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Deposition of CuCdS2 thin film by single step solution process at low temperature as a novel absorber for photovoltaic applications
Superlattices and Microstructures 76 (2014) 125–134
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Deposition of CuCdS2 thin film by single step solution process at low temperature as a novel absorber for photovoltaic applications V. Nirmal Kumar a,b, R. Suriakarthick a, Y. Hayakawa b, Shamima Hussain c, G.M. Bhalerao c, Mukul Gupta d, Vasant Sathe d, R. Gopalakrishnan a,⇑ a
Crystal Research Lab, Department of Physics, Anna University, Chennai, India Research Institute of Electronics, Graduate School of Science and Technology, Shizuoka University, Hamamatsu, Japan c UGC-DAE Consortium for Scientific Research, Kalpakkam, India d UGC-DAE Consortium for Scientific Research, Indore, India b
a r t i c l e
i n f o
Article history: Received 23 September 2014 Accepted 24 September 2014 Available online 12 October 2014 Keywords: CuCdS2 Chemical bath deposition Ternary compound Optical properties Electrical properties
a b s t r a c t Thin film of ternary compound semiconductor, copper cadmium sulphide (CuCdS2) was deposited on glass substrate by low temperature solution process. The deposited CuCdS2 thin film was found to suitable for the fabrication of efficient thin film solar cells at low cost. Hexagonal structure of poly crystalline CuCdS2 was formed from the combination of its binary constituents CuxS and CdS. The film was continuously coated and particles were aggregated like spherical granules. The presence of elements Cu, Cd and S were confirmed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. The absorption spectrum showed that the deposited material had wide range of absorption in visible and near-IR region (300–1100 nm) and band gap was estimated to be 1.42 eV using Tauc’s plot. Vibration modes of CdS and CuxS binary phases were observed and shift between them revealed the existence of binding state between CdS and CuxS. Hall measurement showed p-type conducting nature of charge carriers with higher carrier concentration 3.48 1018 cm3. The mobility and resistivity were measured to be 10.63 cm2/V s and 0.17 O cm, respectively. The analyses revealed that the deposited CuCdS2 thin
⇑ Corresponding author at: Department of Physics, Anna University, Chennai 600 025, Tamilnadu, India. Tel.: +91 44 2235 8710. E-mail addresses: [email protected], [email protected] (R. Gopalakrishnan). http://dx.doi.org/10.1016/j.spmi.2014.09.030 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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film was suitable material for the fabrication of thin film solar cells at low cost. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Development of new semiconductor materials promotes various applications in diverse fields. Apart from elemental semiconductors such as Si and Ge, compound semiconductors made of metal sulphides [1,2], metal oxides [3,4] and metal selenides [5,6] captivated many researchers by virtue of their progressive physical properties. For energy harvesting by means of solar power, ternary and quaternary metal chalcogenides play assuring role emanating from their tuneable physical properties by simply varying the composition of its constituent elements [6,7]. Many research groups working in the field of thin film solar cells (TFSC) have been examining different compound semiconductors [2,8] for effective utilization of solar energy in the past decades, since TFSC have added advantages like availability of low cost fabrication methods, need of small amount of source material and better reproducibility [9]. High efficiency in TFSC was achieved by Cu and Cd based compound semiconductors like CuInGaSe2, CuZnSnS2, CdTe, CuInSe2 and CuxS. CuInGaSe2 recorded the highest efficiency around 20% in TFSCs among all other compound semiconductors [6]. Since CdS has an optimum large band gap of 2.42 eV, it has been used as a window layer with CIGS, CZTS, CIS and CdTe absorbers which recorded higher efficiency [6,10–12]. Since the highest efficiency in TFSCs has been achieved using CdS and copper based absorber materials, fabrication of solar cells by combining these materials could increase the cell efficiency due to enhanced charge transport properties between CuS and CdS owing to less lattice mismatch between them. CuCdS2 is I-II-VI2 compound semiconductor material having properties between its binary counterparts CuxS and CdS. Even though the physical properties of CdS and CuxS are quite opposite, they have similarities in their lattice arrangement which could be revealed by a report by Cook et al. They demonstrated the conversion of CdS single crystal into CuS single crystal by dipping it into CuCl solution [13]. Among the metal chalcogenides family, CuCdS2 is a novel one and yet to understand its properties well. With the course of introducing possible contemporary alternate material for the fabrication of TFSC, we have been synthesised copper cadmium sulphide (CuCdS2) thin films by wet chemical route at low temperature. As for our knowledge this is the first report of CuCdS2 ternary compound semiconductor thin films prepared by chemical bath deposition method aiming low cost production of TFSCs. We are reporting the optical, electrical and morphological properties of CuCdS2 thin film deposited by chemical bath deposition method. Though many physical (thermal evaporation, PLD, sputtering, etc.) and chemical methods (sol–gel, electro deposition, photochemical deposition, etc.) have been employed for the deposition of thin films, chemical bath deposition has many advantages such as, large area of deposition, uniform and continuous coating of thin films, low cost fabrication method and possible reproducibility. Thin film of CuCdS2 has been deposited on microscopic glass substrate from chemical solution containing CuSO4, CdSO4 as cation sources and thiourea as anion source. The deposited film was cleaned ultrasonically for few seconds and dried in oven. The physical properties of deposited CuCdS2 thin films were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) with electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), UV–Vis, Raman spectroscopy and Hall measurement studies. 2. Experiment 2.1. Synthesis and formation of CuCdS2 thin film CuCdS2 thin film was chemically deposited in single step using CuSO45H2O, CdSO48H2O and CS(NH2)2 (thiourea) as cation and anion sources, respectively. The schematic of this simple chemical
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127
Fig. 1. Schematic of simple chemical bath deposition process.
