Transparent Conducting Oxides AsAntireflection Coatings for GaSb TPV Cells

C. M. Ruiz1*, O. Vigil1, C. Algora2, D. Martin2, V. Bermudez1

and E. Dieguez1

Departamento de Fisica de Materiales, Facultad de CienciasUniversidad Autonoma de Madrid, 28049Madrid (SPAIN)

Instituto de Energia Solar. E.T.S.I. Telecommunication — Universidad Politecnica de MadridAvda. Complutense 38; 28040 Madrid (SPAIN)

Abstract. In this work, single layers of TCO thin-film antireflection coatings over GaSb substrates havebeen computer-designed for their use with thermophotovoltaic cells. The optimal parameters of the thinfilm TCO layers over GaSb TPV cell structures, i.e., optimal thickness, refractive index and also thereflectivity of the whole structure as a function of the wavelength have been first theoretically determinedfrom both the refractive index of each TCO and the GaSb substrate. TCOs based on pure and fluorinedoped SnO2 thin films as antireflection coatings have been simulated with specific computer software.The improvements of the cell short-circuit current density for each TCO/GaSb system has been alsocalculated. Regarding the optimal parameters calculated from simulation, TCO films have been grown onGaSb substrates by spray pyrolysis technique. Reflectance characterizations have been performed in themanufactured TCO-GaSb systems in order to compare the experimental reflectivity with the theoreticalresults obtained from the simulations. From these data, we select the system that presents the best relationamong minimum reflectance at the band gap wavelength and maximum reflectance for higherwavelengths.

INTRODUCTIONThe use of antireflection coating (ARC) is an important step for the fabrication of highefficiency thermophotovoltaic (TPV) technology. Single and double layer thin-filmantireflection coatings with different refractive indices, using both the native andtransparent oxides, are the most common technique in solar cells. In the case of GaSbTPV technology double ARC has been used, taken into account the theoreticalconditions for the refractive index of the layers and substrate: ni= (ns)1/4 and n2= (ns)3/4

Where ni and n2 are the refractive index of the layers and ns is the substrate refractiveindex.

For GaSb (ns =3.82), the ARC layers must have a refractive index equal to 1.38 y 2.62respectively. The materials most suitable for these values are MgF2 (1.38) and ZnS(2.32); this is one of the most common ARC systems used in GaSb TPV cells. Usingtwo-layer ARC, the reflectivity losses are reduced to very low values in the spectralrange of interest. However three problems are connected with this system:

i) Its thermal stability, which depends on the deposition technologyii) Low reflectance is obtained for a very wide spectral range. This last means that verylow reflectance for wavelengths higher to the band-gap of GaSb is obtained andtherefore this radiation is not used as back radiation to the emitter.iii) The use of two-layer ARC increases the technological steps and therefore the cost ofthe final system.* corresponding autor e-mail: carmen.herrero@uam.es

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On the other hand, generally, in TPV systems the matching between the emitterradiation and the spectral response of the cells is poor and appropriated selective filtersmust be used to boost the system efficiency. They transmit only the convertibleradiation and reflect the sub-bandgap photons back to the emitter. TCOs can bemanufactured with high transmittance for radiation energies higher than the GaSb band-gap and at the same time with non-zero reflectance for energies lower than this value,with a minimum reflectance at the energy corresponds to the GaSb band-gap edge. Inaddition, TCOs can also be integrated into the device structure. The combination ofthermal and optical properties of TCOs could be an interesting response to the points (/)and (//), while the use of low cost manufacturing methods for growing each single TCOlayer responds to the point (Hi)

Previous reports of TCOs as antirreflection coatings in TPV systems grown by CVD(Chemical Vapour Deposition) showed high absortivity on the TCO film [1],discouraging the use of these materials. Instead of CVD, we propose the use of SprayPyrolysis technique that allows to growth of high quality films with thicknesses ofhundreds of nm, avoiding the problems with absortivity, and being a relative low costtechnique.

