Structural and optical properties of electrodeposited CuInSe2 layers

Solar Energy Materials and Solar Cells 45 (1997) 87-96

ELSEVIER

Structural and optical properties of electrodeposited CuInSe 2 layers N. Stratieva a,*, E. Tzvetkova

a

M. Ganchev

I. T o m o v

a

K. Kochev a

b

a Central Laboratory on Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 blvd. Tzarigradsko Chaussee, 1784 Sofia, Bulgaria Instttute of Physwochemtstrv Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria b

•

.

.

Received 30 September 1994

Abstract

CulnSe 2 thin layers have been obtained from a thiocyanate electrolyte with a complexing agent. The as-deposited layers have been polycrystalline with very small crystallites. The influence of the deposition potential on the composition, on the absorption coefficient, and on gap band energy has been investigated. It has been established that the layers were Cu-rich and crystallized in chalcopyrite phase. The annealing in Ar ambient did not influence the composition of the layers considerably but improved the crystalline structure. This resulted in changes of the absorption coefficient and bandgap energy. Keywords: Electrodeposition; CulnSe2; Thin layers; Structural and optical properties; Annealing in Ar

atmosphere

1. Introduction Electrodeposited CulnSe 2 thin films are a perspective contender for solar cell absorbers because of several advantages such as the possibility for large-scale production, minimum waste of components, and easy monitoring of the deposition process. Several groups reported the preparation of CuInSe 2 thin layers by electrodeposition. S.N. Sahu et al. have deposited CulnSe 2 layers from an aqueous electrolyte [1]. The as-deposited films did not exhibit the correct stoichiometry. X-ray diffraction (XRD) * Corresponding author. 0927-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0927-0248(96)00070-0

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studies established both chalcopyrite and sphalerite phases, as well as binary compounds such as CuSe, CuSe 2, In2Se 3. Bhattacharya et al. [2] have obtained monophase CulnSe 2 but with presence of binary compounds, too. A successful attempt of electrodeposition of stoichiometric CulnSe 2 has been done by D. Pottier and G. Maurin from an acidic sulphate solution containing citrate ions as a complexing agent [3]. However, the solar cells prepared on the base of electrodeposited CulnSe 2 films exhibit efficiency lower as compared to those obtained with evaporated layers. One of the possibilities for increasing the efficiency is to improve CulnSe 2 properties by post annealing treatments. Generally they are carried out in vacuum, inert atmosphere or Se ambient [4-6]. Our paper presents the results from the investigation of composition, of structural and optical properties of CulnSe 2 layers deposited from an aqueous electrolyte. The influence of the deposition potential, temperature and duration of thermal treatment on them is studied.

2. Experimental The CulnSe 2 layers were electrodeposited in potentiostatic regime. A standard three electrode cell was used supplied with a saturated calomel electrode ( + 246 mV versus NHE at 20°C), a Pt foil as anode and a conductive glass substrate (1,5 cm 2 area) as a working electrode. The conductive glass was prepared by a spray technique based on pyrolytic decomposition of SnCIa.5HzO alcohol solution and dopant ions introduced from NH4F and fluorinehydrogen acid [7]. The specific resistance of the deposits was about 50 o h m / c m . The constituents of the electrolyte used for the electrodeposition of CulnSe 2 were CuC1, InzSOa.5H20, SeO 2 or H2SeO 3 or KzSeO 3 and KCNS. They were dissolved in 0,4 M acetate buffer (pH = 5) in the ratio 25,0: 33,0: 42,0. Details about the deposition method could be found elsewhere [8]. The investigated CulnSe 2 layers were prepared at five different deposition potentials: Ud --- - 7 0 0 , - 8 0 0 , - 900, - 1000 and - 1100 mV versus SCE. Their composition was established by EDAX (energy dispersive X-ray fluorescence analysis). The XRD analysis was carried out for phase identification and texture of the deposits. They were made by Philips PW 1010 diffractometer. The C u K a radiation was selected by a secondary graphite monochromator. A comparison with JCPDS file cards was done for the establishing the observed peaks. Phases were considered to present if at least three lines were registrated and the most intensive lines were not absent. The absorption coefficient oL and the gap band energy were calculated from the transmission and reflection spectra. The latter were obtained with U V - V I S spectrophotometer 330 Perkin Elmer in the wavelength range 500-2000 nm at room temperature. An integrating sphere was used to measure the total reflectance and transmittance. The spectra were normalized to BaSO 4. The optical transmittance was corrected for the absorption of the SnOz-glass substrate. The thickness of samples was estimated by measuring of mass deposited and assuming density of layers to be equal to that of bulk CulnSe 2 [9].

