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Synthesis of CuInS2 by chemical route: optical characterization
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Solar Energy Materials & Solar Cells 80 (2003) 115–130
Synthesis of CuInS2 by chemical route: optical characterization P. Guhaa, D. Dasb, A.B. Maityc, D. Gangulia, S. Chaudhuria,* a
Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India b Energy Research Unit, Indian Association for the Cultivation of Science, Kolkata 700 032, India c Department of Physics, Heritage Institute of Technology, Kolkata 700 107, India Received 9 May 2003; accepted 26 May 2003
Abstract CuInS2 powder was prepared by wet chemical route. The chalcopyrite structure of the powder was revealed by XRD studies. Raman measurements of the powder sample indicated four prominent peaks at 292, 305, 340 and 472 cm1. The possible origin of the 305 cm1 peak was investigated and was found to be some local vibration in the structure. The peaks at 292 and 340 cm1 were ascribed to A1 and B2 modes, respectively. The peak at 472 cm1 which was due to the formation of SO2 ion at lower pH value of the precursor solution could be 4 eliminated by using pH>11.0. Photoluminescence (PL) studies of the CuInS2 powder indicated two distinct peaks at 1.49 and 1.42 eV. Post deposition annealing treatment in H2 atmosphere revealed the formation of excess sulphur vacancy leading to the peak at 1.42 eV in the PL spectra while O2 annealing of the powder created a deep defect level at 1.10 eV. Thick CuInS2 films were prepared by Doctor’s blade technique. Optical transmittance studies of these films indicated direct allowed transition at B1.5 eV. r 2003 Elsevier B.V. All rights reserved. Keywords: CuInS2; Raman study; Thin film; Chemical route; Doctor’s blade
1. Introduction Among different semiconductors the chalcopyrite semiconductors represented by Cu(In,Ga)(S,Se)2, CuIn1xGaxSe2 are most attractive absorber layers which received world wide attention for more than two decades for the fabrication of *Corresponding author. Tel.: +1-91-33-473-4971; fax: +1-91-33-473-2805. E-mail address: [email protected] (S. Chaudhuri). 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-0248(03)00138-7
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polycrystalline solar cells due to their high absorption coefficient and nondegradable properties compared to other solar cell materials. Photovoltaic devices based on thin polycrystalline CuInCuIn1xGaxSe2 absorbers reached efficiencies of 18.8% [1]. However, as selenides are toxic materials, with the evolution of ‘‘Green Technology’’ there has been a demand for the use of non-toxic materials instead of the toxic selenides. In this regard CuInS2 is very attractive for device fabrication. In spite of its being a promising alternative of selenides the knowledge gathered so far on CuInS2 is not yet adequate. The demand of a non-toxic solar cell absorber layer brought CuInS2 to the forefront in recent years and various groups are now carrying out research work on this material. The pure sulphur compound CuInS2 has a comparatively large direct band gap of Eg ¼ 1:53 eV which is in the optimum range for terrestrial photovoltaic conversion. The predicted theoretical efficiency with CuInS2 homojunction is as high as 26% [2]. However, till today the highest reported efficiency of solar cells made with CuInS2 as the absorber layer is only close to 12% [3,4]. The control of electronic properties and grain boundary effects along with reproducibility remains an open problem in Cu–In–S system. In spite of these existing problems, the main advantage of CuInS2 is that it can be synthesized in an environment friendly atmosphere because it does not contain any toxic elements like Se, Te, etc. The other advantage of using CuInS2 is that it can be prepared by using a simple chemical route, which is most suitable for commercial application. In this work, Raman scattering studies, a non-destructive characterization technique, have been applied to the analysis of polycrystalline CuInS2 powders prepared by a chemical route as a function of the pH of the precursor solution. In fact, the Raman study was used to obtain the best possible crystalline CuInS2 in powder form. Photoluminescence (PL) study of this powder was performed after the post deposition annealing treatment. Films were prepared by a Doctor’s blade technique using the precipitate of the precursor solution with a suitable pH value which indicated less defect formation in the Raman analysis. Microstructural and optical studies of the films are also presented in this communication.
2. Experimental In the present study copper(II) chloride dihydrate (E. Merck India Limited, 99%), thiourea (E. Merck India Limited), 25% ammonia solution (International Chemicals, India) and indium (III) chloride prepared from indium (metal) and hydrochloric acid (International Chemicals, LR) were used for synthesis. Deionised water was used for preparing the solutions. At first 5 ml aqueous solution of InCl3 and CuCl2 2H2O was prepared and stirred for 1 h. The concentration of each salt in the solution was maintained at 0.3 M l1. This greenish blue solution had a pH of 3.2. Ammonia solution (25%) was added to this solution to change the pH to various values from 7.5 to 11.8. After the addition of NH4OH the color of the solution changed to deep blue. This solution was stirred for almost 2 h. A 2 ml aqueous
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solution of thiourea was prepared in a test tube and slowly added to the above mentioned deep blue solution. The amount of thiourea in the solution was taken in such a way that the Cu:In:S molar ratio in the solution remains at 1:1:3.5. The color of the solution was slowly changed to brown and after 3 h of stirring a black precipitate appeared. The black precipitate was collected and dried at 373 K for 30 min. The precipitate was then annealed at 623–723 K to obtain well-crystallized CuInS2 powder. One part of the crystallized CuInS2 powder was then annealed at 573 K in H2 atmosphere and another part in O2 atmosphere for 30 min. These powders were used in the PL measurements. For the preparation of the films the precipitate of the precursor solution with pHB11.0 was used. CuInS2 films were prepared by Doctor’s blade technique using the slurry like black precipitate of the precursor solution. Doctor’s blade unit consisted of an AC motor, a Doctor’s blade and a substrate holder. The blade was moved by the motor, the moving rate of which could be controlled by the associated electrical circuit. The slurry was kept on the substrate and the Doctor’s blade was moved over the substrate in such a way that a thin layer of chemicals were deposited on the substrate. The substrate with the layer was then annealed in an appropriate temperature inside a graphite box to obtain CuInS2. b-In2S3 was synthesized using InCl3 and thioacetamide as the precursors. Detailed discussion of this synthesis will be published in another paper. X-ray diffraction patterns of powders and the films were carried out by Seifert XDAL 3000 with monochromatic CuKa radiation (Ni filter). The compositions of the materials were determined from EDAX (Kevex Delta Class-I) using ZAF correction. PL spectra were recorded with the excitation B650 nm using a 300 W xenon arc lamp as the emission source. A Hamamatsu photomultiplier was used as the detector along with a 1/4 m monochromator.
