Multistage Degradation Mechanisms in Cu(In, Ga)Se2 Photovoltaic Modules Prepared by Co-Evaporation: Toward High Performances and Enhanced Stability

This study compared the stability and durability of copper indium gallium selenide (CIGS)-type solar cells prepared using one-step and three-step co-evaporation methods by investigating the causes of degradation in each layer in detail. Measurements recorded using a solar simulator showed that the sample prepared using the three-step method had better device performance owing to the large-grained structure of the CIGS absorber layer, which reduced the carrier recombination. Focusing on the discrepancy in grain size, multifarious degradation tests were conducted according to the IEC 61646 standard to evaluate the stability of the cells under harsh environments such as high humidity (85%), high temperature (85 degrees C), and mechanical load. Damp heat (85%/85 degrees C) did not affect the CIGS resistivities in either sample, whereas all the aluminum-doped zinc oxide layers degraded, as determined by confirming the chemisorbed oxygen by exposure to a hot, humid environment. After 200 thermal cycles, the CIGS layers in both samples were mainly degraded while there were no changes in the resistivities of the AZO layer in either sample. The thermal cycling test highlights that the initial resistivities of the one-step sample showed a decisive change before and after thermal cycling compared to the three-step sample. This change might be caused by carriers being scattered at the grain boundaries. Although there were no big differences in the FT-IR spectra before and after thermal cycling, both XRD and XPS results confirmed that not only copper indium sulfide selenium elements of the secondary phase were newly observed by sulfide diffusion from the CdS layer but also that each element (Cu, In, Ga, and Se) was slightly oxidized by the rapid temperature variation from -45 to 85 degrees C. These results prove that the three-step co-evaporation method can produce cells with much higher stability and durability, even when operated under high humidity and temperature conditions.