Preparation, characterization and photoelectrochemical properties of the CuIn0.75Al0.25Se2 alloy

Solar Energy Materials 19 (1989) 167-179 North-Holland, Amsterdam

167

PREPARATION, CHARACTERIZATION AND P H O T O E L E C T R O C H E M I C A L P R O P E R T I E S OF T H E CuIno.75Alo.2sSe 2 A L L O Y

M.L. F E A R H E I L E Y , H.J. L E W E R E N Z and M. F U E N T E S Hahn-Meitner-lnstitut, Bereich Strahlenchemie, D-IO00 Berlin 39, Germany

Received 20 April 1989

The first preparation of the photoactive quaternary chalcopyrite Culn0.75A10.25Se2 is reported. Photoelectrochemical measurements indicate limitations in photoactivity due to inhomogeneities and low contact potentials of p-type samples with the V2+/V 3+ and S:~ 2- /S 22- redox couples. Ultraviolet photoelectron spectra yield an electron affinity of 3.3 eV, explaining the poor performance of p-CuIn0.75A10.25Se2 in electrochemical photovoltaic systems. The energy gap, determined from spectral measurements, is Es =1.33 eV in accordance with Vegard's law. A schematic energy band diagram for semiconductor/redox electrolyte contact formation is derived.

1. Introduction The chalcopyrite CulnSe 2 has shown to be a suitable material for photovoltaic applications. CuInSe2/(Cd , Zn)S devices and CuInSe 2 photoelectrochemical solar cells have consistently obtained power conversion efficiencies above 10% [1-4]. The high absorptivity and steep absorption edge allow thin film applications and explain the high quantum efficiencies. However, the band gap is too small with respect to the theoretical optimum for a single junction device. Therefore, attempts have been made to increase the energy gap [5,6]. They involve the substitution of indium by gallium or selenium by sulfur. However, these sulfur-selenide alloys exhibit comparably p o o r optoelectronic properties. In this work a substitution of indium by aluminum was attempted for the first time and the properties of the new material were analyzed. CuA1Se 2 has been reported to have a room temperature band gap of 2.5 eV [7]. If the variation of the band gap with respect to composition is linear, then it will require the replacement of only 25% of the In to produce a band gap of 1.5 eV. This band gap has been calculated to be the o p t i m u m for homojunction devices [8] whereas the o p t i m u m is between 1.2 and 1.3 eV for heterojunction devices [9]. Therefore, the quantity of aluminum required should not greatly degrade the beneficial optical properties of CuInSe2. 0165-1633/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

16 8

M.L. Fearheil(v et a L /

Photoelectrochemical properties of C u l n , r~A 1~ : ~S e ?

2. Experimental

2.1. Alloy preparation The alloy was grown from the melt using the gradient freeze method. Appropriate amounts of copper, indium and aluminum were placed in a boron nitride boat which was placed at one end of a carbonized quartz ampoule. The selenium, whose amount was varied in an attempt to obtain n- or p-type material, was placed at the other end, and the ampoule evacuated and sealed. The growth end of the two-zone furnace had a m a x i m u m temperature of 1100 ° C and the zone containing the selenium had a maximum temperature of 650 o C. The growth end was cooled at a rate of 50 ° C / h to 700 o C, then at 100 o C / h to room temperature. The lattice constants were determined at room temperature by X-ray powder diffraction (XRD) using Cu K a radiation.

2.2. Analytical studies An attempt to determine the average compositions by energy dispersive analysis of X-ray (EDX) was complicated by the overlap of the S e L a emission with the A1 K a emission line.

2.3. Electrochemical characterization Electrodes were made as previously described [10] from naturally grown material exhibiting (112) facets, whose areas measured between 3 to 15 m m 2. Gold lacquer was smeared on the back of the crystals to form an ohmic contact. Light and dark electrochemical measurements were performed using the standard three-electrode potentiostatic arrangement with a carbon counter electrode and a SCE reference electrode. A 0.5M K2SO 4 solution was used as a supporting electrolyte. Measurements were also performed using different redox couples: V 2 + / V 3+ in HC1, polysulfide and 1 3 / 1 - in HC1. The white light source was a tungsten-iodine lamp adjusted to 80 m W / c m 2. The spectral dependence of the photocurrent was determined in a 0.5 M H2SO4 supporting electrolyte using a potential of - 0 . 4 V versus SCE, and normalized with respect to the spectrum of the lamp.

