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  • JOURNAL ARTICLE
    Alonso-Alvarez D, Ekins-Daukes N, Alonso-Álvarez D, Ekins-Daukes N, Alonso-Alvarez D, Ekins-Daukes N, Alonso Alvarez D, Ekins-Daukes Net al., 2016,

    Photoluminescence-Based Current-Voltage Characterization of Individual Subcells in Multijunction Devices

    , IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 6, Pages: 1004-1011, ISSN: 2156-3381

    © 2016 IEEE. We demonstrate a photoluminescence (PL)-based contactless method to determine the current-voltage (I-V ) characteristics of the individual subcells in a multijunction solar cell. The method relies upon the reciprocity relation between the absorption and emission properties on a solar cell. Laser light with a suitable energy is used to excite carriers selectively in one junction (1J), and the internal voltages are deduced from the intensity of the resulting luminescence. The I-V curves obtained this way on 1J, 2J, and 6J devices are compared with those obtained using electroluminescence (EL). Good agreement is obtained at high-injection conditions, while discrepancies at low injection are attributed to in-plane carrier transport.
  • JOURNAL ARTICLE
    Alonso-Álvarez D, Ekins-Daukes N, Alonso Alvarez D, Ekins-Daukes Net al., 2016,

    SPICE Modelling of Photoluminescence and Electroluminescence Based Current-Voltage Curves of Solar Cells for Concentration Applications

    , Journal of Green Engineering, Vol: 5, Pages: 33-48, ISSN: 1904-4720

    Quantitative photoluminescence (PL) or electroluminescence (EL) experiments can be usedto probe fast and in a non-destructive way the current-voltage (IV) characteristics ofindividual subcells in a multi-junction device, information that is, otherwise, not available.PL-based IV has the advantage that it is contactless and can be performed even in partlyfinished devices, allowing for an early diagnosis of the expected performance of the solarcells in the production environment. In this work we simulate the PL- and EL-based IVcurves of single junction solar cells to assess their validity compared with the true IV curveand identify injection regimes where artefacts might appear due to the limited in-planecarrier transport in the solar cell layers. We model the whole photovoltaic device as anetwork of sub-circuits, each of them describing the solar cell behaviour using the two diodemodel. The sub-circuits are connected to the neighbouring ones with a resistor, representingthe in-plane transport in the cell. The resulting circuit, involving several thousand subcircuits,is solved using SPICE.
  • CONFERENCE PAPER
    Richards RD, Harun F, Cheong JS, Mellor A, Hylton NP, Wilson T, Thomas T, Ekins-Daukes NJ, David JPR, richards RD, Harun F, Cheong JS, Mellor AV, Hylton, wilson T, Thomas T, Ekins-Daukes, David JPRet al., 2016,

    GaAsBi: An Alternative to InGaAs Based Multiple Quantum Well Photovoltaics

    , 43rd IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 1135-1137, ISSN: 0160-8371
  • JOURNAL ARTICLE
    Roucka R, Clark A, Wilson T, Thomas T, Fuhrer M, Ekins-Daukes N, Johnson A, Hoffman R, Begarney D, Roucka R, Clark A, Wilson T, Thomas T, Führer M, Ekins-Daukes N, Johnson A, Hoffman R, Begarney D, Roucka R, Clark A, Wilson T, Thomas T, Fuhrer M, Ekins-Daukes N, Johnson A, Hoffman R, Begarney D, Roucka R, Clark A, Wilson T, Thomas T, Fuhrer M, Ekins-Daukes N, Johnson A, Hoffman R, Begarney Det al., 2016,

    Demonstrating Dilute-Tin Alloy SiGeSn for Use in Multijunction Photovoltaics: Single- and Multijunction Solar Cells With a 1.0-eV SiGeSn Junction

    , IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 6, Pages: 1025-1030, ISSN: 2156-3381

    © 2016 IEEE. SiGeSn ternary alloys offer a means to fabricate a 1.0-eV subcell junction for inclusion in a multijunction solar cell. The main advantage of the SiGeSn alloy is a tuneable bandgap energy and variable lattice parameter, enabling the material to be integrated into the existing lattice-matched multijunction architectures. Recent growth, structural, optical, and device results from SiGeSn material, with energy gaps in the vicinity of 1.0 eV and lattice matched to Ge substrates, are presented. An all lattice-matched InGaP/InGaAs/SiGeSn triple-junction cell is presented and compared with a conventional InGaP/InGaAs/Ge solar cell. Comparable short-circuit current values of 13.9 mA/cm 2 are obtained for both devices under the AM1.5G spectrum, whereas the open-circuit voltage and fill factor are reduced in the device with the SiGeSn subcell. Peak external quantum efficiency in the SiGeSn single junction in excess of 80% is realized, placing a lower limit on the base minority hole diffusion length of 5 μm with surface recombination velocities in close agreement to those found in bulk Ge material.
  • JOURNAL ARTICLE
    Sugiyama M, Fujii H, Katoh T, Toprasertpong K, Sodabanlu H, Watanabe K, Alonso-Alvarez D, Ekins-Daukes NJ, Nakano Y, Sugiyama M, Fujii H, Katoh T, Toprasertpong K, Sodabanlu H, Watanabe K, Alonso-Álvarez D, Ekins-Daukes NJ, Nakano Y, Sugiyama M, Fujii H, Katoh T, Toprasertpong K, Sodabanlu H, Watanabe K, Alonso-Álvarez D, Ekins-Daukes NJ, Nakano Yet al., 2016,