bath deposition process is shown in Fig. 1. The precursor solution was prepared by dissolving 0.08 M of CuSO45H2O and 0.02 M of CdSO48H2O in 100 mL of deionized water. 0.2 M of thiourea was added to this solution under continuous stirring. Ammonia was added drop by drop in the solution until pH of the solution became 10.5. While adding ammonia, ultrasonically cleaned glass substrate was immersed vertically in the solution. Addition of ammonia formed complex cation with copper and cadmium ions, thereby slow and uniform deposition of the compound made possible. The solution was kept under continuous stirring for 40 min. The formation of compound was revealed by the colour change of the solution from dark blue to dark brown. CuCdS2 thin film was formed by chemical reaction between cations (Cu2+, Cd2+) and anions (S2) in the basic region (pH > 7). Basic region of the solution is necessary for the availability of hydroxyl ions that causes hydrolysis of thiourea to form sulphur ions and for the formation of metal hydroxide compound which would be adsorbed on the surface of glass substrate [14]. The formation of CuCdS2 compound is through the formation of its binary counterparts CuxS and CdS. Since copper sulphide exists in five different stable phases at room temperature, it is difficult to distinguish a particular phase in solution [15]. The in-situ study by quartz crystal microbalance technique reported by Borges and Lincot, explained the reaction mechanism of thiourea and ammonia system for the deposition of CdS via the formation of cadmium hydroxide [16]. Rieke and Bentjen reported the formation of cadmium complexes in the deposition of CdS which would be applicable to the other similar systems [17]. The formation of CuxS thin films in similar system via copper complex was reported earlier by Nair and Nair [18]. We used the similar ammonia–thiourea system to deposit CuCdS2 thin films and the reactions of copper (Cu2+) and cadmium (Cd2+) ions with ammonia, as in earlier reports were applicable to our system. The reaction for the formation of CuCdS2 is as follows,
CuSO4 þ CdSO4 þ 8NH3 ! ½CuðNH3 Þ4 2þ þ ½CdðNH3 Þ4 2+
2þ
þ 2SO2 4
ð1Þ
2+
The Cu and Cd ions existing in solution from dissolved CuSO4 and CdSO4 reacted with ammonia to form their complexes tetramine copper ([Cu(NH3)4]2+) and tetramine cadmium ([Cd(NH3)4]2+) cations [18,19]. Since the solution is basic, hydrolysis of thiourea takes place to release sulphur ions as follows,
CSðNH2 Þ2 þ OH ! CH2 N2 þ H2 O þ HS
ð2Þ
HS þ OH ! S2 þ H2 O
ð3Þ 2+
2+
2
As a consequence, cations ([Cu(NH3)4] , [Cd(NH3)4] ) and anions (S nary compound CuCdS2 in the solution as follows,
½CuðNH3 Þ4 2þ þ ½CdðNH3 Þ4
2þ
þ 2S2 ! CuCdS2 þ 8NH3
) will react to form the ter-
ð4Þ
The formation of compound took place via heterogeneous nucleation, along with homogeneous nucleation formed in solution, grown further on glass substrate and container wall. After 40 min the deposited film was taken out from the solution and cleaned ultrasonically for a few seconds to
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remove the loosely adhered particles. Even though the film was kept over 20 s under ultrasonification, film was not pealed out from the substrate which revealed well adhesiveness of deposited material on the substrate. The film was dried in oven at 50 °C for further characterization. 2.2. Characterization of CuCdS2 thin films The deposited CuCdS2 films were subjected to XRD, SEM, TEM with EELS, XPS, UV–Vis, Raman and Hall measurement studies. XRD pattern of the deposited thin film was recorded with Cu Ka X-ray source by Bruker D8 advance XRD system. Surface morphology of Au coated CuCdS2 thin film was analyzed by Supra 55 FESEM – Karl Zeiss system. TEM equipped with EELS was used to identify the crystallinity and elements present. Optical absorbance in the range 200–1100 nm was recorded by Perkin Elmer Lambda 950 system with uncoated glass slides as reference. Raman spectra of the deposited thin films in the range 50–1000 cm1 were recorded by Jobin Yvon Horibra LABRAM-HR visible system. Ar ion laser having excitation wavelength 488 nm, with spot size 1 lm, was used for recording the Raman spectrum. The spectra were recorded at room temperature by CCD camera having 600 lines/mm gratings with spectral resolution of about 1 cm1. Hall effect measurement was carried out using Ecopia Hall effect measurement system (HMS – 3000) to analyse the electrical properties of the deposited thin film. 3. Results and discussion 3.1. Structural properties The X-ray diffraction pattern of the deposited CuCdS2 thin film is shown in Fig. 2. Since the compound is new, there is no data on Joint committee on powder diffraction standards (JCPDS – PDF) is available for this material. Hence the obtained XRD pattern was indexed by comparing with its binary phase materials CuS, Cu2S and CdS. The diffraction peaks from various (h k l) planes were found to matched with peaks of CuS (JCPDS: PDF #75-2233), Cu2S (JCPDS: PDF # 84-0206) and CdS (JCPDS: PDF #77-2306) binary phases. Among the observed peaks, eight of CuS, six of Cu2S and six of CdS were matched with standard data. Three peaks at 2h values 44.01°, 60.77° and 66.58° were observed to be common for (0 0 8), (2 0 4) and (2 0 6) planes of CuS and (1 1 0), (1 0 4) and (2 0 3) planes of CdS. The peak at 60.77° was commonly observed for CuS (2 0 4), Cu2S (2 0 2) and CdS (1 0 4) planes. Atoms of all CuS, Cu2S and CdS phases had arranged to crystalline in hexagonal structure. The close lattice arrangement between CuxS and CdS revealed experimentally by the conversion of CdS single crystal into CuxS single crystal and explained by Cook et al. [13], which strongly support the possibility of co-existence of CuS,
Fig. 2. X-ray diffraction pattern of CuCdS2 thin film.