3. Results and discussion 3.1. Raman study of CuInS2 powder The Raman spectra of chemically synthesized CuInS2 powder prepared from different precursor solutions with different pH values were recorded by a microRaman spectrometer (ILOR-Jobin-Yvon-SPEX) using polarized beam of 632.8 nm with photons from a 20 mW He–Ne laser. The light was focussed on to the sample through a 100 optical microscope objective (Olympus) in a back scattering geometry. The scattered light was collected and dispersed by a grating and the entire spectrum was recorded by a CCD. Fig. 1 shows the Raman spectra of CuInS2 powder samples prepared with different pH of the precursor. It may be observed that there are four prominent peaks at 292, 305, 340 and 472 cm1 for all the samples prepared with different pH values in addition to the peak at 265 cm1. It may also be observed that the intensity of the Raman peak increased with increasing pH of the precursor solution as shown in Fig. 2 for a particular peak (292 cm1). The strong peak at 292 cm1 may be assigned to the A1 mode [5] while the peak at 340 cm1 may
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Raman intensity (a.u.)
292 cm
265 cm
100
-1
305 cm -1 340 cm -1 472 cm
-1
200
pH~ 7.5
pH~ 10.8
pH~ 9
pH~ 11.8
300
400
500
600
-1
Raman shift (cm ) Fig. 1. Raman spectra of CuInS2 powders with different pH value of the precursor solution.
be assigned to the B2 mode similar to that reported by other workers [5,6]. The peak at 265 cm1, which comes only for pHB9.0 was ascribed to B2 mode [6]. It addition, there are two unknown modes at 305 and 472 cm1, respectively which should be identified and for obtaining device quality absorber layer these peaks should be eliminated by controlling the precursor solution as far as possible. Before going into the details of this identification of different modes, a general discussion on chalcopyrite semiconductor may be appropriate. The unit cell of a chalcopyrite semiconductor contains eight atoms, which give rise to 24 vibrational modes. Among these modes only 21 modes are optical and the rest three modes are acoustic. The general vibration of the primitive cell involving optical and acoustic modes may be represented by G ¼ A1 þ 2A2 þ 3B1 þ 4B2 þ 7E:
ð1Þ
The optical modes in the crystal may be written as Gopt ¼ A1 þ 2A2 þ 3B1 þ 3B2 þ 6E
ð2Þ
while the acoustical modes are as follows: Gac ¼ B2 þ E:
ð3Þ
In the optical mode category only three B2 and six E modes are infrared active. On the contrary, all the modes except A2 ; are Raman active. Hence a total of 13 modes are to be expected, in principle, from the Raman measurements. Optical modes having A1 and A2 symmetry involve only displacements of sulpher atoms while the B1 ; B2 and E modes include displacements of Cu and In atoms.
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700
650
600 FWHM of 292 cm-1 peak (cm-1)
Intensity of the peak at 292 cm-1 (a.u.)
750
550
500
450
7.0
6.5
6.0
7
8
9 10 11 pH value
12
400 7
8
9
10
11
12
pH value of the precursor solution Fig. 2. Variation of the peak intensity of the Raman mode at 292 cm1 with pH value of the precursor solution. Inset shows the variation of FWHM of 292 cm1 peak with the pH value of the precursor solution.
The variation of the intensity of the Raman peak was studied with variation of pH of the precursor solution. This showed a gradual increase of intensity, as shown in Fig. 2 for a particular peak (292 cm1). Different peaks of the Raman spectra of the CuInS2 powder (Fig. 1) were fitted to the Gaussian curve and the full-widths at halfmaximum (FWHM) was estimated for each peak. The FWHM of the peak at 292 cm1 decreased with increasing pH value as shown in the inset of Fig. 2 and it becomes steady above a particular pH value (pH>10). Broadening of Raman modes is very much related to the degradation of structural quality [7]. Hence, inset of Fig. 2 clearly shows the existence of worse structural quality of CuInS2 powder samples synthesized with lower pH value of the precursor solution. The peak at 292 cm1 sharpens with the increase of the pH value of the precursor solution. On the other hand, broadening of the Raman peak or increase of FWHM was observed for the peak at 472 cm1 with increasing pH value. A significant variation of the peak width may be observed in Figs. 3(a)–(c) for pH values 7.5, 9.0 and 10.8, respectively. It was noticed that the peak at 472 cm1 which may be ascribed to the internal stretching mode of the SO2 4 ion [8] disappeared above pH=11 (Fig. 3d).
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pH~ 9.0
Intensity (a.u.)
Intensity (a.u.)
pH~ 7.5
440 450 460 470 480 490 500 510
440 450 460 470 480 490 500 510
-1
-1
Wavenumber (cm )
Wavenumber (cm ) 50
pH~ 10.8
Intensity (a.u.)
FWHM of the peak at 472 cm
-1
40
30
20
10
0 8
440 450 460 470 480 490 500 510 -1
10
12
14
pH of the precursor solution
Wavenumber (cm ) 1
Fig. 3. Gaussian fittings of the Raman peak at 472 cm for different pH values of the precursor solution: (a) pHB7.5, (b) pHB9.0, and (c) pHB10.8. (d) Shows the variation of the FWHM of the 472 cm1 mode with the pH value of the precursor solution.
This ion production may decrease with the increasing pH value as excess ammonia reduced the SO2 ions. So, preparing CuInS2 powder at higher pH with excess 4 ammonia could eliminate the unwanted Raman peak at 472 cm1.
