Simultaneous effect of precursor sources and concentration on structural, morphological and optical properties of ZnO nanostructured thin films for photovoltaic applications

The collection and transport of charges at the electrodes are the main factors limiting the efficiency of organic solar cells. Zinc oxide (ZnO) in nanostructured form helps to overcome this problem by introducing a ZnO buffer layer between the photoanode and the donor material. To achieve this, the ZnO thin film must exhibit good crystallinity, along with good electrical conductivity and high optical transparency in the visible range. The aim of this work is to investigate the effect of precursor sources and precursor concentrations on the structural, morphological, and optical properties of ZnO thin films. Three different precursor sources have been used: zinc acetate, zinc chloride and zinc nitrate. In each deposition solution, the precursor concentration varied from 0.1 M to 0.3 M. The ZnO films were deposited on glass substrates and all the films were annealed at 400°C for 3 hours. The structural, morphological and optical properties of deposited films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible spectroscopy respectively. XRD results showed that, regardless of the precursor source, all the ZnO films are polycrystalline with a hexagonal wurtzite structure. ZnO films obtained from acetate and nitrate sources crystallize preferentially along (002) and the peak intensity increases as the precursor concentration increases. SEM images showed that all the ZnO films are homogeneous, but films deposited from zinc acetate and zinc nitrate looked more compact and smoother than those obtained with zinc chloride which looked porous. UV-visible spectroscopy results revealed that the films transmittance depends both on precursor source and concentration. ZnO thin films deposited from zinc acetate at 0.3M concentration exhibit the best transmittance of 95% due to their smooth and uniform surfaces. The band gap of ZnO obtained from the zinc acetate precursor decreases with increasing solution concentration. It is found to be 3.29 eV, 3.26 eV, and 3.22 eV for concentrations of 0.1 M, 0.2 M, and 0.3 M, respectively. It therefore appears that ZnO films obtained from zinc acetate can be used as an electron transport layer for solar cells as they exhibit the best crystallinity and the highest transmittance.