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Cu2S and CdS as CuCdS2 compound. This analysis revealed that the deposited CuCdS2 thin film has hexagonal structure formed from its binary phases CuxS and CdS. The lattice parameter values a = b = 3.788 Å, c = 16.333 Å of CuS, a = b = 3.95 Å, c = 6.75 Å of Cu2S and a = b = 4.136 Å, c = 6.713 Å of CdS are in good agreement with reported values [20,21]. The observed experimental structural data of CuS, Cu2S and CdS binary phases in the deposited CuCdS2 thin film (calculated using powder-X software) are presented in Table.1. Based on the results obtained it is emphasized that the polycrystalline hexagonal structure of CuCdS2 compound semiconductor resulted with coexistence of CuxS (CuS, Cu2S) and CdS binary phases. 3.2. Morphological and elemental analysis Morphological studies were made by SEM, TEM equipped with EELS. SEM micrograph (Fig. 3(a)) revealed the formation of continuous coating of deposited CuCdS2 thin films and particles aggregated like spherical granules. The film was composed of several islands that resulted from the formation of different constituent phases (CuxS and CdS. The average particle size was measured to be 23 nm. Fig. 3(b) and (c) shows TEM image and selected area electron diffraction pattern (SAED) which reveals polycrystalline nature of the deposited material. Inter planar distance (d value) of the deposited material was found to be 1.9 Å from TEM image as shown in Fig. 3(d), which is closely matched with (0 0 8) plane of CuS and (1 1 0) plane of CdS (Table. 1) that revealed the co-existence of CuS and CdS, formed CuCdS2 compound. The kinetic energy of electrons after interaction with specimen was analyzed by EELS equipped with TEM [22]. EELS spectrum revealed Cu L23, Cd M45 and S L23 edges as shown in Fig. 4(a)–(c). X-ray photoelectron spectroscopy of the deposited CuCdS2 thin films is shown in Fig. 5(a)–(d). The obtained binding energy values matched with reported values of CuS, Cu2S and CdS binary phases [23,24]. EELS and XPS spectroscopic studies confirmed the presence of Cu, Cd and S elements in the deposited CuCdS2 thin films. 3.3. Optical properties 3.3.1. UV–Vis spectral analysis The optical property and band gap of the materials are the decisive key factors for any specific application. The recorded absorption spectrum of the deposited CuCdS2 thin film is shown in Fig. 6. Table 1 Structural data of CuS, Cu2S and CdS binary phases in deposited CuCdS2 thin film. Binary phase
Plane
2h (°)
Inter planar spacing (d) (Å)
Lattice parameter
CuS
004 102 103 008 200 202 204 206 002 101 102 111 201 202 002 110 103 112 104 203
21.748 29.316 31.822 44.333 56.020 57.240 60.808 66.499 26.532 29.684 37.772 48.113 55.665 60.777 26.532 44.010 48.113 51.926 60.777 66.583
4.08325 3.04408 2.80984 2.04163 1.64025 1.60814 1.52204 1.40492 3.35687 3.00716 2.37976 1.88968 1.64985 1.52275 3.35687 2.05585 1.88968 1.75953 1.52275 1.40335
a = 3.788 b = 3.788 c = 16.333
Cu2S
CdS
a = 3.95 b = 3.95 c = 6.75
a = 4.136 b = 4.136 c = 6.713
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Fig. 3. Surface morphology (a) SEM image, (b) TEM image, (C) SAED pattern and (d) Inter planar spacing measured from TEM image of CuCdS2 thin film.
Fig. 4. EELS spectrum (a) Cu L23 edge, (b) Cd M45 edge and (c) S L23 edge of CuCdS2 thin film.
It is inferred from absorption spectrum that the material has wide range of absorption in the region 300–1100 nm which is favourable for photovoltaic applications since maximum number of photons coming from solar radiation having wavelength in visible and near-IR regions. The absorption region of this material was relatively high when compared to other absorber layer materials such as CZTS, CIGS and CIS. The optical band gap of the material was found to be 1.42 eV from Tauc’s plot which was shown as inset in Fig. 6. Optical absorption region and band gap of the deposited material were compared with other compound semiconductors as shown in Table 2. This comparison reveals that the deposited CuCdS2 material had adequate absorption resulted from its binary phases CuxS and
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131
Fig. 5. XPS spectra of (a) CuCdS2 thin film a whole, (b) Cu 2p, (c) Cd 3d and (d) S 2p.
Fig. 6. UV–Vis absorption spectrum of CuCdS2 thin films.
CdS. This study revealed that CuCdS2 thin films can be utilized as potential absorber for the fabrication of thin film solar cells.
3.3.2. Raman analysis Raman spectrum is a direct evidence of atoms in binding state. The recorded Raman spectrum of CuCdS2 thin film is shown in Fig. 7. Four peaks were observed at 299.3, 469.8, 604.3 and
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Table 2 Optical response region and band gap of some compound semiconductors compared with CuCdS2 thin films. S. no
Material
Active absorption region (nm)
Optical band gap (eV)
Literature
1. 2. 3. 4. 5. 6.
CdS Cu2S CuInSe2 Cu2ZnSnS4 CuInGaSe2 CuCdS2
>300 350–850 450–900 400–1100 400–900 300–1100
2.36–2.5 2.35 1.37 1.45 1.18 1.42
[25] [26] [27] [28] [29,30] Present work
Fig. 7. Raman spectrum of CuCdS2 thin film.
