Role of Bulk Defect States in Limiting CIGS Device Properties

We have used sustained light-soaking in the near-infrared (780 nm wavelength) to modify the properties of the absorber layer in CIGS solar cells, and thus study the relationship between the absorber electronic properties and the device performance. Through this light-soaking treatment we can increase the hole carrier density in the CIGS absorber, as well as the density of the commonly observed 0.3 eV bulk deep acceptor, by up to a factor of 5. The device performance was periodically recorded under the same 780 nm light, and was found to degrade due to a reduction in fill factor and short circuit current. We demonstrate that these changes can be largely attributed to a reduction in the photogenerated carrier collection, due to a decrease in the depletion width. Based on these data, we estimate the minority carrier diffusion length to be about 0.2 mum. At the same time, no change in open circuit voltage was observed, indicating that the recombination rate through the dominant recombination channel remained nearly constant. This means that it cannot be the 0.3 eV bulk defect. Detailed SCAPS modeling was carried out and reinforces this conclusion