Kuveshni Govender

Understanding the factors that govern the deposition and morphology of thin films of ZnO from aqueous solution

The influence of the choice of complexing ligand, zinc counter-ion, pH, ionic strength, supersaturation, deposition time and substrate on the nature of ZnO films grown from chemical baths (CBD) are discussed. There are significant differences between CBD and similar routes such as hydrothermal methods for ZnO films. Modelling of speciation and experimental results suggest that acicular ZnO morphologies are best obtained by limiting the concentration of one of either Zn2+ or OH− in the presence of a large excess of the other. The presence of a prior ZnO layer can facilitate nucleation at lower levels of supersaturation and enable size tailoring of ZnO columns. The point at which the substrate is introduced into the bath is crucial and can lead to a significant difference in both the width of the rods and optical transparency of the films. HR-TEM has yielded important structural information and a growth mechanism for single crystalline ZnO rods by CBD is described for the first time.

Hybrid polymer/metal oxide solar cells based on ZnO columnar structures

We focus on the preparation of hybrid polymer/zinc oxide (ZnO) solar cells, in which the metal oxide consists of ZnO columnar structures grown perpendicularly on a flat, dense “backing” layer, as a means to provide a direct and ordered path for photogenerated electrons to the collecting electrode. We used scanning electron microscopy, absorption spectroscopy and photovoltaic device measurements to study the morphology and device performance of the prepared structures. Different solution chemical routes were investigated for the synthesis of the inorganic device components, i.e. the ZnO columnar structures and the “backing” layers, which act as a seed-growth layer for the ZnO rods. The growth of the ZnO rods was dependent on the morphological and structural characteristics of the seed layer and moreover, the seed layer itself was also affected by the synthetic conditions for ZnO rod growth. Different polymers (high hole-mobility MEH-PPV based polymer and P3HT) were compared in these structures and power conversion efficiencies of 0.15 and 0.20% were achieved under 1 Sun illumination, respectively. Results are discussed in terms of the optoelectronic properties of the polymers.

Novel low temperature solution deposition of perpendicularly orientated rods of ZnO: substrate effects and evidence of the importance of counter-ions in the control of crystallite growth

Perpendicularly orientated ZnO rods have been grown on thin ZnO templates, from aqueous solutions of zinc acetate and hexamethylenetetraamine (HMT); growth along the c-axis of the ZnO crystallites is promoted by the presence of acetate in the bath.

Novel wet-chemical routes to nano- and microstructured semiconductor layers for improved efficiency photovoltaic devices

Abstract There is increasing interest in using ZnO as an alternative to TiO 2 in photoelectrochemical solar cells that employ high-surface area, nano-structured substrates. Columnar ZnO films may offer fundamental advantages over nanoporous TiO 2 such as improved electrical transport properties. We have grown perpendicularly orientated ZnO crystallites on TOF-glass from aqueous solutions containing a zinc carboxylate salt and hexamethylenetetraamine. These films provide a high-surface area substrate for subsequent overgrowth of semiconductor sensitizer layers of Cu 2− x S and Cu 2− x Se. Films have been characterized by spectroscopic methods (UV–vis, photoluminescence), microscopy (SEM) and powder XRD.

Influence of Pd and Ru additions on stress corrosion cracking of austenitic stainless steels

Abstract The influence of minor alloying platinum group metal additions on stress corrosion cracking (SCC) of austenitic stainless steels has been investigated in simulated pressurised water reactor environments. Crack propagation studies, employing tensile specimens precracked in acidified potassium tetrathionate and subsequently loaded in proving rings revealed that 1 wt-% ruthenium additions improved the resistance of 304 stainless steels to SCC. Analytical transmission electron microscopy was used to characterise crack tips and surface oxides after high temperature SCC tests. Enrichment of ruthenium and molybdenum within the dual oxide surface layers was observed in the ruthenium modified stainless steel, which is likely to have occurred during potassium tetrathionate pre-exposure and promoted intergranular SCC mitigation. Palladium additions (1 wt-%) showed no beneficial effects to SCC, and this was associated with the formation of second phases PdMn particles upon sensitisation, which decreased the availability of Pd.