    Quantum wire-on-well (WoW) cell with long carrier lifetime for efficient carrier transport

    , PROGRESS IN PHOTOVOLTAICS, Vol: 24, Pages: 1606-1614, ISSN: 1062-7995

    A quantum wire-on-well (WoW) structure, taking advantage of the layer undulation of an InGaAs/GaAs/GaAsP superlattice grown on a vicinal substrate, was demonstrated to enhance the carrier collection from the confinement levels and extend the carrier lifetime (220 ns) by approximately four times more than a planar reference superlattice. Strained InGaAs/GaAs/GaAsP superlattices were grown on GaAs substrates under exactly the same conditions except for the substrate misorientation (0 and 6 ° off). The growth on a 6 ° off substrate induced significant layer undulation as a result of step bunching and non-uniform precursor incorporation between steps and terraces, whereas the growth on a substrate without miscut resulted in planar layers. The undulation was the most significant for InGaAs layers, forming periodically aligned InGaAs nanowires on planar wells, a WoW structure. As for the photocurrent corresponding to the sub-bandgap range of GaAs, the light absorption by the WoW was extended to longer wavelengths and weakened as compared with the planar superlattice. Almost the same photocurrent was obtained for both the WoW and the planar superlattice. Open-circuit voltage for the WoW was not affected by the longer-wavelength absorption edge, and the same value was obtained for the two structures. Furthermore, the superior carrier collection in the WoW, especially under forward biases, improved fill factor compared with the planer superlattice.
  • CONFERENCE PAPER
    Wilson T, Thomas T, Fuhrer M, Ekins-Daukes NJ, Roucka R, Clark A, Johnson A, Hoffman R, Begarney D, Wilson T, Thomas T, Führer M, Ekins-Daukes NJ, Roucka R, Clark A, Johnson A, Hoffman R, Begarney D, Wilson T, Thomas TOMOS, Fuhrer M, Ekins-Daukes NJ, Roucka R, Clark A, Johnson A, Hoffman Jnr R, Begarney Det al., 2016,

    Single and Multi-Junction Solar Cells Utilizing a 1.0 eV SiGeSn Junction

    , 12th International Conference on Concentrator Photovoltaic Systems (CPV), Publisher: AMER INST PHYSICS, ISSN: 0094-243X

    © 2016, American Institute of Physics Inc. All rights reserved. Multi-junction photovoltaic technologies lead the way to achieving ultra-high power conversion efficiencies for both space based and terrestrial concentrator applications. However, realizing a lattice matched quad-junction solar cell remains challenging due to a lack of suitable material systems able to achieve the elusive middle sub-cell band-gap energy of 1.0 eV. In this work, we present a potential candidate material to achieve this 1.0 eV sub-cell, the group-IV ternary alloy SiGeSn. Initial simulations of triple and quadruple junction solar cell designs show that this novel material system has the potential to reach efficiencies in excess of 45 and 48% respectively under concentrated illumination. We report on the electrical characterization of both single and triple junction devices based on a 1.0 eV SiGeSn junction. It is shown that the external quantum efficiency performance of both SiGeSn devices is promising given the initial state of development of the alloy, with a fitted minority electron diffusion length in the junction base shown to be limited to 5 μm. However, further material study and growth optimization is required to improve open-circuit voltage performance and the reverse bias breakdown characteristic in the SiGeSn junction. Given these short-comings we believe that group-IV ternary alloys provide a viable alternative approach to achieving highly efficient multi-junction solar cells in future.
  • CONFERENCE PAPER
    Alonso Alvarez D, Lackner D, Philipps SP, Bett AW, Ekins-Daukes NJet al., 2015,

    Photoluminescence-Based Current-Voltage Characterisation of Individual Subcells in Multi-Junction Devices

    , 31st European Photovoltaic Solar Energy Conference and Exhibition, Publisher: European Photovoltaic Solar Energy Conference and Exhibition, Pages: 1509-1513

    We demonstrate a photoluminescence based, contactless method to determine the current-voltage characteristics of the individual subcells in a multi-junction solar cell. The method, furthers known results for single junction devices and relies upon the reciprocity relation between the absorption and emission properties on a solar cell. Laser light with a suitable energy is used to excite carriers selectively in one junction and the internal voltages are deduced from the intensity of the resulting luminescence. The IV curves obtained this way on 1J, 2J and 6J devices are compared to those obtained using electroluminescence. Good agreement is obtained at high injection conditions while discrepancies at low injection are attributed to in-plane carrier transport.
  • JOURNAL ARTICLE
    Emmott CJM, Roehr JA, Campoy-Quiles M, Kirchartz T, Urbina A, Ekins-Daukes NJ, Nelson J, Emmott CJM, Röhr JA, Campoy-Quiles M, Kirchartz T, Urbina A, Ekins-Daukes NJ, Nelson J, Emmott CJM, Roehr JA, Campoy-Quiles M, Kirchartz T, Urbina A, Ekins-Daukes NJ, Nelson Jet al., 2015,

    Organic photovoltaic greenhouses: a unique application for semi-transparent PV?