Consequently, this work deals with the following:

/. The theoretical conditions of the depositions of different TCOs as antireflectioncoatings on GaSb TPV cells and the resulting reduction in the reflectivity loses in eachcase//. An evaluation of the preliminary results depositing different TCOs on GaSb bySpray Pyrolysis technique.

THEORETICAL CONSIDERATIONS

The simplest conditions for designing a single layer ARC for a photovoltaic device arewell known [2]. In order to obtain zero amplitude for the reflected radiation at awavelength (X0), the ARC should have a refractive index (nc) and a thickness (dc) givenby:

«c = k)i

4,-i '"4n,where ns is the substrate refractive index.

Conditions (1) require a layer with an adequate thickness and appropriate refractiveindex. For the evaluation of the optimal conditions it is necessary to know thetheoretical dependence of the refractive index and the extinction coefficient of thesubstrate, as a function of the wavelength. For GaSb, this dependence is known [3] andshown in Figure. 1

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1000 1200 1400

Wavelenght (nm)1800 2000

FIGURE 1. Spectral dependence of refractive index and extinction coefficient of GaSb.

SnCh: F as ARC in GaSb.

As it is well known, the refractive index and extinction coefficient of the TCOs are afunction of the type and doping concentration of impurities. Experimental resultsreported by S. Shanthi and collaborators have been used for the SnO2i F [4] system.Optimal conditions are obtained if the refractive index of the TCO is near of 1.95.Furthermore, we have optimized the minimum reflectance at the band-gap value of theGaSb (1610 nm). Figure 2 shows the spectral dependence of the refractive index of Fdoped SnC>2 thin films on the doping concentration. Taken into account theaforementioned values of the refractive index and the wavelength, the optimal dopingconcentration of F in SnC>2 corresponds to 57 at. %.

2.0-

c» 1.8-X0

— 1.6-0

DC

1.0

-57F/Snat%-38F/Snat%-19F/Snat%- non doped

GaSb (nGaAb=3.82n=1.95)

^gofGaSb(161Qnm)400 800 28001200 1600 2000 2400

Wavelenght (nm)FIGURE 2. Spectral dependence of refractive index for non doped and doped F SnO2 layers.

The spectral reflectance of the SnO2:F /GaSb system for different doping concentrationswas calculated and film thicknesses have been optimized for obtaining the reflectanceminimum at 1610 nm. The results are shown in Fig. 3. The absolute minimum in thereflectance for SnO2i57% F /GaSb is obtained for dc=209 nm.

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DC

—— GaSb (Substrate)— • - • • non doped SnO2(dc=291 nm)—— F/Sn =19% (dc=241nm)—— F/Sn =38% (cf=215nm)

- F/Sn =57% (dC=209nm)

750 1000 1250 1500 1750 2000

Wavelength, X (nm)FIGURE 3. Calculated reflectance of GaSb for different doping concentrations of F in SnO2:F ARC.Theoptimal thickness for each level of doping is given in the figure.

From the spectral radiation SR (X) of the Yb2C>3 and the blackbody at 1735 K, reportedby B. Bitnar et al. [5] both as emitters. Though Ytterbia emitters not are the mostsuitable for GaSb cells, they have been included in this study in order to determine thesensitivity of the theoretical model to different emitters. The internal quantum efficiencyof the GaSb cell QE(A,)[ 3] and the calculated reflectance R(A), the cell short-circuitcurrent density was calculated for each ARC layer with the following equation:

/ _ = . (2)

For the evaluation of the ARC performance, the following figure of merit (FM) hasbeen considered. It is basically the cell short-circuit current density improvement whenthe ARC is included in the cell structure, in percentage terms

FM = (3)

where Jsc and Jsco are the short circuit current densities of the GaSb cells with andwithout the ARC layers, respectively.

