Atomic-Scale Interface Modification Improves the Performance of Cu(In1-xGax)Se2/Zn(O,S) Heterojunction Solar Cells

Cadmium-free buffer layers deposited by a dry vacuum process are mandatory for low-cost and environmentally friendly Cu(In1-xGax)Se-2 (CIGS) photovoltaic in-line production. Zn(O,S) has been identified as an alternative to the chemical bath deposited CdS buffer layer, providing comparable power conversion efficiencies. Recently, a significant efficiency enhancement has been reported for sputtered Zn(O,S) buffers after an annealing treatment of the complete solar cell stack; the enhancement was attributed to interdiffusion at the CIGS/Zn(O,S) interface, resulting in wide-gap ZnSO4 islands formation and reduced interface defects. Here, we exclude interdiffusion or island formation at the absorber/buffer interface after annealing up to 200 degrees C using high-resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX). Interestingly, HR-STEM imaging reveals an epitaxial relationship between a part of the Zn(O,S) buffer layer grains and the CIGS grains induced by annealing at such a low temperature. This alteration of the CIGS/buffer interface is expected to lead to a lower density of interface defects, and could explain the efficiency enhancement observed upon annealing the solar cell stack, although other causes cannot be excluded.

Automated summary

After annealing at 200 degrees C, interdiffusion or island formation at the absorber/buffer interface was excluded using high-resolution scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy.