Composite semiconductors: Selective absorbers of solar energy

Composite semiconductors: Selective absorbers of solar energy

Solar Energy Materials 1 (1979) 93-104 ~)North-Holland Publishing Company COMPOSITE SEMICONDUCTORS: SELECTIVE ABSORBERS OF SOLAR ENERGY J. I. GITTLEM...

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Solar Energy Materials 1 (1979) 93-104 ~)North-Holland Publishing Company

COMPOSITE SEMICONDUCTORS: SELECTIVE ABSORBERS OF SOLAR ENERGY J. I. GITTLEMAN, E. K. SICHEL and Y. ARIE RCA Laboratories Princeton, NJ 08540, USA Received 14 September 1978 We have produced composite semiconductors by cosputtering CaF 2 with either Ge or Si and measured their optical constants. Their optical behavior can be described as being similar to that of the parent semiconductor, with the same energy gap but with a reduced, concentration dependent index of refraction. The normal specular reflectance of films sputtered on mirrored surfaces was measured. These data were used to compute the solar absorptance cq and thermal emittance ~th. It was found that cq ~0.7 and eth ~0.06 with a weak dependence on composition, thickness and operating temperatures. Thus at T = 500°C conversion efficiencies of 50~o are currently possible at solar concentration ratios C of 7-8 and about 70Yo for C ~40 and improved performance can be expected with continuing research.

I. Introduction In order to reduce our dependence on fossil fuels, we are beginning a major research effort to utilize the approximately 1 kW/m 2 of energy which the sun radiates to earth. Conversion of solar radiation to useful heat energy is being explored for a wide variety of domestic, agricultural and industrial applications, each with its own operating temperature and its own requirements for an absorber of radiation. Composite semiconductors which are structurally similar to other composite materials being studied as selective absorbers [ 1 ~ ] have high absorptance, intrinsically high selectivity and high temperature stability, and could prove to be economically attractive materials in many solar thermal applications. The link between the relevant material parameters of the selective absorbers, the solar absorptance ~s and thermal emittance eth,and the system in which they are to be used is the photothermal conversion efficiency ~/. It is defined as ~/=

Solar Power Absorbed-Thermal Power Radiated Incident Solar Power

which an be written as

'1 = [ C~,sEs - ~ E B B ( T)]/CEs,

(1)

where C = concentration ratio, Es = total incident solar power per unit area, EBB(T)= black body radiation power per unit area, =5.67x 10 -12 T#W/cm 2, and T= Absolute temperature. 93

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J. I. Gittleman et al.

C o m p o s i t e semiconductors

It should be noted that r/depends only on the optical properties of the selective absorbing layer and the mirror to which it is bonded. It does not include losses which are characteristic of a particular device design. Eq. (1) can be written as r/=oq[l_

1 (E..'~q ~E~sJJ'

(2)

where fl is the ratio es/eth and measures the selectivity of the absorber. The minimum value of fl is 1 for a black body. Because the maximum value of t/is es, Seraphin [7] has given it the name "Absorptance of Merit". It can be seen from eq. (2) that, for any a,, r/will be a maximum if Cfl >>EBB/E~. The selectivity fl is a materials parameter and is maximized by the choice of absorber; the solar concentration C is maximized by use of parabolic mirrors, heliostats, solar tracking servos, etc. However for obvious economic reasons it is insufficient for Cfl to be large, aq must be large as well. In fact, depending on the device in which the absorber is to be used, appreciable gain can be obtained with a higher ~ and a somewhat lower ft. In table 1 is compared the conversion efficiency at 300°C operation (EBa/E~= 6) for two materials. The first, with ~ = 0.65 and fl = 10, approximates the semiconductor composites discussed below; the second, with ~s =0.9 and fl = 6, has a higher solar absorptance but also a larger thermal emittance. For applications which require unity concentration the more selective material is the only one capable of converting solar energy. For C = 2 the two materials are about equivalent and for larger concentrations the less selective material is clearly superior. It should be noted that for C = 1000 as might be encountered in some of the proposed large, central receiver power stations [8], at an operating temperature of 500°C (EaB/E~= 21) a black body (~ = fl= 1) is very efficient with r/=98~. In the case of solar ponds [9] designed to operate near 100°C (Eaa/Es = 1) and at C = 1, selectivity is very important as can be seen from Eq. (2). Materials will be chosen to maximize r/, cost factors and thermal stability being equal. A layer of silicon (or germanium) deposited on a mirrored surface forms an extremely selective absorber [10]. At wavelengths below 1 #m (about 1.5 #m for Ge), the combination is absorbing and, at greater wavelengths, is only slightly emissive. However its index of refraction n is large (n ~4) and hence its reflection losses are Table 1 r1 [Eq. (2)] Concentration ratio

