David S. Boyle

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 approach to the chemical bath deposition of chalcogenide semiconductors

Abstract Chemical bath deposition (CBD) generates has been successfully employed for the fabrication of II–VI semiconductor thin films. Thin film polycrystalline solar cells, such as the BP Solar ‘Apollo’ CdS:CdTe heterojunction device, offer the potential for low cost solar energy conversion. The large scale exploitation of these devices is partly dependent on a reduction of the potential environmental impact of the technology. The fabrication of CdS window layers by CBD at present generates considerable Cd-containing waste. Use and disposal of cadmium-containing compounds and wastes are highly regulated in the EU and elsewhere. For CdS CBD, the extent of the desired heterogeneous reaction on the substrate surface is limited by two major factors, the competing homogeneous reaction in solution and deposition of material on the CBD reactor walls. In this paper we describe our initial successful efforts to address these problems with the development of a novel high-efficiency CdS CBD system. Chemical modelling and speciation studies have enabled us to develop a process that comprises low cadmium concentrations and eliminates ammonia (which is volatile and undesirable for large scale CBD operations). Films have been characterised by spectroscopic methods (UV–vis, PL and XPS), microscopy (SEM and TEM) and powder XRD.

Novel approach to the deposition of CdS by chemical bath deposition: the deposition of crystalline thin films of CdS from acidic baths

Thin films of cadmium sulfide have been deposited from acidic solutions. The films have been characterised by electronic, photoluminescence (PL) and X-ray photoelectron (XPS) spectroscopies, scanning and transmission electron microscopy (SEM and TEM) and powder X-ray diffraction (XRD). The as-deposited polycrystalline CdS films are hexagonal, the crystallinity of the films is improved by annealing in air at 400 °C. The procedure also leads to a concentration of chloride at the surface of the films. Deposition from acidic baths may open up new simple synthetic routes towards the ternary cadmium-zinc sulfide system.

In situ kinetic studies of the chemical bath deposition of zinc sulfide from acidic solutions

Adherent and uniform films of zinc sulfide have been deposited reproducibly by chemical bath deposition (CBD), onto low iron content microscope slides, under acidic conditions from solutions containing zinc(II) chloride, urea and thioacetamide. In situ measurements using a quartz crystal microbalance suggest that film growth occurs after a short induction period, via an initial rapid phase (deposition rate of 91 nm h−1), followed by a slower process (growth rate of 52 nm h−1). The ZnS films are predominantly of the hexagonal modification (wurtzite structure), as determined by X-ray powder diffraction. Scanning electron micrographs of the deposited layers, obtained on samples removed at regular intervals from the bath, suggest that as the reaction proceeds, uniform film growth is associated with increasing particle size, rather than continuous nucleation and deposition of new primary particles. EDAX spectra of the films are typical of thin CBD-deposited layers of ZnS; signals attributable to elements within the glass substrate are also detected. Grain size distributions have been investigated using computational image analysis, and an average grain growth rate of 33.7 nm h−1 has been calculated.

A study of impurities in some CdS/CdTe photovoltaic cells prepared by wet-chemical methods using secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Quantitative Secondary Ion Mass Spectrometry has been used to determine the elemental profiles and concentrations of isotopes 12C, 16O, 34S and 35Cl within n-CdS/p-CdTe thin film photovoltaic cells. Chemical Bath Deposition (CBD) was used to deposit the CdS window layers. The annealing process induces the formation of a chloride-rich surface layer on CdS as evidenced from X-ray photoelectron spectroscopy measurements. The carbon impurity in heterostructures appears to influence the chloride-promoted recrystallisation of CdTe. High concentrations of 16O, of the order 1020–1021 atoms cm–3 throughout the cells, are consistent with the formation of oxide material in the post-deposition thermal processing. Isotopic profiles for 12C, 34S and 35Cl have similar maxima (≈1019 atoms cm–3) but concentrate at the CdS–CdTe interface. The relatively high tolerance to high concentrations of impurities in our cells suggests that wet chemical methods may have great potential in the fabrication of large area/low cost devices.

Direct evidence for the formation of a passivating layer during chemomechanical polishing of silica by a hydrogen difluoride-based reagent

The sparingly soluble material (the passivating layer) formed during chemomechanical polishing of silica wafers by [HF2]––cerium(IV) oxide–sucrose mixtures at low pH has been identified as K2SiF6 coated with a thin silica layer.

Chemically modified thin organic films supported on polished silica substrates

Organic films, of thickness up toca. 1000 nm, derived from polycyclic aromatic hydrocarbons containing 3–15 rings, have been grown on highly polished silica glass supports by vacuum sublimation. The quality of the films, as judged by their electronic spectra, is very dependent on achieving a near subnanometre surface finish on the silica. Exposure of the films to MoF6 at room temperature results in irreversible adsorption of the latter, as demonstrated by electronic spectroscopy. The fluorides WF6 and AsF5 show similar behaviour, although in many cases their interactions with the films are less marked.

Chemomechanical polishing of silica and silicon by fluoride- and oxide-based reagents: identification of a reaction intermediate

The effect of added hydrogendifluoride anion on the chemomechanical polishing of silica and silicon wafers by aqueous suspensions of cerium(IV) or silicon(IV) oxides has been investigated over a range of solution pH. The effect on silica is marked at very low pH; an intermediate under these conditions has been identified as K2SiF6 coated with a thin silica-like layer. The [HF2]– anion is ineffective for silica polishing above pH 7 and for silicon under all conditions examined. The most effective reagent for the latter substrate is a mixture of cerium(IV) oxide and alkaline silica sol. An explanation for the role of [HF2]– is offered.