Electroluminescence from a single quantum dot within the intrinsic region of a p-i-n junction is shown to act as an electrically driven single-photon source. At low injection currents, the dot electroluminescence spectrum reveals a single sharp line due to exciton recombination, while another line due to the biexciton emerges at higher currents. The second-order correlation function of the diode displays anti-bunching under a continuous drive current. Single-photon emission is stimulated by subnanosecond voltage pulses. These results suggest that semiconductor technology can be used to mass-produce a single-photon source for applications in quantum information technology.

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

Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University's research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html This document may differ from the final, published version of the research and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher's website (a subscription may be required.)Understanding the effects of sand and dust accumulation on photovoltaic modules AbstractNumerical and analytical models of sand and dust particle accumulation on photovoltaic modules in dry regions are presented and supported by a laboratory investigation of sand particle accumulation on a glass surface. Both models and the experimental data indicate that the reduction in the free fractional area can be described by an exponential decay resulting from the formation of clusters of particles. Such clusters can support particles in upper layers which reduce the available area for photon capture by a much smaller amount than particles resting directly on the glass surface. The results qualitatively describe existing field data beyond the linear regime and are developed to account for field conditions, including analysis of photovoltaic module tilt, humidity and wind speed. This investigation is intended as a basis of an engineering design tool to assess the case for including photovoltaics in dry regions.

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Cu 2 ZnSnS 4 (CZTS) nanoparticle inks synthesized by the injection of metal precursors into a hot surfactant offer an attractive route to the fabrication of Earth-abundant Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film photovoltaic absorber layers. In this work it is shown that the chemical reaction conditions used to produce CZTS nanoparticle inks have a fundamental influence on the performance of thin film solar cells made by converting the nanoparticles to large CZTSSe grains in a selenium rich atmosphere and subsequent cell completion. The reaction time, temperature and cooling rate of the nanoparticle fabrication process are found to affect doping level, secondary phases and crystal structure respectively. Specifically, prolonging the reaction offers a new route to increase the concentration of acceptor levels in CZTSSe photovoltaic absorbers and results in higher device efficiency through an increase in the open circuit voltage and a reduction in parasitic resistance. Quenching the reaction by rapid cooling introduces a wurtzite crystal structure in the nanoparticles which significantly degrades the device performance, while elevating the reaction temperature of the nanoparticle synthesis introduces a secondary phase Cu 2 SnS 3 in the nanoparticles and results in the highest cell efficiency of 6.26%. This is correlated with increased doping in the CZTSSe absorber and the results demonstrate a route to controlling this parameter.

The efficiency of a solar cell can be substantially increased by opening new energy gaps within the semiconductor band gap. This creates additional optical absorption pathways which can be fully exploited under concentrated sunlight. Here we report a new approach to opening a sizable energy gap in a single junction GaAs solar cell using an array of InAs quantum dots that leads directly to high device open circuit voltage. High resolution imaging of individual quantum dots provides experimentally obtained dimensions to a quantum mechanical model which can be used to design an optimized quantum dot array. This is then implemented by precisely engineering the shape and size of the quantum dots resulting in a total area (active area) efficiency of 18.3% (19.7%) at 5 suns concentration. The work demonstrates that only the inclusion of an appropriately designed quantum dot array in a solar cell has the potential to result in ultrahigh efficiency under concentration.

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Alternative n-type buffer layer such as In 2 S 3 have been proposed as Cd-free alternative in kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. In this study, optical and electronic characterisation techniques together with device analysis and simulation were used to assess nanoparticle-based CZTSSe absorbers and solar cells with CdS and In 2 S 3 buffers. Photoluminescence spectroscopy indicated CZTSSe absorbers with In 2 S 3 buffer had a lower density of detrimental non-radiative defects and a higher concentration of copper vacancies V Cu + , responsible for p-type conductivity in CZTSSe, in comparison to the absorber with CdS buffer. Capacitance-voltage (C-V) measurements revealed the In 2 S 3 buffer-based CZTSSe devices had a three times higher apparent doping density and a consequently narrower space charge region than devices with a CdS layer. This resulted in poorer collection of photo-generated charge carriers in the near-IR region despite a more favourable band alignment as determined by X-ray photoelectron and inverse photoelectron spectroscopy. The presence of interfacial defect states in In 2 S 3 devices as determined by C-V and biased quantum efficiency measurements are also responsible for the loss in open-circuit voltage compared with reference devices with CdS.