2.4. Surface analysis Ultraviolet photoelectron spectroscopy (UPS) was carried out in a commercial surface analysis system (Vac. Gen. ESCALAB M K I I ) on cleaved samples. The base pressure of the system was in the 10 -11 Torr range and the in-situ cleavage procedure exposed an externally uncontaminated surface. The He I line (hv = 21.2 eV) was used as excitation source.

M.L. Fearheiley et al. / Photoelectrochemicalproperties of Culn o.7~A 10.2~Se 2

169

Fig. 1. Photographs of as-grown Culn 0.75A10.255e2 single crystals exhibiting the (112) facet. 3. Results 3.1. Preparation and material properties

Material grown using the gradient freeze m e t h o d typically exhibited regions possessing large (112) facets (see fig. 1). X R D analysis showed the material to be single phase with chalcopyrite structure (see fig. 2). T h e lattice constants were determined to be: a = 5.746 + 0.002 ,~ and c = 11.495 + 0.008 ,~ with c / a = 2.0005. With regard to the c o m p o u n d s : C u l n S e 2 ( a = 5.814 ,~, c = 11.6344 ,~, c / a = 2.0010 [11]), and CuA1Se 2 ( a = 5.60 ,~, c = 10.90 ,~, c / a = 1.95 [12]), the alloy follows Vegard's law [13]. T h e hot p r o b e m e t h o d showed the material to be p-type. A n a t t e m p t to type convert by annealing under m i n i m u m Se pressure failed as did a t t e m p t s to grow n-type material using excess indium and aluminum. 3.2. Optoelectronic properties

In figs. 3 a - 3 c the dark current and p h o t o c u r r e n t versus voltage curves recorded in 0.5M K 2 S O 4 supporting electrolyte are shown for three representative samples.

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All of the samples studied exhibited a photocurrent and rectifying properties. However, the onset of the decomposition reaction could be shifted to more negative potentials by subsequent etching in 2% Br2/methanol. The spectral dependence of the photocurrent for these samples is shown in figs. 4a-4c. The general features of these spectra varied from sample to sample. M96 exhibited a steep rise in the photocurrent below 1000 nm, and a plot of I~ versus energy (eV) resulted in a band gap of 1.37 eV (see inset). M94 showed a more gradual rise in the photocurrent and a band gap of 1.31 eV. A more complex dependence was found for M90: there appeared to be two contributions to the photocurrent, one arising perhaps from CulnSe 2 regions coexisting at the surface with the alloy, the other one from the alloy itself, with onsets at 1.04 eV and 1.33 eV, respectively. The value for the band gap was smaller than expected, suggesting a bowing of the Eg versus composition curve. We are currently investigating this relationship. The current-voltage characteristics in V 3+/V z+ redox solution are presented in figs. 5a-5c. The samples, M90 and M94, showed photocurrent saturation at a polarization of about - 1 V, i.e. - 0 . 5 V cathodic from the redox potential. It

M.L. Fearheiley et al. / Photoelectrochemical properties of Culno.zsAlo.25Se 2

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Fig. 3. Dark and photocurrent-voltage characteristics of samples M90, M94, and M96 in 0.SM K2SO 4 supporting electrolyte. A W - I lamp was used with a light intensity of 80 m W / c m 2.

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M.L. Fearheiley et aL / Photoelectrochemical properties of Culn o.75Alo.25See

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should be noted that sample M94 (fig. 5b) exhibited a cathodic photocurrent up to - 0 . 2 8 V, whereas for sample M90 (fig. 5a) the photocurrent crossed the dark current at about - 0 . 2 2 V. The height of the photocurrent signal at energy (eV') = Eredo 0 x (V2+/3+) was larger in the latter case. The observed open-circuit voltages were low and ranged from 100 to 270 mV. The saturation current densities were also low and ranged from 1.7 to 2.8 m A / c m 2. An attempt to increase the photovoltages by using a polysulfide redox couple was not successful. The photocurrent onset was shifted to more negative potentials (fig. 6) with respect to the redox potential. Saturation of the photocurrent was not achieved in this potential range. Fig. 7 shows the U P spectrum of a vacuum-cleaved Culn0.vsA10.25Se 2 crystal, compared with the corresponding spectrum from a UHVcleaved CulnSe 2 single crystal. The most obvious difference was the high signal background throughout the investigated spectral range. The survey X-ray photoelectron spectrum (not shown) indicated an unusually high amount of carbon on the surface of Culn 0.75A10.25Se2. There were pronounced differences in the valence band region where peaks a and b differed in relative intensity, and peak c was shifted in position and height. There was an additional structure in Culn0.7~A10.zsSe2 at the Fermi level. The cut-off at high binding was quite similar and was extrapolated at E = - 1 7 eV.

