D. S. Boyle

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.

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.

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.

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.

D.C. conductivity of transparent conductive ZnO:Al films in the temperature range 80–360 K

An experimental study of the temperature dependence of the d.c. conductivity σ as a function of temperature T in the range from 80–360 K on nanocrystalline ZnO:Al films (Al3+ 2%) of thickness 500 nm prepared on glass microscope slides by a dip — coating method is presented. The electrical conductivity σ, which at room temperature varied between 0.1 to 2.7 S/cm, increased almost linearly with T for all the samples. Measurements of the Hall coefficient at room temperature and in a magnetic field of 1.2 T, gave RH=0.53 cm3C−1, from which a carrier concentration of n=1.18×1019 cm−3 and a carrier mobility of μ=1.40 cm2/Vs were deduced.