Publications
293 results found
Pusch A, Ekins-Daukes NJ, 2019, Voltage Matching, Etendue, and Ratchet Steps in Advanced-Concept Solar Cells, PHYSICAL REVIEW APPLIED, Vol: 12, ISSN: 2331-7019
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- Citations: 16
Steiner MA, Barraugh CD, Aldridge CW, et al., 2019, Photoelectrochemical water splitting using strain-balanced multiple quantum well photovoltaic cells, SUSTAINABLE ENERGY & FUELS, Vol: 3, Pages: 2837-2844, ISSN: 2398-4902
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- Citations: 11
Alonso Alvarez D, Weiss C, Fernandez J, et al., 2019, Assessing the operating temperature of multi-junction solar cells with novel rear side layer stack and local electrical contacts, Solar Energy Materials and Solar Cells, Vol: 200, ISSN: 0927-0248
Sub-bandgap sunlight provides a source of heat generation in solar cells that is detrimental to performance, especially in space applications where heat dissipation is limited. In this work we assess the impact that an advanced rear-side contact scheme for multi-junction solar cells has on the cell temperature. Our results show that this scheme reduces the optical power absorption below the bandgap of germanium by 81% compared to a standard, full metallization design. Measurements of the electrical and thermal power fluxes performed in vacuum demonstrate that this lower near-infrared light absorption results in 8% less heat dissipated in the cell with the novel rear-side contact scheme when operating at 25 ºC. Modelling of the operating temperature for both cells when fully encapsulated with glass indicates that this effect will also result in a reduction of the operating temperature of 9 ºC for the novel design.
Cui X, Sun K, Huang J, et al., 2019, Cd-Free Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cell with an efficiency greater than 10% enabled by Al<sub>2</sub>O<sub>3</sub> passivation layers, ENERGY & ENVIRONMENTAL SCIENCE, Vol: 12, Pages: 2751-2764, ISSN: 1754-5692
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- Citations: 85
Ekins-Daukes N, Kay M, 2019, Brighten the dark skies, NATURE ENERGY, Vol: 4, Pages: 633-634, ISSN: 2058-7546
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- Citations: 3
Alonso Alvarez D, Augusto A, Pearce P, et al., 2019, Thermal emissivity of silicon heterojunction solar cells, Solar Energy Materials and Solar Cells, Vol: 201, Pages: 1-7, ISSN: 0927-0248
The aim of this work is to evaluate whether silicon heterojunction solar cells, lacking highly emissive, heavily doped silicon layers, could be better candidates for hybrid photovoltaic thermal collectors than standard aluminium-diffused back contact solar cells. To this end, the near and mid infrared emissivity of full silicon heterojunction solar cells, as well as of its constituent materials – crystalline silicon wafer, indium tin oxide, n-, i- and p-type amorphous silicon – have been assessed by means of ellipsometry and FTIR. The experimental results show that the thermal emissivity of these cells is actually as high as in the more traditional structures, ~80% at 8 μm. Detailed optical modelling combining raytracing and transfer matrix formalism shows that the emissivity in these cells originates in the transparent conductive oxide layers themselves, where the doping is not high enough to result in a reflection that exceeds the increased free carrier absorption. Further modelling suggests that it is possible to obtain lower emissivity solar cells, but that a careful optimization of the transparent conductive layer needs to be done to avoid hindering the photovoltaic performance.
Dimmock JAR, Kauer M, Wu J, et al., 2019, A metallic hot-carrier photovoltaic device, SEMICONDUCTOR SCIENCE AND TECHNOLOGY, Vol: 34, ISSN: 0268-1242
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- Citations: 6
Jiang Y, Keevers MJ, Pearce P, et al., 2019, Design of an intermediate Bragg reflector within triple-junction solar cells for spectrum splitting applications, Solar Energy Materials and Solar Cells, Vol: 193, Pages: 259-269, ISSN: 0927-0248
We investigate the use of distributed Bragg reflectors (DBRs) within triple-junction solar cells (TJSC) for spectrum splitting photovoltaics. An optical model of a lattice-matched (LM) GaInP/GaInAs/Ge TJSC with intermediate DBR is developed, in good agreement with measured reflectance. By modifying the DBR layer number, composition and thickness to broaden the reflectance band, we show that a DBR can provide suitable 900–1050 nm reflectance for spectrum splitting from the LM TJSC to a Si cell, resulting in a more efficient 4-junction receiver. For better practicality and cost effectiveness, we propose that the buffer layers in metamorphic (MM) TJSCs could additionally function as a DBR for spectrum splitting applications. We propose several DBR designs to achieve a suitable spectrum-splitting reflectance band from MM TJSCs to a Si cell, again resulting in a more efficient 4-junction receiver. Finally, we show that our intermediate DBR approach to spectrum splitting has the advantage of a greatly reduced angle-of-incidence dependence compared to a discrete dielectric filter.
