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Solar energy concentrator based on uranyl-doped PMMA
Solar Energy Materials 21 (1991) 327-334 North-Holland
327
Solar energy concentrator based on uranyl-doped P M M A K.K. Pandey and T.C. Pant Photophysics Laboratory, Department of Physics, DSB Campus, Kumaun University, Nainital 263 002, India Received 6 December 1989 A luminescent solar energy concentrator based on uranyl-ion-doped in poly(methyl methacrylate) (PMMA) has been suggested. A qualitative study shows that alkaline species of uranyl ion with high fluorescence intensity can be a good activator for LSC.
I. Introduction
Luminescent solar collectors (LSC) have been proposed for solar energy conversion into electrical energy. Garvin [1], and Weber and Lambe [2] were first to suggest the principle of operation of LSC. The operation of LSC is based on the principle of light trapping by total internal reflection. Transparent materials like PMMA and inorganic glasses having high refractive indices are doped with efficient luminescent molecules or ions having strong absorption bands in the region 400-700 nm. Solar radiation is absorbed and re-emitted as luminescence, a large fraction ( - 75% for PMMA) of which is trapped inside the collector medium due to total internal reflection (fig. 1). After a number of reflections the luminescence radiation
I
FI
F2
~ Coe lco tin
Fig, 1. Principle of luminescence collector. The luminescent molecule interacts with incoming light beam 1. Secondary beams are partly re-emitted (F1) and partly guided in the transparent material (F2, F3, F4) (after ref. [12]). 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
328
K K. Pand,~v. 12 (
t'am
Soklr encr~l c~mcentralor ha.sed un uranv/ d~q~ed t'.I/ M 4
reaches the edge of the plate where a suitable photovoltaic cell converts it irm, electrical energy. The chief advantage of LSC is that the cost of electrical conversion can be reduced provided a viable system comes up which, however, is still far from realisation in practice. One of the important problems in the development of LSC is to find an activator which absorbs strongly in the most intensive part of solar radiation lying in the region 400-900 nm and convert the absorbed energy into luminescence with maximum possible efficiency. It is desired that the luminescence matches the spectral region of sensitivity of the photovoltaic cell used. Thus, the relatively low efficiency of solar cells exposed to white light can be circumvented by coupling a LSC to an appropriate photovoltaic cell and hence solar spectrum can be used more efficiently. The other requirements of activator used in LSC are: (a) separation of absorption from emission band in order to reduce the self absorption and (b) good photochemical stability. Organic dyes are considered good luminescent materials for LSC purpose, because these have large absorption over a wide spectrum range and higher quantum efficiencies than inorganic materials. But disadvantages of existing organic dyes as luminescent collector material are high photodecomposition and large overlap of their absorption and emission spectra. Overlapping of absorption and emission spectra leads to a considerable self absorption of luminescent light along the path of collector resulting in loss of photons in the output edge. Uranyl-ion-doped inorganic glasses have attracted considerable attention for the development of LSC [3-7]. The utility of uranyl ions for LSC is based on the following facts: (i) due to large broad-banded absorption of uranyl ions in the near UV and visible region, a considerable part of the solar spectrum can be absorbed; (ii) uranyl ions fluoresce with reasonably good quantum efficiency and the emission is at longer wavelength where the cell efficiency is higher than at the wavelength of absorbed solar flux though this fluorescence does not fall in the spectral range of maximum sensitivity of the silicon cell ( - 800 nm); (iii) high photostability; and (iv) very small overlap between emission and absorption spectra. Further, incorporation of uranyl ions in PMMA matrix may be more useful because on the one hand PMMA bears all properties required for a good LSC base material (e.g. highly transparent and high refractive index), on the other hand it is easy to prepare PMMA sheets of desired shape and size. From the cost consideration also PMMA collectors are projected to be economical. Therefore, in the present work the fluorescence characteristics of uranyl-ion-doped PMMA has been presented.
