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.