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The heat treatment experiments were carried out in Ar gas flow. The quartz tube with the samples in it was placed in the central zone of a furnace with the temperature preset at a value between 350°-550°C in steps of 50°C. The annealing went on from 15 min to 120 rain. Then the quartz tube was withdrawn and cooled to room temperature for 10 min.

3. Results The EDAX measurements point out that the deposition potential influences considerably In and Se amounts (Table 1). Increasing Ud to more negative values (from - 700 to - 9 0 0 mV) In grows on behalf of Se. In the range - 9 0 0 - 1 0 0 0 mV the composition is near stoichiometric one. Independent of the deposition potential, the ratio C u / I n > 1 in all samples. The layers deposited at - 1100 mV are spongy and poor adherent and have not been considered. The same dependence of the composition versus deposition potential is observed after heat treatment in the temperature range 350°-550°C in Ar flow. The Se amount slightly decreases while the In content increases, but the layers remain still Cu-rich. The data presented in Table 1 relate samples annealed at 550°C for 120 min. Fig. 1 a shows a typical X-ray diffraction pattern of an as-deposited CulnSe 2 layer. It presents a weak broad peak at 2 0 = 26,6°C which is characteristic for a pseudocrystalline CulnSe 2 phase and peaks at 2 0 = 37,9 ° and 51,8 ° belonging to the SnO 2 substrate. As it has been mentioned CulnSe 2 is deposited onto SnO 2 prepared by spray pyrolysis. The temperature of the glass substrate during this deposition has been maintained about 580°C. It is reasonable to assume that the subsequent annealing of such substrates with CulnSe 2 deposits onto them in the temperature range 350°-550°C has not lead to changes in the crystalline structure of the substrates. Hence, the changes observed in the X-ray diffraction pictures of the CulnSe2-SnO: structure after the annealing are not attributed to structure changes of the SnO2 substrate. This means that the thermal treatment influences mainly the (112) peak intensity of CulnSe: but not the (110) peak of SnO 2. This result is favourable in the case, because the (110) peak of SnO 2 overlaps the (112) peak of chalcopyrite CulnSe:.

Table 1 Composition of layers deposited at different deposition potentials before and after annealing at 550°C for 120 min Sample number

35 775 783 735

Deposition potential, mV

700 800 900 1000

Composition before annealing, at.% Composition after annealing, at.% Cu

In

Se

Cu

In

Se

24,3 25,32 25,49 24,8

21,9 22,12 23,76 23,8

53,8 52,52 50,74 51,4

25,32 26,49 27,64 25,1

22,13 21,26 22,58 24,1

52,55 52,24 49,78 50,8

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N. StratieL,a et al. / Solar Energy Materials and Solar CelL~ 45 (1997) 87-96

~ c'~ •

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T ~

T rTT

~ r

F F '

~ r T q l r - r , T

'l

!

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]:I

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::

j

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I I

i ;

'

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L L/)

C

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0,53

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54,

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Fig. 1. X-ray diffraction patterns for: (a) as-deposited CuInSe2 film, (b) CuInSe2 film annealed at 350°C for 120 rain, (c) CuInSe2 film annealed at 450°C for 120 min, and (d) CulnSe2 film annealed at 550°C for 120 rain.

The comparison of the X-ray diffraction patterns of unannealed CulnSe 2 samples deposited for one and the same time but at different potentials ( - 7 0 0 mV, - 8 0 0 mV, - 9 0 0 m V and - 1000 m V ) shows the same structure as m Fig. la. In Fig. l b - l d the X-ray pictures for CuInSe 2 samples annealed at 350 °, 450 ° and 550°C are seen, respectively. All samples have been deposited at the same conditions: deposition potential - 1000 mV and deposition time 90 rain. Films annealed at 350°C display the known peaks of SnO 2 as well as peaks at 2 0 = 52,4 ° at 44,3 °, and at 26,6 ° (Fig. lb). The latter could be ascribed to the chalcopyrite phase of CuInSe:. Increasing the annealing temperature to 450°C, the intensity of the peaks is increased, also, (Fig. lc) and at 550°C (Fig. ld) four relatively sharp peaks of chalcopyrite CuInSe 2 at 52,4 °, 44,3 ° , 35,5 ° and 26,6 ° are seen. The diffraction line at 30,6 ° is identified as the strongest (100) reflection of I n 2 0 3. The X-ray results for CuInSe 2 samples annealed at 550°C and deposited at different potentials: - 7 0 0 mV, - 8 0 0 mV, - 9 0 0 m V and - 1 0 0 0 m V suggest that all the samples contain peaks of the chalcopyrite phase. More than three peaks of this phase are recorded in the diffraction pictures of samples prepared at - 1 0 0 0 mV. The latter exhibit, also, the I n 2 0 3 peak at 30,6 °.