cq = 0.65 fl = 10

cq = 0.9 [3 = 6

1 2 10 25 50

0.26 0.46 0.61 0.63 0.64

0.0 0.45 0.81 0.86 0.88

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95

excessive, leading to a solar absorptance of 30-40%. The reflectance losses can be reduced by the addition of AR coatings to the surface of the absorber. Reflectance losses can also be reduced by dispersing a very finely divided semiconductor in a matrix which is transparent over as wide a wavelength range as possible and has a low index of refraction. We are studying the composite systems Ge-CaF2 and Si-CaF2 as selective absorbers where CaF2 was chosen as the matrix material because of its low index of refraction and low infra-red emittance.

2. Materials

2.1. Preparation and characterization Sputtering was used to produce films on appropriate substrates in a manner similar to that discussed by Abeles, et al. [11] The sputtering targets were made by placing wedges of Si and Ge on a 15 cm diameter disk of hot pressed CaF2 as shown in fig. 1a and lb. The difference in the sizes of the wedges of Si and Ge reflects the difference between the sputtering rates of the two semiconductors. Substrates (1 x 1 crn2) were placed side by side beneath the target, parallel to the diameter which is axial to the wedge. In this way specimens having a wide range of compositions could be obtained in a single sputtering run. Several types of substrates were used, all being squares nominally measuring 1 cm on a side. Quartz substrates were used for samples which were used to determine the optical constants in the wavelength range 0.3 to 2.5 #m and for specimens subjected to electron probe microanalysis. Single crystal Si substrates were used for transmission measurements in the far infra-red. For solar thermal performance determinations quartz substrates were used with either evaporated gold-chrome or aluminum which was subsequently anodized providing the reflecting base for the composite layer. A variety of metal substrates were used for the characterization studies. Auger and ESCA studies provide the best picture we have of the nature of the composite semiconductor film. Results obtained for Si-CaF2 show that bound oxygen exists in a surface layer which is at most 100 ,~ thick. The nature of the bonding has not been determined. Beneath the surface layer there was no oxygen detectable and the silicon present was indistinguishable from bulk silicon. Oxides or fluorides of silicon were either not present or in concentrations too small to be detected. These results are consistent with the hypothesis that the Si-CaF2 and Ge-CaF2 composites are structurally similar to the sputtered cermets [11]. However we were unable to obtain transmission electron micrographs to confirm the structure. The contrast ratio between the semiconductor and the insulator was too small for bright field analysis and the semiconductor grains were either too small or too disordered for dark field analysis. Because of the oxygen in the surface layer of the specimens, quantitative analysis using electron microprobe techniques proved to be inaccurate. Corrections for the interactions of the X-ray emissions from the bulk of a specimen with oxygen in a surface layer are not readily made. The most reliable values of the composition are

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d. 1. Gittleman et al. / Composite semiconductor,s

~L-7"-~ - - ~GERMANIUM-]-M

F

b

Fig. 1. Composite sputtering targets: (a) Si CaFz, (b) Ge~CaF2.