Pusch A, Pearce P, Ekins-Daukes N, 2019, Analytical expressions for the efficiency limits of radiatively coupled tandem solar cells, IEEE Journal of Photovoltaics, Vol: 9, Pages: 679-687, ISSN: 2156-3381
The limiting efficiency for series-connected multijunction solar cells is usually calculated from the assumption that the individual junctions are optically isolated. Here, we develop an analytical formalism to predict efficiencies attainable in the presence of luminescent coupling, i.e. if the individual junctions in a series-connected multi-junction stack are coupled optically, so that luminescence from one junction can be absorbed by the lower bandgap junction below. The formalism deals with non-radiative recombination through the definition of the luminescence extraction efficiency. Using our general formalism we find that the limiting efficiency of a tandem cell becomes much less dependent on exact bandgap combination when luminescent coupling is considered and proceed to consider two technologically important examples of current-mismatched tandem solar cells. We find that a series-connected GaAs on silicon tandem cell can be more efficient than the underlying silicon cell alone, if the luminescence extraction efficiency of the GaAs junction is sufficient. An analysis of luminescent coupling in a perovskite on silicon tandem cell shows that the efficiency penalty for a perovskite bandgap below the optimum value can be mitigated if the luminescence extraction efficiency is high. We suggest that material quality and stability might be more important considerations for perovskite on silicon tandems than engineering the bandgap to achieve precise current matching.
Guarracino I, Freeman J, Ramos A, et al., 2019, Systematic testing of hybrid PV-thermal (PVT) solar collectors in steady-state and dynamic outdoor conditions, Applied Energy, Vol: 240, Pages: 1014-1030, ISSN: 0306-2619
Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of ∼8 min is measured for a commercial PVT module, compared to <
Kamath HG, Ekins-Daukes NJ, Araki K, et al., 2019, The potential for concentrator photovoltaics: A feasibility study in India, PROGRESS IN PHOTOVOLTAICS, Vol: 27, Pages: 316-327, ISSN: 1062-7995
Pearce P, Mellor A, Ekins-Daukes N, 2019, The importance of accurate determination of optical constants for the design of nanometallic light-trapping structures, Solar Energy Materials and Solar Cells, Vol: 191, Pages: 133-140, ISSN: 0927-0248
The optical constants of many metals commonly used in solar cells, e.g. as contacts, rear side planar reflectors, or more complex nanopatterned light-trapping structures, can vary depending on deposition method, thickness and other factors, and as such are not documented consistently in the literature. In the case of nanometallic light-trapping structures specifically designed to improve absorption in a solar cell, the choice of optical constants used in simulations significantly affects the predicted enhancement, as well as the structure's optimal dimensions. The trade-off between coupling into guided modes in the photovoltaic material and the number of photons absorbed parasitically in the metal leads to small differences in the optical constants giving significantly different results for the quantum efficiency and photogenerated current. This work documents several optical constant sources for silver, aluminium, gold and titanium, and the effect this has on plasmon quality factors. The effect of choosing different optical constant sources on modelling outcomes is quantified by considering the optimization of a test structure comprising a grid of metal nanodisks on the front surface of a thinned-down GaAs cell. Finally, we define a new spectrally-integrated figure of merit for comparing the expected performance of metals in light-trapping structures based on their optical constants, which we name the spectral absorption enhancement factor (SAEF).
Kamath HG, Ekins-Daukcs NJ, Araki K, et al., 2019, Performance Analysis and Fault Diagnosis Method for Concentrator Photovoltaic Modules, IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 9, Pages: 424-430, ISSN: 2156-3381
Pusch A, Ekins-Daukes NJ, 2019, The role of etendue and ratchets in spectral conversion and multi-color emission
Spectral conversion can increase solar cell efficiency. We explain the thermo-dynamics of spectral conversion and the role of etendue and electronic ratchets. We also discuss the potential of spectral converters for tunable-color light emitting diodes.