900.6 cm1. The peaks at 299.3, 604.3 and 900.6 cm1 were resulted from 1LO, 2LO and 3LO processes of CdS that was nearly matched with reported values [31]. The peak at 469.8 cm1 was attributed to hexagonal CuS [32]. A shift in Raman peaks was observed from reported CuxS (472 and 474 cm1) and CdS (305, 600 and 904 cm1) binary phases. A small shift to lower frequencies was attributed to size effect or surface phonon mode effect [31]. The more shift in observed Raman peak is due to stress developed in the material [33]. The stress was introduced in the deposited CuCdS2 thin films owing to lattice strain between CuxS and CdS binary phases formed during the deposition process. Since the binding state existed between CuxS and CdS binary phases, it is confirmed that the formation of CuCdS2 compound. Hence, with XRD and Raman results, it is revealed that the existence CuCdS2 compound semiconductor thin films formed from its binary constituents. 3.4. Electrical properties Electrical properties studied by Hall measurement system shows that the deposited CuCdS2 thin film has p-type conductivity with higher carrier concentration of the order of 1018 cm3. The mobility and resistivity of the deposited CuCdS2 thin films were measured to be 10.63 cm2/V s and 0.16 O cm, respectively. A high concentration of p-type carriers was observed in CdS, heavily doped with copper, due to copper 3d levels [34]. The p-type conductivity was due to Cu2+ ions that act as acceptor centres [35]. The carrier concentration, mobility and resistivity values are compared in Table 3, with reported CuS and CdS binary phases and similar compound semiconductors, which are widely used as absorber layer for the fabrication of thin film solar cells. It is observed that the deposited CuCdS2 thin films had higher carrier concentration with lower resistivity. Since the maximum number of photons in the solar spectrum was in visible and NIR regions, with optimum band gap 1.42 eV, higher carrier concentration
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V. Nirmal Kumar et al. / Superlattices and Microstructures 76 (2014) 125–134 Table 3 Electrical properties of some compound semiconductors. S. no.
Compound
Carrier concentration (cm3)
Resistivity (O cm)
Mobility (cm2/ V s)
Type of charge carriers
Literature
1. 2. 3. 4. 5. 6.
CdS Cu2S CuZnSnS CuInGaSe2 CuInSe2 CuCdS2
1.82 1011 1022 7.72 1017 1.49 1016 5 1015–1017 3.48 1018
1.48 104 104 0.01 1.37 103–104 0.16
2.42 103 3 7.45 4.7 103 0.6–7.8 10.63
n-Type p-Type p-Type p-Type p/n-Type p-Type
[36] [23] [37] [38] [39] Present work
and low resistivity, CuCdS2 thin film is a good candidate as absorber layer for the fabrication of thin film solar cells. 4. Conclusion CuCdS2 ternary compound semiconductor thin film was deposited by simple, low temperature solution process in single step. The deposited material had better adherence with substrate. XRD analysis revealed polycrystalline CuCdS2 compound semiconductor with hexagonal structure formed from its binary constituents CuxS and CdS. SEM and TEM micrographs showed continuous coating of the deposited material and particles were aggregated like spherical granules. The average particle size was measured to be 23 nm from SEM image. Inter planar distance (d value) 1.9 Å measured by TEM revealed the existence of CuS and CdS binary phases in same grain resulted in formation of CuCdS2 compound. The elements Cu, Cd and S present in deposited CuCdS2 thin films were confirmed by EELS and XPS spectroscopy. The deposited CuCdS2 had wide range of absorption from 300 to 1100 nm with optimum band gap 1.42 eV. The observed shift in Raman peaks confirmed binding state existed between CuxS and CdS binary phases. Hall measurement study showed p-type conductivity with higher carrier concentration 3.48 1018 cm3 of CuCdS2 thin film having resistivity and mobility values 0.16 O cm and 10.63 cm2/V s, respectively. Having optimum band gap 1.42 eV, p-type charge carriers with high carrier concentration, lower resistivity and wide range of absorption in VIS and near-IR regions, the deposited CuCdS2 thin films could potentially be utilized as absorber layer for effective low cost fabrication of thin film solar cells. References [1] N.R. Pavaskar, C.A. Menezes, A.P.B. Sinha, J. Electrochem. Soc. 124 (1977) 743–748. [2] F.D. Benedetto, I. Bencista, S. Caporali, S. Cinotti, A.D. Luca, A. Lavacchi, F. Vizza, M.M. Miranda, M.L. Foresti, M. Innocenti, Prog. Photovolt. Res. Appl. 22 (2014) 97–106. [3] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451– 6461. [4] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Appl. Phys. Lett. 84 (2004) 3654–3656. [5] R.A. Boudreau, R.D. Rauh, J. Electrochem. Soc. 130 (1983) 513–516. [6] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. 19 (2011) 894–897. [7] L.L. Kazmerski, F.R. White, G.K. Morgan, Appl. Phys. Lett. 29 (1976) 268–270. [8] E. Pentia, V. Draghici, G. Sarau, B. Mereu, L. Pintilie, F. Sava, M. Popescu, J. Electrochem. Soc. 151 (2004) G729–G733. [9] K.L. Chopra, P.D. Paulson, V. Dutta, Prog. Photovolt. Res. Appl. 12 (2004) 69–92. [10] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt. Res. Appl. 21 (2013) 72–76. [11] L. Stolt, J. Hedstrom, J. Kessler, M. Ruckh, K. Velthaus, H. Schock, Appl. Phys. Lett. 62 (1993) 597–599. [12] C. Gretener, J. Perrenoud, L. Kranz, L. Kneer, R. Schmitt, S. Buecheler, A.N. Tiwari, Prog. Photovolt. Res. Appl. 21 (2013) 1580–1586. [13] W.R. Cook, L. Shiozawa, F. Augustine, J. Appl. Phys. 41 (1970) 3058–3063. [14] J.M. Dofia, J. Herrero, J. Electrochem. Soc. 139 (1992) 2810–2814. [15] S.Y. Wang, W. Wang, Z.H. Lu, Mater. Sci. Eng. B 103 (2003) 184–188. [16] R.O. Borges, D. Lincot, J. Electrochem. Soc. 140 (1993) 3464–3473. [17] P.C. Rieke, S.B. Bentjen, Chem. Mater. 5 (1993) 43–53. [18] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 4 (1989) 191–199. [19] N.R. Pavaskar, C.A. Menezes, A.P.B. Sinha, J. EIectrochem. Soc. 124 (1977) 743–748.