obtained using an atomic absorption spectrophotometer. While this technique gives accurate values for the atomic concentration of each of the elements in the composite, it cannot reveal the chemical environment of each element. Thus, in a specimen of Si-CaFz, it cannot distinguish between Si bonded to Si neighbors and Si bonded in some complex with Ca and F. At present we have not determined the semiconducting grain size, the chemistry of the surface layer and whether or not some chemical reaction between the semiconductor and the CaF 2 is taking place. This latter point is of

J. I. Gittleman et al. / Composite semiconductors 1.00

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s

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I

,s

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I

I

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3~,

3~

~

gs

SQOAREF,,

I

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SAMPLE NO.

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L E A S T SQUARE FIT

.2C

I I0

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I 30

I 40

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SAMPLE NO,

Fig. 2. Semiconductor concentration vs. sample position number for (a) Ge~CaF2 and (b) Si~CaF2.

some importance since the presence of compounds of Si, Ca and F could be the source of unwanted emittance in the infra-red. In fig. 2a and b, respectively, are shown the volume fraction of Ge and Si in the composite films as a function of sample position number. The specimen' numbers represent positions along the row of substrates in the sputtering apparatus as measured from a fiducial point, the distance between adjacent numbers being about 2.5 mm. Each point is the concentration of semiconductor averaged over three sample numbers. The volume fraction must be considered nominal since it is computed from the atomic concentration assuming the constituents to be 100~o dense.

98

J. I. Gittleman et al. /' Composite semiconductors 4

~3 0

0

~2

0

0

°

Oq

i .20

"4 0

I 180

'60

I ' lO 0

VOL FRACT.Ge Fig. 3. Index of refraction vs. volume fractions Ge in Ge-CaFz composites.

2.2. Optical constants The index of refraction n and the extinction coefficient k of the composite semiconductor films were determined from measurements of the normal transmittance and reflectance of films deposited on quartz substrates, using a method similar to that described by Bennett and Booty [12]. The measurements were made in the wavelength range 0.3-2.5 #m using a Cary 14 spectrometer. Fig. 3 gives the value ofn vs. volume fraction x of Ge. The index n was found to be a weak function of wavelength so that its average value is given. For x between 0.3 and 0.5, n ~2. Since the reflectance is (1-n)2/(1 +n) 2 for an absorbing medium (n 2 >>k2), Ge-CaF2 (with x ~0.4) should exhibit a reflectance of about 10~o in the visible part of the spectrum and hence an absorptance of about 90~o. The lowering of the index of refraction by dispersing the semiconductor in a low index, transparent matrix is important for the efficient performance for the composite semiconductor as a solar absorber. In fig. 4 is plotted ~/~ vs. photon energy for several specimens of Ge-CaF2, where 71019

1

Ge-CoFz (AS PREPAREDI// 300

o 88 VOL % Ge u 58VOL % Ge

/ / P ~

& 14VOL*/,Ge

"el

d° t/"

°E

200

I00

&

~=

I

1.0

2.0

3.0

PHOTON ENERGY (eV)

Fig. 4. ~/'c( vs. photon energy for Ge-CaF2 composites.

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99

~t= 4r~k/2 the absorption coefficient. The results for Si-CaF2 are similar. It can be seen that e = A ( h v - E o ) 2 where hv is the photon energy, A is a constant and Eo ~0.8 eV and 1.6 eV for Ge and Si, respectively. Eo is independent of semiconductor concentration. In the case of crystalline Ge and Si, when the optical absorption takes place via indirect transitions, the absorption coefficient is given by [13]