2. Experimental Uranyl ions were successfully incorporated in PMMA matrix and it was possible to obtain transparent sheets of uranyl-ion-doped PMMA. For this, a desired quantity of uranyl nitrate dissolved in methanol along with an appropriate quantity of PMMA grains were mixed with chloroform. This uranyl n i t r a t e - P M M A mixture
329
K.K. Pandey, T.C. Pant / Solar energy concentrator based on uranyl-doped PMM,4
in chloroform was slowly heated in an incubator to a temperature of - 60 ° C with intermittant stirring. The homogeneous mass thus obtained was poured in a glass container to obtain transparent sheets of required shape and size and allowed to dry. Thus the transparent sheets of PMMA (thickness - 0 . 1 1 cm) doped with uranyl nitrate (of various concentrations) were made. However, it was found that there was a limit on the maximum concentration (0.5M) that can be incorporated without reducing the transparency of P M M A sheets. Higher concentration ( > 0.5M) results in opaque sheets unsuitable for LSC purpose. Absorption spectra were recorded using a Beckman D K 2A spectrophotometer. Emission spectra were recorded with the help of a Jobin Yvon HRS 2 monochromatot having a dispersion of 12 A / m m , using a R 446 photomultiplier tube. The sample was excited by the 3650 A line from a high-pressure mercury discharge lamp having Wood's filter. 3. Results and discussion
The emission spectra of PMMA show a dependence on uranyl ion concentration. The emission spectra for four uranyl nitrate concentrations are shown in fig. 2. The
50
40
i
i L t
d
IL
I
~
30~
i
T t/)
,/ ¢7 <(
2o c o
i
600
580
540
500
460
Wavelength(nm)
Fig. 2. Emission spectra of uranyl nitrate in PMMA; concentrations of uranyl nitrate are (a) 0.5M, (b) 0.1M, (c) 0.01M and (d) 0.001M.
330
K.K. Pandey. 7~(. Pant , Solar energy concentrator based on uranv/-doped P.{IM.I
most concentrated sample (curve a) shows a series of bands at 4850, 5080 and 566{! ,~ (named a band). This spectrum corresponds to solid uranyl nitrate hexahydrate [8]. At low concentration (curve c) another series of bands (named "y bandL displaced towards the longer wavelength by - 100 ,~, appears and becomes more prominent with dilution. Still further dilution (concentration 0.001 M) of the sample results in a red-shifted spectrum (curve d). These fluorescence spectra in mosz respects are identical to that observed by Pant and Khandelwal [9] in methanol. Changes in the spectra with concentration are attributable to change in p H of the sample and consequent formation of different species of uranyl ion [8,9]. 50
? /
/
/
40
J
30
c
A 2O N m
J
Q/
I
5
10
580
540 ~r
500
460
Wavelength (nm)
Fig. 3. Effect of alkali (NaOH) addition on emission spectrum of uranyl nitrate (concentration 0.08M); quantities of NaOH (concentration 0.1M) are (a) 0.0 ml, (b) 0.001 ml, (c) 0.005 ml, (d) 0.008 ml and (e) 0.02 ml.
K.K. Pandey, T.C. Pant / Solar energy concentrator based on uranyl-doped PMMA
331
The observations were also made by changing the pH of the sample (by addition of NaOH). The emission spectra of uranyl nitrate in PMMA with successive addition of alkali (NaOH) to a 0.08M concentration sample are shown in fig. 3. It can be noted from these spectra that increasing pH of the sample results finally in a entirely new red-shifted broad band spectrum. It can be seen from fig. 3 that this final species fluoresces strongly - the fluorescence intensity increases approximately six times as compared to the sample without alkali in the present case. The matrix PMMA also remains transparent under such condition. Addition of alkali in excess makes the PMMA matrix opaque. The enhancement in fluorescence intensity on addition of alkali appears to be due to the increased value of the molar extinction coefficient (e). It has been established by other workers [8-11] that in moderate concentrations and in acidic nitrate solutions, the uranyl ion exists in the UO 2÷ form which corresponds to a band, while on addition of acid (HNO3) additional species like UO2NO~- are also present; on the other hand dilution or increasing pH results in formation of hydrolysed species like UO3UO:2+ (which corresponds to ~/bands). Sutten [10], and
50
40
t~ C
30
x~
m c
20 c
10
640
600
560
520
Wovelength (nm) Fig. 4. Edge emission spectra of rhodamine 6G in E M M A for various edge distances: (a) 0.0 cm, (b) 0.7 cm, (c) 1.5 cm, (d) 2.5 cm and (e) 5.0 cm.
