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Luminescent solar concentrators—A review
Solar Energy, Vol. 29, No. 4, pp. 323-329, 1982 Printed in Great Britain.
0038-.092XI82/040323--07503.0010 © 1982 Pergamon Press Ltd.
LUMINESCENT SOLAR CONCENTRATORS--A REVIEW A. M. HERMANN Solar Electric Conversion Research Division,Solar Energy Research Institute, 1617 Cole Boulevard,Golden, CO 80401, U.S.A. (Received 6 April 1981; revision accepted 2 January 1982)
kMtraet--A review of the development and an analysis of the principles of operation of Luminescent Solar Concentrator (LSC) devices is presented. State-of-the-artorganic systems are described. The theory of volumedispersed dye systems is discussed, and a new calculationdescribingthe operation of thin-filmdye solid solution systems on undoped substrates is presented. Experimentalpower conversionefficienciesrangingup to 4.5 per cent, air-mass one (AM1), for organic dye thin-film systems are reported, and limitationsof organic devices including photodegradationand loss mechanisms are described. Inorganicluminescentspecies in glass systems are described and are noted to have low luminescentefficienciesat concentrationshigh enough to collect a substantialfraction of solar photons. Proposals to increase these efficienciesby establishinghigher ligand-fieldsymmetry of transition metalions in inorganic host systems are advanced. LSC cost estimates including balance-of-systemscosts are discussed, and expectationsfor the developmentof stable, inexpensiveand efficientLSC devices are justified. 1. INTRODUCTION--HISTORY OF LSC DEVELOPMENT
Lerner appears to have been the first person to construct a solar collecting device using a solution of laser dye between two glass sheets [1]. His ideas were presented in a 1973 proposal to NSF which was rejected. Under Lerner's direction at MIT, Weilmenster submitted a senior thesis entitled "Radiation Transfer Process in Rhodamine----6G Methanol Applied to Solar Energy Conversion". The first publication on luminescent solar collectors in the open literature was that of a Ford Scientific Laboratory group, Weber and Lambe[2]. Weber and Lambe discussed two possible paths toward construction of a practical efficient collector of planar geometry: neodynium-doped glass, and organic dyes from dye lasers. Levitt and Weber[3] constructed devices consisting of Owens-Illinois ED2 neodyniumdoped laser glass and Rhodamine-6G doped polymethylmethacrylate (PMMA). A group in Germany consisting of Goetzberger et aL [4] described a luminescent collector consisting of a stack of dye-doped plastic sheets, each sheet matched to a photovoltaic cell. Swartz et al. [5] described a device consisting of two laser dyes (Rhodamine-rG and Coumarin) dissolved in PMMA (a mixed dye system). Rapp and Boling[6] were the first to report on a luminescent collector consisting of a thin luminescent film deposited on an undoped substrate. They appear to have coined the phrase Luminescent Solar Concentrator (LSC) which will be used generically henceforth in the report for a broad range of luminescent collectors. The literature has subsequently mushroomed; LSC research and development is currently being conducted in at least nine different laboratories in the United States, and in laboratories in Germany, France, Switzerland, Israel and Japan. Indeed, the potential rewards are great. The LSC device is a nontracking concentrator. The materials from which the collector is fabricated are inexpensive using current manufacturing techniques. Infrared radiation is not focused on the solar cells with this type of concentrator,
and the LSC is more efficient at collecting diffuse sunlight than conventional flat-plate panels [7]. 2. LSC DEVICE OPERATION
Most LSC devices are of planar geometry as shown in Fig. 1. Typically a fiate plate of transparent material (host) is impregnated with randomly dispersed guest dye molecules which luminesce efficiently. Photons whose wavelengths lie within the absorption band of the guest are incident on the top of the plate, are absorbed, and are subsequently re-emitted at longer wavelengths (the difference in wavelength between the absorption and emission bands is termed the Stokes shift [8]). The direction of re-emission is, to a first approximation, isotropic, and a large fraction of the re-emitted photons is internally reflected (trapped within the plate). Successive internal reflections carry the luminescent photons to the edge of the plate where they are collected by solar cells (silicon or GaAs). We define the photon flux gain Gp of the LSC as: A fat©
where A~aco is the area of the LSC face upon which sunlight is incident, Aerie is the area of the LSC edge to Luminescent solor c o n c e n t r o t o r
~ 2 5 °/o
For n = l 5
/ Fluorescence " [.,~ A ~, 75%
Fig. ]. Luminescent solar concentrator, LSC.
323
324
A.M.
HERMANN
A
which solar cells are attached, Qa is one fraction of illuminating photons absorbed, ~ is the quantum efficiency for luminescence, and .f is the fraction of the luminescent photons that are trapped in the collector and are eventually transmitted into the solar cell at the LSC edge. To find the fraction .f of luminescent photons trapped by internal reflection at the top surface of the plate, we solve Snell's law as in Fig. 2. Consider a photon reemitted (by a guest molecule) at the critical angle, 0c, the angle of incidence greater than for which the photon is internally reflected at the LSC/air interface. At the critical angle. Sneirs law gives nLSC sin 0c = (1) sin (90°), o r O c = s i n -~
1
(1)
/~ILSC"
In three dimensions, therefore, all photons re-emitted (from the guest) lying on or outside of a cone described by 0c are internally reflected. For isotropic re-emission, we can find the fraction f~ of rays lying on or inside of this cone (refracted out of the plate) by finding the ratio of the area which subtends this cone to the area of a sphere of the same radius. The fraction fo of rays lying on or outside of this cone is then simply [o = 1-.f~. Figure 3 is a sketch of this cone (called the critical cone). The area A is given by A = 2¢rP(1 - cos Oc).
(2)
Hence
Fig. 3. Critical cone described by the critical angle for total internal reflection.
For PMMA, n = 1.5 so that .f~- 0.75, or 75 per cent of the emitted photons are trapped in the plate. We note, however, that if an emitted photon's wavelength should sufficiently overlap the absorption spectrum of the guest, and should this photon subsequently be reabsorbed and re-emitted by another guest prior to its escape from the edge of the plate (into the solar cell), the fraction of photons arriving at the solar cell is reduced by a second critical cone loss to
f= (1---L-lV'2 h
nLSC2 ]
21rrZ(1-cos Oc) = 41rr2 = (1/2)(1 - cos Oc).
'
(3) or, in general, for n absorption and re-emission events,
The fraction of photons refracted out of the plate from both faces is obtained simply by multiplying the result in eqn (3) by a factor of 2. Hence the fraction f of photons trapped in the plate is f = COS Oo
(4)
From eqn (1) and (4), we see that
'----V nLSC2 /
(5)
'
Solar photons
I I
I
I °c/o
I
Emitting/1~
dye molecule
I I
l Air l
f
(1 =
1 ) "/2 - nLSC-'-----~/ .
