Amanda J. Chatten

Luminescent Solar Concentrators - A review of recent results

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp. This paper reviews results mainly obtained within the framework of the Full-spectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

A new approach to modelling quantum dot concentrators

Luminescent collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We have recently proposed a novel concentrator in which the dyes are replaced by quantum dots (QDs). Advantages over dyes include that the absorption threshold can be tuned by choice of dot diameter, and that the red shift between absorption and luminescence is related to the spread of dot sizes. In this paper we discuss how we have developed a self-consistent thermodynamic model for planar concentrators which allows for re-absorption by the QDs.

Quantum dot solar concentrators

The luminescent properties of core-shell quantum dots are being exploited in an unconventional solar concentrator, which promises to reduce the cost of photovoltaic electricity. Luminescent solar collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We present experimental and theoretical results with a novel concentrator in which the dyes are replaced by quantum dots. We have developed a self-consistent thermodynamic model for planar concentrators and find that this three-dimensional flux model shows excellent agreement with experiment.

Luminescent solar concentrators with fiber geometry

The potential of a fibre luminescent solar concentrator has been explored by means of both analytical and ray-tracing techniques. Coated fibres have been found to be more efficient than homogeneously doped fibres, at low absorption. For practical fibres concentration is predicted to be linear with fibre length. A 1 m long, radius 1 mm, fibre LSC doped with Lumogen Red 305 is predicted to concentrate the AM1.5 g spectrum up to 1100 nm at normal incidence by ~35 x. The collection efficiency under diffuse and direct irradiance in London has been analysed showing that, even under clear sky conditions, in winter the diffuse contribution equals the direct.

The luminescent concentrator illuminated

Luminescent concentrator (LC) plates with different dyes were combined with standard multicrystalline silicon solar cells. External quantum efficiency measurements were performed, showing an increase in electrical current of the silicon cell (under AM1.5, 1 sun conditions, at normal incidence) compared to a bare cell. The influence of dye concentration and plate dimensions are addressed. The best results show a 1.7 times increase in the current from the LC/silicon cell compared to the silicon cell alone. To broaden the absorption spectrum of the LC, a second dye was incorporated in the LC plates. This results in a relative increase in current of 5-8% with respect to the one dye LC, giving. Using a ray-tracing model, transmission, reflection and external quantum efficiency spectra were simulated and compared with the measured spectra. The simulations deliver the luminescent quantum efficiencies of the two dyes as well as the background absorption by the polymer host. It is found that the luminescent quantum efficiency of the red emitting dye is 87%, which is one of the major loss factors in the measured LC. Using ray-tracing simulations it is predicted that increasing the luminescent quantum efficiency to 98% would substantially reduce this loss, resulting in an increase in overall power conversion efficiency of the LC from 1.8 to 2.6%.

Quantum Well Solar Cells and Quantum Dot Concentrators

Publisher Summary This chapter focusses on quantum well solar cells and quantum dot concentrators. This chapter reviews the development over the past half a decade of the quantum well solar cell (QWSC) and the quantum dot concentrator (QDC). The study of nanostructures such as quantum wells (QWs) and quantum dots (QDs) has dominated opto-electronic research and development for the past two decades. The chapter also reviews recent advances since then, concentrating in particular on studies of the strain-balanced quantum well solar cell (SB-QWSC) as a concentrator cell and the thermodynamic modelling of the QDC. The SB-QWSC offers a way to extend the spectral range of the highest efficiency single-junction cell, the GaAs cell. It discusses the way this can in principle lead to higher efficiency in both single-junction and multi-junction cells and offers particular advantages in high-concentration systems. The high efficiency, wide spectral range, and small cell size make these systems particularly attractive for high concentration, building-integrated applications using direct sunlight. The chapter also demonstrates a 3D thermodynamic model capable of describing the performance of dye-doped and QD-doped slabs of luminescent concentrators. The model is a powerful tool for analyzing the performance of the luminescent concentrators. The fits show that concentrator performance is currently limited by the quantum efficiency (QE) of the QDs dispersed in the plastics.

Thermodynamic modelling of luminescent solar concentrators

Luminescent solar collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. We have developed self-consistent thermodynamic models for single layer planar concentrators and modules and here we extend the approach to stacks of concentrator slabs that allow better utilization of the solar spectrum. We present experimental and theoretical results on single concentrator slabs and two layer stacks of highly luminescent dyes in a polyacrylate and find that the models show excellent agreement with experiment.

Luminescent and geometric concentrators for building integrated photovoltaics

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 7× 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. Initial results suggest that the combined system can achieve power conversion efficiencies approaching 20% under standard AM1.5g illumination at normal incidence.

Luminescent solar concentrators: Nanorods and raytrace modeling

Nanorods are a novel and promising component for luminescent solar concentrators (LSCs). In particular, their spectra suggest reduced re-absorption losses. We report the incorporation of core-shell nanorods in homogeneous and thin film LSCs. The rods in the solid host appear to retain their spectral features compared to their dissolved state. Short-circuit current measurements with a calibrated solar cell have been compared with the computational simulation of the LSCs. Our raytrace model was applied to fit the fundamental emission spectrum and extract the quantum efficiency (QE) of the nanorods in the concentrators. In the case of the homogeneous LSC, the extracted QE was (67±4)%, which is in good agreement with the quoted value of about 70% for rods in solution. The thin film samples showed noticeably worse performance, which was attributed to possible agglomeration of rods and to macroscopic defects in the film. Finally, the raytrace model was applied to compare the self-absorption between a typical quantum dot concentrator and a nanorod concentrator. The result supported the argument that nanorods exhibit a smaller spectral overlap and consequently less re-absorption losses.

Fabrication, Characterisation & Modelling of Quantum Dot Solar Concentrator Stacks

Quantum dot solar concentrators (QDCs) have been fabricated by the incorporation of quantum dots into highly transparent polymer host materials. UV polymerisation techniques were found to reduce the quantum dot quantum efficiency in comparison with thermally polymerised samples. The sample plates were characterised using photocurrent techniques in individual and stacked configurations. Due to the increase in absorption, stacking the QDC plates results in a 16% increase in photocurrent. This configuration could also reduce thermalisation losses when coupled to solar cells with appropriate band gaps