332
K. K. Panda% 11 ( . Pant , .%'olar energy concentrator based . n uram'/-dopcd ~ ' t i m 4
Betts and Michels [11] have obtained molar extinction coefficient (c) values at 414> for separate species UO~ +, UO3UO~ + and UO2NO f of 7.8, 67.0 and 15.5, respectively. Pant and Khandelwal [8.9] have also concluded that 0.1M aqueou,'; uranyl nitrate solution has ~ = 8.2 which falls to 7.8 on slight acidulation which i,-; due to the UO~ ~ form of uranyl ion. On addition of alkali, ~ increases due to the formation of the hydrolysed species UO: UO~ ~ . The change in the nature of the spectrum (i.e. the broad band spectrum with high intensity) upon further addition of alkali to a still higher content may be due to formation of more complex hydrolysed species like (UO3),,UO,2+ (n = 1, 2, 3 . . . . ). Loss of radiation energy due to self absorption of the luminescent molecule is another important aspect which must be reduced to minimum while developing a viable LSC. Small (as small as possible) overlap between absorption and emission spectra is required to obtain lossless emission at the edge of the LSC. As an illustration to show the effect of self absorption on the output from the edge of an LSC, the emission spectra of a dye (rhodamine 6G) obtained from the edge for various distances of the edge from the centre of the LSC plate where excitation light
50
40 !
30
D
x~
"2 g
20
C
10
580
540 •~
Wavelength(rim
500
460
)
Fig. 5, Edge emission spectra of uranyl nitrate (concentration 0,5M) in P M M A for various edge distances: (A) 0.0 cm, (B) 0.5 cm, (C) 1.5 cm and (D) 3.0 cm.
K.K. Pandey, T.C. Pant
/ Solar
333
energy concentrator based on uranyl-doped PMMA
50 b
T
4O
T
.3
u) E
3O
P
20 '~ (DO
c
.1
10
i
380
420
500
460
540
Wavelength ( n m ) Fig. 6. (a) Emission spectrum of uranyl nitrate (concentration 0.5M). (b) Absorption spectrum of uranyl nitrate in PMMA.
is incident are shown in fig. 4. Similar spectra for uranyl-doped sheet are also shown in fig. 5. It is observed that whereas in the case of the dye the shorter wavelength portion of the spectrum is successively chopped away with increasing distance of the edge, in the case of uranyl ions there is no such effect in the spectrum. This is due to the very small overlap between the emission and absorption spectra of uranyl nitrate (fig. 6) as compared to rhodamine 6G (fig. 7). Thus in the case of uranyl ions the loss of intensity due to self absorption is negligible and this points out its utility for
•4
b
a
40
-3
30
C
c L
-6 .2
20
ul
o
-1
10
480
540
Wclvelength (nm)
600 •
Fig. 7. (a) Emission spectrum of rhodamine 60. (b) Absorption spectrum of rhodamine 6G in PMMA (concentration 5.0 x 10-4M).
334
K.K. Pander, 71 ('r Pant / S o l a r energy concentrator based ml urany/-doped t~MM.4
LSC. It can be m e n t i o n e d that for better efficiency u r a n y l - b a s e d LSC can ht coupled to a C d S - C u 2 S or G a A S photovoltaic cell (having a sensitivity range l'ron~ 400 to 960 n m a n d from 490 to 880 nm, respectively [7]), since the emissiun wavelength ()~,,~x = 530 nm) is close to the b a n d gap of m a x i m u m sensitivity c~l these cells.
4. Conclusion Thus, it is qualitatively c o n c l u d e d that the alkaline species of u r a n y l ion in P M M A having b r o a d b a n d a n d red-shifted fluorescence with high fluorescence intensity can be a good l u m i n e s c e n t material for solar collectors.
Acknowledgement F i n a n c i a l s u p p o r t from CSIR, New Delhi, is gratefully acknowledged.
References [1] R.L. Garwin, Rev. Sci. Instr. 31 (1960) 1010. [2] W.H. Weber and J. Lambe, Optics 15 (1976) 2299. [31 R. Reisfeld and Y. Kalisky, Nature 283 (1980) 281. [4] R. Reisfeld and S. Newman, Nature 274 (1978) 144. [5] R. Reisfeld and C.K. Jorgensen, Struct. Bonding 49 (1982) 1. [6] G. Folcher, N. Keller and J. Paris, Sol. Energy Mater. 10 (1984) 303. [7] N. Neuroth and R. Haspal, Sol. Energy Mater. 16 (1987) 235. [8] D.P. Khandelwal and D.D. Pant, in: Proc. Indian Acad. Sci. Vol. L(5) (1959) 323: L I(2) (1960) 60. [91 D.P. Khandelwal and D.D. Pant, Curr. Sci. 26 (1957) 282. [10] J. Sutton, J. Chem. Soc. Suppl, 2 (1949) $225. [11] R.H. Betts and R.K. Michels, J. Chem. Soc. Suppl. 2 (1949) $286. [12] A. Goetzberger and W. Gruebel, Appl. Opt. 14 (1977) 123.
