Hybrid 3D Finite difference time domain-Monte Carlo ray tracing (FDTD-MCRT) algorithm has been developed to model and optimise small and large scale plasmonically-enhanced luminescent solar concentrator (pLSC) devices for photovoltaic (PV) applications. The configuration parameters (for example, dimensions, shape, and optical properties of metal nanoparticles, luminescent species, and host material) were used to characterise the probability of optical energy transfer and loss processes, as well as reflection, refraction, absorption, emission enhancement, and total internal reflection (TIR) in the pLSC. The algorithm was validated through modelling of various doping concentrations of CdSe/ZnS quantum dots (QD) and gold nano spheres (Au NS) where ∼50% enhancement in optical conversion efficiency (OCE) was observed for a plasmonic composite of 2 ppm Au NS and 0.008 wt. % QD.

Luminescent concentrators have the advantages that they are inexpensive, tracking is unnecessary and both direct and diffuse radiation can be collected. The development of dye-doped concentrators has been limited by the dye stability and the requirements of suitable absorption spectrum, red-shift and high quantum efficiency (QE). We have recently proposed a new approach, the quantum dot concentrator (QDC), in which the dyes are replaced by quantum dots (QDs). QDs are nanometre-sized crystallite semiconductors. The first advantage is that the absorption threshold can be tuned simply by choice of dot diameter. Secondly, high luminescence QE has been observed. Thirdly, being composed of crystalline semiconductor, the dots should be more stable. We describe the application of a thermodynamic model to the QDC which illustrates a further advantage that the red-shift is quantitatively related to the spread of QD sizes, which in turn can be determined during the growth process. We have extended this model to allow for the effects of re-absorption. We find that the overlap of absorption and luminescence spectra has a strong effect on the system efficiency, even in the case of unit QE. We argue that this was a major factor in the disappointing performance of early dye concentrators.

The optimization of optical coupling between photovoltaic (PV) solar cells and luminescent devices such as luminescent solar concentrators (LSC) and luminescent downshifting (LDS) layers is important and can affect their performance significantly. An LSC of 60 × 60 × 3 mm and LDS of 100 × 100 × 0.01 mm both doped with CdSe/ZnS quantum dots (QDs) and coupled to PV solar cells have been modelled. The performance and optical coupling sensitivity of these luminescent PV (LPV) devices were studied by changing the air gap thickness (G) between the luminescent layer and PV solar cell using a Monte Carlo Ray-Tracing (MCRT) algorithm. The host materials were epoxy and poly (methyl methacrylate) (PMMA) polymers for the LSC and LDS, respectively, with a refractive index of 1.5. LPV devices were irradiated by standard AM1.5 global solar radiation. The highest optical efficiency and solar concentration ratio obtained for the LSC device were 2.8% and 56% respectively. Both were decreased to 2.2% and 43% when G was increased from 0 to 0.5 mm. For G = 0.5–2 mm, the optical efficiency and solar concentration ratio decreased to 1.85% and 36%, respectively. In the LDS device, an optical efficiency of 82% was achieved when there was no air gap between the luminescent device and the PV solar cell. Efficiency dropped to 76% when G increased to 0.1 mm and further decreased to 66% for G = 2 mm. The total performance deduction (ΔP) was respectively 37% and 19% for LSC and LDS when G increased from 0 to 2 mm which showed that the LSC was more sensitive than the LDS to optical coupling.

Photon gain, which indicates the breakdown of the Principle of Detailed Balance, has been observed in radiatively dominated quantum well solar cells (QWSCs) at illumination levels consistent with around 200x concentration. Evidence comes from three types of experiment. The radiative dark-current extracted from a short-circuit current versus open circuit voltage plot becomes steeper than expected for radiative recombination. The photoluminescence and electro-luminescence spectra from the lowest exciton broaden as illumination increases. We report evidence demonstrating that photon gain results from hot-carrier recombination. Our results support the hypothesis that hot carriers are responsible for the reduced radiative recombination in QWSCs and the voltage enhancement observed in triple-junction QWSCs.

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 MQWshave 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.

The GaAs/AlGaAs quantum well solar cell (QWSC) shows promise as a novel approach to higher efficiency solar cells but suffers from a poor short circuit current Jsc. We report on efforts to reduce this problem with the use of compositional grading and back surface mirroring. We present experimental quantum efficiency (QE) data on a range of compositionally graded QWSCs and devices in which the back surface of the cell is coated with a mirror, increasing the optical thickness of the quantum well layer in the long wavelength range. The experimental QE spectra are reproduced by a model which deals with arbitrary compositional profiles and optical cavities formed in the mirrored cells. The model is used to design an optimised QWSC, and projected Jsc values given. Applications including II-VI and tandem solar cells are considered.