Table 1 reports the calculated FM values from the optimal thickness for doped andundoped TCO layers. From the analysis of this Table, it is seen that the best results areobtained for the SnO2:57%F. However, from Fig. 3 the reflectance values forwavelengths higher than the GaSb band-gap are also the lowest. Considering the wholeTPV system (TPV emitter and GaSb cell) is also necessary to evaluate how muchradiation is sent back to the emitter. In a TPV system, the radiation losses are not onlyrelated with the reflectivity loss of the cells, but also with the emitter emissivity. Inother words, to calculate the best parameters for the TCO as ARC in a concrete TPVsystem, the back reflection to the emitter of the sub-band wavelength spectral regionmust be taken into account. From this point of view, not necessarily the minimumreflectance determines the total improvement of the complete system. This fact will beevaluated in future works.

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TABLE1

Cell

GaSb

GaSb

GaSb/SnO2

GaSb/Sn02

GaSb/Sn02:19%F

GaSb/SnO2:19%F

GaSb/Sn02:38%F

GaSb/Sn02:38%F

GaSb/Sn02:57%F

GaSb/SnO2:57%F

, Calculated FM for different TCO/GaSb systems.

Emitter

Black-body (1 735 K)

Yb203

Black-body(1735K)

Yb203

Black-body(1735 K)

Yb203

Black-body(1735 K)

Yb203

Black-body(1735 K)

Yb203

J(A/cm2)

10.66

1.35

12.92

1.5

12.83

1.46

13.92

1.57

14.16

1.61

FM%

-

-

21.2

11.1

20.3

8

30.6

16

32.8

19.3

EXPERIMENTAL PROCEDURE

In order to evaluate the ARC effects on GaSb, several layers of undoped and fluorinedoped tin oxide were deposited on GaSb by Spray Pyrolysis technique. GaSb substrateswere obtained from a pure GaSb Bridgman ingot. 1 mm thick and 18mm of diameterwafers were cutted along the perpendicular direction of growth. Then, they weremechanically polished using a colloidal suspension of alumina powder in water andethilenglycol. [6] Before film growth, substrates were rinsed in HC1 to eliminate thenative oxide buffer on the surface.

The metallic salt solution (10 g of SnCl4.5H2O in 90 mL of ethanol, 10 mL of H2O and4 mL of HC1) with F concentration of 57 at.% were obtained adding NH4F to thestarting solution in the required amounts. The solution was sprayed onto a hot substrate(300°C) pyrolitically decompose to oxide films The solution flow and carrier gas (N2)pressure were kept constant at 5mL/min and 0.4 bar, respectively.. The nozzle-to-substrate distance was kept constant (25 cm). The layers thicknesses were calculatedfrom the equation (1), for the minimum of the reflectance measurements using of He-Nelaser and a silicon solar cell coupled to an electrometer. In this form can be calculatedthe thickness of the layer as a function of the spraying time, i.e. the growth rate.

For the X-ray analysis a SIEMENS D5000 difractometer was used. For the reflectancemeasurements a SPECTRO 320, spectrum analyzer from Instrument Systems GmbHwas used in the 320-1700nm range and a FT-IR Bruker IFS60v in the 1400-20000 nmrange. Finally, SEM (Scanning Electron Microscopy) images were obtained with aPhilips XL30. All the instruments are placed in the Investigation Service of the UAM(Sidi) except the SPECTRO 320, located in the IES-UPM, in Madrid

225

RESULTS AND DISCUSSIONFor confirming the formation of SnC>2 films, X-ray measurements were done. TypicalX-ray spectrum corresponding to sample A is shown on figure 4. Peaks of both GaSband SnO2 are identified on the spectrum. The most intense peak corresponds to GaSb,and indicates that the substrate has a dominant orientation. Several peaks with smallerintensities can be identified with SnCh ones. Specially, three orientations are clearlydominating the SnO2 spectrum. These data show the nature of the film grown ispolycrystalline

4 9 0 0 -

l(Arb

itrar

y U

nite

)

0

• SnO 2 D R X p e a k s

m^J

° - 1 ) ( 2 ,

( 2 , 0 , 0 )

