Graphene capping layer for improving the photovoltaic stability of ZnO nanorods solar cells under temperature variations

Abstract

The present study evaluates the role of the graphene capping layer on the optoelectronic performance of ZnO/Si solar cells at different ambient temperatures. The n-ZnO buffer thin film was sputter-deposited on the p-Si substrates to form a p-n heterojunction. Next, ZnO nanostructures were chemically synthesized on top of the ZnO/Si substrates, and few-layer graphene was deposited on ZnO nanostructures using an electrophoretic deposition technique. The planar and cross-sectional electron microscopy analyses exhibited that the graphene capping layer entirely covered the surface of ZnO nanostructures, and their morphology remained intact after graphene transfer. Their photoluminescence (PL) properties before and after graphene deposition were analyzed at various temperatures to evaluate the graphene role in the optical behavior of ZnO nanostructures. The PL intensity was reduced for the graphene-containing sample because of the slight light absorption in each graphene layer. However, the stability of PL spectra at elevated temperatures was improved after graphene transfer. Next, a heterojunction solar cell was developed by depositing top and back contacts to study the photovoltaic performance of nanostructures at various temperatures. The current-voltage results, recorded under a solar light simulator, illustrated that the efficiency of the bare ZnO device was reduced at high temperatures because of the available interstitials and oxygen vacancies. After graphene transferring, this decay was moderated, which enhanced the photovoltaic stability of ZnO-based solar cells. Therefore, the graphene capping layer can act as a stabilizing thin film to enhance the photovoltaic durability of ZnO nanostructures under harsh conditions.