The GaAs/AlGaAs materials system is well suited to multi-bandgap applications such as the multiple quantum well solar cell. GaAs quantum wells are inserted in the undoped AlGaAs active region of a pin structure to extend the absorption range while retaining a higher open circuit voltage than would be provided by a cell made of the well material alone. Unfortunately aluminium gallium arsenide (AlGaAs) suffers from poor transport characteristics due to DX centres and oxygen contamination during growth, which degrade the spectral response. We investigate three mechanisms for improving the spectral response of the MQW solar cell while an experimental study of the open circuit voltage examines the voltage enhancement. An optimised structure for a high efficiency GaAs/AlGaAs solar cell is proposed.

We present a novel way to utilize high-concentration photovoltaic (HCPV) radiative losses and diffuse light, otherwise unused in conventional HCPV systems, to power an Imperial College designed photoelectrochemical reactor (PECR) producing H2 fuel through water splitting. A high efficiency photovoltaic (HEPV) is embedded inside a Luminescent Solar Concentrator (LSC). Edge emission from the radiative recombination loss mechanism in the HEPV is guided within the LSC to the PECR photocathode, whilst the LSC emitted light is guided to the photoanode. The photon streams can be independently optimised in intensity and wavelength. We demonstrate how photon streams with balanced intensity can be achieved.

Multiple-quantum-wells (MQW) in multijunction solar cells allow the absorption edges of the subcells to be tuned individually to maximize the energy yields. In this paper, we present the properties a quantum well material for InGaP quantum well solar cells and the performances of the dual-junction dual-MQW devices. We demonstrate that this dual-junction device can achieve 28% efficiency at 100 suns, which can be further improved by reducing the series resistance in the device.

Luminescent Solar Concentrators (LSCs) offer a way of making Photovoltaic (PV) systems more attractive through reduced energy costs, the possibility of application in cloudy regions, and improved building integration. LSCs collect light over a large area and concentrate it, both spatially and spectrally, onto solar cells at the edges of the device, such that the total cell area required to generate a specific power is reduced. Since the solar cells constitute the more expensive component in the system, this leads to cost reductions. Unlike conventional geometric concentrators, LSCs do not require solar tracking and can collect diffuse as well as direct sunlight. The current research challenges lie in increasing the efficiency of the LSC and extending it to larger areas to make it commercially viable. In this chapter, the authors outline the mode of operation of the LSC, with particular regard to cost considerations and device geometry. They then review recent approaches aiming to increase device efficiency and, finally, introduce their versatile raytrace approach to modeling the LSC. The model is utilised here to investigate tapered LSC designs and rationalise the optimal geometry and configuration for planar LSCs.

The beneficial results of the exponential expansion of photovoltaic installations in Germany and Italy are discussed. Remarkable falls in the peak price of electricity have been observed in both countries. The reasons are discussed in the light of the data from the Kombikraftwerk project. This has demonstrated, in a scaled, real-time experiment, how the demand on the German grid can be met by photovoltaics and wind with back-up from biogas and (pumped hydro) storage. We discuss the implications of the fall in price of photovoltaic cells particularly for 3rd generation technology. Using the specific example of the UK, we demonstrate the advantages of the complementary nature of wind and photovoltaic resources. We demonstrate that the wind and photovoltaic capacity targets for an all renewably powered UK are likely to be significantly lower than in Germany. We conclude by summarising the evidence in favour of a moratorium on all new electricity generation other than by the renewables.

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 In0.01GaAs 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.

We have developed a practical program for assessing the best shape of a luminescent solar concentrator, considering its possible size and orientation constraints. We show that LSC output is highly orientation dependent and that both device reflectance and exposure should be optimised.

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.

The use of nanostructures has been shown to provide practical performance enhancements to high-efficiency III-V based solar cells by permitting sub-bandgap tuneable absorption. Nanostructures present a fertile ground for new solar cell technologies, and an improved understanding of fundamental processes may even lead to functional intermediate band and hot-carrier devices. As the fundamental processes occurring in nanostructured solar cells are complex and not easily observable, the study of such devices often requires the analysis of data derived from experimental characterisation techniques using computer models. Models exist for many individual aspects of these nanostructured solar cells, but as yet no comprehensive modelling solution exists. We report on our progress to produce an extendable abstract modelling framework written in the high-level programming language Python. The framework is intended for deployment both as back-end to a variety of interfaces for specialised modelling purposes, and as a library of methods and classes for use at source-code level, allowing adaptation to a wide variety of research problems. Significant code abstraction, such as sequestering complex materials parameterisation behind a simple material object allows simple scripts to do complex work. Modules underway cover several device simulation tiers, including fundamental processes such as quantum well and dot absorption and recombination, as well as device level simulations such as spatial bias mapping using equivalent circuits and multijunction IV characteristics. These simulations correlate with and derive experimental data from characterisation techniques including spatially and temporally resolved electro- and photoluminescence spectroscopy, fourier-transform infrared spectroscopy, and others.