(6)
For a PMMA/LSC with two absorption re-emission events, f=0.56. It should immediately be clear that a large Stokes shift is important to minimize this "selfabsorption" effect. We also note that calculation leading eqn (5) neglects self-absorption from guests inside the critical cone. We can also estimate the maximum possible power efficiency of an LSC device coupled to a silicon solar cell. Friedman of Owens-Illinos reports that solar cells can be fabricated with power efficiencies ranging up to 43 per cent over narrow spectral regions in the near IR[9]. Recognizing that only 60 per cent of the AM1 solar spectrum is above the bandgap energy of silicon and can be collected, we see that the maximum power efficiency for an LSC plate coupled to silicon solar cells is
m
(rip. . . . )maxim,m = (0.43)(0.60)(0.75) = 20 per cent,
LSC
Fig. 2. Critical angle for total internal reflection in an LSC.
which is lower than the thermodynamic limiting efficiency calculated by Williams of the University of Delaware [9]. The highest efficiencies reported to date for LSC devices[10] are about a factor of five lower than
325
Luminescent solar concentrators--a review this. Stokes-shift energy losses, incomplete solar photon collection and multiple self-absorption effects are largely responsible for reduced efficiency. Other effects which contribute to lower power efficiencies are reduced luminescence quantum efficiencies due to non-radiative losses (e.g. quenching), and host scattering and absorption. The maximum power efficiency measured to date for AMI illumination is 3.2 per cent for coupling to silicon cells, and 4.5 per cent for coupling to GaAs cells. Maximum achievable power efficiencies, assuming a cutoff wavelength of 700nm for absorption, have been estimated to be of the order of 8-12 per cent. The possibility for raising the cut-off wavelength for absorption with concomitant increases in realizable efficiency is related to solar cell bandgap and host absorption. C-H bond absorption occurs at 750nm. Increasing the cut-off beyond 750nm is therefore not productive unless the host can be deuterated or fluorinated, which may be economically feasible in the thin-film approach discussed below but not in the bulk approach. Addition of a taper of material with higher refractive index to the edge of an LSC plate can increase the light concentration [11]. If the boundary between the taper and the collector is convex, a lens effect gives an additional increase in concentration. While the net increase in concentration is limited to less than a factor of two, an analysis[ll] suggests that such tapers are cost beneficial in LSC devices. 3. LSC COST ESTIMATESAND BALANCEOF SYSTEM COSTS
Friedman[10] has made realistic cost estimates for a l m × l m LSC flat plate panel made from PMMA including superstructure and 40 cm 2 of silicon cells mounted along the edges. Using a value of just under $5 for the solar cells ($10/Wp, approximating today's singlecrystal solar cell costs), he arrives at a current cost of $37.30/m 2 for the LSC device. The balance of system (BOS) cost, however, is by no means clear. BOS costs today can be estimated for intermediate to large 10 per cent efficient arrays as about $10,O00/kW~. The earliest application of LSC devices, however, is likely to be in a residential roof-top grid-connected configuration. Cox[12] has projected 1986 BOS costs in the latter case of the order of $2000/kWp on new construction. Thus the total cost of a 10 per cent residential system could be about $2.37• Wp. Arco Solar has experience in field assembly of rooftop photovoltaic panels. Berman and Wildes of Arco Solar estimate[9] that a $37/m 2 LSC device would currently be cost effective in a grid-connected residential configuration if it had a power conversion efficiency of 6 per cent or greater. 4. STATE OF THE ART OF LSC
(a) Organic systems (1) Volume-dispersed dye systems. Organic systems have shown the highest power conversion efficiencies of all LSCs to date. Systems composed of dyes (single and multiple) dissolved in liquid hosts are useful for characterization of the dyes and for the study of their photo-
chemical properties. Zewail of CalTech has reported that liquid LSCs_are not, however, likely to be efficient LSC devices[9]. Zewail[9] has found experimentally that it is possible to absorb up to 70 per cent of the solar photons with a multidye volume dispersed system in solid solution of PMMA. The multidye approach has a number of merits. One advantage, of course, is the ability to span the solar spectrum via absorption with a variety of dyes, each of which absorbs over a different spectral region. Furthermore, if the dye concentrations in the host are high enough (e.g. 10-3 molar), an excitation of one dye molecule in the "cascade" can be transferred to the next type of dye in the cascade via the Forster mechanism of energy transfer (a non-radiative dipole-dipole exciton transfer). An advantage to this cascading effect is a significant reduction in critical cone losses and a reduction in the probability of losing excitation energy from internal processes in the dye molecules. Zweail et al. have achieved up to 2 per cent power conversion efficiencies at AM1 with a 2-dye system at 10-4 molar concentration in PMMA using Rhodamine-575 as the final (longest wavelength emission) dye. There is, however, evidence that LSC plates which are fabricated by polymerization of dye-containing methyl methacrylate are not homogeneous on a molecular level. Zewail et al. [9] attribute shifts in the emission spectrum of Rhodamine 575 (as a function of excitation wavelength) to local inhomogeneities. The role of these inhomogeneities with respect to efficiency limitation is not yet clear. A series of spectroscopic measurements on the relatively obscure dye 4 - dicyanomethylene - 2 - methyl - 6 p - dimethylaminostyryl - 4H - pyran DCM[13] has recently been completed at CalTech. These measurements, coupled to computer calculations on self-absorption, have enabled the CaITech group to predict ten-fold reductions in self-absorption processes in DCMcontaining plates. (2) Thin-film organic dyes on organic substrates. The highest LSC efficiencies reported to date[10] have been for plates one face of which was coated with a relatively thin (25 #m) polymeric film containing dyes, ,as depicted in Fig. 4. A striking feature of such a thin-film organic-dye plate is the angular distribution of the luminescence emanating from the edge of the plate, measured with respect to the plane of the plate. This angular distribution of luminescent radiation can be understood in an approximate fashion as follows. For a thin film LSC shown schematically in Fig. 5, consider rays at slight inclination to the plane of the plate which are refracted into the substrate and reach the plate edge. In the expanded view shown in Fig. 5(b), let 0t = 90° - e, where • is a small angle. Snell's law at the first refraction gives n~ sin 01 = n2 sin 02 or
n~ sin (90° - e) = n2 sin 02.
326
A. M. HERMANN
0' takes on its smallest value when E2[2 is negligible in comparison to unity, for which
Thin Film LSC Solar Radiation
0' = (04)rain = 13 °.
IV
/
Fluorescent ' i ~ Undoped Substrate Dye Film Fig. 4. Thin-film dye-doped host on an undoped index-matched substrate.
Solar radiation
O~,.[Dye-host[
n,
%
I
Substrate (o)
83"- " ~ , ~
(b)
Fig. 5. Simple model for calculation of angular distribution of luminescence in a thin-film LSC. Now ~2 sin (900- ~) = cos ~ = 1 - ~-.