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Deposition of CuCdS2 thin film by single step solution process at low temperature as a novel absorber for photovoltaic applications V. Nirmal Kumar a,b, R. Suriakarthick a, Y. Hayakawa b, Shamima Hussain c, G.M. Bhalerao c, Mukul Gupta d, Vasant Sathe d, R. Gopalakrishnan a,⇑ a
Crystal Research Lab, Department of Physics, Anna University, Chennai, India Research Institute of Electronics, Graduate School of Science and Technology, Shizuoka University, Hamamatsu, Japan c UGC-DAE Consortium for Scientific Research, Kalpakkam, India d UGC-DAE Consortium for Scientific Research, Indore, India b
a r t i c l e
i n f o
Article history: Received 23 September 2014 Accepted 24 September 2014 Available online 12 October 2014 Keywords: CuCdS2 Chemical bath deposition Ternary compound Optical properties Electrical properties
a b s t r a c t Thin film of ternary compound semiconductor, copper cadmium sulphide (CuCdS2) was deposited on glass substrate by low temperature solution process. The deposited CuCdS2 thin film was found to suitable for the fabrication of efficient thin film solar cells at low cost. Hexagonal structure of poly crystalline CuCdS2 was formed from the combination of its binary constituents CuxS and CdS. The film was continuously coated and particles were aggregated like spherical granules. The presence of elements Cu, Cd and S were confirmed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. The absorption spectrum showed that the deposited material had wide range of absorption in visible and near-IR region (300–1100 nm) and band gap was estimated to be 1.42 eV using Tauc’s plot. Vibration modes of CdS and CuxS binary phases were observed and shift between them revealed the existence of binding state between CdS and CuxS. Hall measurement showed p-type conducting nature of charge carriers with higher carrier concentration 3.48 1018 cm3. The mobility and resistivity were measured to be 10.63 cm2/V s and 0.17 O cm, respectively. The analyses revealed that the deposited CuCdS2 thin
⇑ Corresponding author at: Department of Physics, Anna University, Chennai 600 025, Tamilnadu, India. Tel.: +91 44 2235 8710. E-mail addresses: [email protected], [email protected] (R. Gopalakrishnan). http://dx.doi.org/10.1016/j.spmi.2014.09.030 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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film was suitable material for the fabrication of thin film solar cells at low cost. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Development of new semiconductor materials promotes various applications in diverse fields. Apart from elemental semiconductors such as Si and Ge, compound semiconductors made of metal sulphides [1,2], metal oxides [3,4] and metal selenides [5,6] captivated many researchers by virtue of their progressive physical properties. For energy harvesting by means of solar power, ternary and quaternary metal chalcogenides play assuring role emanating from their tuneable physical properties by simply varying the composition of its constituent elements [6,7]. Many research groups working in the field of thin film solar cells (TFSC) have been examining different compound semiconductors [2,8] for effective utilization of solar energy in the past decades, since TFSC have added advantages like availability of low cost fabrication methods, need of small amount of source material and better reproducibility [9]. High efficiency in TFSC was achieved by Cu and Cd based compound semiconductors like CuInGaSe2, CuZnSnS2, CdTe, CuInSe2 and CuxS. CuInGaSe2 recorded the highest efficiency around 20% in TFSCs among all other compound semiconductors [6]. Since CdS has an optimum large band gap of 2.42 eV, it has been used as a window layer with CIGS, CZTS, CIS and CdTe absorbers which recorded higher efficiency [6,10–12]. Since the highest efficiency in TFSCs has been achieved using CdS and copper based absorber materials, fabrication of solar cells by combining these materials could increase the cell efficiency due to enhanced charge transport properties between CuS and CdS owing to less lattice mismatch between them. CuCdS2 is I-II-VI2 compound semiconductor material having properties between its binary counterparts CuxS and CdS. Even though the physical properties of CdS and CuxS are quite opposite, they have similarities in their lattice arrangement which could be revealed by a report by Cook et al. They demonstrated the conversion of CdS single crystal into CuS single crystal by dipping it into CuCl solution [13]. Among the metal chalcogenides family, CuCdS2 is a novel one and yet to understand its properties well. With the course of introducing possible contemporary alternate material for the fabrication of TFSC, we have been synthesised copper cadmium sulphide (CuCdS2) thin films by wet chemical route at low temperature. As for our knowledge this is the first report of CuCdS2 ternary compound semiconductor thin films prepared by chemical bath deposition method aiming low cost production of TFSCs. We are reporting the optical, electrical and morphological properties of CuCdS2 thin film deposited by chemical bath deposition method. Though many physical (thermal evaporation, PLD, sputtering, etc.) and chemical methods (sol–gel, electro deposition, photochemical deposition, etc.) have been employed for the deposition of thin films, chemical bath deposition has many advantages such as, large area of deposition, uniform and continuous coating of thin films, low cost fabrication method and possible reproducibility. Thin film of CuCdS2 has been deposited on microscopic glass substrate from chemical solution containing CuSO4, CdSO4 as cation sources and thiourea as anion source. The deposited film was cleaned ultrasonically for few seconds and dried in oven. The physical properties of deposited CuCdS2 thin films were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) with electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), UV–Vis, Raman spectroscopy and Hall measurement studies. 2. Experiment 2.1. Synthesis and formation of CuCdS2 thin film CuCdS2 thin film was chemically deposited in single step using CuSO45H2O, CdSO48H2O and CS(NH2)2 (thiourea) as cation and anion sources, respectively. The schematic of this simple chemical
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127
Fig. 1. Schematic of simple chemical bath deposition process.
