Effects of the addition of graphene on the defect-related photoluminescent and electrical properties of n-type ZnO thin films

Abstract

This study determines the effect of the addition of graphene on the photoluminescent and electrical properties of ZnO films that are prepared using the sol-gel method. ZnO and graphene-doped ZnO thin films that are annealed at 500 °C for 30 min in nitrogen exhibit n-type conductivity. The correlation between photoluminescence (PL) spectral features and defects of ZnO means that there are oxygen vacancies (VO), zinc vacancies (VZn) and oxygen interstitials (Oi) in a ZnO thin film and there are VO, VZn, Oi in a graphene-doped ZnO thin film. The PL result shows that the incorporation of graphene into ZnO leads to a decrease in the number of VO, VZn and Oi and a transition from VZn to the substitution of carbon for zinc (CZn). The decreased number of VO and Oi is attributed to the substitution of VO by Oi, so equal numbers of acceptors and donors are annihilated and there is no change in the net electron carrier density. The transition from VZn to CZn increases the electron carrier concentration, which is opposite to the electrical observation. The results from PL, X-ray diffraction and X-ray photoelectron spectroscopy measurements demonstrate that a CZn+2Oi complex is formed in a graphene-doped ZnO thin film. The decrease in the electron concentration in a graphene-doped ZnO thin film is associated with the formation of the acceptor-like CZn+2Oi complex.