45 results found
Hu Y, Spies LM, Alonso-Alvarez D, et al., 2018, Identifying and controlling phase purity in 2D hybrid perovskite thin films, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 6, Pages: 22215-22225, ISSN: 2050-7488
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.
Abu Hamed T, Adamovic N, Aeberhard U, et al., 2018, Multiscale in modelling and validation for solar photovoltaics, EPJ Photovoltaics, Vol: 9, ISSN: 2105-0716
Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.
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.
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
Donchev V, Milanova M, Asenova I, et al., 2017, Effect of Sb in thick InGaAsSbN layers grown by liquid phase epitaxy, Journal of Crystal Growth, Vol: 483, Pages: 140-146, ISSN: 0022-0248
Dilute nitride InGaAsSbN layers grown by low-temperature liquid phase epitaxy are studied in comparison with quaternary InGaAsN layers grown at the same growth conditions to understand the effect of Sb in the alloy. The lattice mismatch to the GaAs substrate is found to be slightly larger for the InGaAsSbN layers, which is explained by the large atomic radius of Sb. A reduction of the band gap energy with respect to InGaAsN is demonstrated by means of photoluminescence (PL), surface photovoltage (SPV) spectroscopy and tight-binding calculations. The band-gap energies determined from PL and ellipsometry measurements are in good agreement, while the SPV spectroscopy and the tight-binding calculations provide lower values. Possible reasons for these discrepancies are discussed. The PL spectra reveal localized electronic states in the band gap near the conduction band edge, which is confirmed by SPV spectroscopy. The analysis of the power dependence of the integrated PL has allowed determining the dominant radiative recombination mechanisms in the layers. The values of the refraction index in a wide spectral region are found to be higher for the Sb containing layers.
Riverola A, Mellor AV, Alonso Alvarez D, et al., 2017, Mid-infrared emissivity of crystalline silicon solar cells, Solar Energy Materials and Solar Cells, Vol: 174, Pages: 607-615, ISSN: 0927-0248
The thermal emissivity of crystalline silicon photovoltaic (PV) solar cells plays a role in determining the operating temperature of a solar cell. To elucidate the physical origin of thermal emissivity, we have made an experimental measurement of the full radiative spectrum of the crystalline silicon (c-Si) solar cell, which includes both absorption in the ultraviolet to near-infrared range and emission in the mid-infrared. Using optical modelling, we have identified the origin of radiative emissivity in both encapsulated and unencapsulated solar cells. We find that both encapsulated and unencapsulated c-Si solar cells are good radiative emitters but achieve this through different effects. The emissivity of an unencapsulated c-Si solar cell is determined to be 75% in the MIR range, and is dominated by free-carrier emission in the highly doped emitter and back surface field layers; both effects are greatly augmented through the enhanced optical outcoupling arising from the front surface texture. An encapsulated glass-covered cell has an average emissivity around 90% on the MIR, and dips to 70% at 10 µm and is dominated by the emissivity of the cover glass. These findings serve to illustrate the opportunity for optimising the emissivity of c-Si based collectors, either in conventional c-Si PV modules where high emissivity and low-temperature operation is desirable, or in hybrid PV-thermal collectors where low emissivity enables a higher thermal output to be achieved.
Milanova M, Donchev V, Kostov KL, et al., 2017, Experimental study of the effect of local atomic ordering on the energy band gap of melt grown InGaAsN alloys, Semiconductor Science and Technology, Vol: 32, ISSN: 0268-1242
We present a study of melt grown dilute nitride InGaAsN layers by x-ray photoelectron spectroscopy (XPS), Raman and photoluminescence (PL) spectroscopy. The purpose of the study is to determine the degree of atomic ordering in the quaternary alloy during the epitaxial growth at near thermodynamic equilibrium conditions and its influence on band gap formation. Despite the low In concentration (~3%) the XPS data show a strong preference toward In–N bonding configuration in the InGaAsN samples. Raman spectra reveal that most of the N atoms are bonded to In instead of Ga atoms and the formation of N-centred In3Ga1 clusters. PL measurements reveal smaller optical band gap bowing as compared to the theoretical predictions for random alloy and localised tail states near the conduction band minimum.