bath deposition process is shown in Fig. 1. The precursor solution was prepared by dissolving 0.08 M of CuSO45H2O and 0.02 M of CdSO48H2O in 100 mL of deionized water. 0.2 M of thiourea was added to this solution under continuous stirring. Ammonia was added drop by drop in the solution until pH of the solution became 10.5. While adding ammonia, ultrasonically cleaned glass substrate was immersed vertically in the solution. Addition of ammonia formed complex cation with copper and cadmium ions, thereby slow and uniform deposition of the compound made possible. The solution was kept under continuous stirring for 40 min. The formation of compound was revealed by the colour change of the solution from dark blue to dark brown. CuCdS2 thin film was formed by chemical reaction between cations (Cu2+, Cd2+) and anions (S2) in the basic region (pH > 7). Basic region of the solution is necessary for the availability of hydroxyl ions that causes hydrolysis of thiourea to form sulphur ions and for the formation of metal hydroxide compound which would be adsorbed on the surface of glass substrate [14]. The formation of CuCdS2 compound is through the formation of its binary counterparts CuxS and CdS. Since copper sulphide exists in five different stable phases at room temperature, it is difficult to distinguish a particular phase in solution [15]. The in-situ study by quartz crystal microbalance technique reported by Borges and Lincot, explained the reaction mechanism of thiourea and ammonia system for the deposition of CdS via the formation of cadmium hydroxide [16]. Rieke and Bentjen reported the formation of cadmium complexes in the deposition of CdS which would be applicable to the other similar systems [17]. The formation of CuxS thin films in similar system via copper complex was reported earlier by Nair and Nair [18]. We used the similar ammonia–thiourea system to deposit CuCdS2 thin films and the reactions of copper (Cu2+) and cadmium (Cd2+) ions with ammonia, as in earlier reports were applicable to our system. The reaction for the formation of CuCdS2 is as follows,
CuSO4 þ CdSO4 þ 8NH3 ! ½CuðNH3 Þ4 2þ þ ½CdðNH3 Þ4 2+
2þ
þ 2SO2 4
ð1Þ
2+
The Cu and Cd ions existing in solution from dissolved CuSO4 and CdSO4 reacted with ammonia to form their complexes tetramine copper ([Cu(NH3)4]2+) and tetramine cadmium ([Cd(NH3)4]2+) cations [18,19]. Since the solution is basic, hydrolysis of thiourea takes place to release sulphur ions as follows,
CSðNH2 Þ2 þ OH ! CH2 N2 þ H2 O þ HS
ð2Þ
HS þ OH ! S2 þ H2 O
ð3Þ 2+
2+
2
As a consequence, cations ([Cu(NH3)4] , [Cd(NH3)4] ) and anions (S nary compound CuCdS2 in the solution as follows,
½CuðNH3 Þ4 2þ þ ½CdðNH3 Þ4
2þ
þ 2S2 ! CuCdS2 þ 8NH3
) will react to form the ter-
ð4Þ
The formation of compound took place via heterogeneous nucleation, along with homogeneous nucleation formed in solution, grown further on glass substrate and container wall. After 40 min the deposited film was taken out from the solution and cleaned ultrasonically for a few seconds to
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remove the loosely adhered particles. Even though the film was kept over 20 s under ultrasonification, film was not pealed out from the substrate which revealed well adhesiveness of deposited material on the substrate. The film was dried in oven at 50 °C for further characterization. 2.2. Characterization of CuCdS2 thin films The deposited CuCdS2 films were subjected to XRD, SEM, TEM with EELS, XPS, UV–Vis, Raman and Hall measurement studies. XRD pattern of the deposited thin film was recorded with Cu Ka X-ray source by Bruker D8 advance XRD system. Surface morphology of Au coated CuCdS2 thin film was analyzed by Supra 55 FESEM – Karl Zeiss system. TEM equipped with EELS was used to identify the crystallinity and elements present. Optical absorbance in the range 200–1100 nm was recorded by Perkin Elmer Lambda 950 system with uncoated glass slides as reference. Raman spectra of the deposited thin films in the range 50–1000 cm1 were recorded by Jobin Yvon Horibra LABRAM-HR visible system. Ar ion laser having excitation wavelength 488 nm, with spot size 1 lm, was used for recording the Raman spectrum. The spectra were recorded at room temperature by CCD camera having 600 lines/mm gratings with spectral resolution of about 1 cm1. Hall effect measurement was carried out using Ecopia Hall effect measurement system (HMS – 3000) to analyse the electrical properties of the deposited thin film. 3. Results and discussion 3.1. Structural properties The X-ray diffraction pattern of the deposited CuCdS2 thin film is shown in Fig. 2. Since the compound is new, there is no data on Joint committee on powder diffraction standards (JCPDS – PDF) is available for this material. Hence the obtained XRD pattern was indexed by comparing with its binary phase materials CuS, Cu2S and CdS. The diffraction peaks from various (h k l) planes were found to matched with peaks of CuS (JCPDS: PDF #75-2233), Cu2S (JCPDS: PDF # 84-0206) and CdS (JCPDS: PDF #77-2306) binary phases. Among the observed peaks, eight of CuS, six of Cu2S and six of CdS were matched with standard data. Three peaks at 2h values 44.01°, 60.77° and 66.58° were observed to be common for (0 0 8), (2 0 4) and (2 0 6) planes of CuS and (1 1 0), (1 0 4) and (2 0 3) planes of CdS. The peak at 60.77° was commonly observed for CuS (2 0 4), Cu2S (2 0 2) and CdS (1 0 4) planes. Atoms of all CuS, Cu2S and CdS phases had arranged to crystalline in hexagonal structure. The close lattice arrangement between CuxS and CdS revealed experimentally by the conversion of CdS single crystal into CuxS single crystal and explained by Cook et al. [13], which strongly support the possibility of co-existence of CuS,
Fig. 2. X-ray diffraction pattern of CuCdS2 thin film.