~=~,+ct e

when

hv>(Eg+Ep),

where eta =

A e Ep/kT - -

1 (hv - Eg + Ep)2

A c%- 1 - - e

Ep/k T

and

(hv--Eg--Ep) 2,

and e = e, when (Eg- Ep) < hv < (Eg + Ep). Here the subscripts a and e refer to the absorption and emission of a phonon, Ep is the phonon energy and Eg the gap energy. On the other hand, in amorphous semiconductors hv~toc(hv-Eg) 2 over an appropriate range of photon energies [14]. Because of the high transparency of our specimens near the absorption edge we are unable to obtain accurate values of ct for (by - E o ) < 0.2 Eo, the energy range in which ~t for the crystalline semiconductors exhibits the ( b y - E o +_Ep)2 behavior. Nonetheless the energy dependence of 0t in the composites is highly suggestive. Thus the absorption coefficient of the composite semiconductor films has a dependence on hv which seems intermediate between the behavior of the crystalline and amorphous counterparts. It is as if the semiconductor grains are sufficiently large and ordered to have a characteristic band structure but so small that the boundary conditions at the grain surfaces determine the selection rules. Thus conservation of momentum in an indirect transition involves interactions with the matrix material rather than the emission and absorption of a characteristic phonon. Whatever the reason for the observed dependence of e on by, the optical properties of a composite semiconductor as a function of concentration of the semiconducting phase are qualitatively similar to those of a series of semiconductors, all with the same energy gap but with different densities of (final) states.

3. Solar thermal performance In order to evaluate the composite semiconductors as solar thermal selective absorbers substrates were mirrored with either gold or anodized aluminum. Using a Cary 14 spectrometer for 0.3 #m < 2 < 2.5/~m and a Perkin-Elmer 457 spectrometer for 2.5 #m < 2 < 40/~m, the normal, specular reflectance was measured. It is well known [7] that

es = - -Xfo° ~t(T, 2) W~0.) d2, Es

(3)

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J. I. Gittleman et al. / Composite semiconductors

Es =

WJ~) d2,

,Jo

£th "-~~BBB

(4)

(5)

/3(T, ,E)WB(T, 2) d2,

and EBB=

WB(T,2) d2,

(6)

where WJ2) is the solar energy flux density in the range d2 about the wavelength 2, ct(T, 2) is the hemispherical absorptance at an operating temperature T and wavelength )~, WB(T,2) is the radiated black body energy flux density in the range d2 about 2 and ~(T, 2) is the hemispherical emittance,

c((T,2)= ~(T,~,)= I -R(T, ),),

(7)

where R(T, 2) is the hemispherical reflectance. In order to compute ~,, /~thand r/ we assume that R(T, 2)=R,(To, ).) the normal, specular reflectance measured at room temperature To. While this assumption is not strictly correct it provides a convenient approximation for computing solar thermal performance from our reflectance data, particularly when comparing different specimens and materials. In addition, for computation purposes the integrals were limited to the wavelength range 0.3 #m ~<2 ~<25.0 #m. The truncation involves a negligible error. In Figs. 5 and 6 are shown the measured values of/~(To, 2) for two specimens of Ge-CaF2 containing about 40 volvo Ge. Both specimens were sputtered on substrates having a 1500 ~ layer of evaporated aluminum and differ only in thickness (1.7 #m for fig. 5, 0.5 #m for fig. 6). A layer, several hundred angstroms thick, of A1203 produced by anodization served as a reaction barrier between the AI mirror and the Ge-CaF2 film. There is one major difference to be noted. In fig. 5 there is pronounced absorption at about 12/~m. In fig. 6 this absorption is much reduced because the Ge-CaF2 layer is thinner. A similar absorption band at about the same wavelength is observed for Si-CaF2 [6]. Neither the semiconductor nor CaF2 exhibit vibrational L0

0.8

06

~04 0.2

0 0.3

I0

I00

1 500

WAVELENGTH (MICRONS)

Fig. 5. Normal specular reflectance vs. wavelength for a 1.7/~m thick Ge CaF2 layer containing about 40 vol°; Ge on anodized A1 on Si.

