Physical and chemical degradation behavior of sputtered aluminum doped zinc oxide layers for Cu(In,Ga)Se₂ solar cells

Sputtered aluminum doped zinc oxide (ZnO:Al) layers on borosilicate glass were exposed to damp heat (85 C/85% relative humidity) for 2876 h to accelerate the physical and chemical degradation behavior. The ZnO:Al samples were characterized by electrical, compositional and optical measurements before and after degradation. Hall measurements show that the carrier concentration stayed constant, while the Hall mobility decreased and the overall resistivity thus increased. This can be explained by the increase of potential barriers at the grain boundaries due to the occurrence of space charge regions caused by additional electron trapping sites. X-Ray Diffraction and optical measurements show that the crystal structure and transmission in the range 300-1100 nm do no change, hereby confirming that the bulk structure stays constant. Furthermore, on the surface, white spots appeared, containing elements that migrated from the glass, like silicon and calcium, which reacted with elements from the environment, including oxygen, carbon and chlorine. Depth profiling showed that the increase of the potential barrier is caused by the diffusion of H ₂O/OH^- through the grain boundaries leading to the formation of Zn(OH)₂ or similar species or adsorption of species. They also indicate the presence of chloride and sulfide in the top layer and the possible presence of Zn₅(OH)₈Cl₂·H ₂O and Zn₄SO₄(OH)₆·nH ₂O