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Now, we consider the peak at 305 cm1. The presence of a mode around 305 cm1 (X1) similar to our samples, was reported by other workers [6–7,9] for Cu poor samples only. Three possible origins of the X1 mode as suggested by different workers are discussed below. (i) An intense mode at B305 cm1 has been previously noticed for In rich CuInS2 samples [6,10–11] and also b-In2S3 presents a strong peak close to 305 cm1 with A1 symmetry [12]. These two results indicated that the X1 mode may be related to the presence of b-In2S3 in the In rich CuInS2 samples. To compare the Raman spectra of CuInS2 with that of b-In2S3, we synthesized b-In2S3 powder using InCl3 and thioacetamide as precursors. The compositions of the CuInS2 powders were determined by the energy dispersive X-ray analysis (EDAX) and the results are shown in Table 1. It can be observed that the powders are not only nearly stoichiometric but slightly Cu rich. These results surely did not support the formation of b-In2S3 phase in the powder samples. The XRD spectra of b-In2S3 and CuInS2 powders were also compared and are shown in Fig. 4. It is evident that the XRD of CuInS2 powder shows four prominent peaks due to (1 1 2), (0 0 2), (0 2 4) and (1 3 2) planes while that of b-In2S3 shows strong reflection from (1 1 6), (1 0 9), (0 0 12), (1 0 15), (2 2 12) and (4 1 9) planes. Fig. 4 certainly demonstrated that the strong peaks, which correspond to b-In2S3 were absent in the CuInS2 powder samples. To complete the investigation, we compared the Raman spectrum of CuInS2 with that of b-In2S3. Fig. 5 shows the Raman spectra of b-In2S3 and CuInS2 powder. Although there is an intense Raman mode at B305 cm1, the other peaks which correspond to b-In2S3, are absent in the spectrum of synthesized CuInS2 powder. Also, the EDAX results along with the XRD did not support the presence of b-In2S3 in CuInS2 powder. So, the possibility of the presence of b-In2S3 phase in our samples may be discarded. (ii) Another possibility is the presence of CuInS2 with sphalerite structure but this particular structure is not energetically favorable phase at room temperature [7]. Moreover the XRD studies suggested a chalcopyrite structure with average lattice constants c ¼ 0:552 nm and a ¼ 1:094 nm. In a recent work AlvarezGarcia et al. [13] observed the co-existence of domains having the Cu-Au ordered structure and chalcopyrite structure in a CuInS2 epitaxial layer. They noticed prominent XRD peaks at 2y ¼ 16:0 and 32.3 due to (0 0 1) and (0 0 2) planes respectively in the Cu–Au ordering along with the usual chalcopyrite peaks. The second one at 32.3 is characteristic of both chalcopyrite and Cu–Au structure. After a detailed calculation they showed that the additional band at 305 cm1 in Raman spectrum is related to Cu–Au ordering in CuInS2 crystal structure. The XRD of our CuInS2 sample (Fig. 4) did not indicate any Cu–Au ordering in CuInS2 and also no peak was observed near 2y ¼ 16:0 although a small peak was appeared near 32.2 . The peak at 32.2 must be due to the chalcopyrite structure because for Cu–Au phase there should be another peak at 2y ¼ 16:0 associated with it. Thus, there was no evidence of any structural disorder by which the X1 mode could be explain.
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Table 1 Compositions of CuInS2 powders Sample name
pH value
S (at%)
Cu (at%)
In (at%)
Cu/In
S/(Cu+In)
CISP-9D CISP-9B CISP-9A CISP-9C
7.5 9 10.8 11.8
46.04 46.21 47.19 46.17
27.64 28.77 27.82 28.93
26.33 25.02 25 24.9
1.0497 1.1498 1.1128 1.1618
0.85307 0.85908 0.89341 0.8577
(1 1 2)
(0 2 4) CuInS2 (1 3 2) (2 2 4) (4 1 9)
(2 2 12)
(0 0 12)
(1 0 15)
(1 0 9)
β-In2S3
(1 1 6)
Intensity (a.u.)
(0 0 2)
10
20
30
40
50
60
2θ (degree) Fig. 4. XRD spectra of one CuInS2 powder and one b-In2S3 powder.
(iii) Third possibility was the presence of local vibrational mode as proposed by Kondo et al. [6]. X1 mode can be calculated assuming a virtual force constant corresponding to a Cu1xInxS2 virtual crystal. There is a high possibility that this is the origin of the 305 cm1 mode in the Raman spectra of our powder sample.
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292 cm
-1
CuInS2 302 cm
-1
340 cm
Intensity (a.u.)
303 cm
263 cm 243 cm
-1
-1
-1
323 cm
-1
-1
β-In2S3 363 cm
100
123
200
300
-1
400
500
600
-1
Raman shift (cm ) Fig. 5. Raman spectra of CuInS2 powder and b-In2S3 powder.
3.2. PL study of CuInS2 powder Fig. 6 shows the effect of annealing on the PL spectra of CuInS2 powder at 80 K. A sharp emission peak was observed at 1.49 eV (#1) in the as grown sample along with a small broad emission peak at about 1.42 eV (#2). Annealing of the as grown sample in hydrogen atmosphere drastically changed the intensity of different peaks in the PL spectra. Particularly, the small broad peak around 1.42 eV became significant after annealing. Annealing of the as grown sample in oxygen atmosphere showed a reduction of this emission peak. Moreover, a small peak was found to appear at B1.1 eV (#3) for O2 annealed powder. It is important to note that, regardless of the annealing treatments (in H2 and O2), the sharp peak at 1.49 eV was observed in all cases. This peak may be attributed to the excitonic recombination in the sample. In a relatively small band gap semiconductor, the excitonic binding energy EX (in the effective mass approach) is given by EX ¼ me4 =2h2 e2 n2 ;
ð4Þ
where m is the reduced exciton mass, e is dielectric constant and n an integer (nX1). The photon energy hn of the radiative recombination may be written as hn ¼ Eg EX
ð5Þ
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(a) as grown
1.49 eV
#1
(b) H2 annealed (c) O2 annealed
PL Intensity (a.u.)
T=80K
1.42 eV
#2 (a)
1.10 eV (b)
#3
(c)
1.0
1.2
1.4
1.6
hυ (eV) Fig. 6. PL spectra (at 80 K) of: (a) as grown powder sample; (b) powder annealed in H2 atmosphere; and (c) powder annealed in O2 atmosphere.
with Eg ; the band gap energy of the samples. Using the values m ¼ 0:14m0 ; e ¼ 9:76 [14] and Eg ¼ 1:507 eV at room temperature (as obtained from the film study, discussed later) the expected emission energy obtained from Eq. (5) was found to support predominant excitonic recombination in the powder CuInS2 sample. At relatively low temperature, these emission peaks became more intense probably due to higher stability of excitons at low temperature. According to our observation, the annealing treatments have no significant influence on the excitonic transition. However, annealing treatment of the sample in hydrogen atmosphere strongly enhanced the PL emission peak around 1.42 eV. This emission peak may be attributed to the recombination through donor–acceptor impurity levels in the sample. In general, the energy of the emitted luminescence for a strong donor– acceptor pair (DAP) recombination is given by [15]. hn ¼ Eg EA ED þ e2 =4per;
ð6Þ
where r is the distance between the donor and the acceptor levels within the forbidden gap of the material and EA ; ED are the acceptor and donor activation
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energy states, respectively. The tentative values of EA B70 meV [16], ED B35 meV [17] and rB3:7 nm [14] for our sample fairly support the possibility of impurityinduced recombination peak around 1.42 eV in the PL spectra. The presence of donor level ED for our sample may be due to the existence of sulpher vacancy (VS ) while the acceptor level may be due to the copper vacancy (VCu ). This observation was justified from the studies of PL spectra after annealing in hydrogen and oxygen atmosphere separately. In a simple defect model, we may assume that in the as grown sample oxygen occupies most of the vacant sulfur sites [15]. This reduces the number of sulfur vacancies and virtually exhibits little recombination due to VS and VCu : Annealing in hydrogen atmosphere removes the oxygen from sulfur sites. As a result, the concentration of VS increases and consequently there is increase of the intensity of the corresponding recombination (at 1.42 eV). The annealing of the sample in oxygen atmosphere, on the other hand, reduced VS compared to that of the as grown material and practically this defect-induced transition was completely reduced. It seems that the small peak at 1.1 eV (#3) may be attributed to some deep defect level created from oxygen annealing. Further studies in this respect are in progress. 3.3. Thin films of CuInS2 3.3.1. Microstructural study Figs. 7(a)–(d) show the SEM micrographs of four CuInS2 films synthesized by Doctor’s blade technique at different annealing temperatures (523–823 K). From the SEM it can be observed that discrete grains appeared for annealing temperature greater than 723 K. Films with lower annealing temperature did not indicate any evidence of clear grain growth. In the present work, all the films were prepared using precursor solutions with pH greater than 11 as we observed best possible XRD and Raman spectra for these pH value. Fig. 8 shows the XRD patterns of these CuInS2 films which showed crystalline nature with reflection from (1 1 2), (0 2 4) and (1 3 2) planes. It may be observed that the peak intensity increased with the increasing annealing temperature. The FWHM of these peaks decreased with increasing annealing temperature indicating increase in grain size at higher annealing temperature. 3.3.2. Optical transmittance study Fig. 9 showed the transmittance spectra (Tr % vs. l) of the films. As the annealing temperature increased, the stiffness of the spectra near the band edge increased which is a clear indication of good crystallinity with increasing annealing temperature. In fact, at lower annealing temperature, the grain size remained small which might introduce some defect states in the crystal structure. These defects modulated the fall of Tr vs. l trace due to the transitions occurring through these states. This was reflected in the broadening of the Urbach tail of the absorption spectrum. The Urbach tail width (e) [18] was studied using the following equation: a ¼ const: exp ðhn=eÞ;
ð7Þ
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Fig. 7. SEM micrographs of CuInS2 films produced by Doctor’s blade technique and annealed at: (a) 623 K, (b) 723 K, (c) 823 K and (d) 923 K.