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Cu2S and CdS as CuCdS2 compound. This analysis revealed that the deposited CuCdS2 thin film has hexagonal structure formed from its binary phases CuxS and CdS. The lattice parameter values a = b = 3.788 Å, c = 16.333 Å of CuS, a = b = 3.95 Å, c = 6.75 Å of Cu2S and a = b = 4.136 Å, c = 6.713 Å of CdS are in good agreement with reported values [20,21]. The observed experimental structural data of CuS, Cu2S and CdS binary phases in the deposited CuCdS2 thin film (calculated using powder-X software) are presented in Table.1. Based on the results obtained it is emphasized that the polycrystalline hexagonal structure of CuCdS2 compound semiconductor resulted with coexistence of CuxS (CuS, Cu2S) and CdS binary phases. 3.2. Morphological and elemental analysis Morphological studies were made by SEM, TEM equipped with EELS. SEM micrograph (Fig. 3(a)) revealed the formation of continuous coating of deposited CuCdS2 thin films and particles aggregated like spherical granules. The film was composed of several islands that resulted from the formation of different constituent phases (CuxS and CdS. The average particle size was measured to be 23 nm. Fig. 3(b) and (c) shows TEM image and selected area electron diffraction pattern (SAED) which reveals polycrystalline nature of the deposited material. Inter planar distance (d value) of the deposited material was found to be 1.9 Å from TEM image as shown in Fig. 3(d), which is closely matched with (0 0 8) plane of CuS and (1 1 0) plane of CdS (Table. 1) that revealed the co-existence of CuS and CdS, formed CuCdS2 compound. The kinetic energy of electrons after interaction with specimen was analyzed by EELS equipped with TEM [22]. EELS spectrum revealed Cu L23, Cd M45 and S L23 edges as shown in Fig. 4(a)–(c). X-ray photoelectron spectroscopy of the deposited CuCdS2 thin films is shown in Fig. 5(a)–(d). The obtained binding energy values matched with reported values of CuS, Cu2S and CdS binary phases [23,24]. EELS and XPS spectroscopic studies confirmed the presence of Cu, Cd and S elements in the deposited CuCdS2 thin films. 3.3. Optical properties 3.3.1. UV–Vis spectral analysis The optical property and band gap of the materials are the decisive key factors for any specific application. The recorded absorption spectrum of the deposited CuCdS2 thin film is shown in Fig. 6. Table 1 Structural data of CuS, Cu2S and CdS binary phases in deposited CuCdS2 thin film. Binary phase
Plane
2h (°)
Inter planar spacing (d) (Å)
Lattice parameter
CuS
004 102 103 008 200 202 204 206 002 101 102 111 201 202 002 110 103 112 104 203
21.748 29.316 31.822 44.333 56.020 57.240 60.808 66.499 26.532 29.684 37.772 48.113 55.665 60.777 26.532 44.010 48.113 51.926 60.777 66.583
4.08325 3.04408 2.80984 2.04163 1.64025 1.60814 1.52204 1.40492 3.35687 3.00716 2.37976 1.88968 1.64985 1.52275 3.35687 2.05585 1.88968 1.75953 1.52275 1.40335
a = 3.788 b = 3.788 c = 16.333
Cu2S
CdS
a = 3.95 b = 3.95 c = 6.75
a = 4.136 b = 4.136 c = 6.713
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Fig. 3. Surface morphology (a) SEM image, (b) TEM image, (C) SAED pattern and (d) Inter planar spacing measured from TEM image of CuCdS2 thin film.
Fig. 4. EELS spectrum (a) Cu L23 edge, (b) Cd M45 edge and (c) S L23 edge of CuCdS2 thin film.
It is inferred from absorption spectrum that the material has wide range of absorption in the region 300–1100 nm which is favourable for photovoltaic applications since maximum number of photons coming from solar radiation having wavelength in visible and near-IR regions. The absorption region of this material was relatively high when compared to other absorber layer materials such as CZTS, CIGS and CIS. The optical band gap of the material was found to be 1.42 eV from Tauc’s plot which was shown as inset in Fig. 6. Optical absorption region and band gap of the deposited material were compared with other compound semiconductors as shown in Table 2. This comparison reveals that the deposited CuCdS2 material had adequate absorption resulted from its binary phases CuxS and
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131
Fig. 5. XPS spectra of (a) CuCdS2 thin film a whole, (b) Cu 2p, (c) Cd 3d and (d) S 2p.
Fig. 6. UV–Vis absorption spectrum of CuCdS2 thin films.
CdS. This study revealed that CuCdS2 thin films can be utilized as potential absorber for the fabrication of thin film solar cells.
3.3.2. Raman analysis Raman spectrum is a direct evidence of atoms in binding state. The recorded Raman spectrum of CuCdS2 thin film is shown in Fig. 7. Four peaks were observed at 299.3, 469.8, 604.3 and
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Table 2 Optical response region and band gap of some compound semiconductors compared with CuCdS2 thin films. S. no
Material
Active absorption region (nm)
Optical band gap (eV)
Literature
1. 2. 3. 4. 5. 6.
CdS Cu2S CuInSe2 Cu2ZnSnS4 CuInGaSe2 CuCdS2
>300 350–850 450–900 400–1100 400–900 300–1100
2.36–2.5 2.35 1.37 1.45 1.18 1.42
[25] [26] [27] [28] [29,30] Present work
Fig. 7. Raman spectrum of CuCdS2 thin film.