Hence nl
~2
02 = sin-' [ ~ (1 - ~ - ) ] . Furthermore 90°=02+03, and Snelrs law at the second refraction gives n2 sin 03 = (1) sin 04. Hence the smallest angle 04 at which appreciable radiation appears at the edge is (04)mi, = 0' = sin -1 [n2 sin (90° - 02)] = sin-I [n2 cos 02]. Hence
, :,in'In cos[ in
¢//] /
Specifically, for an acetate butyrate coating (host with n, = 1.475) on PMMA (substrate with nz = 1.492), 0' = sin-I {1.492 cos [sin-' (0.9886 (1 - ¢ ) ) ] } .
We see therefore that, neglecting scattering, no luminescence leaves the plate at angles less than about 13° from the plane of the plate. Friedman has measured[M] the distribution of radiation leaving the plate edge, and his experimental results are in good agreement with these estimates. This angular distribution needs to be addressed in the mounting of solar cells parallel to the plate edges; it has been reported, however, that quarter wavelength AR coatings can be "tuned" to the appropriate angle of incidence [15]. Friedman[10] has achieved AM1 efficiencies of 3.2 per cent with a multilayered mixed-dye thin-film approach using silicon solar cells, and 4.5 per cent using GaAs solar cells. His 0.14m×0.14m×0.003m LSC plates consist of undoped PMMA coated with a thin (25 #m) film of cellulose acetate butyrate which is doped with Coumarin 6 and Rhodamine 101. Each of these dyes is put on in separate layers to form this film. This multilayered approach has a number of significant advantages over volume dispersed mixed-dye systems: • The dye layers can be used on glass as well as on plastic substrates. • Exotic hosts (polar, deuterated, fluorinated, etc.) can be used since only small amounts are needed. • High dye concentrations have been used successfully. Non-radiative energy transfer (cascading) between dyes can be efficient. "Overdoping" of the dye (when possible) may alleviate the photostability problem if the photo-degradation products don't interfere with fluorescence. • Dyes can be UV shielded and/or fluorescence pumped by the substrate by placing the uncoated side away from the sun. • The dyes can be physically separated from one another since they are put on in separate layers. This elimination of dye-dye interactions may enhance dye photostability, a problem discussed at length in a later section. On the other hand, in a mixed-dye volumedispersed system, rapid nonradiative energy transfer may improve stability and efficiency. • Critical cone losses can be reduced in a multilayered film stack, as shown in Fig. 6 for a three-dye system. Photons inside the critical cone of the blue dye may be absorbed by the green or red dye layers. Friedman has shown[9], that 99 per cent of the light emitted by the first dye is collected by the second. His 3.2 per cent efficient plates currently absorb 88 per cent of the solar photons up to 6000 ,~. If this wavelength can be increased to 9000 ~, by development of suitable dyes and hosts, 88 per cent absorption of solar photons would produce LSC devices of 7.5 per cent efficiency. Recent experiments at Owens-Illinois have shown that emission in uniformly-doped thin-film hosts is anisotropic. The apparent degree of anisotropy decreases as the dye concentration increases suggesting the dominance of non-radiative energy transfer in thin-film hosts at high
327
Luminescent solar concentrators--a review Multilayered Film Structure
the fraction of molecules bleached per absorbed photon and the corresponding half-life:
A. Mixed Dye Film-Three CrlUcal Cone Losses for Blue Dye
Molecules Bleached per Photon Absorbed 10-5
c ..I
10 - 6
10-7 10 8
10-9
Half-Life 2 days 2-3 days 5-6 months 4-5 yr 40-50 yr.
B. Multilayer Dye Film-One Critical Cone Loss for Blue Dye Red Dye Green Dye Blue Dye Green Dye Red Dye
,f
Red
E
Wavelength
Fig. 6. (a) Mixed-dye film; (b) multilayer film stack, one type of dye per layer. dye concentrations. These findings further suggest efficiency advantages in cascade dye co-doping in thinfilm hosts. (3) New approaches. Kleinerman of Clark University [9] is currently attempting synthesis of an interesting class of materials for LSC plates. He is building ,r-electron systems linked by short, covalentlyattached, non-conjugating bridges such that the resultant tailor-made multicomponent dye has a high absorption coefficient throughout most of the UV and visible. Solar energy absorbed by the ,r-electron systems is cascaded via intramolecular energy transfer to a chemically bonded emissive center which fluoresces in the near IR. The success of the program relies heavily on the organic synthesis outcome. 4. DIFFICULTIES
(4) DiBiculties. The greatest single difficulty with organic LSC devices is their lack of demonstrated photostabilities. Many laser dyes, for example, degrade in luminescence output approximately exponentially with time. If we define the time required for the luminescence output to reach 1/e of its original value as the half-life, typical half-lives are less than one year at AMI[10]. Progress in long-term stability has been made, however. Wittwer et aL[16] have, in one case, measured a half-life approximating one year under outdoor field conditions. Friedman has increased dye stability by variation of host material [10]. Emphasis on the photo-degradation problem has recently increased. To give some idea of the difficulties encountered in photo-degradation studies, we list below SE Vo[ 29, No 4~E
We see immediately that only trace quantities of degradation products are produced in the range of halflives such as needed for LSCs. Even with the most sophisticated chemical techniques one is hard-pressed to identify the degradation products. Researchers must therefore resort to chemical group substitution procedures and subsequent spectroscopic characterization. These time-consuming procedures are not without merit, however. Great progress has been made in the dye laser field with just such an approach. No systematic approach to degradation studies for LSC devices has yet been reported. Photo-chemical degradation studies are currently being initiated at Owens-Illinois. These studies include degradation from various regions of the solar spectrum (e.g. UV). Dye-host degradation reactions are also of prime concern. Remmant monomer species in polymeric substrates are considered prime suspects as degradationassist centers. Current efforts at producing monomerfree polymers for degradation testing are underway[17]. Degradation in PMMA hosts themselves is currently not considered a significant problem since, according to Cole of Jet Propulsion Laboratory[9], Air Force studies have shown negligible loss in the optical properties of PMMA with seventeen years of desert sunlight exposure. Self-absorption is of course another significant problem with organic LSC devices. The design and synthesis of stable dye molecules with large Stokes shifts is central to the achievement of an efficient LSC. Only recently have systematic efforts to this end been undertaken[9]. (b) Inorganic systems (1) Progress. Inorganic systems are attractive because they offer greater photochemical stability than their organic counterparts, and they often exhibit greater Stokes shifts. Transition metal compounds (d 3 states) have been studied both in fluorescence and in phosphorescence. A simplified energy level diagram for Cr3+ is given in Fig. 7. Ionic and molecular crystals have ligand fields at the Cr3+ site sufficiently high as to cause the zero-phonon *r2 level to lie energetically higher than the zero phonon 2E level. Cr3+ emission from crystalline solids is characterized by a sharp transitions from the 2E excited state as in the case of the 700 nm line of ruby, AI203:Cr3+. Glasses and certain other few crystalline materials, on the other hand, provide low field Cr3+ sites in which the zero phonon 4T2 level lies below the zero phonon 2E state. In these low-field cases, the roomtemperature Cr3+ emission from *r2 consists of a broad, unstructured band centered in the near infrared. Quan-
328
A. M. HERMANN Cr 3+Energy levels
!~4r2 2 E Excited
A
Gla
'
New I mal"eriols [
uby ,, i
,
i
Crystal field
t
, Ground state
Fig. 7. Simplified diagram of chromium-ion energy levels vs crystal field.