where a is the absorption coefficient, hn is incident photon energy and e is Urbach tail width. From this relation, e was calculated by taking inverse of the slope of ln a vs. hn plot (not shown here). A broadening of the tail width (e) was observed with decrease of annealing temperature in Fig. 10 and this was a clear indication of the decrease of defect states with the increasing annealing temperature. It is well known that absorption coefficient (a) is related to the incident photon energy by the following equation: ahn ¼ Aðhn Eg Þm ;
ð8Þ
where A is the constant, Eg is band gap and m is index indicating the transition type. Rearranging this relation we get d½ln ðahnÞ =dðhnÞ ¼ m=ðhn Eg Þ:
ð9Þ
Fig. 11 shows the differential spectra of (plot of d½ln ðahnÞ =dðhnÞvs:hn) which shows a clear discontinuity at B1.50 eV corresponding to the band gap of the film. Considering this value of the band gap we can plot ln (ahn) vs. ln (hn Eg ) (Fig. 11) and the slope of this plot gives the value of m (inset of Fig. 11). For our representative sample, it was found to be 0.52, which indicated a direct allowed transition.
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Annealing Temperature=823K (0 2 4) (1 3 2)
(0 0 2)
(d)
Intensity (a.u.)
Annealing Temperature=723K
(c)
Annealing Temperature=623K
(b)
Annealing Temperature=523K
(a)
20
30
40
50 60 2θ (degree)
70
80
Fig. 8. XRD patterns of the films shown in Fig. 7.
4. Conclusions Raman and photoluminescence (PL) spectra of CuInS2 powder produced by chemical route were studied. Films of CuInS2 were prepared by Doctor’s blade technique and the microstructural and optical properties of these films were investigated. The conclusions derived from these studies are described below. (1) Raman spectra of the powders showed two intense peaks at 292 and 305 cm1. The first one was the characteristic A1 mode, and the second one may be due to some local vibrational mode in the crystal structure. (2) Comparing the Raman and XRD spectra of CuInS2 and b-In2S3 powders it can be said that the unknown mode of 305 cm1 was not due to the formation of b-In2S3 binary phase. (3) Crystal quality of CuInS2 improved with the pH value of the precursor solution and best crystal quality was obtained at pH>11. (4) PL study of CuInS2 powder revealed the formation of sulphur vacancy during H2 annealing. A small PL band was observed at 1.1 eV for the sample annealed in O2 atmosphere due to some deep defect level.
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Ta=823K Ta=723K
16
Ta=623K Ta=523K
Tr%
12
8
4
0 400
600
800
1000
1200
1400
1600
Wavelength (nm) Fig. 9. Transmittance spectra of the films shown in Fig. 7.
30
Urbach tail width
25
Urbach tail width (eV)
128
20
15
10
5
0 500
550
600
650
700
750
800
Annealing temperature (K) Fig. 10. Variation of Urbach tail width with the annealing temperature of the films.
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10
11.2
11.0
ln(αhv)
8 d[ln(αhv)]/d(hv)
129
6
CuInS2 Film m=0.52
10.8
10.6
10.4
4
-0.8
-0.4
0.0
0.4
ln(hv-Eg)
2
CuInS2 Films Eg=1.50 eV
0 1.0
1.5
2.0
2.5
3.0
hv (eV) Fig. 11. Plot of d½ln ðahnÞ =d½hn vs:hn: Inset shows the linear fitting of ln ðahnÞ vs. ln ðhn Eg Þ plot.
(5) CuInS2 films with good crystalline quality in the XRD were produced by low cost Doctor’s blade technique which may be very useful for industrial application. (6) A direct allowed optical transition was observed in the films at B1.50 eV.
Acknowledgements The authors like to thank Mr. K.K. Das for recording the SEM pictures. References [1] M. Contreras, B. Eggas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovoltaics 7 (1999) 311. [2] H.W. Schock, Sol. Energy Mater. Sol. Cells 34 (1994) 19. [3] J. Klaer, J. Bruns, R. Henninger, K. Seimer, R. Klenk, K. Ellmer, D. Br.aunig, Semicond. Sci. Technol. 13 (1998) 1456. [4] T. Walter, R. Menner, H.W. Schock, Proceedings of the 12th E C Photovoltaic Solar Energy Conference, Amsterdam, 1994, p. 1775. [5] D.M. Hwang, C.C. Chen, H.L. Hwang, Chin. J. Phys. 19 (1981) 56. [6] K. Kondo, S. Nakamura, K. Sato, Jpn. J. Appl. Phys. 37 (1998) 5728. ! [7] J. Alvarez-Garcia, J. Macros-Ruzafa, A. P!erez-Rodr!ıguez, A. Romano-Rodr!ıguez, J.R. Morante, R. Scheer, Thin Solid Films 361/362 (2000) 208.