    , ENERGY & ENVIRONMENTAL SCIENCE, Vol: 8, Pages: 1317-1328, ISSN: 1754-5692

    Organic photovoltaics are an emerging solar power technology which embody properties such astransparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique nicheapplications. We report a detailed techno-economic analysis of one such application, namely thephotovoltaic greenhouse, and discuss whether the unique properties of the technology can provideadvantages over conventional photovoltaics. The potential for spectral selectivity through the choice ofOPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typicalgreenhouse crops is used to determine the impact on crop growth of blocking different spectral rangesfrom the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolvedtransparency of a variety of commercially available semi-conducting polymer materials, in addition to anon-commercial low-band-gap material with absorption outside that required for crop growth. Economicanalysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can bemet. Technical analysis shows that semi-transparent OPV devices may struggle to perform better thanopaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap materialPMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improvedperformance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress theimportance of developing new, highly transparent electrode and interlayer materials, along with highefficiency active layers, if the full potential of this application is going to be realised.
  • JOURNAL ARTICLE
    Thomas T, Mellor A, Hylton NP, Fuehrer M, Alonso-Alvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJ, Thomas T, Mellor A, Hylton NP, Fuhrer M, Alonso-Àlvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJ, Thomas T, Mellor A, Hylton NP, Fuhrer M, Alonso-Álvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJ, Thomas T, Mellor A, Hylton NP, Fuehrer M, Alonso-Alvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJet al., 2015,

    Requirements for a GaAsBi 1eV sub-cell in a GaAs-based multi-junction solar cell

    , SEMICONDUCTOR SCIENCE AND TECHNOLOGY, Vol: 30, Pages: 094010-094010, ISSN: 0268-1242

    © 2015 IOP Publishing Ltd. Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs < inf > 0.94 < /inf > Bi < inf > 0.06 < /inf > yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.
  • CONFERENCE PAPER
    Thomas T, Wilson T, Fuhrer M, Hylton N, Ekins-Daukes N, Tan KH, Li D, Wicaksono S, Loke WK, Fatt YS, Johnson Aet al., 2015,

    Use of Double Band Anti-Crossing to Control Optical Absorption of GaAsSbN for Multi-Junction Solar Cells

    , 31st European Photovoltaic Solar Energy Conference and Exhibition
  • JOURNAL ARTICLE
    Wilson T, Thomas T, Führer M, Ekins-Daukes NJ, Wilson T, Thomas T, Führer M, Ekins-Daukes NJ, Wilson T, Thomas T, Führer M, Ekins-Daukes NJet al., 2015,

    Addressing reflectivity losses in multijunction solar cells to achieve 50% power conversion efficiency

    , Materials Research Innovations, Vol: 19, Pages: 503-507, ISSN: 1432-8917

    © W. S. Maney & Son Ltd. 2016. As we progress towards the next generation of multi-junction solar cell technology, incorporating four junctions or more, current dual-layer coating technology applied to commercially available triple-junction solar cells will become inadequate as the current matching between the lower sub cells, absorbing in the infrared, becomes more critical. Here we present some single, double and triple layer anti-reflection coatings for comparison, optimised using a genetic algorithm, for both triple-and quad-junction solar cell architectures. It is found that there is a small increase in power conversion efficiency between the dual and triple layer coatings applied to the triple-junction structure (0.7%), however this is more than doubled when optimised dual and triple layer coatings are applied to the quad-junction device (1.5%). This work demonstrates the effectiveness of a well-optimised multi-layer anti-reflection coating in realising an appreciable increase in device performance in the next generation of multi-junction solar cells, highlighting a realistic power conversion efficiency of up to 50.35%.
  • CONFERENCE PAPER
    Alonso-Alvarez D, Fuehrer M, Thomas T, Ekins-Daukes N, Alonso-Alvarez D, Fuhrer M, Thomas T, Ekins-Daukes Net al., 2014,

