Structural and Optical Properties of Solution Processed ZnO Nanostructures

Abstract

Aluminium-doped zinc oxide (AZO) is a low-cost alternative to indium tin oxide (ITO) for applications in optoelectronic devices as a transparent conducting thin film. Typically, AZO thin films are deposited using expensive, high vacuum equipment with high energy cost and materials wastage. This research initially focuses on slot-die coating as an inexpensive alternative to physical vapour deposition (PVD) to form AZO nanoparticle thin films under ambient laboratory conditions. These nanoparticle films, when compared with their PVD counterpart, appeared to be lower in band gap energy due to low incorporation of Al dopants. Further, due to the large number of grain boundaries, the electrical resistivity of these films is much larger than the PVD counterpart, as measured using a non-contact THz spectroscopy method.
Following this, hydrothermal synthesis is used to synthesise ZnO nanowires grown on slot-die AZO nanoparticle seed films, and address the differences using ammonium hydroxide and hexamethylenetetramine (HMTA) as bases to react with zinc nitrate in an aqueous solution. From electron microscopy images, ammonium hydroxide produces pointed tips whereas HMTA forms flat tips, caused by the chelating properties of HMTA. Time-dependent growth analysis shows that the nanowires form in the later stages of growth and with strong c-axis preferred orientation. The optical band gap is observed to decrease with solution temperature for both types of bases, however surface oxygen vacancy defects are observed in photoluminescence measurements of the ammonium hydroxide grown nanowires.
The ammonium hydroxide synthesised nanowires were further examined for their light scattering properties. For relatively thin AZO nanoparticle seed films, strain-induced hemi-spherical arches form in the seed film, creating a flower-like morphology. The quantity of light scattered by the nanowires, referred to as the haze, approaches 90% for these hemi-spherical nanowire arrays when coated with CdS. Modelling the optical path showed that the nanowires scatter light through a combination of total internal reflection and refraction through neighbouring nanowires. The haze data was also used to theoretically investigate light trapping in a model CdS/CdTe solar cell, as an example of light trapping. The external quantum efficiency (EQE) is simulated and enhanced by scattered light obtained from the haze measurement. This resulted in a 42% increase in current density for a 0.2 μm thick absorber on a nanowire array, compared to the planar counterpart. Although the simulation is qualitative and requires refinement for real devices, the results suggest that low-cost hemispherical nanowire scatterers can be beneficial for light trapping in solar cells.

Date of Award	22 May 2025
Original language	English
Awarding Institution	Northumbria University
Supervisor	Neil Beattie (Supervisor) & Vincent Barrioz (Supervisor)