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[8] H.K. Liu, M.L. Hu, W.S. Tse, D.P. Wong, S.J. Lin, Chin. J. Phys. 36 (1998) 542. ! [9] J. Alvarez-Garcia, A. P!erez-Rodr!ıguez, A. Romano-Rodr!ıguez, T. Jawhari, J.R. Morante, R. Scheer, W. Calvet, Thin Solid Films 387 (2001) 216. [10] G. Morrel, R.S. Katiyar, S.Z. Weisz, T. Walter, W. Shocj, I. Balberg, Appl. Phys. Lett. 69 (1996) 987. [11] R. Hunger, M. Wilhelm, K. Diesner, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, Vienna, Austria, July 1998. [12] K. Kambas, J. Spyridelis, M. Balkanski, Phys. Stat. Sol. B 105 (1981) 29. ! [13] J. Alvarez-Garcia, A. P!erez-Rodr!ıguez, B. Barcones, A. Romano-Rodr!ıguez, A. Janotti, S. Wei, R. Scheer, Appl. Phys. Lett. 80 (2002) 562. [14] J.J.M. Binsma, L.J. Giling, M. Bloem, J. Lumin. 27 (1982) 55. [15] K. Topper, J. Krauser, J. Bruns, R. Scheer, A. Weinger, D. Brauning, Sol. Energy Mater. Sol. Cells 49 (1997) 383. [16] K. Topper, J. Bruns, R. Scheer, M. Weber, A. Weinger, D. Brauning, Appl. Phys. Lett. 71 (1997) 482. [17] H. Matsuhita, T. Mihira, T. Takizawa, Jpn. J. Appl. Phys. 40 (2001) 4789. [18] F. Urbach, Phys. Rev. 92 (1953) 1324.
Solar Energy Materials & Solar Cells 80 (2003) 115–130
Synthesis of CuInS2 by chemical route: optical characterization P. Guhaa, D. Dasb, A.B. Maityc, D. Gangulia, S. Chaudhuria,* a
Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India b Energy Research Unit, Indian Association for the Cultivation of Science, Kolkata 700 032, India c Department of Physics, Heritage Institute of Technology, Kolkata 700 107, India Received 9 May 2003; accepted 26 May 2003
Abstract CuInS2 powder was prepared by wet chemical route. The chalcopyrite structure of the powder was revealed by XRD studies. Raman measurements of the powder sample indicated four prominent peaks at 292, 305, 340 and 472 cm1. The possible origin of the 305 cm1 peak was investigated and was found to be some local vibration in the structure. The peaks at 292 and 340 cm1 were ascribed to A1 and B2 modes, respectively. The peak at 472 cm1 which was due to the formation of SO2 ion at lower pH value of the precursor solution could be 4 eliminated by using pH>11.0. Photoluminescence (PL) studies of the CuInS2 powder indicated two distinct peaks at 1.49 and 1.42 eV. Post deposition annealing treatment in H2 atmosphere revealed the formation of excess sulphur vacancy leading to the peak at 1.42 eV in the PL spectra while O2 annealing of the powder created a deep defect level at 1.10 eV. Thick CuInS2 films were prepared by Doctor’s blade technique. Optical transmittance studies of these films indicated direct allowed transition at B1.5 eV. r 2003 Elsevier B.V. All rights reserved. Keywords: CuInS2; Raman study; Thin film; Chemical route; Doctor’s blade
1. Introduction Among different semiconductors the chalcopyrite semiconductors represented by Cu(In,Ga)(S,Se)2, CuIn1xGaxSe2 are most attractive absorber layers which received world wide attention for more than two decades for the fabrication of *Corresponding author. Tel.: +1-91-33-473-4971; fax: +1-91-33-473-2805. E-mail address: [email protected] (S. Chaudhuri). 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-0248(03)00138-7
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polycrystalline solar cells due to their high absorption coefficient and nondegradable properties compared to other solar cell materials. Photovoltaic devices based on thin polycrystalline CuInCuIn1xGaxSe2 absorbers reached efficiencies of 18.8% [1]. However, as selenides are toxic materials, with the evolution of ‘‘Green Technology’’ there has been a demand for the use of non-toxic materials instead of the toxic selenides. In this regard CuInS2 is very attractive for device fabrication. In spite of its being a promising alternative of selenides the knowledge gathered so far on CuInS2 is not yet adequate. The demand of a non-toxic solar cell absorber layer brought CuInS2 to the forefront in recent years and various groups are now carrying out research work on this material. The pure sulphur compound CuInS2 has a comparatively large direct band gap of Eg ¼ 1:53 eV which is in the optimum range for terrestrial photovoltaic conversion. The predicted theoretical efficiency with CuInS2 homojunction is as high as 26% [2]. However, till today the highest reported efficiency of solar cells made with CuInS2 as the absorber layer is only close to 12% [3,4]. The control of electronic properties and grain boundary effects along with reproducibility remains an open problem in Cu–In–S system. In spite of these existing problems, the main advantage of CuInS2 is that it can be synthesized in an environment friendly atmosphere because it does not contain any toxic elements like Se, Te, etc. The other advantage of using CuInS2 is that it can be prepared by using a simple chemical route, which is most suitable for commercial application. In this work, Raman scattering studies, a non-destructive characterization technique, have been applied to the analysis of polycrystalline CuInS2 powders prepared by a chemical route as a function of the pH of the precursor solution. In fact, the Raman study was used to obtain the best possible crystalline CuInS2 in powder form. Photoluminescence (PL) study of this powder was performed after the post deposition annealing treatment. Films were prepared by a Doctor’s blade technique using the precipitate of the precursor solution with a suitable pH value which indicated less defect formation in the Raman analysis. Microstructural and optical studies of the films are also presented in this communication.
2. Experimental In the present study copper(II) chloride dihydrate (E. Merck India Limited, 99%), thiourea (E. Merck India Limited), 25% ammonia solution (International Chemicals, India) and indium (III) chloride prepared from indium (metal) and hydrochloric acid (International Chemicals, LR) were used for synthesis. Deionised water was used for preparing the solutions. At first 5 ml aqueous solution of InCl3 and CuCl2 2H2O was prepared and stirred for 1 h. The concentration of each salt in the solution was maintained at 0.3 M l1. This greenish blue solution had a pH of 3.2. Ammonia solution (25%) was added to this solution to change the pH to various values from 7.5 to 11.8. After the addition of NH4OH the color of the solution changed to deep blue. This solution was stirred for almost 2 h. A 2 ml aqueous
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solution of thiourea was prepared in a test tube and slowly added to the above mentioned deep blue solution. The amount of thiourea in the solution was taken in such a way that the Cu:In:S molar ratio in the solution remains at 1:1:3.5. The color of the solution was slowly changed to brown and after 3 h of stirring a black precipitate appeared. The black precipitate was collected and dried at 373 K for 30 min. The precipitate was then annealed at 623–723 K to obtain well-crystallized CuInS2 powder. One part of the crystallized CuInS2 powder was then annealed at 573 K in H2 atmosphere and another part in O2 atmosphere for 30 min. These powders were used in the PL measurements. For the preparation of the films the precipitate of the precursor solution with pHB11.0 was used. CuInS2 films were prepared by Doctor’s blade technique using the slurry like black precipitate of the precursor solution. Doctor’s blade unit consisted of an AC motor, a Doctor’s blade and a substrate holder. The blade was moved by the motor, the moving rate of which could be controlled by the associated electrical circuit. The slurry was kept on the substrate and the Doctor’s blade was moved over the substrate in such a way that a thin layer of chemicals were deposited on the substrate. The substrate with the layer was then annealed in an appropriate temperature inside a graphite box to obtain CuInS2. b-In2S3 was synthesized using InCl3 and thioacetamide as the precursors. Detailed discussion of this synthesis will be published in another paper. X-ray diffraction patterns of powders and the films were carried out by Seifert XDAL 3000 with monochromatic CuKa radiation (Ni filter). The compositions of the materials were determined from EDAX (Kevex Delta Class-I) using ZAF correction. PL spectra were recorded with the excitation B650 nm using a 300 W xenon arc lamp as the emission source. A Hamamatsu photomultiplier was used as the detector along with a 1/4 m monochromator.
