfield, Dana Sulas, Jun Liu, Steve Johnston, Mowafak Al-Jassim
NREL, GOLDEN, CO, United States
Today it is increasingly important to investigate the reliability of thin-film solar modules before their large penetration in the photovoltaic market. Sodium (Na) is known to improve Cu(In,Ga)Se2 (CIGS) solar cell efficiency; however, Na is also suspected to degrade the module performance via potential-induced degradation (PID). Because PID could cause devastating power reduction in solar modules, it is essential to understand the PID failure mechanism and address this problem. In this paper, we extensively investigated the role of Na in a PID study of CIGS solar cells by Kelvin probe force microscopy and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). In-situ microscopy characterizations on an atomic force microscopy platform were performed on two stressed CIGS device under room temperature (RT) and high temperature (HT) at 85°C. During PID stressing, we observed that the depletion region widens as Na migrates; also, rather than making CIGS more p-doped, a large amount of Na may become traps for free carriers and cause CIGS and CdS structural defects. On the other hand, the p-n junction becomes leaky at RT for over a month; similar junction evolution was observed for the HT-stressed sample, with the junction eventually collapsing after 18 hours. The diode behaviors were confirmed by dark current-voltage measurement. TOF-SIMS reveals that Na accumulates on the ZnO and CdS side, as well as on the upper layer of CIGS. The results indicate that Na drifted due to the voltage applied on the soda-lime glass, then diffused through the whole device. The sodium profiles have different points of evolution due to the temperature differences between the two stressed samples. The consistent results show unambiguously how Na from the substrate glass causes PID in CIGS solar cells.