Structural modification and band gap tailoring of zinc oxide thin films using copper impurities

Abstract

The doping effects of Cu on structural, morphological and optical properties of ZnO thin films and their PEC properties have been investigated via chemical bath deposition (CBD) technique at 353 K bath temperature and a pH of 11.5 with post-deposition annealing at 673 K. The concentration of Cu in ZnO varied between 1 and 5 at.%. X-ray diffraction analysis revealed that the synthesized Cu-doped ZnO (CZO) thin films were highly crystalline with hexagonal wurtzite structure, showing strong preferential growth along the c-axis for 3 at.% Cu concentration. A shift in angular peak position of 0.545o in 2θ towards higher angle was observed for CZO films which is an indication of effective substitution of Cu atoms on Zn lattice. Crystallite sizes were enhanced from 28 to 32 nm in the (002) crystal plane. Optical analysis indicates a red shift in the absorption band edge up to 450 nm upon Cu doping. Transmittance characteristics increased slightly from 80 to 90% in the visible range at optimum Cu concentration of 3 at.%. Optical energy band gap was found to decrease from 3.03 eV for undoped ZnO to 2.7 eV upon Cu doping. The morphological structures of the CZO thin films were strongly influenced by Cu impurities and its concentration. The water contact angles showed strong dependence on Cu impurities in ZnO and decreased considerably from 71.3 to 15.2°. The synthesized CZO films showed enhanced photoelectrochemical properties, giving a short circuit current (I sc) of 0.098 mAcm−2 and open circuit voltage (V oc) of 796 mV for an optimum Cu concentration of 3 at.% with photoconversion efficiency of 0.062% and fill factor of 63%.