15 results found
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.
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.
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).
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.
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.
Cleveland ER, Hirst LC, Brittman S, et al., 2018, Enhanced Optical Absorption in an Ultra-thin Textured Solar Cell Using Nanosphere Natural Photolithography, Pages: 2878-2881
© 2018 IEEE. We recently demonstrated an ultra-thin solar cell with increased radiation tolerance as compared to a traditionally thick absorber counterpart. However, as the active region of the device was reduced so was the absorption with respect to state of the art devices. Therefore, we have implemented nanosphere natural photolithography to fabricate ordered and random arrays of micropillar structures leading to enhanced light absorption within an ultra-thin solar cell. We will discuss the fabrication process and illustrate the effectiveness of integrating light trapping structures within an ultra-thin solar cell design.
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.
Pearce P, Slade R, 2018, Feed-in tariffs for solar microgeneration: Policy evaluation and capacity projections using a realistic agent-based model, Energy Policy, Vol: 116, Pages: 95-111, ISSN: 0301-4215
Since 2010, over 700,000 small-scale solar photovoltaic (PV) systems have been installed by households in Great Britain and registered under the feed-in tariff (FiT) scheme. This paper introduces a new agent-based model which simulates this adoption by considering decision-making of individual households based on household income, social network, total capital cost of the PV system, and the payback period of the investment, where the final factor takes into account the economic effect of FiTs. After calibration using Approximate Bayesian Computation, the model successfully simulates observed cumulative and average capacity installed over the period 2010–2016 using historically accurate FiTs; setting different tariffs allows investigation of alternative policy scenarios. Model results show that using simple cost control measures, more installation by October 2016 could have been achieved at lower subsidy cost. The total cost of supporting capacity installed during the period 2010–2016, totalling 2.4 GW, is predicted to be £14 billion, and costs to consumers significantly exceed predictions. The model is further used to project capacity installed up to 2022 for several PV cost, electricity price, and FiT policy scenarios, showing that current tariffs are too low to significantly impact adoption, and falling PV costs are the most important driver of installation.
Barker AJ, Sadhanala A, Deschler F, et al., 2017, Defect-assisted photoinduced halide segregation in mixed-halide perovskite thin films, ACS Energy Letters, Vol: 2, Pages: 1416-1424, ISSN: 2380-8195
Solution-processable metal halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example, in CH3NH3Pb(I1-xBrx)3, 0 < x < 1) is however hampered by the reversible photoinduced formation of sub-bandgap emissive states. We find that ion segregation takes place via halide defects, resulting in iodide-rich low-bandgap regions close to the illuminated surface of the film. This segregation may be driven by the strong gradient in carrier generation rate through the thickness of these strongly absorbing materials. Once returned to the dark, entropically driven intermixing of halides returns the system to a homogeneous condition. We present approaches to suppress this process by controlling either the internal light distribution or the defect density within the film. These results are relevant to stability in both single- and mixed-halide perovskites, leading the way toward tunable and stable perovskite thin films for photovoltaic and light-emitting applications.
Sadhanala A, Ahmad S, Zhao B, et al., 2015, Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications., Nano Lett, Vol: 15, Pages: 6095-6101
Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425-570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1-x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride-bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.
Silverberg GJ, Pearce P, Vecitis CD, 2015, Controlling self-assembly of reduced graphene oxide at the air-water interface: quantitative evidence for long-range attractive and many-body interactions., ACS Appl Mater Interfaces, Vol: 7, Pages: 3807-3815
Industrial-scale applications of two-dimensional materials are currently limited due to lack of a cost-effective and controlled synthesis method for large-area monolayer films. Self-assembly at fluid interfaces is one promising method. Here, we present a quantitative analysis of the forces governing reduced graphene oxide (rGO) assembly at the air-water interface using two unique approaches: area-based radial distribution functions and a theoretical Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction potential for disks interacting edge-to-edge. rGO aggregates at the air-water interface when the subphase ionic strength results in a Debye screening length equal to the rGO thickness (∼1 mM NaCl), which is consistent with the DLVO interaction potential. At lower ionic strengths, area-based radial distribution functions indicate that rGO-rGO interactions at the air-water interface are dominated by long-range (tens of microns) attractive and many-body repulsive forces. The attractive forces are electrostatic in nature; that is, the force is weakened by minor increases in ionic strength. A quantitative understanding of rGO-rGO interactions at the air-water interface may allow for rational synthesis of large-area atomically thin films that have potential for planar electronics and membranes.
Van Hattem B, Corfdir P, Brereton P, et al., 2013, Photoluminescence in tilted magnetic field of triply negatively charged excitons hybridized with a continuum, Acta Physica Polonica A, Vol: 124, Pages: 798-800, ISSN: 0587-4246
We analyse the magneto-photoluminescence of triply negatively charged excitons coupled to a continuum of states. The excitonic complex is confined to a Stranski-Krastanow InAs/GaAs quantum dot embedded in a Schottky diode. different orientations of the magnetic field have been investigated. A modelling of the Coulomb blockade together with the calculation of the electron Fock-Darwin spectrum has allowed us to predict the magnetic fields of anticrossing between the quantum dot energy states and the wetting layer Landau levels. Good agreement between the theoretical model and the experimental results has been obtained.
Van Hattem B, Corfdir P, Brereton P, et al., 2013, From the artificial atom to the Kondo-Anderson model: Orientation-dependent magnetophotoluminescence of charged excitons in InAs quantum dots, Physical Review B - Condensed Matter and Materials Physics, Vol: 87, ISSN: 1098-0121
We present a magnetophotoluminescence study on neutral and charged excitons confined to InAs/GaAs quantum dots. Our investigation relies on a confocal microscope that allows arbitrary tuning of the angle between the applied magnetic field and the sample growth axis. First, from experiments on neutral excitons and trions, we extract the in-plane and on-axis components of the Landé tensor for electrons and holes in the s shell. Then, based on the doubly negatively charged exciton magnetophotoluminescence, we show that the p-electron wave function spreads significantly into the GaAs barriers. We also demonstrate that the p-electron g factor depends on the presence of a hole in the s shell. The magnetic field dependence of triply negatively charged excitons photoluminescence exhibits several anticrossings, as a result of coupling between the quantum dot electronic states and the wetting layer. Finally, we discuss how the system evolves from a Kondo-Anderson exciton description to the artificial atom model when the orientation of the magnetic field goes from Faraday to Voigt geometry. © 2013 American Physical Society.
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