Material Property Changes in Defects Caused by Reverse Bias Exposure of CIGS Solar Cells

Abstract

Partial shading of Cu(In,Ga)Se2 modules can lead to the formation of reverse bias induced wormlike defects. These wormlike defects act as local shunts and permanently decrease module output. A good understanding of the formation and propagation mechanisms of these defects is needed in order to mitigate the negative effects, or to prevent these defects from forming. In this study wormlike defects were formed on small non encapsulated cells by exposing them to reverse bias conditions. SEM-EDX measurements showed a rearrangement of elements: indium, gallium and copper were replaced by cadmium while selenium was replaced by sulfur in the area around the defect. Moreover, additional electronic-defect levels were found in that area with spectrally resolved photoluminescence spectroscopy. Based on the material changes in the area close to the wormlike defects, a propagation mechanism is proposed. The model assumes a chemical reaction as the driving force for propagation instead of melting due to ohmic heating.

Propagation mechanism of reverse bias induced defects in Cu(In,Ga)Se2 solar cells

“…Besides changes in the buffer layer, also indications for compositional changes in the CIGSe absorber layer in the border region of the defect were found. SEM EDX measurements showed a redistribution of elements, a more detailed description of the EDX results is presented in [14]. The redistribution of indium and gallium as observed in EDX was confirmed by Raman measurement in the form of a shift in A1 CIGSe peak position.…”

“…In the first stage a 1 wt.% acetic acid (HAc) solution was used to remove the TCO by dipping for 3 and 6 minutes for the sample with 1 and 2 µm thick TCO, respectively. The samples were subsequently etched using a 3 wt.% hydrochloric acid (HCl) solution for 15 minutes to completely remove the remaining ZnO and CdS [14].…”

“…Therefore, the indium signal in Figure 6 (c) is enhanced by the presence of cadmium. More details on quantitative analyses of the elements in the border region can be found in reference [14].The defect shows large areas with lower copper content. Since SEM is a surface technique, it is possible that the copper has migrated towards the back contact.…”

“…Cadmium is known to diffuse from the buffer layer into the CIGSe absorber layer under the influence of heat. Bakker et al [14] described defect levels indicating CdCu antisites in the border regions of wormlike defects. Furthermore, Guthrey et al [6] showed cadmium diffusion into the absorber in the propagation front of wormlike defects.…”

Extraction and microscopic analysis of partial shading‐induced defects in a commercial CIGS PV module

“…Partial shading can occur in PV fields (e.g., from self-shading or cleaning equipment) and in domestic roof installations (e.g., trees), but is even more likely in integrated applications (e.g., BIPV). [13][14][15] Apart from the temporary power loss that such shading can induce, 16,17 it can also lead, in very specific scenarios, to wormlike defects, [13][14][15][18][19][20][21] reducing irreversibly the performance of the modules. These defects must be better understood to be mitigated or even eliminated.…”