3. Results and discussion 3.1. Raman study of CuInS2 powder The Raman spectra of chemically synthesized CuInS2 powder prepared from different precursor solutions with different pH values were recorded by a microRaman spectrometer (ILOR-Jobin-Yvon-SPEX) using polarized beam of 632.8 nm with photons from a 20 mW He–Ne laser. The light was focussed on to the sample through a 100 optical microscope objective (Olympus) in a back scattering geometry. The scattered light was collected and dispersed by a grating and the entire spectrum was recorded by a CCD. Fig. 1 shows the Raman spectra of CuInS2 powder samples prepared with different pH of the precursor. It may be observed that there are four prominent peaks at 292, 305, 340 and 472 cm1 for all the samples prepared with different pH values in addition to the peak at 265 cm1. It may also be observed that the intensity of the Raman peak increased with increasing pH of the precursor solution as shown in Fig. 2 for a particular peak (292 cm1). The strong peak at 292 cm1 may be assigned to the A1 mode [5] while the peak at 340 cm1 may
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Raman intensity (a.u.)
292 cm
265 cm
100
-1
305 cm -1 340 cm -1 472 cm
-1
200
pH~ 7.5
pH~ 10.8
pH~ 9
pH~ 11.8
300
400
500
600
-1
Raman shift (cm ) Fig. 1. Raman spectra of CuInS2 powders with different pH value of the precursor solution.
be assigned to the B2 mode similar to that reported by other workers [5,6]. The peak at 265 cm1, which comes only for pHB9.0 was ascribed to B2 mode [6]. It addition, there are two unknown modes at 305 and 472 cm1, respectively which should be identified and for obtaining device quality absorber layer these peaks should be eliminated by controlling the precursor solution as far as possible. Before going into the details of this identification of different modes, a general discussion on chalcopyrite semiconductor may be appropriate. The unit cell of a chalcopyrite semiconductor contains eight atoms, which give rise to 24 vibrational modes. Among these modes only 21 modes are optical and the rest three modes are acoustic. The general vibration of the primitive cell involving optical and acoustic modes may be represented by G ¼ A1 þ 2A2 þ 3B1 þ 4B2 þ 7E:
ð1Þ
The optical modes in the crystal may be written as Gopt ¼ A1 þ 2A2 þ 3B1 þ 3B2 þ 6E
ð2Þ
while the acoustical modes are as follows: Gac ¼ B2 þ E:
ð3Þ
In the optical mode category only three B2 and six E modes are infrared active. On the contrary, all the modes except A2 ; are Raman active. Hence a total of 13 modes are to be expected, in principle, from the Raman measurements. Optical modes having A1 and A2 symmetry involve only displacements of sulpher atoms while the B1 ; B2 and E modes include displacements of Cu and In atoms.
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700
650
600 FWHM of 292 cm-1 peak (cm-1)
Intensity of the peak at 292 cm-1 (a.u.)
750
550
500
450
7.0
6.5
6.0
7
8
9 10 11 pH value
12
400 7
8
9
10
11
12
pH value of the precursor solution Fig. 2. Variation of the peak intensity of the Raman mode at 292 cm1 with pH value of the precursor solution. Inset shows the variation of FWHM of 292 cm1 peak with the pH value of the precursor solution.
The variation of the intensity of the Raman peak was studied with variation of pH of the precursor solution. This showed a gradual increase of intensity, as shown in Fig. 2 for a particular peak (292 cm1). Different peaks of the Raman spectra of the CuInS2 powder (Fig. 1) were fitted to the Gaussian curve and the full-widths at halfmaximum (FWHM) was estimated for each peak. The FWHM of the peak at 292 cm1 decreased with increasing pH value as shown in the inset of Fig. 2 and it becomes steady above a particular pH value (pH>10). Broadening of Raman modes is very much related to the degradation of structural quality [7]. Hence, inset of Fig. 2 clearly shows the existence of worse structural quality of CuInS2 powder samples synthesized with lower pH value of the precursor solution. The peak at 292 cm1 sharpens with the increase of the pH value of the precursor solution. On the other hand, broadening of the Raman peak or increase of FWHM was observed for the peak at 472 cm1 with increasing pH value. A significant variation of the peak width may be observed in Figs. 3(a)–(c) for pH values 7.5, 9.0 and 10.8, respectively. It was noticed that the peak at 472 cm1 which may be ascribed to the internal stretching mode of the SO2 4 ion [8] disappeared above pH=11 (Fig. 3d).
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pH~ 9.0
Intensity (a.u.)
Intensity (a.u.)
pH~ 7.5
440 450 460 470 480 490 500 510
440 450 460 470 480 490 500 510
-1
-1
Wavenumber (cm )
Wavenumber (cm ) 50
pH~ 10.8
Intensity (a.u.)
FWHM of the peak at 472 cm
-1
40
30
20
10
0 8
440 450 460 470 480 490 500 510 -1
10
12
14
pH of the precursor solution
Wavenumber (cm ) 1
Fig. 3. Gaussian fittings of the Raman peak at 472 cm for different pH values of the precursor solution: (a) pHB7.5, (b) pHB9.0, and (c) pHB10.8. (d) Shows the variation of the FWHM of the 472 cm1 mode with the pH value of the precursor solution.
This ion production may decrease with the increasing pH value as excess ammonia reduced the SO2 ions. So, preparing CuInS2 powder at higher pH with excess 4 ammonia could eliminate the unwanted Raman peak at 472 cm1.