Alonso Alvarez D, Ferre Llin, L LFL, et al., 2017, ITO and AZO films for low emissivity coatings in hybrid photovoltaic-thermal applications, Solar Energy, Vol: 155, Pages: 82-92, ISSN: 0038-092X
We report on the electrical and optical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 μm) to obtain a consistent model for the permittivity of each of the films. The results are then used to design an optimized, single layer, high transparency, high conductivity film, suitable as front transparent electrode and low thermal emissivity coating for silicon based solar cells. The best performance is found using the properties of the ITO film grown at 250 °C, with a state of the art resistivity of 0.2 mΩ cm and an optimized thickness of 75 nm which leads to 0.79 average absorptivity in the solar range (300–2000 nm) and 0.21 average emissivity in the thermal range (5–20 μm). The structural characterization of the films using X-Ray diffraction, and the Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties.
Alonso Alvarez D, Ferre Llin L, Mellor A, et al., 2017, Comparative study of annealed and high temperature grown ITO and AZO films for solar energy applications, MRS Advances, Vol: 2, Pages: 3117-3122, ISSN: 2059-8521
We present the optical and electrical properties of ITO and AZO films fabricated directly on silicon substrates under several growth and annealing temperatures, as well as their potential performance when used as low emissivity coatings in hybrid photovoltaic-thermal systems. We use broadband spectroscopic ellipsometry measurements (from 300 nm to 20 μm) to obtain a consistent model for the permittivity of each of the films. The best performance is found using the properties of the ITO film grown at 250 °C, with a state of the art resistivity of 0.2 mΩ-cm and an optimized thickness of 75 nm which leads to an estimated 50% increase in the extracted power compared to a standard diffused silicon solar cell. The Hall mobility and resistivity measurements of all the films are also provided, complementing and supporting the observed optical properties.
Lee K-H, Barnham KWJ, Roberts JS, et al., 2017, Investigation of carrier recombination dynamics of InGaP/InGaAsP multiple quantum wells for solar cells via photoluminescence, IEEE Journal of Photovoltaics, Vol: 7, Pages: 817-821, ISSN: 2156-3381
The carrier recombination dynamics of InGaP/InGaAsP quantum wells is reported for the first time. By studying the photoluminescence (PL) and time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well (MQW) heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very low nonradiative recombination rate and high radiative efficiency compared with the control InGaP sample. Along with the analyses of PL emission spectrum and external quantum efficiencies, it suggests that this is due to small confinement potentials in the conduction band but high confinement potentials in the valence band. These results explain several features found in InGaP/InGaAsP MQW solar cells previously.
Donchev V, Asenova I, Milanova M, et al., 2017, Optical properties of thick GaInAs(Sb)N layers grown by liquid-phase epitaxy, Journal of Physics: Conference Series, Vol: 794, ISSN: 1742-6588
We present an experimental and theoretical study of GaInAs(Sb)N layers withthickness around 2 μm, grown by liquid-phase epitaxy (LPE) on n-type GaAs substrates. Thesamples are studied by surface photovoltage (SPV) spectroscopy and by photoluminescencespectroscopy. A theoretical model for the band structure of Sb-containing dilute nitrides isdeveloped within the semi-empirical tight-binding approach in the sp3d5s*sN parameterisationand is used to calculate the electronic structure for different alloy compositions. The SPVspectra measured at room temperature clearly show a red shift of the absorption edge withrespect to the absorption of the GaAs substrate. The shifts are in agreement with theoreticalcalculations results obtained for In, Sb and N concentrations corresponding to theexperimentally determined ones. Photoluminescence measurements performed at 300K and 2 Kshow a smaller red shift of the emission energy with respect to GaAs as compared to the SPVresults. The differences are explained by a tail of slow defect states below the conduction bandedge, which are probed by SPV, but are less active in the PL experiment.
Mellor AV, Guarracino I, Llin LF, et al., 2016, Specially designed solar cells for hybrid photovoltaic-thermal generators, 43rd IEEE Photovoltaic Specialists Conference, Publisher: IEEE
The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low-emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near-infrared part of the solar spectrum for heat generation.
Galleano R, Zaaiman W, Alonso Alvarez D, et al., 2016, Results of the fifth international spectroradiometer comparison for improved solar spectral irradiance measurements and related impact on reference solar cell calibration, IEEE Journal of Photovoltaics, Vol: 6, Pages: 1587-1597, ISSN: 2156-3381
This paper reports on the results of the fifth spectral irradiance measurement intercomparison and the impact these results have on the spread of spectral mismatch calculations in the outdoor characterization of reference solar cell and photovoltaic (PV) devices. Ten laboratories and commercial partners with their own instruments were involved in the comparison. Solar spectral irradiance in clear sky condition was measured with both fast fixed and slow rotating grating spectroradiometers. This paper describes the intercomparison campaign, describes different statistical analysis used on acquired data, reports on the results, and analyzes the impact these results would have on the primary calibration of a c-Si PV reference cell under natural sunlight.
