Effect of Cd diffusion on the electrical properties of the Cu(In,Ga)Se2 thin-film solar cell

Cu(In,Ga)Se2 (CIGSe)-based solar cells are promising candidates for efficient sunlight harvesting. However, their complex composition and microstructure can change under operation conditions, for instance heating from sun light illumination can lead to a degradation in performance. Here, we investigate the thermally-induced degradation processes in a set of CIGSe-based solar cells that were annealed at temperatures between 150 degrees C and 300 degrees C. Using correlative atom probe tomography (APT)/transmission electron microscope (TEM), we found that the buffer/absorber interface is not sharp but consists of an interfacial zone (2-6.5 nm wide) where a gradient of constituent elements belonging to the CdS buffer and CIGSe absorber appears. An enhanced shortrange Cd in-diffusion inside the CIGSe was observed whenever a low Ga/(Ga + In) ratio (<= 0.15) occurred at the interface. This might indicate the presence of Ga vacancies as a channeling defect for Cd in-diffusion inside the CIGSe layer leading to a buried p/n-homojunction. We evidence that a considerable amount of Cd is found inside the CIGSe layer at annealing temperatures higher than 150 degrees C. Further investigations of the elemental redistribution inside the CIGSe layer combined with C-V measurements support the formation of CdCu + donor like defects deep inside the p-type CIGSe which lead to a strong compensation of the CIGSe layer and hence to strong deterioration of cell efficiency at annealing temperatures higher than 200 degrees C. Hence, understanding the degradation processes in Cu(In,Ga)Se2 (CIGSe)-based solar cells opens new opportunities for further improvement of the long-term device performance.

Automated summary

The study reveals that thermally-induced degradation processes in Cu(In,Ga)Se2 (CIGSe)-based solar cells involve enhanced short-range Cd diffusion and the formation of CdCu + donor-like defects deep inside the p-type CIGSe layer, leading to a significant deterioration of cell efficiency. Understanding these degradation processes opens up new opportunities for further improvement of long-term device performance.