Luminescence solar collectors (LSCs) or concentrators were first introduced in the late 1970s (see, for example ref. 16-20). Following intense research activity in the 1980s, the area has received renewed interest in the past decade or so due to the availability of new materials and the advent of photonics, leading to more optimistic theoretical predictions.48’49
Figure 9.9 (a) Chemical structure of donor (DiO) and acceptor (Dil) carbocyanine
dyes used for (b) energy transfer between dye monolayers. (c) Fluorescence decay curves for the DiO in the absence and presence of the acceptor dye (DiI) showing the significant shortening of the decay curve. The decay curve has been fitted with a multi-exponential and the energy transfer was calculated to be 80%. The excitation wavelength was 440 nm.
A fluorescent solar collector usually consists of a flat plate, doped with a luminescent species, which absorbs the incident sunlight (direct or diffuse). A large fraction of the emitted light is then trapped within the collector by total internal reflection (TIR) and is directed to a solar cell at the edge of the collector where the remained edges of the collector can be covered by mirrors. A schematic of the operation of the luminescent solar collector is shown in Figure 9.11.
An increase in the photon flux reaching the solar cell is achieved by virtue of the large area difference between the front face of the collector and the edge area covered by solar cells. The ultimate aim is to produce a sizeable concentration gain ratio Aent to Aexit, where Aent and Aexit are the areas of the top and edge surfaces, respectively (Figure 9.12).
Figure 9.10 (a) Schematic overview of an artificial photonic antenna system. The
image on the left shows the chromophores being embedded in the channels of the host material (zeolite). The dyes act as donor molecules that absorb the incoming light and transport the electronic excitation energy via resonance energy transfer to the acceptors shown at the ends of the channels on the right. The process can be analysed by measuring the emission of the acceptors and comparing it with that of the donors. The double arrows indicate the orientation of the electronic transition dipole moment (ETDM). The image on the right shows a bunch of such strictly parallel channels: a schematic view of some channels in a hexagonal zeolite crystal with cylindrical morphology. Reproduced from ref. 10 with permission from the European Society for Photobiology, the European Photochemistry Association, and the Royal Society of Chemistry. (b) Schematic of brickstone arrangement of dyes to form an aggregate. Reproduced with permission from ref. 45.
Figure 9.11 (a) Schematic diagram of fluorescent collector, A and B. llustrate the
path length of rays emitted with an angle в # ec and в > ec respectively. Reprinted with permission from ref. 28, Copyright © Swiss Chemical Society: CHIMIA. (b) Perspective view of
a fluorescent solar collector. Reprinted with permission from ref. 24, Copyright 2009, AIP Publishing LLC.
Figure 9.12 Schematic diagram of a generic fluorescent collector defining the areas Aent and Aexit. Reprinted with permission from ref. 49, Copyright 2006, AIP Publishing LLC.
The collector may be composed of a transparent matrix such as PMMA,50 glass51 or liquid27 which is doped with a mixture of dyes,35 quantum dots52 or rare earth ions.53 The choice of the luminescent species can be modified to suit the bandgap of the solar cell. For efficient operation, the collector has to absorb a substantial part of the incident light and this usually necessitates the use of several different dyes. Efficient absorption, however, implies losses through re-absorption of the emitted light and a careful understanding of the re-absorption losses in the collector is therefore paramount.44