Optimisation of ZnO nanorods for improved carrier extraction in organic-inorganic hybrid Photovoltaic cells

Abstract

Zinc Oxide (ZnO) is a promising semiconductor material suitable for the application in electronic and optoelectronic systems. ZnO tailored into nanorods has a potential to be used in photovoltaic cells due to its large surface area to volume ratio that allows for maximum light harvesting and improved electron transportation efficiency. ZnO nanorods that are less compact, thin and long with fewer defects can be used for organic/inorganic solar cells application. In this study chemical bath deposition (CBD) was used to grow ZnO nanorods on a glass substrate coated with a seed layer. The seed layer was deposited using the ultrasonic spray pyrolysis (USP) technique prior to growth of nanorods. The influence of seed layer thickness on the alignment, distribution and homogeneity of the subsequently grown nanorods was explored. A KLA Tencor D-100 surface profilometer was used to measure the seed layer thickness. The effects of varying the growth time, temperature and precursor concentration on the morphological, structural, optical, vibrational and electrical properties of the subsequently grown nanorods were explored. X-ray diffraction (XRD) revealed structural properties under different growth parameters. Scanning Electron Microscopy (SEM) was used to observe the arrangements, distribution and uniformity of the ZnO nanorods as the growth conditions were varied. Confirmation of Zn and O elements in the seed layers and nanorods was done by Energy Dispersive X-ray Spectroscopy (EDX). UV/Vis/NIR spectrophotometer measurements revealed an optical transmittance of between 50 – 70 % for the nanorods. Optical properties such as the band gap (Eg), the extinction coefficient (𝑘), the refractive index (𝑛), and the real (ε1) and imaginary (ε2) dielectric constants were derived from the transmittance spectra. Raman spectroscopy results confirmed the presence of phonon modes belonging to the hexagonal ZnO phase in consistency with the XRD results. The Four Point Probe set up was used to determine electrical resistivity (𝜌) and figure of merit (FOM) of the grown nanorods. This work shows that the orientation, structural, optical, vibrational and electrical properties of the grown nanorods are controlled by alteration of the growth parameters. Well aligned, less compact and long ZnO nanorods with high aspect ratio suitable for working as a photoanode in solar cells were obtained under optimized growth conditions of: a precursor concentration of 25 mM, bath temperature of 90 °C and growth time of 2 hrs.