    Elements of modelling and design of multi-quantum well solar cells

    , 40th IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 2865-2870

    © 2014 IEEE. Multi-quantum wells structures provide some flexibility for adjusting the absorption edge of multi-junction sub-cells. In order to obtain the desirable performance, it is essential to have an accurate model of the MQW properties and of the solar cell as a whole, including the light absorption, carrier extraction and carrier collection mechanisms. In this work, we show that Shockley-Read-Hall (SRH) recombination and insufficient light absorption are the main limiting factors for achieving high currents. The former can be reduced by a smart placement of the QWs inside the structure. By leaving a gap in the MQW stack, where SRH recombination is maximum, an improvement of the current at the maximum power point can be achieved without adding QWs. Increasing their number enhances light absorption but also the thickness of the device and the difficulty for carrier transport across the QW region. In this case, knowing the background doping and the carrier mobilities help to make an optimum solar cell design. In particular, we find than an intentional, low doping might lead to higher currents with short QW stacks than using a longer ones on an intrinsic region.
  • JOURNAL ARTICLE
    Alonso-Alvarez D, Thomas T, Fuehrer M, Hylton NP, Ekins-Daukes NJ, Lackner D, Philipps SP, Bett AW, Sodabanlu H, Fujii H, Watanabe K, Sugiyama M, Nasi L, Campanini M, Alonso-Álvarez D, Thomas T, Führer M, Hylton NP, Ekins-Daukes NJ, Lackner D, Philipps SP, Bett AW, Sodabanlu H, Fujii H, Watanabe K, Sugiyama M, Nasi L, Campanini M, Alonso-Álvarez D, Thomas T, Führer M, Hylton NP, Ekins-Daukes NJ, Lackner D, Philipps SP, Bett AW, Sodabanlu H, Fujii H, Watanabe K, Sugiyama M, Nasi L, Campanini M, Alonso-Alvarez D, Thomas T, Fuehrer M, Hylton NP, Ekins-Daukes NJ, Lackner D, Philipps SP, Bett AW, Sodabanlu H, Fujii H, Watanabe K, Sugiyama M, Nasi L, Campanini Met al., 2014,

    InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

    , APPLIED PHYSICS LETTERS, Vol: 105, Pages: 083124-083124, ISSN: 0003-6951

    Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times w eaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells. © 2014 AIP Publishing LLC.
  • JOURNAL ARTICLE
    Dimmock JAR, Day S, Kauer M, Smith K, Heffernan J, Dimmock JAR, Day S, Kauer M, Smith K, Heffernan J, Dimmock JAR, Day S, Kauer M, Smith K, Heffernan Jet al., 2014,

    Demonstration of a hot-carrier photovoltaic cell

    , PROGRESS IN PHOTOVOLTAICS, Vol: 22, Pages: 151-160, ISSN: 1062-7995

    We demonstrate a new hot-carrier photovoltaic cell based on the resonant tunnelling of hot electrons from a narrow-band-gap semiconductor to a wider-band-gap semiconductor. Hot electrons are photogenerated at a variety of wavelengths in a GaAs absorber followed by resonant tunnelling through a double-barrier quantum well into an AlGaAs collector, forming an energy-selective interface in the centre of the device. We show theoretically the presence of a tunnel current from the absorber to the collector under illumination, offering a method to extract carriers from a hot-electron distribution at zero bias. We experimentally demonstrate a hot-carrier photovoltaic cell based on this concept. Two features of its measured current-voltage characteristic, namely the peak to valley current ratio and the current peak voltage, are shown to vary with the wavelength of illumination in a way that clearly demonstrates hot-carrier extraction. Copyright © 2013 John Wiley & Sons, Ltd. We demonstrate a new hot-carrier photovoltaic cell based on the resonant tunnelling of hot electrons from a narrow-band-gap semiconductor to a wider-band-gap semiconductor. We outline the theory of hot-carrier tunnelling between semiconductors with offset conduction bands, and we experimentally demonstrate a hot-carrier photovoltaic cell based on this concept. Copyright © 2013 John Wiley & Sons, Ltd.
  • JOURNAL ARTICLE
    Piper RB, Yoshida M, Farrell DJ, Khoury T, Crossley MJ, Schmidt TW, Haque SA, Ekins-Daukes NJ, Piper RB, Yoshida M, Farrell DJ, Khoury T, Crossley MJ, Schmidt TW, Haque SA, Ekins-Daukes NJet al., 2014,

    Kinetic insight into bimolecular upconversion: experiment and simulation

    , RSC ADVANCES, Vol: 4, Pages: 8059-8063, ISSN: 2046-2069
  • JOURNAL ARTICLE
    Yoshida M, Amrania H, Farrell DJ, Browne B, Yoxall E, Ekins-Daukes NJ, Phillips CC, Yoshida M, Amrania H, Farrell DJ, Browne B, Yoxall E, Ekins-Daukes NJ, Phillips CC, Yoshida M, Amrania H, Farrell DJ, Browne B, Yoxall E, Ekins-Daukes NJ, Phillips CCet al., 2014,

    Progress Toward Realizing an Intermediate Band Solar Cell-Sequential Absorption of Photons in a Quantum Well Solar Cell

    , IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 4, Pages: 634-638, ISSN: 2156-3381

