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

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.