327
Solar energy concentrator based on uranyl-doped P M M A K.K. Pandey and T.C. Pant Photophysics Laboratory, Department of Physics, DSB Campus, Kumaun University, Nainital 263 002, India Received 6 December 1989 A luminescent solar energy concentrator based on uranyl-ion-doped in poly(methyl methacrylate) (PMMA) has been suggested. A qualitative study shows that alkaline species of uranyl ion with high fluorescence intensity can be a good activator for LSC.
I. Introduction
Luminescent solar collectors (LSC) have been proposed for solar energy conversion into electrical energy. Garvin [1], and Weber and Lambe [2] were first to suggest the principle of operation of LSC. The operation of LSC is based on the principle of light trapping by total internal reflection. Transparent materials like PMMA and inorganic glasses having high refractive indices are doped with efficient luminescent molecules or ions having strong absorption bands in the region 400-700 nm. Solar radiation is absorbed and re-emitted as luminescence, a large fraction ( - 75% for PMMA) of which is trapped inside the collector medium due to total internal reflection (fig. 1). After a number of reflections the luminescence radiation
I
FI
F2
~ Coe lco tin
Fig, 1. Principle of luminescence collector. The luminescent molecule interacts with incoming light beam 1. Secondary beams are partly re-emitted (F1) and partly guided in the transparent material (F2, F3, F4) (after ref. [12]). 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
328
K K. Pand,~v. 12 (
t'am
Soklr encr~l c~mcentralor ha.sed un uranv/ d~q~ed t'.I/ M 4
reaches the edge of the plate where a suitable photovoltaic cell converts it irm, electrical energy. The chief advantage of LSC is that the cost of electrical conversion can be reduced provided a viable system comes up which, however, is still far from realisation in practice. One of the important problems in the development of LSC is to find an activator which absorbs strongly in the most intensive part of solar radiation lying in the region 400-900 nm and convert the absorbed energy into luminescence with maximum possible efficiency. It is desired that the luminescence matches the spectral region of sensitivity of the photovoltaic cell used. Thus, the relatively low efficiency of solar cells exposed to white light can be circumvented by coupling a LSC to an appropriate photovoltaic cell and hence solar spectrum can be used more efficiently. The other requirements of activator used in LSC are: (a) separation of absorption from emission band in order to reduce the self absorption and (b) good photochemical stability. Organic dyes are considered good luminescent materials for LSC purpose, because these have large absorption over a wide spectrum range and higher quantum efficiencies than inorganic materials. But disadvantages of existing organic dyes as luminescent collector material are high photodecomposition and large overlap of their absorption and emission spectra. Overlapping of absorption and emission spectra leads to a considerable self absorption of luminescent light along the path of collector resulting in loss of photons in the output edge. Uranyl-ion-doped inorganic glasses have attracted considerable attention for the development of LSC [3-7]. The utility of uranyl ions for LSC is based on the following facts: (i) due to large broad-banded absorption of uranyl ions in the near UV and visible region, a considerable part of the solar spectrum can be absorbed; (ii) uranyl ions fluoresce with reasonably good quantum efficiency and the emission is at longer wavelength where the cell efficiency is higher than at the wavelength of absorbed solar flux though this fluorescence does not fall in the spectral range of maximum sensitivity of the silicon cell ( - 800 nm); (iii) high photostability; and (iv) very small overlap between emission and absorption spectra. Further, incorporation of uranyl ions in PMMA matrix may be more useful because on the one hand PMMA bears all properties required for a good LSC base material (e.g. highly transparent and high refractive index), on the other hand it is easy to prepare PMMA sheets of desired shape and size. From the cost consideration also PMMA collectors are projected to be economical. Therefore, in the present work the fluorescence characteristics of uranyl-ion-doped PMMA has been presented.