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Now, we consider the peak at 305 cm1. The presence of a mode around 305 cm1 (X1) similar to our samples, was reported by other workers [6–7,9] for Cu poor samples only. Three possible origins of the X1 mode as suggested by different workers are discussed below. (i) An intense mode at B305 cm1 has been previously noticed for In rich CuInS2 samples [6,10–11] and also b-In2S3 presents a strong peak close to 305 cm1 with A1 symmetry [12]. These two results indicated that the X1 mode may be related to the presence of b-In2S3 in the In rich CuInS2 samples. To compare the Raman spectra of CuInS2 with that of b-In2S3, we synthesized b-In2S3 powder using InCl3 and thioacetamide as precursors. The compositions of the CuInS2 powders were determined by the energy dispersive X-ray analysis (EDAX) and the results are shown in Table 1. It can be observed that the powders are not only nearly stoichiometric but slightly Cu rich. These results surely did not support the formation of b-In2S3 phase in the powder samples. The XRD spectra of b-In2S3 and CuInS2 powders were also compared and are shown in Fig. 4. It is evident that the XRD of CuInS2 powder shows four prominent peaks due to (1 1 2), (0 0 2), (0 2 4) and (1 3 2) planes while that of b-In2S3 shows strong reflection from (1 1 6), (1 0 9), (0 0 12), (1 0 15), (2 2 12) and (4 1 9) planes. Fig. 4 certainly demonstrated that the strong peaks, which correspond to b-In2S3 were absent in the CuInS2 powder samples. To complete the investigation, we compared the Raman spectrum of CuInS2 with that of b-In2S3. Fig. 5 shows the Raman spectra of b-In2S3 and CuInS2 powder. Although there is an intense Raman mode at B305 cm1, the other peaks which correspond to b-In2S3, are absent in the spectrum of synthesized CuInS2 powder. Also, the EDAX results along with the XRD did not support the presence of b-In2S3 in CuInS2 powder. So, the possibility of the presence of b-In2S3 phase in our samples may be discarded. (ii) Another possibility is the presence of CuInS2 with sphalerite structure but this particular structure is not energetically favorable phase at room temperature [7]. Moreover the XRD studies suggested a chalcopyrite structure with average lattice constants c ¼ 0:552 nm and a ¼ 1:094 nm. In a recent work AlvarezGarcia et al. [13] observed the co-existence of domains having the Cu-Au ordered structure and chalcopyrite structure in a CuInS2 epitaxial layer. They noticed prominent XRD peaks at 2y ¼ 16:0 and 32.3 due to (0 0 1) and (0 0 2) planes respectively in the Cu–Au ordering along with the usual chalcopyrite peaks. The second one at 32.3 is characteristic of both chalcopyrite and Cu–Au structure. After a detailed calculation they showed that the additional band at 305 cm1 in Raman spectrum is related to Cu–Au ordering in CuInS2 crystal structure. The XRD of our CuInS2 sample (Fig. 4) did not indicate any Cu–Au ordering in CuInS2 and also no peak was observed near 2y ¼ 16:0 although a small peak was appeared near 32.2 . The peak at 32.2 must be due to the chalcopyrite structure because for Cu–Au phase there should be another peak at 2y ¼ 16:0 associated with it. Thus, there was no evidence of any structural disorder by which the X1 mode could be explain.
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Table 1 Compositions of CuInS2 powders Sample name
pH value
S (at%)
Cu (at%)
In (at%)
Cu/In
S/(Cu+In)
CISP-9D CISP-9B CISP-9A CISP-9C
7.5 9 10.8 11.8
46.04 46.21 47.19 46.17
27.64 28.77 27.82 28.93
26.33 25.02 25 24.9
1.0497 1.1498 1.1128 1.1618
0.85307 0.85908 0.89341 0.8577
(1 1 2)
(0 2 4) CuInS2 (1 3 2) (2 2 4) (4 1 9)
(2 2 12)
(0 0 12)
(1 0 15)
(1 0 9)
β-In2S3
(1 1 6)
Intensity (a.u.)
(0 0 2)
10
20
30
40
50
60
2θ (degree) Fig. 4. XRD spectra of one CuInS2 powder and one b-In2S3 powder.
(iii) Third possibility was the presence of local vibrational mode as proposed by Kondo et al. [6]. X1 mode can be calculated assuming a virtual force constant corresponding to a Cu1xInxS2 virtual crystal. There is a high possibility that this is the origin of the 305 cm1 mode in the Raman spectra of our powder sample.
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292 cm
-1
CuInS2 302 cm
-1
340 cm
Intensity (a.u.)
303 cm
263 cm 243 cm
-1
-1
-1
323 cm
-1
-1
β-In2S3 363 cm
100
123
200
300
-1
400
500
600
-1
Raman shift (cm ) Fig. 5. Raman spectra of CuInS2 powder and b-In2S3 powder.
3.2. PL study of CuInS2 powder Fig. 6 shows the effect of annealing on the PL spectra of CuInS2 powder at 80 K. A sharp emission peak was observed at 1.49 eV (#1) in the as grown sample along with a small broad emission peak at about 1.42 eV (#2). Annealing of the as grown sample in hydrogen atmosphere drastically changed the intensity of different peaks in the PL spectra. Particularly, the small broad peak around 1.42 eV became significant after annealing. Annealing of the as grown sample in oxygen atmosphere showed a reduction of this emission peak. Moreover, a small peak was found to appear at B1.1 eV (#3) for O2 annealed powder. It is important to note that, regardless of the annealing treatments (in H2 and O2), the sharp peak at 1.49 eV was observed in all cases. This peak may be attributed to the excitonic recombination in the sample. In a relatively small band gap semiconductor, the excitonic binding energy EX (in the effective mass approach) is given by EX ¼ me4 =2h2 e2 n2 ;
ð4Þ
where m is the reduced exciton mass, e is dielectric constant and n an integer (nX1). The photon energy hn of the radiative recombination may be written as hn ¼ Eg EX
ð5Þ
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(a) as grown
1.49 eV
#1
(b) H2 annealed (c) O2 annealed
PL Intensity (a.u.)