J. I. Gittleman et al. / Composite semiconductors

101

0.8

~06 ~: o~4

0.2

0

0'3

1.0

10'.0

10;.0

WAVELENGTH { MICRONS)

Fig. 6. Normal specular reflectance vs. wavelength for a 0.6 # m thick Ge~CaF2 layer containing about 40 volvo Ge on anodized AI on SiO2.

modes which would be expected to give rise to an absorption at 12/~m. Because of the reproducibility of the absorption it is not likely to be due to chance contamination. Further, in the case of Si it occurs at too long a wavelength to be caused by an oxide. While there is no direct evidence concerning the source of the absorption, we conjecture that it arises from the formation of bonds between the semiconductor and the matrix, perhaps at the surface of the semiconductor grains. Although it is not obvious by inspection of the figures, the solar absorptance of the thicker film is somewhat greater than the thinner. This is because the wavelengths within which the reflectance changes from near zero (high absorptance) to near unity (low emittance) shifts to slightly larger values as the thickness increases. Using eqs. (3)-(7) the solar thermal performance parameters can be computed from the data in figs. 5 and 6. In table 2 these parameters are compared at two operating temperatures. The following points should be noted: (1) the solar absorptance is larger for the thicker film, (2) the thermal emittance is larger for the thicker film leading to a smaller selectivity (0t/8), (3) the emittance is smaller and hence the selectivity is larger at the higher operating temperature. As was noted above, the larger solar absorptance for the thicker film is due to the shift of the absorption edge to longer wavelengths. The greater thermal emittance results from the increased prominence of the 12 #m absorption in the thicker film. Finally it is interesting to note that both films are more selective at 500°C than at 300°C. This is due to the fact that at 500°C the black body radiation peak is at a shorter wavelength (3.75/~m) than at 300°C (5.0 #m). Hence at Table 2 T= 300° C

1.7/~m Ge-CaF2/A1 0.6 # m Ge-CaF2/A1

T= 500° C

~sa

~th

0~/~

0tsa

F~th

0~/~3

0.721 0.648

0.101 0.061

7.14 10.6

0.721 0.648

0.0778 0.053

9.26 12.2

~Computed for 1 Standard Airmass.

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[.0 0.8

E

0.6

0.4

~ 0.2~ Ill

2

I

I

I

I

4

6

8

I0

CONCENTRATIONRATIO

Fig. 7. Solar thermal conversion efficiency (Air M a s s = 1) vs. solar concentration ratio for Ge-CaF2 composites on anodized AI mirrors; - o - 0.6 #m layer, - A - 1.7 #m layer.

the higher temperature a smaller fraction of the total radiation is lost in the wavelength range near 12 #m where ~(T, 2) peaks. It should also be noted that the solar absorptance at the operating temperatures is expected to be somewhat larger than those values given in table 2. This is because the given values are based on room temperature measurements. At the elevated temperatures the energy gap will be smaller and hence the absorption edge will appear at longer wavelengths than are shown in figs. 5 and 6. Using the values in table 2 and eq. (2) the concentration ratio required to obtain a conversion efficiency of 50~ can be computed. For both specimens this occurs at about C = 7.5. At lower concentration ratios (and lower efficiencies), the thinner film is superior because of its higher selectivity; at higher concentration ratios the thicker film is superior because of its higher solar absorptance. This is illustrated in fig. 7 where r/is plotted vs. C at both 300°C and 500°C and for both films. A black body requires a concentration ratio of 42 for 50~o efficiency at 500°C while for the thicker film q ~ 70~o at this value of C. On the other hand for C ~>70 the black body has superior performance because of its high solar absorptance. The dependence of the performance parameters on Ge concentration is very weak, and while the peak performance is expected to occur around 40 volvo Ge (using measured values of n and k for calculations) no significant variations were noted for values between 35 and 50 vol~. The results for Si-CaF2 were similar to those of Ge-CaF2. Using room temperature reflectance data the performance of the Si composites were slightly poorer than that of the Ge. On the other hand at elevated operating temperatures the reverse might be true both because of the temperature variation of the energy gaps and because the thermal generation of carriers tends to increase free carrier absorption more in Ge than Si.