M.L. Fearheiley et al. / Photoelectrochemical properties of Culn o 75Alo : sSe :

174

4. Discussion The results in figs. 1 and 2 show that it is possible to grow the new material CuIn0.75A10.25Se 2 as a single phase with chalcopyrite structure. However, semiconductor optimization involves detailed control in the growth process as a deviation from stoichiometry, a n d / o r impurities in the ppm range can drastically alter and deteriorate electronic properties even if only a small percentage of the defects is electrically active. Because of these restrictions our optoelectronic characterization, monitored by photoelectrochemical behavior, displayed varying properties for different samples. The results in fig. 3 show that the as-grown material is photoactive but exhibits

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175

M.L. Fearheiley et al. / P hotoelectrochemical properties of Culn o 75A Io.25Se 2

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considerable dark current in reverse direction, which is indicative of a poor barrier with reduced rectifying properties. The samples showed predominantly p-type behavior, i.e. cathodic photocurrents at potentials cathodic from the flatband situation. The accurate flatband potential has not been determined since the samples varied substantially in their electronic properties, such as conductivity, doping and homogeneity. All samples shown in fig. 3 also exhibited anodic photocurrents in a region of high anodic dark currents. The material is either inhomogeneous, and consists of adjacent p- and n-type regions as observed on WSe 2 [15], or almost intrinsic showing predominantly photoconductor behavior [16,17]. However, the results in redox electrolyte (see fig. 5) on the same samples indicate that the characteristics shown in figs. 3a and 3b are due to p- and n-type regions on the same sample since saturation photocurrents were reached at relatively small cathodic potentials (figs. 5a and 5b). In a photoconductor, no dark current and only a linearly increasing photocurrent with applied potential are expected [16]. The highest photoactivity was found for the sample M96 (fig. 3b). In redox electrolyte, the same sample exhibited exclusively p-type conductivity and the dark current was zero at the redox potential (fig. 5b). The sample shown in fig. 3a showed a cross-over in the 1 - V characteristics indicating photocathode as well as photoanode behavior. The small cathodic dark current at E 0 originates from an unknown dark reaction (possibly H 2 evolution) inhibiting equilibration of electrochemical potentials [18].

176

M . L . Fearheiley et al. / Photoelectrochemical properties o f C u l n o 7~A 1. _~~Se :

The photocurrent spectra displayed in fig. 4 show quite a variation in band gap energy. From the actual stoichiometry of the compound, the application of Vegard's law [13,19] predicts an almost linear relationship of the band gap with composition. with an expected gap energy of

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M. L~ Fearheiley et a L / Photoelectrochemical properties of Culn o. zsA lo. 25See

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shift in flatband potential created by the adsorption of sulfur anions, which resulted in a reduced contact potential. Such effects have been observed earlier [20,21]. The photoelectron spectra shown in fig. 7 allow the determination of the electron affinity of the new compound. With X = h ~ , - X - [ E c - EF I, where X denotes electron affinity and X the spectral width (from E F to the cut-off energy), we obtain X = 21.2 - 16.8 - 1.1 eV, hence X = 3.3 eV. In addition it can be seen, by comparison with a UP-spectrum of single crystalline CulnSe2, that most features of CulnSe 2 band structure were maintained, in particular the peaks labelled a, b and c. The high background signal of Culn0.75A10.25Se 2 was probably due to carbon impurities. This indicates that carbon was incorporated, in substantial amounts, since any exterior contamination has been excluded by cleaving the sample in UHV. This contamination resulted from the graphitization of the ampoule, which was done in order to avoid the incorporation of any silicon and oxygen from the quartz into the sample