    In order to realize an intermediate band solar cell, which promises high photovoltaic energy conversion efficiency, achieving higher photocurrent while maintaining the cell voltage is essential. We report on a transient photocurrent due to the sequential absorption of photons in a single quantum well by continuously pumping to stimulate interband transitions (from a valence band to an intermediate band) and showing an intersubband transition (from an intermediate band to a conduction band) with a pulsed infrared laser. We demonstrate the extent to which multiple-photon absorption can be achieved in quantum well devices and propose that a quantum well is a suitable candidate for an intermediate band solar cell. From the combination of this and other sequential absorption results, it is clear that enhancing the short lifetime of a carrier in the intermediate band is the next step toward achieving a working intermediate band solar cell. In light of this, we enhance our previous suggestion, the photon ratchet intermediate band solar cell, as a means of increasing the electron lifetime. © 2014 IEEE.
  • JOURNAL ARTICLE
    Chan NLA, Young TB, Brindley HE, Ekins-Daukes NJ, Araki K, Kemmoku Y, Yamaguchi M, Chan NLA, Young TB, Brindley HE, Ekins-Daukes NJ, Araki K, Kemmoku Y, Yamaguchi M, Chan N, Young T, Brindley H, Ekins-Daukes N, Araki K, Kemmoku Y, Yamaguchi M, Chan NLA, Young TB, Brindley HE, Ekins-Daukes NJ, Araki K, Kemmoku Y, Yamaguchi Met al., 2013,

    Validation of energy prediction method for a concentrator photovoltaic module in Toyohashi Japan

    , PROGRESS IN PHOTOVOLTAICS, Vol: 21, Pages: 1598-1610, ISSN: 1062-7995

    III–V concentrator photovoltaic systems attain high efficiency through the use of series connected multi-junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20-cell InGaP/In0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency.
  • JOURNAL ARTICLE
    Ekins-Daukes NJ, Lee K-H, Hirst L, Chan A, Fuehrer M, Adams J, Browne B, Barnham KWJ, Stavrinou P, Connolly J, Roberts JS, Stevens B, Airey R, Kennedy K, Ekins-Daukes NJ, Lee KH, Hirst L, Chan A, Führer M, Adams J, Browne B, Barnham KWJ, Stavrinou P, Connolly J, Roberts JS, Stevens B, Airey R, Kennedy K, Ekins-Daukes NJ, Lee K-H, Hirst L, Chan A, Führer M, Adams J, Browne B, Barnham KWJ, Stavrinou P, Connolly J, Roberts JS, Stevens B, Airey R, Kennedy Ket al., 2013,

    Controlling radiative loss in quantum well solar cells

    , JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 46, Pages: 264007-264007, ISSN: 0022-3727

    The inclusion of quantum well layers in a solar cell provides a means for extending the absorption and therefore increasing the photocurrent of the cell. In 2009, a single-junction GaAsP/InGaAs quantum well solar cell attained a peak efficiency of 28.3% under solar concentration. Since then InGaP/MQW/Ge quantum well devices have attained efficiencies in excess of 40% under concentration and over 30% under AM0. The principle motivation for incorporating a quantum well stack into a multi-junction solar cell is to increase the photocurrent delivered by the middle junction over the conventional In 0.01GaAs bulk junction. This enables additional current to flow through the top and middle cells, resulting in a sharp rise in efficiency. However, quantum wells also provide some freedom to manipulate the radiative recombination in the quantum well solar cell. We show that under radiatively dominated, anisotropic emission, strong radiative coupling between sub-cells takes place, resulting in a multi-junction solar cell that is tolerant to daily and seasonal changes to the solar spectrum. © 2013 IOP Publishing Ltd.
  • JOURNAL ARTICLE
    Hylton NP, Li XF, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SA, Hylton NP, Li XF, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SA, Hylton NP, Li XF, Giannini V, Lee KH, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SA, Hylton NP, Li XF, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SA, Hylton NP, Li XF, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SAet al., 2013,

    Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes

    , SCIENTIFIC REPORTS, Vol: 3, ISSN: 2045-2322

    We illustrate the important trade-off between far-field scattering effects, which have the potential to provide increased optical path length over broad bands, and parasitic absorption due to the excitation of localized surface plasmon resonances in metal nanoparticle arrays. Via detailed comparison of photocurrent enhancements given by Au, Ag and Al nanostructures on thin-film GaAs devices we reveal that parasitic losses can be mitigated through a careful choice of scattering medium. Absorption at the plasmon resonance in Au and Ag structures occurs in the visible spectrum, impairing device performance. In contrast, exploiting Al nanoparticle arrays results in a blue shift of the resonance, enabling the first demonstration of truly broadband plasmon enhanced photocurrent and a 22% integrated efficiency enhancement.
  • JOURNAL ARTICLE
    Li X, Hylton NP, Giannini V, Lee K-H, Ekins-Daukes NJ, Maier SA, Li X, Hylton NP, Giannini V, Lee KH, Ekins-Daukes NJ, Maier SA, Li X, Hylton NP, Giannini V, Lee K-H, Ekins-Daukes NJ, Maier SAet al., 2013,

    Multi-dimensional modeling of solar cells with electromagnetic and carrier transport calculations