UL i,1)

IL . dIrnlW X Awfto™ kkhj i*i iJwnta A*«tfffl™tiiliiilbljliiw

FIGURE 4. X-ray spectrum for the system non doped SnO2/GaSb

In order to determine the size of the film crystals and the chemical uniformity, SEMimages were obtained. An image of the topography of non doped SnO2 system surfaceis shown on figure 5.a. A large quantity of crystals all over the image with a uniformdistribution can be observed. It is worth noting that the size of the crystals is veryconstant all over the film. Average size of the crystal is about some tens of nanometers.This size indicates us that we are working with a large distribution of nanocrystals like atexturized film. From BackScattering Electrons, a map of chemical distribution isobtained. From this sample, an image from the frontier film- substrate was made.Difference among substrate (upper side on figure 5.b.) and film (lower side) is clearlyobserved. Darker colour on lower side indicates de presence of Sn on this part of thesample. It is also important to check the colour uniformity on the film side, showing aconstant-like distribution of the chemical species

FIGURE 5. SEM image of the film, showing the average size of the crystals (5.a.) and BSE map (5.b) ,showing a high uniformity of chemical composition on the film

226

Also, reflectance measurements were done for comparing with theoretical results.For this, spectral reflectance was measured on the range of 250-2500nm, althoughcalculations were only performed in the central range of 680-2000nm. Figure.6 showsthe experimental reflectance of the undoped and fluorine doped SnO2/GaSb systems Asit can be shown, experimental data reproduce the shape of the calculated functions butthey are shifted. This clearly means that the oxide films have different thickness thanexpected. The theoretical reflectance on each figure has been calculated for the optimalthickness, while the real film thicknesses have been extracted from the theoretical fit tothe experimental data. It is worth noting that, as minimum and maximum places can bereproduced from the calculations, relative values of the function do not match with theexperimental ones. It is due to the fact that both SnO2 and GaSb refractive index used inthe calculations have been extracted from the literature, so variations on these values areexpected among different experiments. It is also important to observe that, the biggerthe F concentration is, the better the fits calculated are. Furthermore the formation of athin layer of native oxide during the TCO growth process could contribute to theformation of the two layer ARC Regarding the functions, it seems that the bestantirreflecting coating for TPV systems is the SnO2 57%F doped, as it has a minimumnear the GaSb bandgap, and increase its reflectance for higher wavelengths

1200

A,(nm)2000 400 1600 2000

1200 1600 2000 400 1200

JL(nm)

FIGURE 6.Experimental (solid line) and best theoretical fit (dashed line) for non doped (6.a.), 19%Fdoped (6.b.), 38%F doped (6.c.) and 57%F doped (6.d.) SnO2 films. Respectively optimal thicknesses are695 nm, 260nm, 315nm and 162 nm

227

Finally, short-circuit current densities and FM have been calculated for all the samplesfrom their experimental reflectance data. Results are summarized on table 2. In almostall the cases a decrease on current density is observed comparing with the theoreticaldata on table I, except on the last case. Taken into account the thickness layer differencebetween the calculated and the measured reflectance short-circuit current densities andFM were calculated for the theoretical fit of each case. The calculated densities are alsoon table 2 respectively.

In the three first cases, Jflt is smaller than the optimal calculated, but higher than theexperimental one. Probably the cause of these differences is the shift in reflectancebetween theoretical and experimental data. While the calculated data always have analmost zero reflectance on the minimum, on the experimental ones the minimum is atalmost 10%.Also the existence of a native oxide layer over the substrate should havemodified the behaviour of the system. Special attention has to be paid to the SnO2 38%Fwhen it is used with black-body emitter and SnCh 19%F with Yb2Oa emitter cases. Onboth cases, FM is negative respect the simple GaSb cell and fitted values aresignificantly smaller than optimal previously calculated. This is mainly due to the factthat films grown have different values than theoretical ones, being maxima and minimashifted from their best position.

