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The long-term performance of solar modules is of key importance to achieve profitable solar power installations. In this work, the degradation mechanism potential-induced degradation (PID) was investigated for CuIn1-xGaxSe2 (CIGS) thin film solar cells. PID is caused by a combination of certain system voltage situations and environment conditions, such as temperature and humidity. The conditions for PID were reproduced in the lab, using small test cells. A voltage was applied between the solar cell back contact and the rear side of the glass substrate while heating the samples to a temperature of 85°C. Similar to crystalline silicon technology, CIGS solar cells were found to be susceptible to PID. One critical parameter for the degradation behavior is the choice of substrate and its ability to release Na during applied bias. The degradation was found to be linked with Na migration from the substrate into the devices. Solar cells, which were fully deteriorated in terms of electrical performance by PID, were found to have a substantially increased Na concentration. However, solar cells grown on Na free and high resistivity substrates were observed to be PID-resilient. The degradation was shown to partly be non-permanent. Fully degraded CIGS solar cells could recover electrical performance to a certain degree. Three different recovery methods were applied (i) a passive recovery in darkness at room temperature, (ii) accelerated recovery with a reversed bias as compared to the PID treatment and (iii) etching and replacement of the top window layers followed by reversed bias. Recovery to over 90% of the initial efficiency was possible. However, the recovery rate varied depending on the recovery method. The accelerated method was found to reduce the concentration of Na in the buffer layer and interface volumes. The etch recovery method, which consists of renewing window and buffer layers further strengthen the hypothesis that a major part of the degradation could be attributed to the buffer layer and its interface to CIGS. The importance of the buffer layer in PID was further highlighted in the experiment where the standard CdS buffer layer was substituted with Zn(O,S). Both types of solar cells degrade in the PID conditions. Zn(O,S) cells exhibited ohmic current-voltage relationship (no diode characteristics) in the degraded state, while the CdS counterpart had some degree of diode behavior. During recovery with the accelerated method, the CdS cells restored both current-voltage and capacitance-voltage behavior to larger extent than the Zn(O,S) cells. For the latter, the efficiency stayed close to zero throughout the recovery period. |