Reinders A, Ekins-Daukes N, Schmidt T, et al., 2019, Designing with Luminescent Solar Concentrator Photovoltaics, IEEE 46th Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 221-226, ISSN: 0160-8371
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- Citations: 1
Cleveland ER, Kotulak N, Tomasulo S, et al., 2019, Interstitial Light Trapping and Optical Confinement in Multijunction Solar Cells, IEEE 46th Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 2826-2828, ISSN: 0160-8371
Pusch A, Ekins-Daukes NJ, 2019, Thinking Beyond Limiting Efficiencies Of Advanced Concepts: Design Rules And Material Requirements For Realistic Devices, IEEE 46th Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 8-11, ISSN: 0160-8371
Lehmann AG, Ekins-Daukes N, Keevers M, 2019, Numerical flowline concentrator design in 3D, Conference on Nonimaging Optics - Efficient Design for Illumination and Solar Concentration XVI, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X
Pearce P, Wilson T, Johnson A, et al., 2018, Characterization of SiGeSn for use as a 1 eV sub-cell in multi-junction solar cells, 7th IEEE World Conference on Photovoltaic Energy Conversion (WCPEC) / A Joint Conference of 45th IEEE PVSC / 28th PVSEC / 34th EU PVSEC, Publisher: IEEE, Pages: 0943-0948
Four-junction solar cells require a sub-cell which absorbs across a 1 eV transition for optimal performance. Due to a lack of available lattice-matched materials with the correct bandgap, current high-efficiency 4J devices use lattice-mismatched sub-cells, complicating the fabrication process. Group IV ternary SiGeSn alloys are a promising material system for achieving a lattice-matched material with a 1 eV direct transition, with functional devices having already been demonstrated. However, further investigation of the fundamental properties of relevant SiGeSn alloys is key to fabricating an efficient 4J device. Results from steady-state photoluminescence and spectroscopic ellipsometry are presented for three different compositions compositions of SiGeSn grown lattice-matched to Ge/GaAs on GaAs substrates. The results show the expected blueshift in the fundamental indirect gap, measured through photoluminescence, and the lowest indirect gap around 1 eV, calculated through analysis of the ellipsometry data. The higher energy transitions also show the expected shifts.
Pearce P, Mellor A, Ekins-Daukes N, 2018, Quantifying parasitic losses from metal scattering structures in solar cells: How uncertainty in optical constants affects simulation results, 7th IEEE World Conference on Photovoltaic Energy Conversion (WCPEC) / A Joint Conference of 45th IEEE PVSC / 28th PVSEC / 34th EU PVSEC, Publisher: IEEE, Pages: 2918-2923, ISSN: 0160-8371
The optical constants of many metals commonly used in solar cells, e.g. as contacts or for light trapping structures, are not documented consistently in the literature, with different sources giving different values. In the case of metallic structures designed to improve absorption in a solar cell junction, the use of data from different sources can give strongly varying results for the effectiveness of nanophotonic light-trapping structures. The trade-off between diffraction into more oblique orders in the junction, enhancing absorption in the photovoltaic material, and the number of photons absorbed parasitically in the metal means small differences in the optical constants can lead to different very conclusions about the EQE and J SC . This work documents the different optical constants for silver, aluminium, gold and titanium from several sources, the effect this has on plasmon quality factors, and quantifies the effect on modelling outcomes by considering the optimization of a test structure using a grid of metal nanodisks on the front surface of a thinned-down GaAs cell. Finally, we consider the effect for a structure previously predicted to give a very high J SC for a solar cell with an ultra-thin GaAs layer.
Pearcef P, Wilsonf T, Johnson A, et al., 2018, Characterization of SiGeSn for Use as a 1 eV Sub-Cell in Multi-Junction Solar Cells, Pages: 943-948
Four-junction solar cells require a sub-cell which absorbs across a 1 eV transition for optimal performance. Due to a lack of available lattice-matched materials with the correct bandgap, current high-efficiency 4J devices use lattice-mismatched sub-cells, complicating the fabrication process. Group IV ternary SiGeSn alloys are a promising material system for achieving a lattice-matched material with a 1 eV direct transition, with functional devices having already been demonstrated. However, further investigation of the fundamental properties of relevant SiGeSn alloys is key to fabricating an efficient 4J device. Results from steady-state photoluminescence and spectroscopic ellipsometry are presented for three different compositions compositions of SiGeSn grown lattice-matched to Ge/GaAs on GaAs substrates. The results show the expected blueshift in the fundamental indirect gap, measured through photoluminescence, and the lowest indirect gap around 1 eV, calculated through analysis of the ellipsometry data. The higher energy transitions also show the expected shifts.
Pusch A, Hylton NP, Ekins-Daukes NJ, 2018, Comparison of possible realizations of quantum ratchet intermediate band solar cells, Pages: 1841-1844
Three fundamentally different approaches for the realization of a quantum ratchet intermediate band solar cell are compared. The quantum ratchet is a mechanism by which an energy loss of excited electrons is turned into an improvement in the efficiency of intermediate band solar cells by reducing carrier recombination in the system. The main requirement for this is to engineer forbidden transitions which can be achieved through vanishing spatial overlap of the wave-functions, spinselection rules or a momentum mismatch.
