The characterisation techniques and principal results can be conveniently illustrated on the example of single-dye collectors. Studies carried out so far on the effect of mirrors on the fluorescent collectors indicate that a small air gap between the mirror and the edge of the collector is needed for better collector efficiency39 and any attempt on optical coupling disturbs the TIR structure and limits the efficiency. The effect of re-absorption losses also puts a limit on the concentration gain factor, which cannot assume very high values, and so most collectors to date have been fabricated with gain factors up to 50, and is much lower than the high gain ratios near the thousands initially reported.29 This restriction applies only to TIR configurations and is lifted when a photonic band stop is used together with a near unity fluorescence efficiency dye49 as seen in Section 9.6. Also, differences between a fluorescent solar concentrator surrounded by four edge mounted solar cells versus a single solar cell configuration have shown that the re-absorption probability of trapped photons in a four solar cell configuration is improved.54
Despite the improvements of fluorescent collectors over the past 30 years, the overall experimental power conversion efficiencies for fabricated devices (collector and solar cell) remain well below 10% (Table 9.1). Higher efficiencies of fluorescent collectors using GaAs solar cells (Table 9.2) are reported due to a better match of the dye emission spectra to the GaAs bandgap.
The highest power conversion efficiency for a single-dye collector for silicon solar cells reported is 2.4%, increasing to 2.7% (optical efficiency of 14.5%) if combined with a second dye. Recently an efficiency of 2.8% has been reported with CdSe quantum dots.52 Using the same two dye mixture,
Table 9.1 Efficiencies of fluorescent collectors coupled to a c-Si cell, as reported in the literature for different collector gain; the highest efficiency for a single plate collector is about 3%
Table 9.2 Efficiencies of fluorescent collectors coupled to GaAs cells for different collector gain, as reported in the literature
the power conversion efficiency using a single GaAs solar cell increases to 4.6% and increases further to 7.1% when used with four GaAs solar cells connected parallel, albeit with a reduced collector gain. The best results so far have been obtained with collectors gain in the region of 10-15 due to severe restrictions posed by re-absorption.
The possibility of using many luminescent species together to extend the absorption range in the collector was proposed originally by Zewail and co – workers.19’20 Making use of Forster resonance transfer (Section 9.3) the light absorption is maximised while the acceptor concentration is kept to a minimum to reduce re-absorption. This approach has been employed successfully in LB films45 and dye-loaded zeolite channels55 (Section 9.3), which can lead to collectors absorbing a substantial part of the incoming radiation spectrum (high QA) and an emission close to the bandgap of the solar cell (high QC). The spectral characteristics of such a collector are shown schematically in Figure 9.14(a), where DE denotes the width of the photon
Figure 9.14 (a) Schematic diagram of the incident and emitted fluxes and (b) the
absorption spectrum for an optimum fluorescent collector with light trapping by TIR. Reprinted with permission from ref. 31, Copyright 2012, AIP Publishing LLC.
transport channel in energy units. Broad absorption of the incident light is achieved by the use of an appropriate mixture of dyes. For optimal absorption of the solar radiation, the emission region should be spectrally narrow and close to the semiconductor wavelength bandgap. At the same time, it is important to ensure that this emission region is absorption-free as shown in Figure 9.14(b). The absorption coefficients for absorption and emission should therefore satisfy:
aabs $ 1/d, aem < 1/L (9.22)
where d and L are the thickness and length of the collector, respectively.
Recently, an LSC scheme has been proposed that mimics a four-level laser design, making use of Forster energy transfer and phosphorescence in thin film organic coatings on glass substrates.56 Collector optical efficiencies near 50% were measured and device efficiencies of the order of 6.8% when used in tandem have been claimed but were not experimentally verified. Other multidye fluorescent collector studies included three dyes with a near-unit efficient energy transfer resulting in high optical efficiencies.57 The above studies show that it is possible to create multi-dye mixtures in fluorescent collectors
Figure 9.15 Normalised absorption and emission spectra for a three dye collector (Y083, O240 and R305) with donor : acceptor ratios (30 : 20 : 1). The individual dye absorption bands have been deconvoluted in the spectra.
that absorb a significant part of the incident solar spectrum and exhibit efficient photon management.
We have fabricated a three dye collector based on the BASF Lumogen F dyes yellow (Y083), orange (O240) and red (R305). The dyes were dissolved in PMMA and spin coated on glass substrates. In this mixture the Y083 and O240 act as the donor dyes and the R305 acts as the acceptor dye to which all the excitation energy is transferred and emission occurs. By increasing the donor to acceptor ratio we can reduce the re-absorption losses in the collector. An example of the absorption and emission characteristics of a collector is shown in Figure 9.15. The combined absorption spectrum has been deconvoluted to the absorption of the individual dyes. A significant portion of the incident photon flux is absorbed in the donor dyes (Y083 and O240) while the absorption of the acceptor dye (R305) remains low, which is necessary to reduce re-absorption losses. We expect fluorescent collectors with these spectral characteristic to be able to reach significant optical efficiencies.