Ian Forbes

Inorganic photovoltaic cells

The inorganic semiconductor materials used to make photovoltaic cells include crystalline, multicrystalline, amorphous, and microcrystalline Si, the III-V compounds and alloys, CdTe, and the chalcopyrite compound, copper indium gallium diselenide (CIGS). We show the structure of the different devices that have been developed, discuss the main methods of manufacture, and review the achievements of the different technologies.

Chemical bath deposition of zinc sulfide based buffer layers using low toxicity materials

ZnS thin films that are used as buffer layers in thin film solar cells are usually deposited by chemical bath deposition using hydrazine hydrate as a complementary complexing agent. This material is, however, highly hazardous and its replacement by a less-hazardous material is desirable. In this work thin films of zinc sulfide have been deposited using sodium citrate as a novel complementary complexing agent and experiments performed to determine the optimum conditions for producing conformal films, free from pinholes. The layers deposited were found to consist of Zn and S but with a small concentration of O present. Data relating to how the deposition conditions affected the physical properties of the deposited layers are reported.

Optical properties of high quality Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

Highly oriented and conducting ZnO:Ga layers grown by chemical spray pyrolysis

Highly conducting and transparent gallium doped zinc oxide (ZnO:Ga) layers have been deposited using a spray pyrolysis process. The films were deposited on Corning 7059 glass substrates at different temperatures and gallium doping concentrations, keeping the other deposition parameters constant. The ZnO:Ga films grown at a substrate temperature of 350 °C with a gallium doping concentration of 5.0 at.% had the best physical properties. These layers were highly oriented along the 〈002〉 with a grain size of 98 nm. The films were n-conductivity type with an electrical conductivity of 1.32×103 Ω−1cm−1. The transmittance of these films was higher than 85% in the visible region with a high reflectance in the infra-red region. The figure of merit evaluated was 3.4×10−2 Ω−1.

Characterization of CuInSe2 material and devices: comparison of thermal and electrochemically prepared absorber layers

CuInSe2-based absorber layers for photovoltaic devices have been fabricated using two different scalable processes, electrodeposition and sputtering, both followed by thermal annealing. The structural properties of the absorber layers were studied by SEM, XRD and MiniSIMS. Sputtered absorber layers exhibit larger grain sizes than electrodeposited layers, but both types of film consist of randomly orientated crystallites. Electrodeposited layers appear to have a uniform composition with evidence of a MoSe2 layer at the back contact, whilst sputtered layers show no evidence for a MoSe2 layer. The external quantum efficiency spectrum of films and completed devices was measured, and the band gap and broadening parameters were obtained using electroreflectance spectroscopy. A device based on electrodeposited CuInSe2 achieved an AM 1.5 efficiency of 6.6%, whilst a device based on sputtered CuInSe2 had an efficiency of 8.3%. Impedance measurements were used to calculate doping densities of 2 ? 1016 and 4 ? 1015?cm?3 for the electrodeposited and sputtered devices, respectively.

The potential of thermophotovoltaic heat recovery for the glass industry

This paper aims to provide an overview of heat recovery by thermophotovoltaics (TPV) from industrial high‐temperature processes and uses the glass industry in the UK as an example. The work is part of a study of potential industrial applications of TPV in the UK being carried out by the Northumbria Photovoltaics Applications Centre. The paper reviews the relevant facts about TPV technology and the glass industry and identifies locations of use for TPV. These are assessed in terms of glass sector, furnace type, process temperature, impact on the existing process, power scale and development effort of TPV. Knowledge of these factors should contribute to the design of an optimum TPV system. The paper estimates possible energy savings and reductions of CO2 emissions using TPV in the glass industry.

Growth of high-quality CuInSe2 films by selenising sputtered Cu–In bilayers using a closed graphite box

High-quality CuInSe2 thin films have been prepared using a two-stage process. Multiple bilayers of Cu and In were deposited onto Mo-coated glass substrates by magnetron sputtering to produce a predominant Cu11In9 phase, and the layers were selenised using elemental selenium in a closed graphite box at different temperatures in the range of 250–550°C. The films were characterised to evaluate their physical behaviour by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray analysis. Polycrystalline and single-phase CuInSe2 films with a strong (112) preferred orientation were obtained at an annealing temperature of about 500°C. These layers were uniform, and the crystals were densely packed with a grain size of about 2.0 μm. At lower annealing temperatures, the presence of different binary phases of Cu–In, Cu–Se and In–Se were observed in the films. It was found that these binary phases react with each other, resulting in the formation of the ternary CuInSe2 phase.

The potential of thermophotovoltaic heat recovery for the UK industry

SYNOPSIS Thermophotovoltaics (TPV) is the use of the photovoltaic effect to generate electricity from a high-temperature thermal source. This paper aims to provide an overview of heat recovery by TPV from industrial high-temperature processes. The paper reviews the relevant facts about TPV technology and the high-temperature industry and identifies three principle locations for TPV heat recovery. For each location, one example process is assessed in terms of applicability of TPV, impact on the existing process and power scale. Knowledge of these factors should contribute to the design of an optimum TPV system. The paper estimates the range of possible energy savings and reductions of CO, emissions using TPV in the high-temperature industry.

A solar cell stacked slot-loaded suspended microstrip patch antenna with multiband resonance characteristics for WLAN and WiMAX systems

In this paper, a novel self-complementary shaped multiple- L slot loaded suspended microstrip patch antenna stacked with a polycrystalline silicon (poly-Si) solar cell is presented for 2.4/5.2 GHz band WLAN and 2.5/3.3/5.8 GHz band WiMAX networks. While the proposed self-complementary shaped multiple-L slot loaded suspended patch enables the propagation of multiple TMmn modes to be present, the poly-Si solar cell works as an RF parasitic patch element in addition to its photovoltaic function. The proposed stacked solar antenna combination topology enables the radiating patch to be easily modified by slot-loading to achieve multiband resonance characteristics and the poly-Si solar cell to operate without being shaded by any RF components of the antenna ensuring an optimum solar operation performance.

Formation of CuInSe2 by the selenization of sputtered Cu/In layers

Single layers of Cu and In were deposited onto Mo coated glass substrates by radio frequency sputtering. The Cu11In9 phase has been found to be the majority phase in these precursors. The selenization of the sputtered layers has been achieved by depositing a 1 μm Se layer onto the precursor by thermal evaporation followed by an annealing in vacuum. Samples were annealed at different temperatures varying between 100–600°C at intervals of 50°C. The chalcopyrite structured ternary phase of CuInSe2 with a significant amount of preferential orientation in the (112) direction was obtained in samples annealed at 400°C or above. Morphological, compositional and structural analyses of the samples annealed at different temperatures were performed using a variety of complementary techniques. The results were analysed to explain the growth of CuInSe2 on the selenization of sputtered Cu-In precursors. The occurrence of various binary phases of Cu-In, Cu-Se and In-Se in different annealing temperature ranges has also been investigated. The phenomenon of volume expansion in CuInSe2 on selenization has been found to manifest itself as a shift in the characteristic (110) X-ray diffraction peak of Mo.