Alonso Alvarez D, Ekins-Daukes N, 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: 2245-4586
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.
Alonso Alvarez D, Ekins-Daukes N, 2016, Photoluminescence-Based Current-Voltage Characterisation Of Individual Subcells In Multi-Junction Devices, IEEE Journal of Photovoltaics, Vol: 6, Pages: 1004-1011, ISSN: 2156-3381
We demonstrate a photoluminescence based,contactless method to determine the current-voltagecharacteristics of the individual subcells in a multi-junctionsolar cell. The method relies upon the reciprocity relationbetween the absorption and emission properties on a solarcell. Laser light with a suitable energy is used to excitecarriers selectively in one junction and the internal voltagesare deduced from the intensity of the resultingluminescence. The IV curves obtained this way on 1J, 2Jand 6J devices are compared to those obtained usingelectroluminescence. Good agreement is obtained at highinjection conditions while discrepancies at low injection areattributed to in-plane carrier transport.
Alonso Alvarez D, Ekins-Daukes N, Quantum wells for high-efficiency photovoltaics, SPIE OPTO 2016: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V
Sugiyama M, Fujii H, Katoh T, et 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: 1099-159X
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.
Alonso Alvarez D, Lackner D, Philipps SP, et 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.
Sugiyama M, Fujii H, Katoh K, et al., 2015, Quantum Wire-on-Well (WoW) Cell With Long Carrier Lifetime for Efficient Carrier Transport, 31st European Photovoltaic Solar Energy Conference and Exhibition, Publisher: European Photovoltaic Solar Energy Conference and Exhibition, Pages: 42-47
A quantum wire-on-well (WoW) structure, taking advantage of the layer undulation of an In- GaAs/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 4 times as compared with a planar reference superlattice. Strained InGaAs/GaAs/GaAsP superlattices were grown on GaAs substrates under exactly the same condition except for the substrate misorientation (0o- and 6o- off). The growth on a 6o-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 wire-on-well 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, and 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.
Thomas T, Mellor A, Hylton NP, et al., 2015, Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell, Semiconductor Science and Technology, Vol: 30, ISSN: 1361-6641
Alonso-Alvarez D, Ross D, Klampaftis E, et al., 2015, Luminescent down-shifting experiment and modelling with multiple photovoltaic technologies, PROGRESS IN PHOTOVOLTAICS, Vol: 23, Pages: 479-497, ISSN: 1062-7995
Toprasertpong K, Fujii H, Thomas T, et al., 2015, Absorption threshold extended to 1.15eV using InGaAs/GaAsP quantum wells for over-50%-efficient lattice-matched quad-junction solar cells, Progress in Photovoltaics, Vol: 24, Pages: 533-542, ISSN: 1099-159X
Alonso-Alvarez D, Klampaftis E, Ross D, et al., 2014, External Thermalization of Carriers With Luminescent Down Shifting for Lower Operating Solar Cell Temperature, IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 4, Pages: 1532-1537, ISSN: 2156-3381
Alonso-Alvarez D, Thomas T, Fuehrer M, et al., 2014, InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells, Applied Physics Letters, Vol: 105, ISSN: 1077-3118
Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs  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  GaAs substrates, of up to 1 μs, 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 weaker 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.
Ross D, Alonso-Alvarez D, Klampaftis E, et al., 2014, The Impact of Luminescent Down Shifting on the Performance of CdTe Photovoltaics: Impact of the Module Vintage, IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 4, Pages: 457-464, ISSN: 2156-3381
Alonso-Alvarez D, Fuehrer M, Thomas T, et al., 2014, Elements of modelling and design of multi-quantum well solar cells, 40th IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 2865-2870
Taboada AG, Llorens JM, Alonso-Alvarez D, et al., 2013, Effect of Sb incorporation on the electronic structure of InAs quantum dots, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950
Alonso-Alvarez D, Alen B, Ripalda JM, et al., 2013, Strain driven migration of In during the growth of InAs/GaAs quantum posts, APL MATERIALS, Vol: 1, ISSN: 2166-532X
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