A type of solar concentrator for photovoltaics utilizing light pipe trapping of luminescence is described. Total collector efficiencies of 3. 2% have been measured, and efficiencies of 10% appear theoretically possible. The photodegradation lifetime of the dyes presently used is about one year under optimal conditions

Batchelder, J. S.

Cole, T.

Zewail, A. H.

Caltech Authors - Main

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Luminescent solar concentrators 
J. S. Batchelder, A.H. Zewail, T. Cole 
Department of Applied Physics 
and 
Arthur Amos Noyes Laboratory of Chemical Physics 
California Institute of Technology, Pasadena, California 91125 
Abstract 
A type of solar concentrator for photovoltaics utilizing light pipe trapping of luminescence is described. 
Total collector efficiencies of 3. 2% have been measured, and efficiencies of 10% appear theoretically 
possible. The photodegradation lifetime of the dyes presently used is about one year under optimal 
conditions. 
Introduction 
The Luminescent Solar Concentrator, or LSC, is a solar concentrator based on light pipe trapping of 
molecular or ionic luminescence. The conceptual operation of an LSC is illustrated by the diagram of a 
Planar Solar Concentrator (PSC} shown in Figure 1; a transparent material like polymethyl methacrylate 
THE PLANAR SOLAR CONCENTRATOR <PSC). 
L 
HIRROREO 
BACK 
/ 
Figure 1. A planar solar concentrator, or PSC, which is a particular embodiment of an LSC. Sunlight 
enters from above and passes twice through the plate thickness D, the backing mirror being separated 
from the plate by an air gap. Part of this flux is absorbed by a dye or inorganic ion, and the ensuing lum-
inescence can either escape back out of the face (A) or be trapped by total internal reflection (B). This 
trapped light will then propagate to the photovoltaic cells (PVC) where it is absorbed and converted into 
electricity. 
(PMMA) is impregnated with guest luminescent absorbers, such as organic dye molecules having strong 
absorption bands in the visible and UV regions of the spectrum, and also having an efficient quantum yield 
of emission. Solar photons entering the upper face of the plate are absorbed, and luminescent photons are 
then emitted. Snell's law dictates that the e,robability that the luminescent photons will be trapped in the 
plate by total internal reflection is (1-1/n2)Y2, where n is the index of the host material. For example, 
about 75% of the emission is trapped in a PMMA plate with an index of 1. 49. Successive reflections 
transport the luminescence to the edge of the plate, where they can enter an edge-mounted array of 
Photovoltaic Cells, or PVCs. 
Theoretical performance 
An LSC 's performance can be broken down into four factors: initial absorption of the solar flux, energy 
transfer mechanisms between the absorbing dyes or ions, transport of the energy to the edge of the plate, 
SPIE Vol. 248 Role of Electro-Optics in Photovoltaic Energy Conversion (1980) I 105 
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BATCHELDER. ZEWAIL. COLE 
and the response of the attached cells to the concentrated light. The first LSC device reported 1 contained 
a single type of organic laser dye in a PMMA host. Typically a single dye can absorb about 15% of the 
incident solar flux above the bandgap energy of silicon, so that a single dye plate has a limited efficiency. 
We proposed the use of many different types of dye molecules in the same plate. 2 These are chosen such 
that the emission band of one dye strongly overlaps the absorption band of the next, allowing an excitation 
to cascade from one dye to the next, undergoing final emission by the reddest dye. In this way, the effec-
tive absorption bandwidth can be extended over the entire visible spectrum. Another technique to extend 
the effective absorption band is to use a series of parallel plates, each containing a different dye, and each 
plate outputting light to cells with different bandgaps. 3 This effectively splits the sunlight into wavelength 
bands and concentrates it for the optimal cells in each band. 
In a system which contains a mixture of different dyes, an excitation can be transferred from one dye to 
a similar dye molecule, as well as from one type of dye to another. This is possible because there is 
typically a significant overlap between the emission and extinction spectra of a dye, as shown in Figure 2. 
RHDDAMINE-575 SPECTRA IN PMMA. 
EMISSrnN 
1.39 1.69 
WRVENUMBERS 
ABSDRPTIDN 
2.39 
E I! 
Figure 2. The absorption and emission spectra of the laser dye rhodamine- 575 in PMMA A sample was 
prepared with an effective dye concentration of 1. 3E-5 molar, as determined by the measured peak optical 
density and the known peak extinction coefficient of R-575 in methanol. The absorption was measured 
through a pathlength of 2mm, the measured peak optical density was . 27. The emission spectrum was the 
surface emission due to a 20, 492 wavenumber (4880A) excitation. 
Each time a dye molecule absorbs the emission from a similar dye molecule, or undergoes self-
absorption, there is a chance that the excitation will not cause an emission, or that the emission will not 
be internally trapped within the plate. For example, while the probability for trapping a single emission 
in a PMMA plate is 75%, including the effect of self-absorption drops the probability to (. 75)**ns, where 
ns is the number of self-absorptions undergone by the emission. 
In a zero-order model of the performance of a LSC- PVC system, we assume that the absorbed solar 
flux is cascaded to the lowest energy dye, that the average number of self-absorptions by this dye is ns, 
and that the resulting emission is transported without loss through the plate into the cell. Under these 
assumptions, the collector efficiency is: 
LSC Collector Efficiency = AMl Cell Efficiency 
* (Dye Quantum Eff. * Trapping Eff. )ns 
* (Solar Flux above bandgap absorbed by LSC) 
/ (Solar Flux above bandgap of the cell) 