High efficiency quantum well GaAs solar cells have been successfully applied in commercial multijunction concentrator cells to increase the absorption in the infrared and provide variability of the absorption edge to optimise energy harvesting. Multiple quantum well (MQW) top cells can further improve the performance of multijunction solar cells since the absorption edge of top and middle subcells can be tuned with the MQWs to maximize the efficiency. Also, our simulations show that photon coupling resulting from the radiative dominance of the MQW top cell can make the multijunction cell less sensitive to variations in the incoming spectrum, thus further improving energy harvesting. New results on the characterisation of a novel MQW top cell will be presented along with electro- and photo-luminescence studies relevant to the photonic coupling.

The role of hot carriers in enhancing the radiation resistance of GaAs solar cells has been investigated. The laser-pulse induced, short-circuit current response method was used to study solar cell degradation caused by radiation damage. Samples were subject to radiation doses in the range 1×1014–4×1016electroncm−2 and then probed with laser pulses with 7ns duration and 3.7eV energy. We have developed a non-stationary theory of minority carrier flow in the active region of the solar cell. The theory allows a description of the temporal evolution of the short-circuit current and the dependence on irradiation dose. The model agrees well with experimental results using a single fitting parameter. This parameter is the carrier capture cross-section of radiation center E5 in the p-region emitter of the solar cell. The value of the cross-section was determined to be 0.1×10−12cm2 from the results under non-stationary condition. This is seven times lower than that deduced from the current–voltage characteristics in the dark for a similar solar cell. The difference can be explained by a strong reduction of capture cross-section with increased carrier energy. Our results suggest that the observed cross-section reduction is caused by carrier accumulation at energies comparable with the optical phonon energy.

In developed countries 60% of the electricity consumed is attributable to commercial and public buildings. Even in the UK, the solar energy incident on buildings is more than 7x the electrical energy they consume. This represents a problem (the management of solar heat gain and glare) but also an opportunity that may be taken advantage of using complementary concentrator technologies. We are investigating conventional geometric and luminescent concentrators that may be combined to optimally harvest the direct and diffuse components of sunlight within a double glazed window unit. The conventional geometric concentrator employs an array of linear Fresnel lenses in the form of a power generating blind focusing the light onto light-bars, which serve as secondary concentrators, to couple the light to high efficiency solar cells located in the window frame. The modules resemble Venetian blinds as illustrated in Figure 1, which eliminate the direct component of sunlight, but also provide a view through the window. In addition they transmit a significant fraction of the diffuse sunlight, which has been scattered in the atmosphere and is incident over a wide range of angles, for glare-free natural interior illumination, even when the sun faces the window. This eliminates the need for interior lights when the blind is working. Furthermore, these solar tracking, transparent, building integrated solar concentrators (BISCs) can be considered as highly effective ““solar blinds”” since they shield the interior from direct sunlight, which is converted to electricity, thereby protecting the building from excessive heating and reducing the need for expensive air-conditioning. However, this concentrator can only harvest the direct component of sunlight and it has long been recognised that in many developed countries the diffuse component can be greater than the direct [1]. The highest building integrated photovoltaic power generation density will therefore be achieved by making optimal use of both the diffuse and direct components of sunlight. The conventional BISCs described above only harvest the direct sunlight and we have therefore developed a transparent luminescent solar concentrator (LSC), see e.g. [2], which can be added to the window glass to harvest the significant short wavelength fraction (< 450nm) of the diffuse sunlight to which the eye is insensitive. A prototype is illustrated in Figure 2. We have developed ray-tracing software, which utilizes constructive solid geometry to allow the interrogation of complex geometries, to optimise the designs of the light-bar and transparent LSC. Initial results suggest that optical efficiencies around 50% are achievable for the light-bar and that the combined system can achieve power conversion efficiencies of over 25% under standard AM1.5g illumination. It is expected that the full integrated system would produce 2 to 3 times the electricity of current building integrated PV systems based on 2 generation thin or semitransparent a-Si or CIS cells [3], in addition to making more effective use of the solar spectrum. Our calculations suggest that the system would enable a typical office building behind a south facing wall, even in London, to be electrically self sufficient for much of the year. NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7138-6 Vol. 3, 2011 669 Figure 1. Impression of a BISC Venetian blind Figure 2. A prototype transparent LSC References [1] A Goetzberger, ““Fluorescent Solar Energy Collectors: Operating Conditions with Diffuse Light””, Appl. Phys. 16, 399, (1978). [2] W G J H M Van Sark, K W J Barnham, L H Sloof, A J Chatten, A Büchtemann et al., ““Luminescent Solar Conentrators –– A Review of Recent Results””, Optics Express 16, 21773 (2008). [3] K W J Barnham, M Mazzer and B Clive, ““Resolving the energy crisis: nuclear or photovoltaics?””, Nature Materials, 5, 161, (2006). NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7138-6 Vol. 3, 2011 670