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Increased efficiency of luminescent solar concentrators after application of organic wavelength selective mirrors

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Abstract

Organic wavelength-selective mirrors are used to reduce the loss of emitted photons through the surface of a luminescent solar concentrator (LSC). A theoretical calculation suggests that application of a 400 nm broad reflector on top of an LSC containing BASF Lumogen Red 305 as a luminophore can reflect 91% of all surface emitted photons back into the device. Used in this way, such broad reflectors could increase the edge-emission efficiency of the LSC by up to 66%. Similarly, 175 nm broad reflectors could increase efficiency up to 45%. Measurements demonstrate more limited effectiveness and dependency on the peak absorbance of the LSC. At higher absorbance, the increased number of internal re-absorption events reduces the effectiveness of the reflectors, leading to a maximum increase in LSC efficiency of ~5% for an LSC with a peak absorbance of 1. Reducing re-absorption by reducing dye concentration or the coverage of the luminophore coating results in an increase in LSC efficiency of up to 30% and 27%, respectively.

©2012 Optical Society of America

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Figures (10)

Fig. 1
Fig. 1 The working principle of wavelength selective mirrors. Green photons in the solar spectrum are transmitted by the reflector and absorbed by the dye molecules within the waveguide. Red emitted photons are reflected back into the device.
Fig. 2
Fig. 2 The simulated reflective properties of 150 nm broad reflectors made from gradient pitch cholesterics. (a) The reflectivity of a full reflector made by stacking a right- and a left-handed cholesteric on top of each other and (b) the reflectivity of a full reflector made by right handed cholesterics on both sides of a half wave retarder centered at 560 nm are used to make a full reflector. The color in these plots represents the reflectivity of the cholesteric reflectors; dark blue is 0% reflection and dark red is 100% reflection.
Fig. 3
Fig. 3 Calculated efficiency of cholesterics in reflecting light emitted from the top surface of a Red 305 containing LSC for narrowband reflectors (white squares), 175 nm broad gradient pitch reflectors (grey squares), 400 nm broad gradient pitch cholesteric reflectors (black squares), layered reflectors made from 2 narrowbands (filled red circles for stacked right and left handed reflectors and open red circles for stacked right handed reflectors on both sides of a half wave retarder centered at 560 nm) as a function of the onset wavelength of the cholesteric reflectors.
Fig. 4
Fig. 4 Normalized absorption and emission spectrum of Red 305 in polycarbonate.
Fig. 5
Fig. 5 The fraction of the incoming sunlight within the absorption band of the dye that could be absorbed by the luminophore (Red 305) that passes through the cholesterics (Fea) made from narrowband reflectors (white squares), 175 nm broad gradient pitch reflectors (grey squares), 400 nm broad gradient pitch cholesteric reflectors (black squares), layered reflectors made from 2 narrow bands (filled red circles for stacked right and left handed reflectors and open red circles for stacked right handed reflectors on both sides of a half wave retarder centered at 560 nm).
Fig. 6
Fig. 6 The calculated maximum possible increase in LSC efficiency after application of cholesteric reflectors to an LSC containing Red 305 as a luminophore. The reflectors are made from narrowband cholesterics (white squares), 175 nm broad (grey squares) and 400 nm broad gradient pitch cholesteric (black squares), layered cholesteric and reflectors made from 2 narrowbands (red circles for stacked right and left handed reflectors and open red circles for stacked right handed reflectors on both sides of a half wave retarder centered at 560 nm). The main graph is an enlargement of the graph region which gave an increase in efficiency; the inset shows all data.
Fig. 7
Fig. 7 Schematic depiction of the measurement setup.
Fig. 8
Fig. 8 Reflection spectra of a broadband reflector with an onset wavelength of 730 nm made from 2 layered right handed narrowband reflectors on both sides of a half wave retarder centered at 560 nm, both experimental (black) and calculated (gray).
Fig. 9
Fig. 9 Relative LSC efficiency after application of broadband reflectors with respect to the LSC without the broadband reflector. The LSCs contain Red 305 with different peak absorbance: calculated (black), 0.05 (red), 1.01 (green), 2.36 (blue).
Fig. 10
Fig. 10 Relative efficiency of patterned LSCs after application of broadband reflectors with respect to the patterned LSC without the broadband reflector. The LSCs are topped with a coating containing Red 305 with a peak absorbance of 1.0 with different pattern coverage of the surface: calculated (black), 20% (green), 30% (cyan), 50% (red), 70% (yellow), 100% (blue).

Tables (2)

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Table 1 Maximum Reflection Efficiency of the Cholesteric for Surface Emitted Light of LSCs Containing Red 305 as Luminophore

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Table 2 Maximum Calculated Increase in LSC Efficiency after Addition of the Cholesteric Reflectors to LSCs Containing Red 305 as Luminophore

Equations (9)

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λ ¯ = n ¯ p
λ θ ¯ = λ 0 ¯ cos[ sin 1 ( sinθ n ¯ ) ]
Δλ=pΔn
η refl = photon s surface E s ( λ )r( θ,λ ) E p ( θ )dθdλ photon s surface E s ( λ ) E p ( θ )dθdλ
f chol EA = I( λ )*( 1r( λ ) )*A( λ )dλ I( λ )*A( λ )dλ
η LSC,max = n edge,chol n edge,bare = f chol EA n edge,bare n edge,bare + n edge,SL,chol n edge,bare
n edge,SL,chol = f chol EA *QE* ϕ SL * η chol
n edge,bare =QE* ϕ wm
η LSC,max = f chol EA ( 1+ η chol )
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