Category Materials Challenges

Advanced Photonic Concepts

Photon frequency management can benefit from recent advances in photonics and this section provides a brief outline of different ways to increase the efficiency of collectors beyond the TIR efficiency limit.

Spectrally selective filters have been proposed for application to lumines­cent solar collectors in order to reduce the light losses through the TIR escape cone. The filters can be fabricated from photonic crystals. A photonic crystal has a spatial periodic variation in its dielectric constant and prevents light of certain energy propagating in certain directions.65 The top face of the collector can be covered by a photonic structure (a band stop) that reflects the fluorescent light (Figure 9.20) blocking much of the escaping light and reducing photon transport losses to a minimum...

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LDS Examples

Typical external quantum efficiency (EQE) curves for thin film solar cells fabricated in the PV21 consortium are shown in Figure 9.17 for three different platforms—amorphous silicon (a-Si), CdTe and CIGS—together with the AM1.5G solar spectrum photon flux. The short wavelength region (300-500 nm), which is of interest in the down-shifting process, shows a reduced spectral response.

From an inspection of the EQE curves at wavelengths below 500 nm it can be seen that the most promising improvements in relative efficiency increase could be achieved with CdTe solar cells. Recent down-shifting studies have also been carried out on c-Si cells but with only small incre­ments observed in efficiencies.61 The two CdTe EQE curves shown in Figure 9.17 are fabricated from different methods...

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Luminescence Down-Shifting (LDS)

9.5.1 Introduction

Luminescence down-shifting (LDS) of the incident solar spectrum was orig­inally proposed by Hovel et al.58 in order to overcome the low spectral response in the blue region of the solar spectrum in some types of solar cells.

Figure 9.16 (a) A typical down-shifting structure for a CdTe solar cell. (b) A similar

structure as a fluorescent concentrator. Reproduced from ref. 27.

Proposed during the same period as luminescent solar collectors, both technologies share many similarities; a down-shifting structure for CdTe solar cells Figure 9.16(a) can be compared with a similar structure acting as a fluorescent collector in Figure 9.16(b). The solar cells in Figure 9...

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LSC Examples

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...

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Spectroscopic Characterisation of LSCs

In this section we introduce a simple characterisation technique based on the absorption and emission spectra of fluorescent collectors used for a spectral-based analysis of the performance of the collectors using the two – flux model outlined in Section 9.2.3. The analysis is based on careful measurements of the edge fluorescence of the collector and comparing that with the ‘first’ generation fluorescence spectra obtained from samples with no re-absorption at low dye concentrations. The re-absorption loss that occurs from a partial overlap of the absorption and emission bands is eval­uated by scaling the measured edge fluorescence to the first generation fluorescence.

The photon flux emitted from the edge of the collector is usually observed with a fibre optic or an integrating spher...

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Luminescent Solar Collectors

9.4.1 Introduction

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%...

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Examples of Energy Transfer for Efficient Photon Management

The good overlap of the absorption and emission spectra of the BASF dyes shown in Figure 9.5 forecasts efficient energy transfer in a mixture of these dyes. This will lead to an increase of the absorption efficiency in the dye layer and, at the same time, the emission wavelength can be shifted towards the red end of the spectrum, helping improve the operation of fluorescent collectors and down-shifting.

Examples of combined absorption and fluorescence spectra for different BASF dye doped PMMA mixtures fabricated by spin coating on glass substrates are shown in Figure 9.8. These mixtures of dyes can be used in fluorescent or down-shifting collectors. The normalised absorption and

Figure 9...

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Estimation of Quantum Yield

The fluorescence spectra and intensities of fluorescent samples are depen­dent on the optical density of the samples and the geometry of fluorescence detection.34 Of particular importance is fluorescence quenching at high dye concentrations, and the fluorescence emitted by the collectors provides a good vehicle how this quenching can be quantified. A simple method is to observe the fluorescence intensities at the red end of the spectrum where we can ensure that the re-absorption is negligible.42 At that wavelength range the fluorescence intensity is proportional to concentration except for any exci­tation energy loss due to quenching...

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