    , PROGRESS IN PHOTOVOLTAICS, Vol: 21, Pages: 109-120, ISSN: 1062-7995

    We present a multi-dimensional model for comprehensive simulations of solar cells (SCs), considering both electromagnetic and electronic properties. Typical homojunction and heterojunction gallium arsenide SCs were simulated in different spatial dimensions. When considering one-dimensional problems, the model performs carrier transport calculations following a Beer-Lambert optical absorption approximation. We show that the results of such simulations exhibit excellent agreement with the standard PC1D one-dimensional photovoltaic simulation. Photonic and plasmonic attempts to enhance SC efficiency demand comprehensive electromagnetic calculations to be undertaken in order to acquire accurate carrier generation profiles in two and three-dimensional systems. Our model provides complete spectral and spatial information of typical optical and electronic behavior. Furthermore, our approach permits the detailed investigation of complex systems, including plasmonic SCs, which cannot be simulated using low-dimensional modeling tools. We present the results of numerical simulations of an optically thin plasmonic gallium arsenide SC and observe improved device performance arising from the application of plasmonic nanostructures, which agree well with previous experimental findings. Copyright © 2012 John Wiley & Sons, Ltd.
  • JOURNAL ARTICLE
    Farrell DJ, Yoshida M, Farrell DJ, Yoshida Met al., 2012,

    Operating regimes for second generation luminescent solar concentrators

    , PROGRESS IN PHOTOVOLTAICS, Vol: 20, Pages: 93-99, ISSN: 1062-7995
  • JOURNAL ARTICLE
    Lee K-H, Barnham KWJ, Connolly JP, Browne BC, Airey RJ, Roberts JS, Fuehrer M, Tibbits TND, Ekins-Daukes NJ, Lee KH, Barnham KWJ, Connolly JP, Browne BC, Airey RJ, Roberts JS, Führer M, Tibbits TND, Ekins-Daukes NJ, Lee K-H, Barnham KWJ, Connolly JP, Browne BC, Airey RJ, Roberts JS, Fuhrer M, Tibbits TND, Ekins-Daukes NJet al., 2012,

    Demonstration of Photon Coupling in Dual Multiple-Quantum-Well Solar Cells

    , IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 2, Pages: 68-74, ISSN: 2156-3381

    Multiple-quantum-well (MQW) top cells can enhance the performance of multi-junction solar cells since the absorption edge of top and middle subcells can be tuned with the MQWs to maximize the efficiency. The radiative dominance of MQW top cells can enhance photon coupling, which can potentially reduce the spectral sensitivity of the device and, thus, raise the energy harvest. We present experimental results on photon coupling in dual-junction cells with GaInP top cells containing GaInAsP quantum wells along with theoretical calculation based on a detailed balance model. It is observed that at high concentration, approximately 50 of the dark current of an MQW top cell is transferred to the photocurrent of the cell in the bottom, which is much higher than any previously reported values. © 2011 IEEE.
  • JOURNAL ARTICLE
    Yoshida M, Ekins-Daukes NJ, Farrell DJ, Phillips CC, Yoshida M, Ekins-Daukes NJ, Farrell DJ, Phillips CC, Yoshida M, Ekins-Daukes NJ, Farrell DJ, Phillips CC, Yoshida M, Ekins-Daukes NJ, Farrell DJ, Phillips CCet al., 2012,

    Photon ratchet intermediate band solar cells

    , APPLIED PHYSICS LETTERS, Vol: 100, Pages: 263902-263902, ISSN: 0003-6951

    In this paper, we propose an innovative concept for solar power conversion-the photon ratchet intermediate band solar cell (IBSC)-which may increase the photovoltaic energy conversion efficiency of IBSCs by increasing the lifetime of charge carriers in the intermediate state. The limiting efficiency calculation for this concept shows that the efficiency can be increased by introducing a fast thermal transition of carriers into a non-emissive state. At 1 sun, the introduction of a ratchet band results in an increase of efficiency from 46.8% to 48.5%, due to suppression of entropy generation. © 2012 American Institute of Physics.
  • CONFERENCE PAPER
    Adams JGJ, Browne BC, Ballard IM, Connolly JP, Chan NLA, Ioannides A, Elder W, Stavrinou PN, Barnham KWJ, Ekins-Daukes NJ, Adams JGJ, Browne BC, Ballard IM, Connolly JP, Chan NLA, Ioannides A, Elder W, Stavrinou PN, Barnham KWJ, Ekins-Daukes NJ, Adams JGJ, Browne BC, Ballard IM, Connolly JP, Chan NLA, Ioannides A, Elder W, Stavrinou PN, Barnham KWJ, Ekins-Daukes NJ, Adams JGJ, Browne BC, Ballard IM, Connolly JP, Chan NLA, Ioannides A, Elder W, Stavrinou PN, Barnham KWJ, Ekins-Daukes NJet al., 2011,

    Recent results for single-junction and tandem quantum well solar cells

    , 25th European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC)/5th World Conference on Photovoltaic Energy Conversion (WCPEC-5), Publisher: WILEY-BLACKWELL, Pages: 865-877, ISSN: 1062-7995

    The band gap of the quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single-junction strain-balanced quantum well solar cell (SB-QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB-QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain-balanced quantum wells in multi-junction solar cells can current match the sub-cells without the introduction of dislocations. Calculations are shown which predict efficiency limits as a function of QW a bsorption and band gap for such cells. A dual-junction InGaP/GaAs solar cell with QWs in the bottom sub-cell has been grown and characterized. Laboratory and calculated efficiencies relative to control cells are presented for the reported cell and a modeled device, respectively. © 2011 John Wiley & Sons, Ltd.
  • CONFERENCE PAPER
    Barnham KWJ, Adams JGJ, Elder W, Stavrinou P, Ekins-Daukes NJet al., 2009,