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4. Summary We have produced composite semiconductors by cosputtering Ge and Si with CaF2 and measured their optical constants. Their optical behavior can be described as being similar to that of the parent semiconductor, with the same energy gap, but with a reduced, concentration dependent index of refraction. The normal specular reflectance of films sputtered on mirrored surfaces was measured. These data were used to compute the solar absorptance as and thermal emittance e-th. It was found that ~s ~0.7 and eth =0.06 with a weak dependence on composition, thickness and operating temperature. Thus at T= 500°C conversion efficiencies of 50% are possible at solar concentration ratios C of 7-8 and about 70% for C ~40. By inspection ofeq. (2) it can be seen that there is always a value of C above which a black body will exhibit a conversion efficiency larger than that of any other absorber. On the other hand the question of the choice of absorber is an economic one depending not only on r/but also on the cost of land, capital costs of different types of systems, etc. Thus a parabolic trough, distributed receiver with a concentration ratio of 50 or less could be more economical for some applications than a central receiver for which C > 500. While it is clearly a task for future research to seek ways of increasing the solar absorptance of the composite semiconductors and hence their versatility, the possibility of using these and other highly selective absorbers for the economical conversion of solar energy should be considered.

Acknowledgements We thank P. Zanzucchi and D. A. Kramer for their assistance in obtaining the far infra-red reflectance data, J. H. Thomas and G. O. Fowler for the Auger/ESCA analyses, E. P. Bertin for the electron probe microanalysis, H. H. Whitaker for the atomic absorption spectra analyses, and A. P. Pica and S. Bozowski for assistance in processing the optical data. This research was sponsored by the U.S. Department of Energy under Contract No. EG-77-C-02-4557.

References [1] H. Tabor, in: Low Temperature Engineering Applications of Solar Energy, ed. R. C. Jordan (The American Society of Heating Refrigerating and Air-conditioning Engineers, New York, 1967) p. 41. 1-2] G. E. McDonald, Solar Energy 17 (1975) 119. [3] H. Mar, L. H. Liu, P. B. Zimmer, R. E. Peterson and J. S. Gross, Honeywell, Inc., Contract No. NSF-C-957(AER-74-09104) (1975). 1-4] D. M. Mattox, J. Vac. Sci. Technol. 13 (1976) 127. [-5] J. C. C. Fan and P. M. Zavracky, Appl. Phys. Lett. 29 (1976) 478. [6] J. I. Gittleman, B. Abeles, P. Zanzucchi and Y. Arie, Thin Solid Films 45 (1977) 9. [7] B. O. Seraphin, Solar Energy Conversion-Solid State Physics Aspects, in: Topics in Applied Physics, ed. B. O. Seraphin (Springer-Verlag, Berlin, to be published in 1979).

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[8] A number of studies of various central receiver systems is being supported by DOE. Representative reports in which this work is described are SAND-77-8513, SAN/1108-8 and SAN/1109-77. System analyses are to be found in SAN/1101-1. All reports are available through NTIS. [9] See, for example, DOE report UCRL-52385 available from NTIS. [10] B. O. Seraphin and A. B. Meinel, in: Optical Properties of Solids-New Developments, ed. B. O. Seraphin (North-Holland, Amsterdam, 1975) p. 928. [11] B. Abeles, P. Sheng, M. D. Coutts and Y. Arie, Adv. Phys. 24 (1975) 407. [12] J. M. Bennett and M. J. Booty, Appl. Opt. 5 (1966) 41. [13] J. I. Pankove, Optical Processes in Semiconductors (Dover Publications, New York, 1975) p. 37. [14] J. Tauc, Optical Properties of Non-crystalline Solids, in: Optical Properties of Solids, ed. F. Abeles (North-Holland, Amsterdam, 19721 p. 277.