N. Stratieva et al. / Solar Energy Materials and Solar Cells 45 (1997) 87-96

91

The X-ray studies of samples annealed from 15 min to 6 hours at 550°C show that when the annealing time is increased from 15 min to 120 min, new lines appear in the XRD spectra. Essential changes are not observed for thermal treatment longer than 120 min. The texture of the layers was studied according to the following equation: (1)

li = li~(1 - e-2~a/sin°'),

where I i is the intensity of the /-diffraction line, Ii~ the intensity of the same line of an "infinitely" thick sample, /~ the linear absorption coefficient of the material, d the thickness of the layer, and 0i the diffraction angle of the /-line. The 1i intensity is related to the pole density Pi in the respective /-direction by a normalization factor N/ [10]:

li~

= PiN~.

(2)

The normalization factor N~ is equivalent to the integrated intensity in /-direction of an infinitely thick random sample of the same substance. The calculations according to the Eqs. (1) and (2) indicate that the CuInSe 2 layers represent textures with (112) predominant orientation. The ( l l 2 ) - p o l e density is about 1,5 times higher than (220, 204)-pole density. Transmission spectra of unannealed and of annealed at different temperatures samples are shown in Fig. 2. The samples have been deposited at - 1000 mV for 60 min and annealed for 2 h. As it is seen, the spectra are characterized by considerable changes in the region 1 0 0 0 - 1350 nm. While the spectrum of the unannealed layer smoothly decreases at short wavelengths, the spectra of the annealed layers exhibit a sharp edge indicating a region of strong absorption with the strongest absorption edge observed in samples annealed at 550°C (Fig. 2d).

30--~ 25 .

.

.

.

20o

.=.

b

d

5

_J-

500

1000 Wavelength [rim]

1500

Fig. 2. Transmission spectra of CulnSe 2 films: (a) unannealed, (b) annealed at 350°C for 120 min, (c) annealed at 450°C for 120 min, and (d) annealed at 550°C for 120 min.

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In order to find out the optimum annealing conditions different annealing time has been used for each temperature. It has been revealed that strong changes in the transmission spectra are observed after 15 min annealing. When the heat treatment duration increases they go on slower and the strongest ones occur for 120 min. Further increase of the annealing time does not cause essential changes. These results indicate that the most appropriate annealing conditions improving the crystalline structure of layers correspond to a temperature of 550°C and to a duration of 120 min. The determination of the absorption coefficient and, hence, of the forbidden gap energy from the transmission and reflectance spectra is done under the following consideration: 1. a strongly absorbing thin film and a weakly absorbing substrate, 2. no interference effects in the films spectra. In this case the absorption coefficient can be calculated by means of the equation: a = ln( 1 - R ) 2 / T d ,

(3)

where R is the reflectance, T the transmission, and d the thickness of the layers. As it is

105

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o

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c

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i,

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86i --4

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2

10 3 - - ~ 0.7

1.0

1.3

1.6

1.9

2.2

2.5

hv (eV) Fig. 3. Absorption coefficient as a function of photon energy incident for films deposited at 1000 mV and annealed at: (a) 350°C, (b)450°C, and (c) 550°C.

N. Stratieva et al. / Solar Energy Materials and Solar Cells 45 (1997) 87-96

93

known CulnSe 2 is a semiconductor of direct optical transitions. For that the absorption coefficient in the region of strong absorption obeys: a = a/hv(hv

- Eg) 1/2,

(4)

where A is a parameter that depends upon the transition probability and the refractive index. Therefore, from the plot of ( a h v ) 2 versus h v the gap energy can be defined by extrapolating the linear portion of the curve to the intercept of the photon energy axes. A least-squares fitting subroutine was used to define the intercept with the energy-axes. Fig. 3 presents the absorption coefficient as a function of the annealing temperature. As it is seen the absorption coefficient is slightly influenced by the temperature in the region 1,4-2,5 eV. Its values are from 2.104 to 7.104 for 550°C to 3.104 to 1.105 cm -~ for 350°C. All the samples exhibit substantial sub-band absorbance that decreases with the increase of annealing temperature. The gap energy calculated from the dependence ( a h v ) 2 versus h v in the range 0 , 8 - ! ,4 eV for the different annealing temperatures amounts 0,92 eV for 350°C, 0,96 eV for 450°C and 1 eV for 550°C. The transmission spectra for samples deposited at different deposition potentials have maximum transmittance in the range 1350-2000 nm, as the highest is for samples deposited at - 1000 mV. The calculated absorption coefficient is shown in Fig. 4 and it

i 10 5 a

~ -

_, oOOoO°

/

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0 0 0

."