Also it is very interesting to observe the SnO2 57%F film. In both cases, experimentaland fitted, short current circuit density is higher than in the theoretical case calculatedfor situate the reflectance minimum in 1610 nm. It can be explained by the fact that notonly absolute minimum is important, but also bigger reflectance values for the sub-bandwavelengths. These conditions are achieved on both cases, especially on theexperimental case, allowing the important increase in the FM.

TABLE2. Calculated FM for the experimental TCO/GaSb systems.

Cell

GaSb/SnO2

GaSb/Sn02

GaSb/SnO2:19%F

GaSb/SnO2:19%F

GaSb/Sn02:38%F

GaSb/Sn02:38%F

GaSb/SnO2:57%F

GaSb/SnO2:57%F

Emitter

Black-body(1735 K)

Yb203

Black-body(1735 K)

Yb203

Black-body(1735K)

Yb203

Black-body(1735K)

Yb203

Jexp(A/cm2)

10.86

1.43

11.35

1.18

10.55

1.36

18.40

1.44

FMexp %

1.8

5.9

6.5

-12.6

-1

0.7

72.6

6.7

JnttA/cm5)

12.79

1.63

13.23

1.49

12.02

1.53

15.12

1.88

FMflt %

19

20.7

18.8

1.0

12.8

13.3

41.8

39.2

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CONCLUSIONS

TCOs are promising antirreflecting coatings for GaSb cells. In this work preliminarystudy of pure and non doped SnO2 films have been carried out. Initial calculationsshowed an increase of short-circuit current density for all the compositions studied, andan optimal thickness for each doping concentration has been found. On a second stage,pure and doped SnO2 films have been grown over GaSb substrates by Spray Pyrolysistechnique. X-ray and SEM measurements demonstrated the formation of a thin,homogeneous and polycrystalline film of SnO2 over the substrate. Reflectancemeasurements clearly show antirreflecting behaviour of the system, tough the thicknessof the films were not the theoretically estimated. Current density was calculated forthese samples. All the systems showed a decrease in the FM, due to the difference of thereal thickness from the optimal one, except the SnO2 57%F doped one that presents agood reflectance spectrum and has the highest current density, being more than 50%higher than the calculated for this composition, showing that is a very good candidate tobe an antirreflecting coating for GaSb. The use of vacuum deposition techniques, withthe TCO deposited at room temperature and a more control of the layers thickness in thedeposition of Spray pyrolysis-TCO thin films are in progress

ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of the Spanish Education andScience Ministery under proyect "MAT-2003-09873". Carmen M Ruiz alsoacknowledges the grant of the UAM. This work has been partially supported by theEuropean Commission through the FULLSPECTRUM project (Ref. N.: SES6-CT-2003-502620).

REFERENCES

1.- A Comparison of CompositeTransparent Conducting Oxides Based on the BinaryCompounds CdO and SnO2; X. Li, T. Gessert, C. DeHart, T. Barnes,H. Moutinho, Y.Yan, D. Young, M. Young,J. Perkins, and T. Coutts. NREL/CP-520-31017 October20012..-J.Thomas and G.Hass, in Physics of Thin Films, edited by G. Hass and R.E. Thun(Academic New York, 1964), Vol.23.- D. Martin and C. Algora in Thermophotovoltaic Generation of Electricity, edited byTJ. Couts, G. Guazzoni and J. Luther (American Institute of Physics,Melville,NewYork, 2003)p.4424.- S. Shanthi, C. Subramanian and P. Ramasamy, Crys. Res. Technol., vol. 34,(1999),p. 1037.5.- B.Bitnar, J.C. Mayor, W. Durisch, A.Meyer, G. Palfinger, F. von Roth and H. Sigg.,in Thermophotovoltaic Generation of Electricity, edited by TJ. Couts, G. Guazzoni andJ. Luther (American Institute of Physics,Melville,New York, 2003)p. 18.6. P.S. Dutta, H.L. Bath, V. Kumar, J. Appl. Phys. 81 5821 (1997)

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