tum yields for luminescences are low in glass hosts, e.g. the highest luminescent quantum yield of Cr~÷ found to date in glass is 17 per cent (silicate glass)[18]. The low quantum yields are thought to be due to lack of perfect octohedral symmetry around the Cr 3÷. Ways of providing a higher symmetry environment are currently being studied. Andrews and Lempicki[19] are incorporating the Cr 3+ ions into aluminum phosphate glass ceramics. Maintaining optical transparency in such systems which show promising absorption and emission characteristics is exceedingly difficult. They have also conducted co-doping studies in glass hosts. Since Cr 3+ is an excellent absorber of solar radiation, experiments in doping glass with both Cr3+ and Nd 3÷ (an excellent emitter) have been performed[19]. This approach to circumvent the low Cr 3+ fluorescence efficiency has been found to be successful for the luminescent Cr 3+ ions in phosphate and silicate glass. Up to 80 per cent transfer efficiency to Nd 3+ can be achieved for this subpopulation of Cr3+ ions at moderate Nd 3+ ion densities. Unfortunately, the existance of a large subpopulation of non-luminescent Cr 3+ ions severely limits this approach in its applicability to LSC devices. Another class of transition metal compounds in which the transition metal ion (typically d s) is bonded to an organic porphyrin ring is currently under investigation[20]. In this case the phosphorescence is associated with a porphyrin ring. Recent preliminary experiments[20] have shown phosphorescent quantum efficiencies in excess of 0.6 at room temperature. (2) DiBiculties. The prime difficulty is low luminescent quantum yields due to low symmetry ligand fields in all the glass hosts examined to date. It is now clear from theory and experiment that chromium in glass (independent of glass composition) is not an efficient enough emitter for LSC applications. This clarification has, in fact, produced important scientific findings[21] related to the absorption spectra of Cr 3+ doped glasses. Inhomogeneous line broadening in glass has been observed to shift sharp levels due to interaction with the quasicontinuum. But the difficulty remains: if higher symmetry ligand fields cannot be achieved in optically suitable glass, LSC applications will have to be ruled out. As for the d s compounds, one may also have to face the photostability problem in the transition metalion/porphyrin system.
(c) Hybrid devices Some attention has been paid to the possibility of construction of a hybrid LSC exploiting the best capabilities of organic and inorganic systems. A distinct possibility now being investigated by Friedman at Owens-Illinois [9] a UV-blue absorbing luminescent glass substrate to provide UV protection for and to pump radiatively an organic thin-film dye in a polymeric host on the bottom face of the glass substrate. Current studies center on glasses doped with U022*, Eu 3÷ and Nd 3÷. Quantum yields as high as 0.71 (for Cu 3+, 0.5 mole, in sodium potassium phosphate) have been achieved to date. 5. CONCLUSION Enormous progress in the understanding and quantitative characterization of LSC devices has been made since the initial studies less than ten years ago. Power conversion efficiencies ranging up to 4.5 per cent and low cost-projections speak favorably for increased research in the area. It appears that, without a major breakthrough, inorganic systems will continue to suffer from low quantum yields at luminescent species concentrations high enough to collect a large fraction of solar photons. Continued research in inorganic systems is justified, however, particularly in view of the possibility of the construction of hybrid devices. Photodegradation of organic dye systems seems to be the central problem to be addressed in that area. The stability problem has been largely sidestepped to the present. Commercially available dyes of usually unknown purity have been used and characterized for photochemical stability, but no systematic investigations of dye selection, dye synthesis, and characterization have been made. With a suitable effort, it should be possible to achieve half-lives of four to five years in the near term. A similar problem in the dye-laser project has been solved with about two man-years of effort. Self-absorption processes have not been adequately studied. Only recently have attempts at characterizing quantitatively this and other loss mechanisms been initiated. Numerical calculations for efficiency estimates based on spectroscopic data are currently being undertaken. With research directed at solving the aforementioned problems, stable, inexpensive, and efficient LSC devices can be anticipated.
Note added in proof. This group has reported recently the achievementof a luminescentquantum efficiencyof 7070 for Cr3+ in mullite(an alumina-silicateceramic). REFERENCES
1. J. S. Batchelder, A. H. Zewail and T. Cole, AppL Optics 18, 3090 (1979). 2. W. H. Weber and John Lambe, AppL Optics 15, 2299 (1976). 3. J. A. Levitt and W. H. Weber, AppL Optics 16, 2684 (1977). 4. A. Goetzbergerand W. Gruebel, Appl. Optics 14, 123 (1977). 5. B. A. Swartz, T. Cole and A. H. Zewail, Optics Lett. 1, 73 (1977). 6. C. F. Rapp and N. L. Baling, Proc. 13th IEEE Photovoltaic Spec. Conf., p. 690. IEEE, New York (1978).
Luminescent solar concentrators--a review 7. P. S. Friedman, Owens-Illinois Techn. Status Rep. 1 (11/14/80). 8. I. N. Levine, Molecular Spectroscopy. Wiley Interscience, New York (1975). 9. Luminescent Solar Concentrator Rev., Solar Energy Research Institute (Oct. 1980). 10. P. S. Friedman, Owens-Illinois Rep., July 1979-March 1980. ll. A. Goetzberger and O. Schirmer, Appl. Phys. 19, 53 (1979). 12. C. H. Cox, III, DOE/ET/20279-%Rep. available from NTIS, 5285 Port Royal Redd, Springfield,Virginia (1980). 13. This dye was suggested for study by P. Hammond at the SERI LSC Rev., Oct. 1980. It is available from Exciton Corp, and its characterization is discussed in P. R. Hammond, Optics Communications 29, 331 (1979). 14. P. S. Friedman, Owens-Illinois Final Report, Jan. 1979-June 1979.
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15. P. S. Friedman, Owens-Illinois, 1981 (private communication). 16. V. Wittwer, K. Heidler, A. Zastrow and A. Goetzberger, Proc. 14th IEEE Photovoltaic Conf. p. 760, IEEE, San Diego (198o). 17. A. Gupta, Jet Propulsion Laboratory. Private communication (1981). 18. L. J. Andrews, A. Lempicki and B. C. McCollum(preprint). 19. A. Lempicki, L. Andrews and B. C. McCollum, GTE Ann. Progress Rep., 1 Sept. 1979-31 Dec. 1980. 20. P. O'D. Offenhartz and R. H. Micbeels, EIC Project Status Rep. June 1980-Dec. 1980. 21. A. Lempicki, L. Andrews, S. J. Nettel and B. C. McCoUum, Phys. Rev. Lett. 44, 1234(1980).