Riverola A, Mellor A, Alvarez DA, et al., 2018, Experimental and theoretical study of the infrared emissivity of crystalline silicon solar cells, IEEE 44th Photovoltaic Specialist Conference (PVSC), Publisher: IEEE, Pages: 1339-1341, ISSN: 0160-8371
Mellor A, Alonso Alvarez D, Guarracino I, et al., 2018, Roadmap for the next-generation of hybrid photovoltaic-thermal solar energy collectors, Solar Energy, Vol: 174, Pages: 386-398, ISSN: 0038-092X
For hybrid photovoltaic-thermal collectors to become competitive with other types of solar energy converters, they must offer high performance at fluid outlet temperatures above 60 °C, as is required for space heating and domestic hot water provision, which together account for nearly 50% of heat demand. A roadmap is presented of the technological advances required to achieve this goal. Strategies for reducing convective, radiative and electrical losses at elevated temperature are discussed, and an experimental characterisation of a novel transparent low-emissivity coating for photovoltaic solar cells is presented. An experimentally-validated simulation formalism is used to project the performance of different combinations of loss-reduction strategies implemented together. Finally, a techno-economic analysis is performed to predict the price points at which the hybrid technologies along the roadmap become competitive with non-hybrid photovoltaic and solar thermal technologies. The most advanced hybrid technology along the roadmap employs an evacuated cavity, a transparent low-emissivity coating, and silicon heterojunction photovoltaic cells.
France RM, Espinet-Gonzalez P, Ekins-Daukes NJ, et al., 2018, Multijunction Solar Cells With Graded Buffer Bragg Reflectors, IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 8, Pages: 1608-1615, ISSN: 2156-3381
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- Citations: 14
Alonso Alvarez D, Wilson T, Pearce P, et al., 2018, Solcore: a multi-scale, Python-based library for modelling solar cells and semiconductor materials, Journal of Computational Electronics, Vol: 17, Pages: 1099-1123, ISSN: 1569-8025
Computational models can provide significant insight into the operation mechanisms and deficiencies of photovoltaic solar cells. Solcore is a modular set of computational tools, written in Python 3, for the design and simulation of photovoltaic solar cells. Calculations can be performed on ideal, thermodynamic limiting behaviour, through to fitting experimentally accessible parameters such as dark and light IV curves and luminescence. Uniquely, it combines a complete semiconductor solver capable of modelling the optical and electrical properties of a wide range of solar cells, from quantum well devices to multi-junction solar cells. The model is a multi-scale simulation accounting for nanoscale phenomena such as the quantum confinement effects of semiconductor nanostructures, to micron level propagation of light through to the overall performance of solar arrays, including the modelling of the spectral irradiance based on atmospheric conditions. In this article, we summarize the capabilities in addition to providing the physical insight and mathematical formulation behind the software with the purpose of serving as both a research and teaching tool.
Yan C, Huang J, Sun K, et al., 2018, Cu2ZnSnS4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment, NATURE ENERGY, Vol: 3, Pages: 764-772, ISSN: 2058-7546
Richards RD, Harun F, Cheong JS, et al., 2018, GaAsBi: an alternative to InGaAs based multiple quantum well photovoltaics, Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 1135-1137
A series of GaAsBi/GaAs multiple quantum well p-i-n diodes are characterized using IV, photocurrent and illuminated IV measurements. The results are compared to an InGaAs/GaAsP multiple quantum well control device of a design that has demonstrated excellent performance in triple junction photovoltaics. The extended absorption of the GaAsBi/GaAs devices, compared to that of the InGaAs/GaAsP device, suggests that GaAsBi/GaAs could present a viable alternative to InGaAs/GaAsP for quad junction photovoltaics.
Wilson T, Hylton NP, Harada Y, et al., 2018, Assessing the nature of the distribution of localised states in bulk GaAsBi, Scientific Reports, Vol: 8, ISSN: 2045-2322
A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented usingpower and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift inthe peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrierlocalisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to thefilling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperaturepresents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore,a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates thepresence of two Gaussian-like distributions making up the density of localised states. These components are attributed to thepresence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formationof Bi related point defects, resulting from low temperature growth.
Donchev V, Milanova M, Asenova I, et al., 2018, Effect of Sb in thick InGaAsSbN layers grown by liquid phase epitaxy (vol 483, pg 140, 2018), JOURNAL OF CRYSTAL GROWTH, Vol: 486, Pages: 178-178, ISSN: 0022-0248
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