2. Experimental Uranyl ions were successfully incorporated in PMMA matrix and it was possible to obtain transparent sheets of uranyl-ion-doped PMMA. For this, a desired quantity of uranyl nitrate dissolved in methanol along with an appropriate quantity of PMMA grains were mixed with chloroform. This uranyl n i t r a t e - P M M A mixture
329
K.K. Pandey, T.C. Pant / Solar energy concentrator based on uranyl-doped PMM,4
in chloroform was slowly heated in an incubator to a temperature of - 60 ° C with intermittant stirring. The homogeneous mass thus obtained was poured in a glass container to obtain transparent sheets of required shape and size and allowed to dry. Thus the transparent sheets of PMMA (thickness - 0 . 1 1 cm) doped with uranyl nitrate (of various concentrations) were made. However, it was found that there was a limit on the maximum concentration (0.5M) that can be incorporated without reducing the transparency of P M M A sheets. Higher concentration ( > 0.5M) results in opaque sheets unsuitable for LSC purpose. Absorption spectra were recorded using a Beckman D K 2A spectrophotometer. Emission spectra were recorded with the help of a Jobin Yvon HRS 2 monochromatot having a dispersion of 12 A / m m , using a R 446 photomultiplier tube. The sample was excited by the 3650 A line from a high-pressure mercury discharge lamp having Wood's filter. 3. Results and discussion
The emission spectra of PMMA show a dependence on uranyl ion concentration. The emission spectra for four uranyl nitrate concentrations are shown in fig. 2. The
50
40
i
i L t
d
IL
I
~
30~
i
T t/)
,/ ¢7 <(
2o c o
i
600
580
540
500
460
Wavelength(nm)
Fig. 2. Emission spectra of uranyl nitrate in PMMA; concentrations of uranyl nitrate are (a) 0.5M, (b) 0.1M, (c) 0.01M and (d) 0.001M.
330
K.K. Pandey. 7~(. Pant , Solar energy concentrator based on uranv/-doped P.{IM.I
most concentrated sample (curve a) shows a series of bands at 4850, 5080 and 566{! ,~ (named a band). This spectrum corresponds to solid uranyl nitrate hexahydrate [8]. At low concentration (curve c) another series of bands (named "y bandL displaced towards the longer wavelength by - 100 ,~, appears and becomes more prominent with dilution. Still further dilution (concentration 0.001 M) of the sample results in a red-shifted spectrum (curve d). These fluorescence spectra in mosz respects are identical to that observed by Pant and Khandelwal [9] in methanol. Changes in the spectra with concentration are attributable to change in p H of the sample and consequent formation of different species of uranyl ion [8,9]. 50
? /
/
/
40
J
30
c
A 2O N m
J
Q/
I
5
10
580
540 ~r
500
460
Wavelength (nm)
Fig. 3. Effect of alkali (NaOH) addition on emission spectrum of uranyl nitrate (concentration 0.08M); quantities of NaOH (concentration 0.1M) are (a) 0.0 ml, (b) 0.001 ml, (c) 0.005 ml, (d) 0.008 ml and (e) 0.02 ml.
K.K. Pandey, T.C. Pant / Solar energy concentrator based on uranyl-doped PMMA
331
The observations were also made by changing the pH of the sample (by addition of NaOH). The emission spectra of uranyl nitrate in PMMA with successive addition of alkali (NaOH) to a 0.08M concentration sample are shown in fig. 3. It can be noted from these spectra that increasing pH of the sample results finally in a entirely new red-shifted broad band spectrum. It can be seen from fig. 3 that this final species fluoresces strongly - the fluorescence intensity increases approximately six times as compared to the sample without alkali in the present case. The matrix PMMA also remains transparent under such condition. Addition of alkali in excess makes the PMMA matrix opaque. The enhancement in fluorescence intensity on addition of alkali appears to be due to the increased value of the molar extinction coefficient (e). It has been established by other workers [8-11] that in moderate concentrations and in acidic nitrate solutions, the uranyl ion exists in the UO 2÷ form which corresponds to a band, while on addition of acid (HNO3) additional species like UO2NO~- are also present; on the other hand dilution or increasing pH results in formation of hydrolysed species like UO3UO:2+ (which corresponds to ~/bands). Sutten [10], and
50
40
t~ C
30
x~
m c
20 c
10
640
600
560
520
Wovelength (nm) Fig. 4. Edge emission spectra of rhodamine 6G in E M M A for various edge distances: (a) 0.0 cm, (b) 0.7 cm, (c) 1.5 cm, (d) 2.5 cm and (e) 5.0 cm.
