Effects of Zn Diffusion from (Zn,Mg)O Buffer to CIGS Film on the Performance of Cd-Free Cu(In,Ga)Se_2 Solar Cells

摘要

A diffusion of Zn into the CIGS film and its effects on the device performance were investigated when the atomic layer deposited (Zn,Mg)0 film was used as a buffer layer of CIGS solar cells. SIMS and ICP mass analysis showed that the Zn concentration in the CIGS film was increased with increasing ALD process time. The hole concentration in the CIGS film was decreased by Zn incorporation. It was suggested that open circuit voltage degradation in CIGS cell with Zn incorporation was mainly due to enhanced recombination by Zn defects instead of Fermi-level change. The results suggest that the ALD process should be as short as possible to avoid Zn diffusion. On the other hand, a small amount of Zn incorporation on the surface of the CIGS film by post rapid thermal annealing of a (Zn,Mg)O/CIGS cell at 200°C showed an increased cell performance. However, excess Zn incorporation on the surface of the CIGS film caused degradation of cell performance due to the generation of a new defect. The generation of a deep-level defect and the change of the minority carrier lifetime were characterized by low-temperature PL and time-resolved PL, respectively. The cell performance was correlated with these results.