0038-.092XI82/040323--07503.0010 © 1982 Pergamon Press Ltd.
LUMINESCENT SOLAR CONCENTRATORS--A REVIEW A. M. HERMANN Solar Electric Conversion Research Division,Solar Energy Research Institute, 1617 Cole Boulevard,Golden, CO 80401, U.S.A. (Received 6 April 1981; revision accepted 2 January 1982)
kMtraet--A review of the development and an analysis of the principles of operation of Luminescent Solar Concentrator (LSC) devices is presented. State-of-the-artorganic systems are described. The theory of volumedispersed dye systems is discussed, and a new calculationdescribingthe operation of thin-filmdye solid solution systems on undoped substrates is presented. Experimentalpower conversionefficienciesrangingup to 4.5 per cent, air-mass one (AM1), for organic dye thin-film systems are reported, and limitationsof organic devices including photodegradationand loss mechanisms are described. Inorganicluminescentspecies in glass systems are described and are noted to have low luminescentefficienciesat concentrationshigh enough to collect a substantialfraction of solar photons. Proposals to increase these efficienciesby establishinghigher ligand-fieldsymmetry of transition metalions in inorganic host systems are advanced. LSC cost estimates including balance-of-systemscosts are discussed, and expectationsfor the developmentof stable, inexpensiveand efficientLSC devices are justified. 1. INTRODUCTION--HISTORY OF LSC DEVELOPMENT
Lerner appears to have been the first person to construct a solar collecting device using a solution of laser dye between two glass sheets [1]. His ideas were presented in a 1973 proposal to NSF which was rejected. Under Lerner's direction at MIT, Weilmenster submitted a senior thesis entitled "Radiation Transfer Process in Rhodamine----6G Methanol Applied to Solar Energy Conversion". The first publication on luminescent solar collectors in the open literature was that of a Ford Scientific Laboratory group, Weber and Lambe[2]. Weber and Lambe discussed two possible paths toward construction of a practical efficient collector of planar geometry: neodynium-doped glass, and organic dyes from dye lasers. Levitt and Weber[3] constructed devices consisting of Owens-Illinois ED2 neodyniumdoped laser glass and Rhodamine-6G doped polymethylmethacrylate (PMMA). A group in Germany consisting of Goetzberger et aL [4] described a luminescent collector consisting of a stack of dye-doped plastic sheets, each sheet matched to a photovoltaic cell. Swartz et al. [5] described a device consisting of two laser dyes (Rhodamine-rG and Coumarin) dissolved in PMMA (a mixed dye system). Rapp and Boling[6] were the first to report on a luminescent collector consisting of a thin luminescent film deposited on an undoped substrate. They appear to have coined the phrase Luminescent Solar Concentrator (LSC) which will be used generically henceforth in the report for a broad range of luminescent collectors. The literature has subsequently mushroomed; LSC research and development is currently being conducted in at least nine different laboratories in the United States, and in laboratories in Germany, France, Switzerland, Israel and Japan. Indeed, the potential rewards are great. The LSC device is a nontracking concentrator. The materials from which the collector is fabricated are inexpensive using current manufacturing techniques. Infrared radiation is not focused on the solar cells with this type of concentrator,
and the LSC is more efficient at collecting diffuse sunlight than conventional flat-plate panels [7]. 2. LSC DEVICE OPERATION
Most LSC devices are of planar geometry as shown in Fig. 1. Typically a fiate plate of transparent material (host) is impregnated with randomly dispersed guest dye molecules which luminesce efficiently. Photons whose wavelengths lie within the absorption band of the guest are incident on the top of the plate, are absorbed, and are subsequently re-emitted at longer wavelengths (the difference in wavelength between the absorption and emission bands is termed the Stokes shift [8]). The direction of re-emission is, to a first approximation, isotropic, and a large fraction of the re-emitted photons is internally reflected (trapped within the plate). Successive internal reflections carry the luminescent photons to the edge of the plate where they are collected by solar cells (silicon or GaAs). We define the photon flux gain Gp of the LSC as: A fat©
where A~aco is the area of the LSC face upon which sunlight is incident, Aerie is the area of the LSC edge to Luminescent solor c o n c e n t r o t o r
~ 2 5 °/o
For n = l 5
/ Fluorescence " [.,~ A ~, 75%
Fig. ]. Luminescent solar concentrator, LSC.
323
324
A.M.
HERMANN
A
which solar cells are attached, Qa is one fraction of illuminating photons absorbed, ~ is the quantum efficiency for luminescence, and .f is the fraction of the luminescent photons that are trapped in the collector and are eventually transmitted into the solar cell at the LSC edge. To find the fraction .f of luminescent photons trapped by internal reflection at the top surface of the plate, we solve Snell's law as in Fig. 2. Consider a photon reemitted (by a guest molecule) at the critical angle, 0c, the angle of incidence greater than for which the photon is internally reflected at the LSC/air interface. At the critical angle. Sneirs law gives nLSC sin 0c = (1) sin (90°), o r O c = s i n -~
1
(1)
/~ILSC"
In three dimensions, therefore, all photons re-emitted (from the guest) lying on or outside of a cone described by 0c are internally reflected. For isotropic re-emission, we can find the fraction f~ of rays lying on or inside of this cone (refracted out of the plate) by finding the ratio of the area which subtends this cone to the area of a sphere of the same radius. The fraction fo of rays lying on or outside of this cone is then simply [o = 1-.f~. Figure 3 is a sketch of this cone (called the critical cone). The area A is given by A = 2¢rP(1 - cos Oc).
(2)
Hence
Fig. 3. Critical cone described by the critical angle for total internal reflection.
For PMMA, n = 1.5 so that .f~- 0.75, or 75 per cent of the emitted photons are trapped in the plate. We note, however, that if an emitted photon's wavelength should sufficiently overlap the absorption spectrum of the guest, and should this photon subsequently be reabsorbed and re-emitted by another guest prior to its escape from the edge of the plate (into the solar cell), the fraction of photons arriving at the solar cell is reduced by a second critical cone loss to
f= (1---L-lV'2 h
nLSC2 ]
21rrZ(1-cos Oc) = 41rr2 = (1/2)(1 - cos Oc).
'
(3) or, in general, for n absorption and re-emission events,
The fraction of photons refracted out of the plate from both faces is obtained simply by multiplying the result in eqn (3) by a factor of 2. Hence the fraction f of photons trapped in the plate is f = COS Oo
(4)
From eqn (1) and (4), we see that
'----V nLSC2 /
(5)
'
Solar photons
I I
I
I °c/o
I
Emitting/1~
dye molecule
I I
l Air l
f
(1 =
1 ) "/2 - nLSC-'-----~/ .
