Neodymium-chromium doped phosphate glasses as luminescent solar concentrators

Solar Energy Materials 13 (1986) 267-277 North-Holland, A m s t e r d a m

NEODYMIUM-CHROMIUM

267

DOPED

PHOSPHATE

GLASSES

AS

L U M I N E S C E N T SOLAR C O N C E N T R A T O R S B. JEZOWSKA-TRZEBIATOWSKA, E. L U K O W I A K and W. STRI~K Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw. Poland

A. BUCZKOWSKI, S. PATELA and J. RADOJEWSKI Institute of Electron Technology, Technical University of Wroctaw, Wroctaw, Poland J.

SARZYI~/SKI

Jenenia Gbra Optical Works, Jelenia Gbra, Poland Received 16 July 1985; in revised form 16 December 1985 A luminescent solar concentrator based on N d 3+ and Cr 3 + doubly doped lithium aluminium phosphate glass for solar energy conversion applications has been presented. Absorption and fluorescence spectra and q u a n t u m yield of lithium aluminium phosphate glasses are reported. The dependence of the light concentration coefficient versus dopant concentration and attenuation of the guided light in the luminescent solar concentrator have been studied. The best results were obtained for glass doped with 0.07 x 1020 i o n s / c m 3 of Cr 3 + and 6.97 × 10 20 i o n s / c m 3 of Nd 3+. In addition, technology and properties of silicon solar cells operating with luminescent solar concentrators have been briefly outlined.

1. I n t r o d u c t i o n

Recent research in a number of laboratories in France, Israel, Germany, UK, USSR and USA was directed toward utilizing the luminescent solar concentrators (LSCs) to enhance the conversion efficiency of solar conversion devices (for reviews see refs. [1-3]). In the planar LSC containing fluorescent dopants the solar radiation absorption band does not overlap (or only to a small extent) the emission band matching the Si bandgap. A schematic diagram of the LSC with solar cells at its edge is shown in fig. 1. Potential advantages of the LSC are numerous: - no need for sun tracking, - concentration of both direct and diffuse incident light, - infrared radiation is not focused on the solar cell, - luminescent centres can be chosen to match the spectral characteristics of the solar cells. The gain of photon flux of the LSC is defined as [1]: A face Gf = -A -edge '17trap'l~abs~ lum~ par,

0165-1633/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

268

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incident

mirrored edge '

/•

\~....

/ ..........

.\e,)dvmium

~hr,~mmm dopcd pho~piunc ~la~c~

reflected tight

~.1 I

:

. ..

I/I/""

I

luminescent centre

...

~ untrapped re- emitted

,

" .......

~u ~

photovoituic celts

guided trapped and tight re-emitted

I,g~t Fig. 1. Schematic diagram of the LSC with a solar cell.

where rl,,b~ is the absorption efficiency, being the fraction of solar flux absorbed by luminescent centres, ~ltr,p = cos 0~ is the trapping efficiency where @ is the critical angle for total reflection corresponding to the critical cone loss of the isotropic luminescence Tlj..... which denotes the centre luminescence efficiency being the ratio between the number of photons emitted and the number of points absorbed, and Tip~r is the parasitic efficiency due to inter alia self-absorption of luminescence radiation by the emitting centres or absorption and scattering by imperfections in the LSC. The typicals values of the above efficiencies are the following (Goetzberger et al. [3]): TI,r,p= 75%, "l~abs 10-20%, "q~u,,,= 70-95%, Tip~r= 30-40%. Af .... and A~dge are the areas of the LSC face and edge respectively. In order to compare the different LSC glasses, the light concentration efficiency, C, has also been defined, which is the ratio of the output power density to the incident power density for a unit LSC. Various materials have been used as matrices and fluorescent centres for LSCs. Organic and inorganic glasses are the most common ones. However, organic LSCs though cheaper, are less photochemically stable and vulnerable to surface damage [2]. Among various inorganic planar LSCs, the glasses containing Cr 3-~ are the most promising candidates. The Cr -~÷ possesses broad spin-allowed absorption bands in the visible and UV regions assuring high absorption efficiency. Due to the weak ligand field the Cr 3+ luminescence in glasses is attributed mainly to a T 2 -*4A 2 fluorescence with 7, ...... = 850 nm well matching the Si bandgap. Its practical application is, however, greatly limited because the quantum yield of fluorescence is rather low. Another attractive candidate for the LSC is Nd 3 + ions, also fluorescing in the infrared region. The fluorescent quantum yield in glasses is high in comparison with that for Cr 3+. However, its absorption efficiency is much lower. An idea to improve the photon flux Gf in case of Nd 3~ ions in the glass LSC devices by Cr ~ ~ --, Nd 3+ energy transfer was earlier postulated by Andrews et al. [3]. Recently the problem of energy transfer Cr 3+ ~ Nd 3 + has been investigated in phosphate glasses in ref. [4]. =

