Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 13, Issue 37, Pages 44207-44213Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.1c10251
Keywords
thin film solar cells; CIGS; buffer layer; STEM-EDX; high-resolution STEM
Funding
- FCT [028917]
- ERDF through COMPETE2020 [028917]
- European Commission [713640]
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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.
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.
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