(6)
For a PMMA/LSC with two absorption re-emission events, f=0.56. It should immediately be clear that a large Stokes shift is important to minimize this "selfabsorption" effect. We also note that calculation leading eqn (5) neglects self-absorption from guests inside the critical cone. We can also estimate the maximum possible power efficiency of an LSC device coupled to a silicon solar cell. Friedman of Owens-Illinos reports that solar cells can be fabricated with power efficiencies ranging up to 43 per cent over narrow spectral regions in the near IR[9]. Recognizing that only 60 per cent of the AM1 solar spectrum is above the bandgap energy of silicon and can be collected, we see that the maximum power efficiency for an LSC plate coupled to silicon solar cells is
m
(rip. . . . )maxim,m = (0.43)(0.60)(0.75) = 20 per cent,
LSC
Fig. 2. Critical angle for total internal reflection in an LSC.
which is lower than the thermodynamic limiting efficiency calculated by Williams of the University of Delaware [9]. The highest efficiencies reported to date for LSC devices[10] are about a factor of five lower than
325
Luminescent solar concentrators--a review this. Stokes-shift energy losses, incomplete solar photon collection and multiple self-absorption effects are largely responsible for reduced efficiency. Other effects which contribute to lower power efficiencies are reduced luminescence quantum efficiencies due to non-radiative losses (e.g. quenching), and host scattering and absorption. The maximum power efficiency measured to date for AMI illumination is 3.2 per cent for coupling to silicon cells, and 4.5 per cent for coupling to GaAs cells. Maximum achievable power efficiencies, assuming a cutoff wavelength of 700nm for absorption, have been estimated to be of the order of 8-12 per cent. The possibility for raising the cut-off wavelength for absorption with concomitant increases in realizable efficiency is related to solar cell bandgap and host absorption. C-H bond absorption occurs at 750nm. Increasing the cut-off beyond 750nm is therefore not productive unless the host can be deuterated or fluorinated, which may be economically feasible in the thin-film approach discussed below but not in the bulk approach. Addition of a taper of material with higher refractive index to the edge of an LSC plate can increase the light concentration [11]. If the boundary between the taper and the collector is convex, a lens effect gives an additional increase in concentration. While the net increase in concentration is limited to less than a factor of two, an analysis[ll] suggests that such tapers are cost beneficial in LSC devices. 3. LSC COST ESTIMATESAND BALANCEOF SYSTEM COSTS
Friedman[10] has made realistic cost estimates for a l m × l m LSC flat plate panel made from PMMA including superstructure and 40 cm 2 of silicon cells mounted along the edges. Using a value of just under $5 for the solar cells ($10/Wp, approximating today's singlecrystal solar cell costs), he arrives at a current cost of $37.30/m 2 for the LSC device. The balance of system (BOS) cost, however, is by no means clear. BOS costs today can be estimated for intermediate to large 10 per cent efficient arrays as about $10,O00/kW~. The earliest application of LSC devices, however, is likely to be in a residential roof-top grid-connected configuration. Cox[12] has projected 1986 BOS costs in the latter case of the order of $2000/kWp on new construction. Thus the total cost of a 10 per cent residential system could be about $2.37• Wp. Arco Solar has experience in field assembly of rooftop photovoltaic panels. Berman and Wildes of Arco Solar estimate[9] that a $37/m 2 LSC device would currently be cost effective in a grid-connected residential configuration if it had a power conversion efficiency of 6 per cent or greater. 4. STATE OF THE ART OF LSC
(a) Organic systems (1) Volume-dispersed dye systems. Organic systems have shown the highest power conversion efficiencies of all LSCs to date. Systems composed of dyes (single and multiple) dissolved in liquid hosts are useful for characterization of the dyes and for the study of their photo-
chemical properties. Zewail of CalTech has reported that liquid LSCs_are not, however, likely to be efficient LSC devices[9]. Zewail[9] has found experimentally that it is possible to absorb up to 70 per cent of the solar photons with a multidye volume dispersed system in solid solution of PMMA. The multidye approach has a number of merits. One advantage, of course, is the ability to span the solar spectrum via absorption with a variety of dyes, each of which absorbs over a different spectral region. Furthermore, if the dye concentrations in the host are high enough (e.g. 10-3 molar), an excitation of one dye molecule in the "cascade" can be transferred to the next type of dye in the cascade via the Forster mechanism of energy transfer (a non-radiative dipole-dipole exciton transfer). An advantage to this cascading effect is a significant reduction in critical cone losses and a reduction in the probability of losing excitation energy from internal processes in the dye molecules. Zweail et al. have achieved up to 2 per cent power conversion efficiencies at AM1 with a 2-dye system at 10-4 molar concentration in PMMA using Rhodamine-575 as the final (longest wavelength emission) dye. There is, however, evidence that LSC plates which are fabricated by polymerization of dye-containing methyl methacrylate are not homogeneous on a molecular level. Zewail et al. [9] attribute shifts in the emission spectrum of Rhodamine 575 (as a function of excitation wavelength) to local inhomogeneities. The role of these inhomogeneities with respect to efficiency limitation is not yet clear. A series of spectroscopic measurements on the relatively obscure dye 4 - dicyanomethylene - 2 - methyl - 6 p - dimethylaminostyryl - 4H - pyran DCM[13] has recently been completed at CalTech. These measurements, coupled to computer calculations on self-absorption, have enabled the CaITech group to predict ten-fold reductions in self-absorption processes in DCMcontaining plates. (2) Thin-film organic dyes on organic substrates. The highest LSC efficiencies reported to date[10] have been for plates one face of which was coated with a relatively thin (25 #m) polymeric film containing dyes, ,as depicted in Fig. 4. A striking feature of such a thin-film organic-dye plate is the angular distribution of the luminescence emanating from the edge of the plate, measured with respect to the plane of the plate. This angular distribution of luminescent radiation can be understood in an approximate fashion as follows. For a thin film LSC shown schematically in Fig. 5, consider rays at slight inclination to the plane of the plate which are refracted into the substrate and reach the plate edge. In the expanded view shown in Fig. 5(b), let 0t = 90° - e, where • is a small angle. Snell's law at the first refraction gives n~ sin 01 = n2 sin 02 or
n~ sin (90° - e) = n2 sin 02.
326
A. M. HERMANN
0' takes on its smallest value when E2[2 is negligible in comparison to unity, for which
Thin Film LSC Solar Radiation
0' = (04)rain = 13 °.
IV
/
Fluorescent ' i ~ Undoped Substrate Dye Film Fig. 4. Thin-film dye-doped host on an undoped index-matched substrate.
Solar radiation
O~,.[Dye-host[
n,
%
I
Substrate (o)
83"- " ~ , ~
(b)
Fig. 5. Simple model for calculation of angular distribution of luminescence in a thin-film LSC. Now ~2 sin (900- ~) = cos ~ = 1 - ~-.