B. Jekowska-Trzebiatowska et al. / Neodymium- chromium doped phosphate glasses

269

Table 1 Quantum yields of Cr 3+ and Nd 3+ doped glasses Concentration [10 20 i o n s / c m 3] Cr s+

Nd 3+

0.012 0.071 2.4

2.4 2.4 2.4

Quantum yields Nd3+

Cr 3+

4F3/2 __.4 i9/2

4F3/2 __.4 i9/2

0.05 0.06 0.07

0.28 0.22 0.15

4T 2 ---~4A. ~ 0.07 0.09 0.08

0.40 0.36 0.30

In this paper we report on the properties of LSCs made of Cr 3+ and Nd 3+ doubly doped lithium aluminium phosphate glasses.

1.1. Glass preparation The lithium aluminium phosphate glass was prepared first from the base glass with the composition of 80.53 mol% P205, 11.16 mol% Li20 and 8.31 mol% A1203. Then the glass was powdered and melted, adding appropriate amounts of N d 2 0 s and C r 2 0 3 in a platinum crucible at temperatures of 1200-1300°C. The dopant concentrations are given in table 1.

1.2. Spectroscopic properties Absorption spectra were recorded at room temperature on a Specord M 40 spectrophotometer. The spectrum for 0.03 mol% Cr203 and 3.00 mol% Nd203 doubly doped glass is shown in fig. 2. For other concentrations the spectra look similar. In the region 25 000-11 000 cm-~ it consists of two broad bands attributed to the 4A 2 _._.~4T2 (~max = 15300 cm -1) a n d 4A 2 ___+4TI (~max = 21 800 cm -1) transitions. The sharp peaks located at the contours of the Cr(III) transitions arise from the Nd(III) transitions. Fluorescence spectra were measured with the apparatus and are described elsewhere [4]. The typical luminescence spectrum of the doubly doped glass is shown in fig. 3. It consists of the broad 4T2 - * 4 A 2 fluorescence band of Cr(III) centered at 11 500 cm-1. The two fluorescence bands of Nd(III) correspond to 4F3-.-.~4 Ill/2 and 4F3/2 ---~4 I9/2 transitions located at 9400 and 11400 cm -~, respectively. Only the fluorescence bands located at the region 11000-12000 cm-1 are useful for the LSC, so only the resonant 4F3/2 ._,4 i9/2 transition although less intensive, is effective for solar energy conversion using silicon cells. We determined the quantum yields for Cr(III) and Nd(III) doped glasses for three selected samples. They are listed in table 1. The fluorescence quantum yields for Cr(III) and Nd(III) doped glasses were determined using a modified Vavil0v method of measurement by a direct comparison between the number of emitted and absorbed quanta. The measurements were performed on powdered samples of doped glass and the reference (undoped base glass) in a standard system for fluorescence measurement by the reflection method.

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/-,00 J

500

chrommm dopc,d phov~hatc ~/ax ~cs

600 i

700 [ nm] i

A

1,0

0,5

0,0 25000

s6oo

Fig. 2. Absorption spectrum for 0,03 mol% Cr203 and 3.00 mol% Nd203 doubly doped glass.

