FABRICATION, CHARACTERISATION & MODELLING OF QUANTUM DOT SOLAR CONCENTRATOR STACKS Daniel J Farrell, Amanda J Chatten, Andreas Büchtemann1 & Keith W J Barnham Blackett Laboratory, Physics Department, Imperial College London, London SW7 2BW, UK. Email: daniel.farrell@imperial.ac.uk 1Fraunhofer Institute for Applied Polymer Research, Golm D-14476, Germany. ABSTRACT 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. INTRODUCTION The luminescent solar concentrator (LSC) was originally proposed over 20 years ago [1-3] and attracted much interest as an inexpensive, non-tracking approach that collects both direct and diffuse sunlight. Sunlight incident on a dye-doped plastic sheet is absorbed by the luminescent dye and then re-emitted over all solid angles in the sheet. A fraction is wave-guided, via
Fig. 1 Stokes’ shift predicted from the thermodynamic model [5,9-11]. As the quantum dot distribution is broadened the corresponding luminescence is red-shifted. This reduces the spectral overlap between the absorption and the luminescence, thus reducing re-absorption losses.
total internal reflection (TIR), to the sheet edges, where the light is concentrated and absorbed by a solar cell. In addition, a fraction of the photons emitted at angles less than the critical angle can be trapped in the sheet by the appropriate use of mirrors. One practical problem with the LSC is that luminescent dyes photo-degrade [4]. Our group [5,6] has suggested replacing organic dyes with semiconductor quantum dots (QDs) creating the Quantum Dot Concentrator (QDC). QDs have several advantages over organic dyes; (i) semiconductor QDs are inherently less prone to bleaching in sunlight, (ii) they absorb over a wider spectral range, (iii) the absorption threshold can be tuned over the solar spectrum simply by the choice of dot diameter and (iv) the Stokes’ shift of an ensemble of dots can be engineered by controlling the spread in dot size [5], as illustrated in Fig. 1. In addition to the luminescent species, the properties of the transparent host material are of critical importance for making a large, practical sized concentrator. As the plate dimensions increase, so does the average path-length of the luminescent photons, which in turn increases the fraction of the photons absorbed by the host matrix. Various transparent polymer materials have been investigated by varying the monomer blend, the initiators and polymerisation technique (see Fabrication). Like tandem solar cells, a stack arrangement of QDC plates with different absorption thresholds means that less of the incident energy is lost in down conversion to the energy of the luminescence [2]. Stacking QDCs with different band-gaps can also lead to a better utilization of the incident solar spectrum and, in addition some of the luminescence escaping one plate can be absorbed in another. QDC plates containing QDs of different band-gap have been fabricated and characterised for this study. MODELLING Our group has developed radiative-transfer thermodynamic models for single planar luminescent concentrators [7,8] and stacks [9] by applying a detailed balance argument (equation 1) to relate the absorbed light to the spontaneous emission using self-consistent three dimensional fluxes.
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