Hence nl
~2
02 = sin-' [ ~ (1 - ~ - ) ] . Furthermore 90°=02+03, and Snelrs law at the second refraction gives n2 sin 03 = (1) sin 04. Hence the smallest angle 04 at which appreciable radiation appears at the edge is (04)mi, = 0' = sin -1 [n2 sin (90° - 02)] = sin-I [n2 cos 02]. Hence
, :,in'In cos[ in
¢//] /
Specifically, for an acetate butyrate coating (host with n, = 1.475) on PMMA (substrate with nz = 1.492), 0' = sin-I {1.492 cos [sin-' (0.9886 (1 - ¢ ) ) ] } .
We see therefore that, neglecting scattering, no luminescence leaves the plate at angles less than about 13° from the plane of the plate. Friedman has measured[M] the distribution of radiation leaving the plate edge, and his experimental results are in good agreement with these estimates. This angular distribution needs to be addressed in the mounting of solar cells parallel to the plate edges; it has been reported, however, that quarter wavelength AR coatings can be "tuned" to the appropriate angle of incidence [15]. Friedman[10] has achieved AM1 efficiencies of 3.2 per cent with a multilayered mixed-dye thin-film approach using silicon solar cells, and 4.5 per cent using GaAs solar cells. His 0.14m×0.14m×0.003m LSC plates consist of undoped PMMA coated with a thin (25 #m) film of cellulose acetate butyrate which is doped with Coumarin 6 and Rhodamine 101. Each of these dyes is put on in separate layers to form this film. This multilayered approach has a number of significant advantages over volume dispersed mixed-dye systems: • The dye layers can be used on glass as well as on plastic substrates. • Exotic hosts (polar, deuterated, fluorinated, etc.) can be used since only small amounts are needed. • High dye concentrations have been used successfully. Non-radiative energy transfer (cascading) between dyes can be efficient. "Overdoping" of the dye (when possible) may alleviate the photostability problem if the photo-degradation products don't interfere with fluorescence. • Dyes can be UV shielded and/or fluorescence pumped by the substrate by placing the uncoated side away from the sun. • The dyes can be physically separated from one another since they are put on in separate layers. This elimination of dye-dye interactions may enhance dye photostability, a problem discussed at length in a later section. On the other hand, in a mixed-dye volumedispersed system, rapid nonradiative energy transfer may improve stability and efficiency. • Critical cone losses can be reduced in a multilayered film stack, as shown in Fig. 6 for a three-dye system. Photons inside the critical cone of the blue dye may be absorbed by the green or red dye layers. Friedman has shown[9], that 99 per cent of the light emitted by the first dye is collected by the second. His 3.2 per cent efficient plates currently absorb 88 per cent of the solar photons up to 6000 ,~. If this wavelength can be increased to 9000 ~, by development of suitable dyes and hosts, 88 per cent absorption of solar photons would produce LSC devices of 7.5 per cent efficiency. Recent experiments at Owens-Illinois have shown that emission in uniformly-doped thin-film hosts is anisotropic. The apparent degree of anisotropy decreases as the dye concentration increases suggesting the dominance of non-radiative energy transfer in thin-film hosts at high
327
Luminescent solar concentrators--a review Multilayered Film Structure
the fraction of molecules bleached per absorbed photon and the corresponding half-life:
A. Mixed Dye Film-Three CrlUcal Cone Losses for Blue Dye
Molecules Bleached per Photon Absorbed 10-5
c ..I
10 - 6
10-7 10 8
10-9
Half-Life 2 days 2-3 days 5-6 months 4-5 yr 40-50 yr.
B. Multilayer Dye Film-One Critical Cone Loss for Blue Dye Red Dye Green Dye Blue Dye Green Dye Red Dye
,f
Red
E
Wavelength
Fig. 6. (a) Mixed-dye film; (b) multilayer film stack, one type of dye per layer. dye concentrations. These findings further suggest efficiency advantages in cascade dye co-doping in thinfilm hosts. (3) New approaches. Kleinerman of Clark University [9] is currently attempting synthesis of an interesting class of materials for LSC plates. He is building ,r-electron systems linked by short, covalentlyattached, non-conjugating bridges such that the resultant tailor-made multicomponent dye has a high absorption coefficient throughout most of the UV and visible. Solar energy absorbed by the ,r-electron systems is cascaded via intramolecular energy transfer to a chemically bonded emissive center which fluoresces in the near IR. The success of the program relies heavily on the organic synthesis outcome. 4. DIFFICULTIES
(4) DiBiculties. The greatest single difficulty with organic LSC devices is their lack of demonstrated photostabilities. Many laser dyes, for example, degrade in luminescence output approximately exponentially with time. If we define the time required for the luminescence output to reach 1/e of its original value as the half-life, typical half-lives are less than one year at AMI[10]. Progress in long-term stability has been made, however. Wittwer et aL[16] have, in one case, measured a half-life approximating one year under outdoor field conditions. Friedman has increased dye stability by variation of host material [10]. Emphasis on the photo-degradation problem has recently increased. To give some idea of the difficulties encountered in photo-degradation studies, we list below SE Vo[ 29, No 4~E
We see immediately that only trace quantities of degradation products are produced in the range of halflives such as needed for LSCs. Even with the most sophisticated chemical techniques one is hard-pressed to identify the degradation products. Researchers must therefore resort to chemical group substitution procedures and subsequent spectroscopic characterization. These time-consuming procedures are not without merit, however. Great progress has been made in the dye laser field with just such an approach. No systematic approach to degradation studies for LSC devices has yet been reported. Photo-chemical degradation studies are currently being initiated at Owens-Illinois. These studies include degradation from various regions of the solar spectrum (e.g. UV). Dye-host degradation reactions are also of prime concern. Remmant monomer species in polymeric substrates are considered prime suspects as degradationassist centers. Current efforts at producing monomerfree polymers for degradation testing are underway[17]. Degradation in PMMA hosts themselves is currently not considered a significant problem since, according to Cole of Jet Propulsion Laboratory[9], Air Force studies have shown negligible loss in the optical properties of PMMA with seventeen years of desert sunlight exposure. Self-absorption is of course another significant problem with organic LSC devices. The design and synthesis of stable dye molecules with large Stokes shifts is central to the achievement of an efficient LSC. Only recently have systematic efforts to this end been undertaken[9]. (b) Inorganic systems (1) Progress. Inorganic systems are attractive because they offer greater photochemical stability than their organic counterparts, and they often exhibit greater Stokes shifts. Transition metal compounds (d 3 states) have been studied both in fluorescence and in phosphorescence. A simplified energy level diagram for Cr3+ is given in Fig. 7. Ionic and molecular crystals have ligand fields at the Cr3+ site sufficiently high as to cause the zero-phonon *r2 level to lie energetically higher than the zero phonon 2E level. Cr3+ emission from crystalline solids is characterized by a sharp transitions from the 2E excited state as in the case of the 700 nm line of ruby, AI203:Cr3+. Glasses and certain other few crystalline materials, on the other hand, provide low field Cr3+ sites in which the zero phonon 4T2 level lies below the zero phonon 2E state. In these low-field cases, the roomtemperature Cr3+ emission from *r2 consists of a broad, unstructured band centered in the near infrared. Quan-
328
A. M. HERMANN Cr 3+Energy levels
!~4r2 2 E Excited
A
Gla
'
New I mal"eriols [
uby ,, i
,
i
Crystal field
t
, Ground state
Fig. 7. Simplified diagram of chromium-ion energy levels vs crystal field.