The glass samples were pulverized to a fine powder of granulation of about 0.01 mm. The number of absorbed quanta per unit time was determined from the known power of the exciting beam. The error of measurement is less than 15%. The measurement method is described elsewhere [5]. The highest yields were found for the lowest Cr(III) concentration. It is seen from table 1 that with an increase of Cr(III) concentration the fluorescence quantum yield of Nd(III) changes in a different way with respect to 4F3/2 _.__~419/2 and 4F3/2 __,4 ill/2 transitions. For the first we have observed a strong decrease of quantum yield whereas for the second a slight increase was noticed. An enhancement of 4F3/2 ~4 ill/2 fluorescence may be elucidated with the energy transfer Cr ---, Nd. For the resonant 4Fs/2 ~4 i9/2 transition a quite different process was observed. This transition overlaps the absorption and linked with the 4A 2 _..~4 72 transition of Cr(III) so its result is strongly quenched due to the Nd--* Cr energy transfer. For the Cr(III) fluorescence the effect of chromium concentration does not affect significantly the quantum yield within the experimental error. So the total quantum yield decreases with increasing Cr(III) concentration in a less extent than the resonant neodymium 4E~/~ ___~4i9/2 transition.

272

B. Je?.onska-Trzehtatowxka et al. / N e o d y m i u m chromium d.ped plu~vphatc ~/a~ ~c,

incident light

AI mirrors

mirror

k ~

solar ceil s

glass Fig. 4. The right-angle triangle LSC with solar cells.

coefficients of the guided light. In the latter case the trapped and guided re-emitted light beams reflected from the reflected edge have been excluded. The more optimal shape of the LSC is employed, the more light intensity from the unit LSC area reaches the solar cell. For each glass the characteristic length called the maximal LSC dimension, L . . . . has been calculated as the distance at which the intensity of the guided light decreases by 6 dB (i.e. twice as low). This dimension should not be exceeded in order to avoid excess losses of optical energy before it reaches the LSC edge. One of the best shapes was found among right-angle triangles with solar cells attached to the hypotenuse, and the two other sides covered with aluminium (fig. 4). Since solar energy absorption in the glasses is relatively now, a mirror should be placed under the glass plate. Aluminium mirror-film must not be deposited directly onto the back surface of the concentrator because the metallic film would cause high attenuation of the guided light.

3. Design and properties of silicon solar cells Solar cells connected with the LSC should have a different construction from that designed for AM1 insolation [6]. The specific requirements are determined by a higher illumination level, narrower spectral band and changed reflection conditions, since a cell is no longer surrounded by air but attached to chromium-neodymium glass. The solar cell under investigation was designed including a minimizing

B. Jekowska-Trzebiatowska et al. / Neodymium- chromium doped phosphate glasses

.4

273

A\

R .3

2 .2

\

.4

.6

.8

1.0 % ~ml

Fig, 5. The calculated spectra reflectivity for various antireflection coatings: (1) Si-Ta205 107 nm, air, (2) Si-Ta205 107 nm, glass, (3) Si-Ta205 90 nm, SiO 2 70 nm, air.

procedure of reflectance at LSC wavelengths, selection of the appropriate siliconsubstrate doping concentration, optimization of the junction depth and front electrode grid. A cell was optimized for an illumination intensity equivalent to 10 suns at a wavelength of 900 nm. The optimal antireflection-coating parameters calculated for a single layer of Ta205 on the silicon cell attached to the LSC are presented in fig. 5. For comparison the relations of the reflectance for the cell covered with a single and double-layered antireflection coating and surrounded by air are also included therein.

1.0 R colt .8

.6

.4

.2

.4

.6

.8

n~3 1.0

[~m]

Fig. 6, The carrier collection efficiency as a function of wavelength for various junction depths.

274

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cm-3

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0,8

0,6

0,~

O 12

0,~

I

i

I-

0,6

0,8

I~0 x [~m]

Fig. 7. The carrier collection efficiency as a function of wavelength for various substrate doping concentrations.

As mentioned above the proper choice of the p - n junction depth is one of the most important problems in solar cell design. A shallow junction depth makes it possible to lower the losses due to electron-hole recombination processes inside the

1.0

.6

.4

Rs= 10 ohm/ct IF: 10x30 mA/cm 2 15 = 10-11 A/cm 2

i6

n k:

Fig. 8. The relationship between the power transfer coefficient and the number of collecting units with the width of finger as a parameter.