Stress corrosion cracking of Ru doped 304 stainless steel in high temperature water

Abstract In pressurised water reactor primary water, there is generally growing concern that stress corrosion cracking (SCC) may occur in occluded locations where residual oxygen and other impurities may be trapped. For these critical components, the deployment of more SCC resistant materials is desirable. In this paper, the effect of 1 wt-%Ru additions on the SCC susceptibility of 304 austenitic stainless steels was investigated in high temperature water. Slow strain rate tensile tests were performed on standard and 1 wt-%Ru modified 304 stainless steels in both sensitised and cold worked conditions. Preliminary results showed that, although both ruthenium doped and standard 304 stainless steels exhibited intergranular SCC, the former was less susceptible as indicated by a greater strain to failure. The results obtained suggest an improved performance of the Ru doped 304 stainless steel towards SCC susceptibility in these environments.

Electrodeposition of mesoporous CdTe films with the aid of citric acid from lyotropic liquid crystalline phases

Nanostructured CdTe films were electrodeposited on CdS substrates from lyotropic liquid crystalline phases of non-ionic surfactants using citric acid. The deposition mechanism was studied using cyclic voltammetry. The effect of citric acid on the deposition properties of CdTe was investigated. The CdTe films were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, polarized optical microscopy, and UV–vis spectroscopy. In addition quartz crystal microbalance studies were carried out to provide further evidence for the mesoporosity of the CdTe films. The results indicated that the films exhibited a mesoporous structure and good optical properties.

Developing cadmium-free window layers for solar cell applications: some factors controlling the growth and morphology of β-indium sulfide thin films and related (In,Zn)S ternaries

Suitable conditions for reproducible deposition of good quality In2S3 thin films on tin oxide coated (TO) glass substrates by low temperature solution deposition have been identified. Baths containing various carboxylic acids yield adherent, specular and crystalline films but with discernible morphological differences, within a defined pH range. Films have been characterised by XRD, SEM, XPS and electronic spectroscopy. The as-deposited films were found to be tetragonal β-In2S3; no evidence for the incorporation of hydroxy-indium species was detected by XPS measurements. The formation of ternary (In,Zn)S material, structurally similar to β-In2S3, appears to be possible via incorporation of zinc salts in similar deposition baths.

Characterization of thin film cadmium sulfide grown using a modified chemical bath deposition process

Cadmium sulfide polycrystalline films with potential for application as a solar cell window layer have been grown by a modified chemical bath deposition process, using ethylenediamine as a complexing agent and employing direct heating of the substrate. Films have been characterized using atomic force microscopy, scanning electron microscopy, grazing incidence X-ray diffraction, photoluminescence, photoconductivity, and optical absorption. Both as-grown films and films processed by annealing using cadmium chloride (CdCl/sub 2/) exhibit promising properties for this use. As-grown films are crystalline, possess low surface roughness (4.4 nm RMS), small grain size (18 nm), large direct bandgap (2.54 eV) and are highly textured with some degree of hexagonal phase present. On annealing with CdCl/sub 2/ there is an increase in surface roughness (24.5 nm RMS), modest grain growth (23 nm), a decrease in bandgap (2.44 eV) and diffraction data are consistent with increasing hexagonal character. There is also evidence of better crystalline quality and a large reduction in electron trapping states upon CdCl/sub 2/ annealing.

High efficiency solution infiltration routes to thin films with photonic properties

A chemical bath deposition (CBD) method has been developed to prepare three-dimensionally-ordered macroporous films of CdS and TiO2, using colloidal crystals as templates. A series of sequential, short infill/rinse/anneal steps are employed to effect complete infiltration of SiO2 (opal) thin films with CdS or TiO2. Removal of templates allows fabrication of macroporous inverse replica structures that exhibit periodic modulation of dielectric behaviour and have potential for use in photonic applications. A study of the photonic properties of films indicates that the multi-step CBD method is a useful approach for infiltration of opal interstices.