900.6 cm1. The peaks at 299.3, 604.3 and 900.6 cm1 were resulted from 1LO, 2LO and 3LO processes of CdS that was nearly matched with reported values [31]. The peak at 469.8 cm1 was attributed to hexagonal CuS [32]. A shift in Raman peaks was observed from reported CuxS (472 and 474 cm1) and CdS (305, 600 and 904 cm1) binary phases. A small shift to lower frequencies was attributed to size effect or surface phonon mode effect [31]. The more shift in observed Raman peak is due to stress developed in the material [33]. The stress was introduced in the deposited CuCdS2 thin films owing to lattice strain between CuxS and CdS binary phases formed during the deposition process. Since the binding state existed between CuxS and CdS binary phases, it is confirmed that the formation of CuCdS2 compound. Hence, with XRD and Raman results, it is revealed that the existence CuCdS2 compound semiconductor thin films formed from its binary constituents. 3.4. Electrical properties Electrical properties studied by Hall measurement system shows that the deposited CuCdS2 thin film has p-type conductivity with higher carrier concentration of the order of 1018 cm3. The mobility and resistivity of the deposited CuCdS2 thin films were measured to be 10.63 cm2/V s and 0.16 O cm, respectively. A high concentration of p-type carriers was observed in CdS, heavily doped with copper, due to copper 3d levels [34]. The p-type conductivity was due to Cu2+ ions that act as acceptor centres [35]. The carrier concentration, mobility and resistivity values are compared in Table 3, with reported CuS and CdS binary phases and similar compound semiconductors, which are widely used as absorber layer for the fabrication of thin film solar cells. It is observed that the deposited CuCdS2 thin films had higher carrier concentration with lower resistivity. Since the maximum number of photons in the solar spectrum was in visible and NIR regions, with optimum band gap 1.42 eV, higher carrier concentration
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V. Nirmal Kumar et al. / Superlattices and Microstructures 76 (2014) 125–134 Table 3 Electrical properties of some compound semiconductors. S. no.
Compound
Carrier concentration (cm3)
Resistivity (O cm)
Mobility (cm2/ V s)
Type of charge carriers
Literature
1. 2. 3. 4. 5. 6.
CdS Cu2S CuZnSnS CuInGaSe2 CuInSe2 CuCdS2
1.82 1011 1022 7.72 1017 1.49 1016 5 1015–1017 3.48 1018
1.48 104 104 0.01 1.37 103–104 0.16
2.42 103 3 7.45 4.7 103 0.6–7.8 10.63
n-Type p-Type p-Type p-Type p/n-Type p-Type
[36] [23] [37] [38] [39] Present work
and low resistivity, CuCdS2 thin film is a good candidate as absorber layer for the fabrication of thin film solar cells. 4. Conclusion CuCdS2 ternary compound semiconductor thin film was deposited by simple, low temperature solution process in single step. The deposited material had better adherence with substrate. XRD analysis revealed polycrystalline CuCdS2 compound semiconductor with hexagonal structure formed from its binary constituents CuxS and CdS. SEM and TEM micrographs showed continuous coating of the deposited material and particles were aggregated like spherical granules. The average particle size was measured to be 23 nm from SEM image. Inter planar distance (d value) 1.9 Å measured by TEM revealed the existence of CuS and CdS binary phases in same grain resulted in formation of CuCdS2 compound. The elements Cu, Cd and S present in deposited CuCdS2 thin films were confirmed by EELS and XPS spectroscopy. The deposited CuCdS2 had wide range of absorption from 300 to 1100 nm with optimum band gap 1.42 eV. The observed shift in Raman peaks confirmed binding state existed between CuxS and CdS binary phases. Hall measurement study showed p-type conductivity with higher carrier concentration 3.48 1018 cm3 of CuCdS2 thin film having resistivity and mobility values 0.16 O cm and 10.63 cm2/V s, respectively. Having optimum band gap 1.42 eV, p-type charge carriers with high carrier concentration, lower resistivity and wide range of absorption in VIS and near-IR regions, the deposited CuCdS2 thin films could potentially be utilized as absorber layer for effective low cost fabrication of thin film solar cells. References [1] N.R. Pavaskar, C.A. Menezes, A.P.B. Sinha, J. Electrochem. Soc. 124 (1977) 743–748. [2] F.D. Benedetto, I. Bencista, S. Caporali, S. Cinotti, A.D. Luca, A. Lavacchi, F. Vizza, M.M. Miranda, M.L. Foresti, M. Innocenti, Prog. Photovolt. Res. Appl. 22 (2014) 97–106. [3] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451– 6461. [4] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Appl. Phys. Lett. 84 (2004) 3654–3656. [5] R.A. Boudreau, R.D. Rauh, J. Electrochem. Soc. 130 (1983) 513–516. [6] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. 19 (2011) 894–897. [7] L.L. Kazmerski, F.R. White, G.K. Morgan, Appl. Phys. Lett. 29 (1976) 268–270. [8] E. Pentia, V. Draghici, G. Sarau, B. Mereu, L. Pintilie, F. Sava, M. Popescu, J. Electrochem. Soc. 151 (2004) G729–G733. [9] K.L. Chopra, P.D. Paulson, V. Dutta, Prog. Photovolt. Res. Appl. 12 (2004) 69–92. [10] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt. Res. Appl. 21 (2013) 72–76. [11] L. Stolt, J. Hedstrom, J. Kessler, M. Ruckh, K. Velthaus, H. Schock, Appl. Phys. Lett. 62 (1993) 597–599. [12] C. Gretener, J. Perrenoud, L. Kranz, L. Kneer, R. Schmitt, S. Buecheler, A.N. Tiwari, Prog. Photovolt. Res. Appl. 21 (2013) 1580–1586. [13] W.R. Cook, L. Shiozawa, F. Augustine, J. Appl. Phys. 41 (1970) 3058–3063. [14] J.M. Dofia, J. Herrero, J. Electrochem. Soc. 139 (1992) 2810–2814. [15] S.Y. Wang, W. Wang, Z.H. Lu, Mater. Sci. Eng. B 103 (2003) 184–188. [16] R.O. Borges, D. Lincot, J. Electrochem. Soc. 140 (1993) 3464–3473. [17] P.C. Rieke, S.B. Bentjen, Chem. Mater. 5 (1993) 43–53. [18] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 4 (1989) 191–199. [19] N.R. Pavaskar, C.A. Menezes, A.P.B. Sinha, J. EIectrochem. Soc. 124 (1977) 743–748.
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