If the number of self-absorptions is assumed to be 2, and a silicon cell with 18% AMl efficiency is 
attached to a multi-dye plate, then the predicted efficiency should be between 9% and 12%. 
Observed performance 
(1) 
The highest efficiency plate produced to day had a total collector efficiency of 3. 2%. 4 These LSCs are 
formed by applying thin films containing the dye to the back of an acrylic plate. A promising alternative 
106 / SPIE Vol. 248 Role of Electro-Optics in Photovoltaic Energy Conversion (1980) 
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LUMINESCENT SOLAR CONCENTRATORS 
is to fill the gap between two parallel glass plates with a mixture of dyes in a solvent such as ethylene 
glycol. We have obtained efficiences of 2% on a preliminary prototype liquid system. 6 
Principle problems 
The outstanding questions to be answered to realize a practical power system are efficiency and life-
time. The best present devices are about a factor of three less efficient than calculations initially predic-
ted they should be. The predominant cause of this loss appears to be a higher rate of self-absorption than 
was expected. Two avenues are being pursued to reduce this loss. One is to design a specialized molecule 
which has both a large Stoke's shift and a high quantum efficiency. 7 Our approach has been to deliberately 
modify the environment of the dyes to narrow their absorption and emission peaks, so as to reduce their 
overlap. For example, in Figure 3 we show the emission spectra for rhodamine- 575 in PMMA as a func-
z 
0 
,_. 
(f) 
(f) 
,_. 
:L: 
w 
RH~DRMINE-575 EMISSI~N IN R PMMA R~D 
.16, .4. 1 .• 4 .• AND 10.CM. PATHLENGTHS. 
WRVENUMBERS 
2.12 
E ll 
Figure 3. The emission spectra of a PMMA rod containing R-575 for five differ!_int self-absorption path-
lengths. The same plastic of Figure 2 was excited by 22, 941 wavenumber (4359A) light at five different 
distances from the end of the rod illuminating a spectrometer. The spectrum with a bluest emission 
travelled the shortest pathlength. Notice that the self-absorption effect lowers the fraction of light collec-
ted as the pathlength increases, and that for the longest pathlengths the red tail becomes stronger due to 
contributions from the emission of self-absorbed excitation. 
tion of the pathlength that the emission travelled through the plastic. Figure 4 is a similar series of 
z 
0 
,_. 
(f) 
CJ) 
....... 
:L: 
w 
RH~DRMINE-575 EMISSl~N IN METHAN~L 
.3. 1 .• 3 .• 10 .• AND 30.CM. PATHLENGTHS 
1.32 1.72 
WRVENUMBERS 
2.12 
E ll 
SPIE Vol. 248 Role of Electro-Optics in Photovoltaic Energy Conversion (1980) / 107 
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BATCHELDER. ZEWAIL. COLE 
Figure 4. The emission spectra of a glass tube containing R- 575 dissolved in methanol for five different 
pathlengths. In contrast to Figure 3, the dye in the methanol solution is at 9. 2E- 5 molar, or seven times 
the concentration of the dye in the plastic, but the total loss due to self-absorption is similar. While the 
lost flux is principly on the blue emission tail in the methanol solution, the loss is fairly uniform in the 
plastic, indicating that the absorption spectrum of the dye has much more broad tails in the plastic than in 
the liquid. 
spectra of the same laser dye dissolved in methanol. The absorption tail of the dye in methanol appears 
to dye away faster going to lower energy than it does in the PMMA sample, and the spectra appear more 
peaked, implying that there is considerably more inhomogeneous broadening in the plastic sample than in 
the methanol. This suggests that the liquid might prove to be a more efficient system due to reduced 
self-absorption. 
While the photodegradation half-life varies orders of magnitudes from one dye to another, typical life-
times under solar exposure for these dyes is about a year under optimal conditions. Plastics such as 
cellulose acetate buterate form a more stable host than PMMA, and in general dyes in liquid solvents are 
more stable than they are in plastics. An alternative to laser dye molecules are inorganic ions in glass. 
Such systems are comparably rugged,undergoing little or no degradation, but their quantum efficiency of 
luminescence is typically low. 
Here at Caltech we are studying the thermal and temporal properties of the dye spectra in a variety of 
hosts to try and increase the overall efficiency. 
Cost per watt 
The principle advantage of the LSC is its high geometric gain. If the plate was perfectly efficient, light 
incident on the edge-mounted cells would have a flux density greater than the incident solar flux 
by a factor which is the area of the face of the LSC divided by the area of the edge, a factor called the 
geometric gain. The geometric gain of a typical plate is around 100, allowing great reductions in the 
required area of solar cells. If a 10% system efficiency is assumed, estimates find that an environ-
mentally protected .LSC i;pould produce power at about $ 0. 37 per peak watt. 5 
Acknowledgments 
A. H. Zewail is an Alfred P. Sloan Fellow and a Camille and Henry Dreyfus Foundation teacher-
scholar. This work was supported in part by a contract from the U. S. Department of Energy/Solar 
Energy Research Institute, and in part by ARCO Solar, Inc. 
References 
1. Weber, W. H. , and Lambe, J. , Appl Opt. 15, 2299 (1976). 
2. Swartz, B. A., Cole, T., and Zewail, A. H., Opt. Lett. 1, 73 {1977). 
3. Goetzberger, A. , and Greubel, W. , Appl. Phys. 14, 123 {1977). 
4. Batchelder, J. S., Zewail, A. H. , and Cole, T., 'Appl. Opt. 18, 3090 {1979). 
5. Friedman, P. S., Advanced Concentrator Review Meehng, March 31, 1980, Solar Energy Research 
Institute, 1617 Cole Boulevard, Golden, Colorado 80401. 
6. Zewail, A. H. , op. cit 
7. Offenhartz, P. O'D., op. cit. 
708 I SPIE Vol. 248 Role of Electro-Optics in Photovoltaic Energy Conversion (1980) 


Luminescent Solar Concentrators

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