332
K. K. Panda% 11 ( . Pant , .%'olar energy concentrator based . n uram'/-dopcd ~ ' t i m 4
Betts and Michels [11] have obtained molar extinction coefficient (c) values at 414> for separate species UO~ +, UO3UO~ + and UO2NO f of 7.8, 67.0 and 15.5, respectively. Pant and Khandelwal [8.9] have also concluded that 0.1M aqueou,'; uranyl nitrate solution has ~ = 8.2 which falls to 7.8 on slight acidulation which i,-; due to the UO~ ~ form of uranyl ion. On addition of alkali, ~ increases due to the formation of the hydrolysed species UO: UO~ ~ . The change in the nature of the spectrum (i.e. the broad band spectrum with high intensity) upon further addition of alkali to a still higher content may be due to formation of more complex hydrolysed species like (UO3),,UO,2+ (n = 1, 2, 3 . . . . ). Loss of radiation energy due to self absorption of the luminescent molecule is another important aspect which must be reduced to minimum while developing a viable LSC. Small (as small as possible) overlap between absorption and emission spectra is required to obtain lossless emission at the edge of the LSC. As an illustration to show the effect of self absorption on the output from the edge of an LSC, the emission spectra of a dye (rhodamine 6G) obtained from the edge for various distances of the edge from the centre of the LSC plate where excitation light
50
40 !
30
D
x~
"2 g
20
C
10
580
540 •~
Wavelength(rim
500
460
)
Fig. 5, Edge emission spectra of uranyl nitrate (concentration 0,5M) in P M M A for various edge distances: (A) 0.0 cm, (B) 0.5 cm, (C) 1.5 cm and (D) 3.0 cm.
K.K. Pandey, T.C. Pant
/ Solar
333
energy concentrator based on uranyl-doped PMMA
50 b
T
4O
T
.3
u) E
3O
P
20 '~ (DO
c
.1
10
i
380
420
500
460
540
Wavelength ( n m ) Fig. 6. (a) Emission spectrum of uranyl nitrate (concentration 0.5M). (b) Absorption spectrum of uranyl nitrate in PMMA.
is incident are shown in fig. 4. Similar spectra for uranyl-doped sheet are also shown in fig. 5. It is observed that whereas in the case of the dye the shorter wavelength portion of the spectrum is successively chopped away with increasing distance of the edge, in the case of uranyl ions there is no such effect in the spectrum. This is due to the very small overlap between the emission and absorption spectra of uranyl nitrate (fig. 6) as compared to rhodamine 6G (fig. 7). Thus in the case of uranyl ions the loss of intensity due to self absorption is negligible and this points out its utility for
•4
b
a
40
-3
30
C
c L
-6 .2
20
ul
o
-1
10
480
540
Wclvelength (nm)
600 •
Fig. 7. (a) Emission spectrum of rhodamine 60. (b) Absorption spectrum of rhodamine 6G in PMMA (concentration 5.0 x 10-4M).
334
K.K. Pander, 71 ('r Pant / S o l a r energy concentrator based ml urany/-doped t~MM.4
LSC. It can be m e n t i o n e d that for better efficiency u r a n y l - b a s e d LSC can ht coupled to a C d S - C u 2 S or G a A S photovoltaic cell (having a sensitivity range l'ron~ 400 to 960 n m a n d from 490 to 880 nm, respectively [7]), since the emissiun wavelength ()~,,~x = 530 nm) is close to the b a n d gap of m a x i m u m sensitivity c~l these cells.
4. Conclusion Thus, it is qualitatively c o n c l u d e d that the alkaline species of u r a n y l ion in P M M A having b r o a d b a n d a n d red-shifted fluorescence with high fluorescence intensity can be a good l u m i n e s c e n t material for solar collectors.
Acknowledgement F i n a n c i a l s u p p o r t from CSIR, New Delhi, is gratefully acknowledged.
References [1] R.L. Garwin, Rev. Sci. Instr. 31 (1960) 1010. [2] W.H. Weber and J. Lambe, Optics 15 (1976) 2299. [31 R. Reisfeld and Y. Kalisky, Nature 283 (1980) 281. [4] R. Reisfeld and S. Newman, Nature 274 (1978) 144. [5] R. Reisfeld and C.K. Jorgensen, Struct. Bonding 49 (1982) 1. [6] G. Folcher, N. Keller and J. Paris, Sol. Energy Mater. 10 (1984) 303. [7] N. Neuroth and R. Haspal, Sol. Energy Mater. 16 (1987) 235. [8] D.P. Khandelwal and D.D. Pant, in: Proc. Indian Acad. Sci. Vol. L(5) (1959) 323: L I(2) (1960) 60. [91 D.P. Khandelwal and D.D. Pant, Curr. Sci. 26 (1957) 282. [10] J. Sutton, J. Chem. Soc. Suppl, 2 (1949) $225. [11] R.H. Betts and R.K. Michels, J. Chem. Soc. Suppl. 2 (1949) $286. [12] A. Goetzberger and W. Gruebel, Appl. Opt. 14 (1977) 123.