    Efficiency enhancement in strain-balanced quantum well solar cells via anisotropic emission

    , 24th European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Barnham KWJ, Ekins-Daukes NJ, Browne B, 2009,

    Radiative limits in quantum well solar cells

    , 24th European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Chatten AJ, Bose R, Farrell DJ, Pardo-Sanchez C, Pravettoni M, Mazzer M, Barnham WJet al., 2009,

    Luminescent Solar Concentrators: cylindrical design

    , 24th European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Chatten AJ, Pravettoni M, Farrell DJ, Bose R, Kenny RP, Barnham Ket al., 2009,

    External quantum efficiency measurements of luminescent solar concentrators: a study of the impact of backside reflector size and shape

    , 24th European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Adams JGJ, Barnham KWJ, Connolly JP, Hill G, Roberts JS, Tibbits TND, Geen M, Pate M, Barnham KWJ, Adams JGJ, Connolly JP, Hill G, Roberts JS, Tibbits TND, Geen M, Pate Met al., 2008,

    Strain-balanced quantum well concentrator cells from multiwafer production

    , 33rd IEEE Photovoltaic Specialists Conference, ISSN: 0160-8371

    Strain-balanced quantum well solar cells with 50 quantum well layers of In 0.11 GaAs in the intrinsic region of a GaAs p-i-n solar cell have been studied. Devices have been taken from different positions across 4' wafers from two manufacturing runs on a multi-wafer production reactor. Growth has been compared for devices with and without distributed Bragg reflectors (DBRs), and for devices with and without stepped p-region emitters. Device quantum efficiencies and dark currents have been modeled and compared to experimental data in order to predict efficiencies at 500 suns in an AM1.5D spectrum. Suppression of the radiative recombination component of the dark current has been observed in devices with DBRs, indicating the occurrence of photon recycling. A decrease in the Shockley injection component of the dark current has been found in devices with stepped p-region emitters, as minority carriers are deflected from the surface of the device. Good uniformity of performance has been observed in devices from across the wafer despite varying DBR position, and high device yields are to be expected during manufacture. © 2008 IEEE.
  • CONFERENCE PAPER
    Barnham KWJ, Adams JGJ, Connolly JP, Calder C, Hill G, Roberts JS, Tibbits TND, Geen M, Pate Met al., 2008,

    Characterisation of Strain-Balanced Quantum Well Concentrator Cells Enhanced By Photon Recycling

    , PVSAT-4
  • CONFERENCE PAPER
    Barnham KWJ, Adams JGJ, Ginige R, Connolly JP, Ballard IM, Ekins-Daukes NJ, Hill G, Roberts JSet al., 2008,

    Single-Junction Concentrator Quantum Well Solar Cells Enhanced By Photon Recycling

    , 23rd European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Barnham KWJ, Browne B, Ioannides A, Connolly J, Hill G, Roberts J, Airey R, Hill G, Smekens G, Van Begin Jet al., 2008,

    Efficiency of Tandem Quantum Well Solar Cells

    , PVSAT-4
  • CONFERENCE PAPER
    Barnham KWJ, Fuhrer MF, Connolly JP, Mazzer M, Ballard IM, Ekins-Daukes NJ, Johnson DC, Bessiere A, Roberts JS, Airey R, Calder C, Hill G, Tibbits TNDet al., 2008,

    Exciton Broadening in Quantum Well Solar Cells

    , 23rd European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Bose R, Farrell DJ, Chatten AJ, Pravettoni M, Buechtemann A, Quilitz J, Fiore A, Manna L, Barnham KWJet al., 2008,

    LUMINESCENT SOLAR CONCENTRATORS: NANORODS AND RAYTRACE MODELING

    , 33rd IEEE Photovoltaic Specialists Conference, Publisher: IEEE, Pages: 24-+, ISSN: 0160-8371
  • CONFERENCE PAPER
    Browne B, Ekins-Daukes N, Ioannides A, Connolly J, Ballard I, Barnham K, Tibbits T, Pate M, Geen M, Roberts J, Hill G, Calder C, Airey R, Smekens G, Van Begin Jet al., 2008,

    Dual Multiple Quantum Well Solar Cells

    , 23rd European Photovoltaic Solar Energy Conference
  • CONFERENCE PAPER
    Browne B, Ioannides A, Connolly J, Barnham K, Roberts J, Airey R, Hill G, Smekens G, Van Begin J, Barnham KWJ, Browne B, Ioannides A, Connolly J, Roberts J, Airey R, Hill G, Smekens G, Van Begin Jet al., 2008,