tum yields for luminescences are low in glass hosts, e.g. the highest luminescent quantum yield of Cr~÷ found to date in glass is 17 per cent (silicate glass)[18]. The low quantum yields are thought to be due to lack of perfect octohedral symmetry around the Cr 3÷. Ways of providing a higher symmetry environment are currently being studied. Andrews and Lempicki[19] are incorporating the Cr 3+ ions into aluminum phosphate glass ceramics. Maintaining optical transparency in such systems which show promising absorption and emission characteristics is exceedingly difficult. They have also conducted co-doping studies in glass hosts. Since Cr 3+ is an excellent absorber of solar radiation, experiments in doping glass with both Cr3+ and Nd 3÷ (an excellent emitter) have been performed[19]. This approach to circumvent the low Cr 3+ fluorescence efficiency has been found to be successful for the luminescent Cr 3+ ions in phosphate and silicate glass. Up to 80 per cent transfer efficiency to Nd 3+ can be achieved for this subpopulation of Cr3+ ions at moderate Nd 3+ ion densities. Unfortunately, the existance of a large subpopulation of non-luminescent Cr 3+ ions severely limits this approach in its applicability to LSC devices. Another class of transition metal compounds in which the transition metal ion (typically d s) is bonded to an organic porphyrin ring is currently under investigation[20]. In this case the phosphorescence is associated with a porphyrin ring. Recent preliminary experiments[20] have shown phosphorescent quantum efficiencies in excess of 0.6 at room temperature. (2) DiBiculties. The prime difficulty is low luminescent quantum yields due to low symmetry ligand fields in all the glass hosts examined to date. It is now clear from theory and experiment that chromium in glass (independent of glass composition) is not an efficient enough emitter for LSC applications. This clarification has, in fact, produced important scientific findings[21] related to the absorption spectra of Cr 3+ doped glasses. Inhomogeneous line broadening in glass has been observed to shift sharp levels due to interaction with the quasicontinuum. But the difficulty remains: if higher symmetry ligand fields cannot be achieved in optically suitable glass, LSC applications will have to be ruled out. As for the d s compounds, one may also have to face the photostability problem in the transition metalion/porphyrin system.
(c) Hybrid devices Some attention has been paid to the possibility of construction of a hybrid LSC exploiting the best capabilities of organic and inorganic systems. A distinct possibility now being investigated by Friedman at Owens-Illinois [9] a UV-blue absorbing luminescent glass substrate to provide UV protection for and to pump radiatively an organic thin-film dye in a polymeric host on the bottom face of the glass substrate. Current studies center on glasses doped with U022*, Eu 3÷ and Nd 3÷. Quantum yields as high as 0.71 (for Cu 3+, 0.5 mole, in sodium potassium phosphate) have been achieved to date. 5. CONCLUSION Enormous progress in the understanding and quantitative characterization of LSC devices has been made since the initial studies less than ten years ago. Power conversion efficiencies ranging up to 4.5 per cent and low cost-projections speak favorably for increased research in the area. It appears that, without a major breakthrough, inorganic systems will continue to suffer from low quantum yields at luminescent species concentrations high enough to collect a large fraction of solar photons. Continued research in inorganic systems is justified, however, particularly in view of the possibility of the construction of hybrid devices. Photodegradation of organic dye systems seems to be the central problem to be addressed in that area. The stability problem has been largely sidestepped to the present. Commercially available dyes of usually unknown purity have been used and characterized for photochemical stability, but no systematic investigations of dye selection, dye synthesis, and characterization have been made. With a suitable effort, it should be possible to achieve half-lives of four to five years in the near term. A similar problem in the dye-laser project has been solved with about two man-years of effort. Self-absorption processes have not been adequately studied. Only recently have attempts at characterizing quantitatively this and other loss mechanisms been initiated. Numerical calculations for efficiency estimates based on spectroscopic data are currently being undertaken. With research directed at solving the aforementioned problems, stable, inexpensive, and efficient LSC devices can be anticipated.
Note added in proof. This group has reported recently the achievementof a luminescentquantum efficiencyof 7070 for Cr3+ in mullite(an alumina-silicateceramic). REFERENCES
1. J. S. Batchelder, A. H. Zewail and T. Cole, AppL Optics 18, 3090 (1979). 2. W. H. Weber and John Lambe, AppL Optics 15, 2299 (1976). 3. J. A. Levitt and W. H. Weber, AppL Optics 16, 2684 (1977). 4. A. Goetzbergerand W. Gruebel, Appl. Optics 14, 123 (1977). 5. B. A. Swartz, T. Cole and A. H. Zewail, Optics Lett. 1, 73 (1977). 6. C. F. Rapp and N. L. Baling, Proc. 13th IEEE Photovoltaic Spec. Conf., p. 690. IEEE, New York (1978).
Luminescent solar concentrators--a review 7. P. S. Friedman, Owens-Illinois Techn. Status Rep. 1 (11/14/80). 8. I. N. Levine, Molecular Spectroscopy. Wiley Interscience, New York (1975). 9. Luminescent Solar Concentrator Rev., Solar Energy Research Institute (Oct. 1980). 10. P. S. Friedman, Owens-Illinois Rep., July 1979-March 1980. ll. A. Goetzberger and O. Schirmer, Appl. Phys. 19, 53 (1979). 12. C. H. Cox, III, DOE/ET/20279-%Rep. available from NTIS, 5285 Port Royal Redd, Springfield,Virginia (1980). 13. This dye was suggested for study by P. Hammond at the SERI LSC Rev., Oct. 1980. It is available from Exciton Corp, and its characterization is discussed in P. R. Hammond, Optics Communications 29, 331 (1979). 14. P. S. Friedman, Owens-Illinois Final Report, Jan. 1979-June 1979.
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15. P. S. Friedman, Owens-Illinois, 1981 (private communication). 16. V. Wittwer, K. Heidler, A. Zastrow and A. Goetzberger, Proc. 14th IEEE Photovoltaic Conf. p. 760, IEEE, San Diego (198o). 17. A. Gupta, Jet Propulsion Laboratory. Private communication (1981). 18. L. J. Andrews, A. Lempicki and B. C. McCollum(preprint). 19. A. Lempicki, L. Andrews and B. C. McCollum, GTE Ann. Progress Rep., 1 Sept. 1979-31 Dec. 1980. 20. P. O'D. Offenhartz and R. H. Micbeels, EIC Project Status Rep. June 1980-Dec. 1980. 21. A. Lempicki, L. Andrews, S. J. Nettel and B. C. McCoUum, Phys. Rev. Lett. 44, 1234(1980).