B. Je~owska-Trzebiatowska et al. / Neodymium- chromium doped phosphate glasses

1,0 ,

275

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0,8

0,6

w=50 0,/,

urn

I F = 1 0 x 30

Is = 10-I} 0,2

I

,

I

10

mA/cm 2 A/cm 2 ,

t

100 n[cm -1]

Fig. 9. The relationship between the power transfer coefficient and the number of collecting units with the sheet resistance as a parameter.

surface "dead" layer, but causes an increase of the electric power losses due to the series resistance of the cell, since sheet resistance of the emitter layer is higher in this case [7]. The influence of the p - n junction depth on the carrier collection efficiency is illustrated in fig. 6. A rise in the junction depth decreases the collection efficiency significantly. Abrupt decrease of this efficiency for wavelengths longer than 1 ~m due to the absorption threshold is considerable, therefore, it is not possible to take advantage of the emission peak at 1.06 ~tm. Selection of the substrate doping concentration is another important problem. Low substrate doping concentration may cause an increase of short-circuit current of the cell because of longer carrier lifetime and carrier diffusion length but also may cause significant losses due to series resistance and an increase in dark saturation current. The relationship between the carrier collection efficiency and the light wavelength is presented in fig. 7 with substrate doping concentration as a parameter. The influence of the series resistance connected with the sheet resistance of the emitter layer and metallization cover shape on the power transfer coefficient is illustrated in fig. 9. This coefficient may be defined as: k=l

Rs I2

PM

PMmax -- PMmax '

where R s is the series resistance of the cell, I M is the maximum power transfer

276

B. JeSowska-Tr:ehiatow~ka et a/ / N e o d v m m m . ~hromtum doped ph~).whatc ~/a.sw~

3O c = IOxR

in

[%1

2O

J

/

J x : ,9 ~-n P

1010%

1015

= loo mW/cm2

101-6 N [crn -3 ]o

Fig. 10. The theoretical efficiency as a function of s u b s t r a t e d o p i n g c o n c e n t r a t i o n for X = 900 n m and p o w e r c o n c e n t r a t i o n factor C = 0.1, 1., 10.

current, P M m a x is the theoretial maximum power for R~ = 0 and completely transparent contact layer. The relationship between the power transfer coefficient and the number of collecting units is presented in fig. 8 with the width of fingers as a parameter, and with the sheet resistance as a parameter in fig. 9. Fig. 10 presents the influence of the substrate doping concentration on the power efficiency including all effects mentioned earlier. In the calculations, bandgap narrowing effects and Auger recombination processes were taken into consideration.

4. Fabrication

The typical process was applied for experimental silicon solar cells fabrication. This process consists of silicon oxidation and the first photolithography for selective doping, the diffusion of phosphorus with POC13 as a dopant source, the second " l i f t - o f f ' photolithography, T i - P t rnetallization on both sides, and the soldering of the cells and completing the fabrication. On the basis of the spectral response and current-voltage measurements of the cell, we calculated the efficiency of the cell to exceed 20% for monochromatic 10-fold concentrated light at a wavelength of 0.9 ~tm.

5. Conclusion

Since an 0.25 m 2 triangular plate LSC receives 8-fold concentrated power at the edge, it seems reasonable to obtain a power of about 30 W and a light concentration

B. Je2owska-Trzebiatowska et al. / Neodymium -chromium doped phosphate glasses

277

efficiency estimated to be 3% for the 1 m2 rectangular solar panel consisting of four triangular plates made of the investigated chromium-neodymium glasses.

References [1] [2] [3] [4]

R. Reisfeld and Ch.K. Jorgensen, Struct. and Bonding 49 (1982) 1. A.M. Hermann, Solar Energy 29 (1982) 323. L. Andrews, B.C. McCollum and A. Lempicki, J. Lumin. 24 (1981) 877. E. Lukowiak, W. Str~k, M. Szymaflski, J. Szarzyflski, E. Mugehski, R. Cywiflski and B. JeC,owskaTrzebiatowska, Rare Earth Spectroscopy, Eds. B. Je~,owska-Trzebiatowska, J. Legendziewicz and W. StrCk (World Scientific, Singapore, 1985). [5] M. Szymaflski, Acta Phys. Polon. A63 (1983) 59. [6] C.M. Garner, F.W. Sexton and R.D. Nasby, Solar Cells 4 (1981) 37. [7] A. Buczkowski, Ph.D. Thesis, Technical University of Wroclaw (1978).