~

,

c

~ "

/

5_

i

3~

i

! 0.7

1.0

1.3

1.6

1.9

2.2

2.5

hv (eV) Fig. 4. Absorption coefficient versus photon energy incident for films annealed at 550°C for 120 min and deposited at: (a) -800 mV, (b) -900 mV, and (c) - 1000 mV.

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N. Stratieva et al. / Solar Energy Materials and Solar Cells 45 (1997) 87-96 10

~ -

a

fi~

%

b

27

c a

0.8

1.2

hv (eV)

1.6

2.0

Fig. 5. (oLh t,)2 versus h u lbr films annealed at 550°C for 120 min and deposited at: (a) - 800 mV, (b) - 900 mV, and (c) - 1000 mV.

does not exceed 2.105 cm 1 Significant values of the absorption coefficient are obtained below the absorption edge. They decrease with the increase of the cathodic potential. From the upper values of o~, the gap energy of the samples deposited at different potentials are calculated (Fig. 5). Slight differences are established: 0,978 eV for Ud = - 8 0 0 mV, 0,98 for Ud = - 9 0 0 mV and 1 eV for Uo = - 1000 inV.

4. Discussion The X-ray diffraction patterns of as-deposited CulnS% layers present broad peaks in the region 2 0 = 250-28 °. Such peaks are supposed to be due to pseudocrystalline CuInSe 2 phase. Similar results have been obtained by G.D. Mooney et al. for thin films formed by rapid thermal processing [11]. The appearance of sharp peaks in case of annealed samples and the decrease of halfwidth of the peaks with the increase of the annealing temperature (Fig. l b - l d ) mean that the grain sizes enlarge and hence more compact layers are obtained. The availability of CuInSe 2 peaks of chalcopyrite phase in the X-ray diffraction patterns of the annealed

At. Stratieva et al. / Solar Energy Materials and Solar Cells" 45 (1997) 87-96

95

samples is characteristic for Cu-rich CulnS% [12,13]. This is confirmed in our case, also, as the investigations of composition of the layers have shown that the ratio C u / I n > 1 (Table 1). The peak at 2 0 = 30,8 ° in the X-ray pictures shown in Fig. 1 d is observed only for samples treated at temperatures higher than 450°C and deposited at high electronegative potentials (above - 1000 mV). Further investigations have shown, as it must be awaited, that high cathodic potentials urge hydrogen evolution. The latter results in alkalization of the electrolyte near the cathodic space and to the formation of indium hydroxide, respectively. Probably, the annealing at temperatures above 450°C leads to its dehydratation to In203. The appearance of a steep edge in the transmission spectra (in the range 1150-1350 nm, Fig. 2d) after the thermal treatment indicates improvement of the crystallinity of the layers. The magnitude of this steep edge is increasing with the increase of annealing time and temperature, Nevertheless the thermal treatment duration, a tail towards the short-wave region is observed. It points out that the structure of the layers has remained still imperfect. Similar significant sub-gap absorption has been observed in Cu-rich films by J.R. Tuttle et al. The authors attributed it to the availability of Cu 2-6 Se phase [14]. The removal of this secondary phase by NaCN chemical treatment resulted in a drop in s u b - bandgap absorption and an increase in extrapolated transition energy values, as well as in a strong change in film composition [15]. The band gap values for annealed samples deposited at - 1 0 0 0 mV are in a fair agreement with the reported values for electrodeposited thin film CuInSe 2 [16,17]. The increase of these values from 0,92 eV to 1 eV with the increase of the temperature from 350°C to 550°C could be due either to variations of the composition or to a decrease of the imperfections in the crystalline structure. As the EDAX results have shown, the composition of the layers is not influenced considerably by the annealing (Table l ). So, the observed increase of gap energies is conditioned by the improvement of the crystallinity of the layers, most probably. The slight differences obtained for gap energies of samples deposited at different deposition potentials might be due to the small variation of stoichiometry.

5. Conclusions The results obtained show that the deposition method we use gives the opportunity to prepare by one-step process Cu-rich chalcopyrite CulnS% layers. The increase of the deposition potential to more negative values results in a decrease of C u / I n ratio and in the range 900-1000 mV the composition of the layers is near to the stoichiometric one. The annealing in Ar ambient does not affect considerably C u / I n ratio but leads to a decrease of Se.

Acknowledgements The authors would like to thank Dr. E. Nikolova for the EDAX analysis. This work has been partially supported by the Bulgarian Ministry for Science and Education, contracts No. F-221 and No. H-253 and EC contract JOU2-CT92-0141.

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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