    Tandem quantum well solar cells

    , 33rd IEEE Photovoltaic Specialists Conference, ISSN: 0160-8371

    Quantum wells offer advantages in conventional bulk tandem solar cells since they allow the independent tailoring of the absorption edge of either cell with no lattice mismatch and subsequent relaxation. We describe progress in the band gap engineering of InGaP/GaAs solar cells using strain balanced quantum wells and present a tandem quantum well structure which has achieved 30.6% efficiency under 54 suns AM1.5g. This is a record for photovoltaic nanostructured devices. We predict realistic efficiencies of 34% under 600suns, AM1.5d low AOD for optimized devices. Finally, the possibility and potential gains of introducing quantum wells into both cells of an InGaP/GaAs device are discussed. © 2008 IEEE.
  • CONFERENCE PAPER
    Chatten AJ, bose, farrell, pravettoni, Buchtemann A, Quiltz J, Fiore A, Manna L, Nelson J, Alivisatos AP, Barnhamet al., 2008,

    The Effect of Size and Dopant Concentration of the Performance of Nanorod Luminescent Solar Concentrators

    , The 23rd European Photovoltaic Solar Energy Conference and Exhibition
  • CONFERENCE PAPER
    Chatten AJ, bose R, farrell D, Buchtemann A, Barnhamet al., 2008,

    The Luminescent Concentrator: Thin Films and Large Area Modelling

    , PVSAT-4
  • CONFERENCE PAPER
    Fuhrer MF, Connolly JP, Mazzer M, Ballard IM, Johnson DC, Barnham KWJ, Bessiere A, Roberts JS, Airey R, Calder C, Hill G, Tibbits TND, Pate M, Geen M, Fuhrer MF, Connolly JP, Mazzer M, Ballard IM, Johnson DC, Barnham KWJ, Bessiere A, Roberts JS, Airey R, Calder C, Hill G, Tibbits TND, Pate M, Geen Met al., 2008,

    HOT CARRIERS IN STRAIN BALANCED QUANTUM WELL SOLAR CELLS

    , 33rd IEEE Photovoltaic Specialists Conference, Publisher: IEEE, Pages: 1016-+, ISSN: 0160-8371

    The incorporation of strain-balanced quantum wells into a GaAs solar cell extends the spectral response resulting in a photocurrent increase that can exceed the reduction in voltage performance, leading to higher overall efficiencies [1]. At concentrator current levels the main carrier loss mechanism is radiative recombination from the quantum wells. We have recently reported on evidence of hot carrier effects in the quantum well regions of GaAs based strain-balanced cells incorporating InGaAs quantum wells and GaAsP barriers [2] . This paper extends this work to a greater range of samples, and reports on a bias-dependent broadening in exciton luminescence observed in all samples at high biases. We present two possible interpretations of the data using different parts of the generalised Planck equation. © 2008 IEEE.
  • JOURNAL ARTICLE
    Johnson JC, Ballard IM, Barnham KWJ, Connolly JP, Mazzer M, Bessiere A, Calder C, Hill G, Roberts JSet al., 2007,

    Observation of photon recycling in strain-balanced quantum well solar cells

    , Applied Physics Letters, Vol: 90, Pages: 213505-213507

    Photon recycling in strain-balanced quantum well solar cells grown on distributed Bragg reflectors has been observed as a suppression of the dark current and a change in electroluminescence spectra.Comparing devices grown with and without distributed Bragg reflectors we have demonstrated up to a 33% reduction in the ideality n=1 reverse saturation current. Furthermore, to validate the observations we demonstrate how both the measured dark currents and electroluminescence spectra fit very well to a photon recycling model. Verifying our observations with the model then allows us to calculate optimized device designs.
  • JOURNAL ARTICLE
    Rohr C, Abbott P, Ballard I, Connolly JP, Barnham KWJ, Mazzer M, Button C, Nasi L, Hill G, Roberts JS, Clarke G, Ginige R, Rohr Cet al., 2006,

    InP-based lattice-matched InGaAsP and strain-compensated InGaAs/InGaAs quantum well cells for thermophotovoltaic applications

    , Journal of Applied Physics, Vol: 100, ISSN: 0021-8979

    Quantum well cells (QWCs) for thermophotovoltaic (TPV) applications are demonstrated in the InGaAsP material system lattice matched to the InP substrate and strain-compensated InGaAsInGaAs QWCs also on InP substrates. We show that lattice-matched InGaAsP QWCs are very well suited for TPV applications such as with erbia selective emitters. QWCs with the same effective band gap as a bulk control cell show a better voltage performance in both wide and erbialike emission. We demonstrate a QWC with enhanced efficiency in a narrow-band spectrum compared to a bulk heterostructure control cell with the same absorption edge. A major advantage of QWCs is that the band gap can be engineered by changing the well thickness and varying the composition to the illuminating spectrum. This is relatively straightforward in the lattice-matched InGaAsP system. This approach can be extended to longer wavelengths by using strain-compensation techniques, achieving band gaps as low as 0.62 eV that cannot be achieved with lattice-matched bulk material. We show that strain-compensated QWCs have voltage performances that are at least as good as, if not better than, expected from bulk control cells. © 2006 American Institute of Physics.
  • JOURNAL ARTICLE
    Bushnell D, Tibbits T, Barnham K, Connolly J, Mazzer M, Ekins-Daukes N, Roberts J, Hill G, Airey Ret al., 2005,

    Effect of well number on the performance of quantum-well solar cells

    , Journal of Applied Physics, Vol: 97

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