T=80K
1.42 eV
#2 (a)
1.10 eV (b)
#3
(c)
1.0
1.2
1.4
1.6
hυ (eV) Fig. 6. PL spectra (at 80 K) of: (a) as grown powder sample; (b) powder annealed in H2 atmosphere; and (c) powder annealed in O2 atmosphere.
with Eg ; the band gap energy of the samples. Using the values m ¼ 0:14m0 ; e ¼ 9:76 [14] and Eg ¼ 1:507 eV at room temperature (as obtained from the film study, discussed later) the expected emission energy obtained from Eq. (5) was found to support predominant excitonic recombination in the powder CuInS2 sample. At relatively low temperature, these emission peaks became more intense probably due to higher stability of excitons at low temperature. According to our observation, the annealing treatments have no significant influence on the excitonic transition. However, annealing treatment of the sample in hydrogen atmosphere strongly enhanced the PL emission peak around 1.42 eV. This emission peak may be attributed to the recombination through donor–acceptor impurity levels in the sample. In general, the energy of the emitted luminescence for a strong donor– acceptor pair (DAP) recombination is given by [15]. hn ¼ Eg EA ED þ e2 =4per;
ð6Þ
where r is the distance between the donor and the acceptor levels within the forbidden gap of the material and EA ; ED are the acceptor and donor activation
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energy states, respectively. The tentative values of EA B70 meV [16], ED B35 meV [17] and rB3:7 nm [14] for our sample fairly support the possibility of impurityinduced recombination peak around 1.42 eV in the PL spectra. The presence of donor level ED for our sample may be due to the existence of sulpher vacancy (VS ) while the acceptor level may be due to the copper vacancy (VCu ). This observation was justified from the studies of PL spectra after annealing in hydrogen and oxygen atmosphere separately. In a simple defect model, we may assume that in the as grown sample oxygen occupies most of the vacant sulfur sites [15]. This reduces the number of sulfur vacancies and virtually exhibits little recombination due to VS and VCu : Annealing in hydrogen atmosphere removes the oxygen from sulfur sites. As a result, the concentration of VS increases and consequently there is increase of the intensity of the corresponding recombination (at 1.42 eV). The annealing of the sample in oxygen atmosphere, on the other hand, reduced VS compared to that of the as grown material and practically this defect-induced transition was completely reduced. It seems that the small peak at 1.1 eV (#3) may be attributed to some deep defect level created from oxygen annealing. Further studies in this respect are in progress. 3.3. Thin films of CuInS2 3.3.1. Microstructural study Figs. 7(a)–(d) show the SEM micrographs of four CuInS2 films synthesized by Doctor’s blade technique at different annealing temperatures (523–823 K). From the SEM it can be observed that discrete grains appeared for annealing temperature greater than 723 K. Films with lower annealing temperature did not indicate any evidence of clear grain growth. In the present work, all the films were prepared using precursor solutions with pH greater than 11 as we observed best possible XRD and Raman spectra for these pH value. Fig. 8 shows the XRD patterns of these CuInS2 films which showed crystalline nature with reflection from (1 1 2), (0 2 4) and (1 3 2) planes. It may be observed that the peak intensity increased with the increasing annealing temperature. The FWHM of these peaks decreased with increasing annealing temperature indicating increase in grain size at higher annealing temperature. 3.3.2. Optical transmittance study Fig. 9 showed the transmittance spectra (Tr % vs. l) of the films. As the annealing temperature increased, the stiffness of the spectra near the band edge increased which is a clear indication of good crystallinity with increasing annealing temperature. In fact, at lower annealing temperature, the grain size remained small which might introduce some defect states in the crystal structure. These defects modulated the fall of Tr vs. l trace due to the transitions occurring through these states. This was reflected in the broadening of the Urbach tail of the absorption spectrum. The Urbach tail width (e) [18] was studied using the following equation: a ¼ const: exp ðhn=eÞ;
ð7Þ
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Fig. 7. SEM micrographs of CuInS2 films produced by Doctor’s blade technique and annealed at: (a) 623 K, (b) 723 K, (c) 823 K and (d) 923 K.
where a is the absorption coefficient, hn is incident photon energy and e is Urbach tail width. From this relation, e was calculated by taking inverse of the slope of ln a vs. hn plot (not shown here). A broadening of the tail width (e) was observed with decrease of annealing temperature in Fig. 10 and this was a clear indication of the decrease of defect states with the increasing annealing temperature. It is well known that absorption coefficient (a) is related to the incident photon energy by the following equation: ahn ¼ Aðhn Eg Þm ;
ð8Þ
where A is the constant, Eg is band gap and m is index indicating the transition type. Rearranging this relation we get d½ln ðahnÞ =dðhnÞ ¼ m=ðhn Eg Þ:
ð9Þ
Fig. 11 shows the differential spectra of (plot of d½ln ðahnÞ =dðhnÞvs:hn) which shows a clear discontinuity at B1.50 eV corresponding to the band gap of the film. Considering this value of the band gap we can plot ln (ahn) vs. ln (hn Eg ) (Fig. 11) and the slope of this plot gives the value of m (inset of Fig. 11). For our representative sample, it was found to be 0.52, which indicated a direct allowed transition.
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Annealing Temperature=823K (0 2 4) (1 3 2)
(0 0 2)
(d)
Intensity (a.u.)
Annealing Temperature=723K
(c)
Annealing Temperature=623K
(b)
Annealing Temperature=523K
(a)
20
30
40
50 60 2θ (degree)
70
80
Fig. 8. XRD patterns of the films shown in Fig. 7.
4. Conclusions Raman and photoluminescence (PL) spectra of CuInS2 powder produced by chemical route were studied. Films of CuInS2 were prepared by Doctor’s blade technique and the microstructural and optical properties of these films were investigated. The conclusions derived from these studies are described below. (1) Raman spectra of the powders showed two intense peaks at 292 and 305 cm1. The first one was the characteristic A1 mode, and the second one may be due to some local vibrational mode in the crystal structure. (2) Comparing the Raman and XRD spectra of CuInS2 and b-In2S3 powders it can be said that the unknown mode of 305 cm1 was not due to the formation of b-In2S3 binary phase. (3) Crystal quality of CuInS2 improved with the pH value of the precursor solution and best crystal quality was obtained at pH>11. (4) PL study of CuInS2 powder revealed the formation of sulphur vacancy during H2 annealing. A small PL band was observed at 1.1 eV for the sample annealed in O2 atmosphere due to some deep defect level.
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Ta=823K Ta=723K
16
Ta=623K Ta=523K
Tr%
12
8
4
0 400
600
800
1000
1200
1400
1600
Wavelength (nm) Fig. 9. Transmittance spectra of the films shown in Fig. 7.
30
Urbach tail width
25
Urbach tail width (eV)
128
20
15
10
5
0 500
550
600
650
700
750
800
Annealing temperature (K) Fig. 10. Variation of Urbach tail width with the annealing temperature of the films.
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10
11.2
11.0
ln(αhv)
8 d[ln(αhv)]/d(hv)
129
6
CuInS2 Film m=0.52
10.8
10.6
10.4
4
-0.8
-0.4
0.0
0.4
ln(hv-Eg)
2
CuInS2 Films Eg=1.50 eV
0 1.0
1.5
2.0
2.5
3.0
hv (eV) Fig. 11. Plot of d½ln ðahnÞ =d½hn vs:hn: Inset shows the linear fitting of ln ðahnÞ vs. ln ðhn Eg Þ plot.
(5) CuInS2 films with good crystalline quality in the XRD were produced by low cost Doctor’s blade technique which may be very useful for industrial application. (6) A direct allowed optical transition was observed in the films at B1.50 eV.
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