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178

M . L . Fearheiley et al. / Photoelectrochemical properties o f C u l n o 75A l o , ~S e :

during growth, as well as to prevent the reaction of a l u m i n u m with the quartz of the ampoule. In comparison to C u l n S e 2, the quaternary c o m p o u n d showed a pronounced peak near the Fermi energy, indicating a specific change in b a n d structure a n d / o r transition probability. Since only Cu 3d b a n d s are admixed into the valence b a n d top, it is likely that the addition of A1 resulted in a splitting of the degenerate hybridized Cu 3d levels. This is a matter of further investigation with optoelectronically improved crystals. In fig. 8 the junction energies of p-Culn075A1025Se 2 with respect to some redox couples are shown. F r o m the negative value of E v ( - 4 . 4 eV with respect to vacuum) it becomes clear that only very limited photovoltages can be expected even in contact with c o m p a r a b l y negative redox couples such as v a n a d i u m ( I I ) / ( I I I ) or polysulfide. Efficient photoelectrochemical systems can only be expected with n-type samples.

5. Conclusion The first preparation of the photoactive quaternary chalcopyrite C u l n 0.75A1 o.25Se2 is reported. The photoactivity was c o m p a r a b l y p o o r and the material exhibited inhomogeneities and impurities. The electron affinity of X = 3.3 eV does not readily permit the formation of wet photovoltaic systems with p-type conductivity samples. The energy gap is Eg---1.33 eV and fits well with calculation f r o m Vegard's law. Surface analysis reveals an u n k n o w n structure near the valence b a n d top possibly related to a Cu 3d level.

Acknowledgements The authors thank Dr. C. Pettenkofer for the U P spectrum. M.L.F. would like to thank the Alexander von H u m b o l d t F o u n d a t i o n for its support.

References [1] J.L. Shay, S. Wagner and H.M. Kasper, Appl. Phys. Letters 27 (1975) 89. [2] R.A. Michelsen, Proc. Polycrystalline Thin Film Review Meeting, 16-18 May 1983, Golden, CO, SERI Publication CP211-1985. [3] W.E. Devaney, R.A. Michelsen, W.S. Chen, Y.R. Hsiao, J.M. Stewart, L.C. Olson and A. Rothwarf, Semiannual Report, Boeing Engineering and Construction Company, Seattle, WA, May 1984. [4] S. Menezes, H.J. Lewerenz and K.J. Bachmann, Nature 305 (1983) 615. [5] H.W. Schock, B. Dimmler, H. Dittrich, J. Kimmerle and R. Menner, in: Proc. 7th European Photovoltaic Solar Energy Conf., Sevilla, October 1986. [6] P. Lange, H. Neff, M.L. Fearheiley and K.J. Bachmann, J. Electron. Mater. 14 (6) (1985) 667. [7] W.N. Honeyman, J. Phys. Chem. Solids 30 (1969) 1935. [8] J.J. Loferski, J. Appl. Phys. 27 (1956) 777. [9] J. Lindmayer, in: Proc. 13th IEEE Photovoltaic Specialists Conf. (IEEE, New York, 1978) p. 1096. [10] H. Goslowsky, K.-D. Husemann, J. Luck, W.W. Szacki and H.J. Lewerenz, Mater. Letters 4 (1986) 198.

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179

[11] W.N. Honeyman, J. Phys. Chem. Solids 30 (1969) 1935. [12] M.L. Fearheiley, K.J. Bachmann, C. Herrington and S. Vasquez, J. Electron. Mater. 14(6) (1985) 677. [13] K. Nakjima, J. Appl. Phys. 49 (1979) 5944. [14] M. Sander, H.J. Lewerenz, W. Jaegermann and D. Schmeisser, Fresenius Z. Anal. Chem. 329 (1987) 367. [15] S. Menezes, L.F. Schneemeyer and H.J. Lewerenz, Appl. Phys. Letters 38 (1981) 949. [16] H. Gosiowsky, H.M. Kiihne, H. Neff, R. K~Stzand H.J. Lewerenz, Surface Sci. 149 (1985) 191. [17] J. Stumper and H.J. Lewerenz, J. Electroanal. Chem., in press. [18] H.J. Lewerenz and H. Goslowsky, J. Appl. Phys. 63 (1988) 2420. [19] H.J. Lewerenz and R. Michaelis, J. Electrochem. Soc. 135 (1988) 913. [20] R.H. Wilson, J. Electrochem. Soc. 126 (1979) 1187. [21] H.J. Lewerenz, H. Gerischer and M. Liibke, J. Electrochem. Soc. 131 (1984) 100.