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Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol–gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency
Solar Energy Materials & Solar Cells 95 (2011) 838–842
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol–gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency Kunihiko Tanaka , Yuki Fukui, Noriko Moritake, Hisao Uchiki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka, Niigata 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2010 Received in revised form 25 October 2010 Accepted 31 October 2010 Available online 20 November 2010
The properties of Cu2ZnSnS4 (CZTS) thin films deposited by sol–gel sulfurization were investigated as a function of the chemical composition of the sol–gel solutions used. The chemical composition ratio Cu/(Zn+ Sn) of the sol–gel solution was varied from 0.73 to 1.00, while the ratio Zn/Sn was kept constant at 1.15. CZTS films deposited using sol–gel solutions with Cu=ðZn þ SnÞ o 0:80 exhibited large grains. In addition, the band gaps of these Cu-poor CZTS thin films were blue shifted. Solar cells with the structure Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass were fabricated under non-vacuum conditions. The solar cell with the CZTS layer deposited using the sol–gel solution with Cu/(Zn +Sn) ¼ 0.80 exhibited the highest conversion efficiency of 2.03%. & 2010 Elsevier B.V. All rights reserved.
Keywords: Cu2ZnSnS4 Solar cell Sol–gel sulfurizing Non-vacuum process
1. Introduction Cu2ZnSnS4 (CZTS) is a promising material for the absorber layer of thin-film solar cells. CZTS has a direct band gap energy of about 1.5 eV and an absorption coefficient that could potentially be greater than 104 cm 1. Furthermore, CZTS does not contain any toxic or low abundance elements. Because of their superior optoelectronic characteristics, CZTS thin films have been investigated as the absorber layer of thin-film solar cells. Katagiri et al. fabricated CZTS-based solar cells with the structure of Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass (SLG) [1–5]. The CZTS layers were prepared by sulfurizing the precursors deposited by electron-beam evaporation or radio-frequency magnetron co-sputtering. The solar cells had efficiencies of 1.08%, 3.93%, and 6.77% for CZTS chemical compositions of Cu/(Zn+Sn)¼0.99 and Zn/Sn¼1.01, Cu/(Zn+Sn)¼0.73 and Zn/Sn¼1.70, and Cu/(Zn+Sn)¼0.87 and Zn/Sn¼1.15, respectively. Thus, higher efficiencies were obtained using Cu-poor, Zn-rich CZTS absorber layers. These CZTS absorber layers were prepared in a vacuum, which entails high costs and complex processes. Several research groups have prepared CZTS absorber layers under non-vacuum conditions. Araki et al. prepared CZTS absorber layers by sulfurizing electroplated precursors [6,7]. The most efficient solar cell exhibited an
Corresponding author.
E-mail address: [email protected] (K. Tanaka). 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.10.031
efficiency ðZÞ of 3.16%, an open-circuit voltage (Voc) of 540 mV, a short-circuit current density (Jsc) of 12.6 mA/cm2, and a fill factor (FF) of 46.4%. This solar cell was Cu poor and Zn rich. Ennaoui et al. prepared CZTS layers by sulfurization of Cu–Zn–Sn precursors deposited by one-step electrodeposition [8]; the most efficient cell had Z ¼ 3:4%, Voc ¼563 mV, Jsc ¼14.8 mA/cm2, and FF¼41.0%. Its CZTS chemical composition was Cu/(Zn+Sn)¼0.98 and Zn/Sn¼1.00. All the semiconductor layers of the CZTS solar cells fabricated in our previous studies were deposited under non-vacuum conditions [9–11]. CZTS absorber, CdS buffer, and ZnO:Al window layers were deposited by sol–gel sulfurization, chemical bath deposition, and the sol–gel method, respectively. The CZTS absorber layer that exhibited the highest efficiency of 1.6% was Cu poor and Zn rich (Cu/(Zn+Sn) ¼ 0.96 and Zn/Sn¼1.17) [11]. Chen et al. found that Cu-poor and Zn-rich conditions improved the efficiency of CZTS solar cells because Cu-poor conditions enhance the formation of Cu vacancies, which give rise to shallow acceptors in CZTS, while Zn-rich conditions suppress the substitution of Cu at Zn sites, which give rise to relatively deep acceptors [12]. The above-mentioned results indicate that using Cu-poor and Zn-rich CZTS layers results in high-efficiency solar cells. Therefore, it is critical to control the chemical composition of the CZTS absorber layer to produce high-efficiency CZTS solar cells. In this report, we investigated whether the chemical composition of CZTS thin films deposited by sol–gel sulfurization can be controlled by varying the chemical composition of the sol–gel solution. We also investigated the influence of the chemical composition of the sol–gel solutions on several properties of CZTS thin films.
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
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Mo
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In this study, CZTS thin films were prepared under non-vacuum conditions by sol–gel sulfurization [9]. Mo-coated SLG (Mo/SLG) and SLG were used as substrates for these thin films. The SLG substrate was used for observing the optical properties of the thin films. The metal sources of the sol–gel solutions were copper (II) acetate monohydrate, zinc (II) acetate dihydrate, and tin (II) chloride dihydrate. The sol–gel solutions were Cu poor and Zn rich with chemical compositions of Cu/(Zn+Sn)¼0.73–1.00 and Zn/Sn¼1.15. In addition, a stoichiometric sol–gel solution of Cu/(Zn+Sn)¼1.00 and Zn/Sn ¼ 1.00. Table 1 lists the names and the chemical compositions of the sol–gel solutions employed. Sol–gel solutions with two metal ion concentrations (0.35 and 1.75 M) were prepared. The metal sources were dissolved in 2-methoxyethanol. Deionized water and ammonium acetate were added as a stabilizer for the 0.35 M sol–gel solution and monoethanolamine was added as a stabilizer for the 1.75 M sol–gel solution. Because high-concentration sol–gel solutions erode Mo, the Mo/SLG substrates were precoated with the low-concentration (0.35 M) sol–gel solution before being coated with the 1.75 M sol–gel solution [10]. The same coating procedure was applied to SLG substrates, which were used to characterize the optical properties of the thin films. The sol–gel solution coated substrates, which were precursors, were then annealed in an atmosphere of N2 and H2S (5%) at 500 1C for 1 h. Further details are given in Refs. [10,11]. Solar cells with the structure of Al/ZnO:Al/CdS/CZTS/Mo/SLG and a cell area of 0.12 cm2 were prepared in the present study. All the semiconductor layers were deposited under non-vacuum conditions. The CdS buffer layer was grown by chemical bath deposition method from an aqueous solution of CdI2, thiourea, and ammonia. The CdS buffer layer was a few tens of nanometers thick. The ZnO:Al window layer was deposited by the sol–gel method. Zinc (II) acetate dihydrate, aluminum chloride hexahydrate, 2-methoxyethanol, and monoethanolamine were used in the sol–gel solution. The window layer was 150 nm thick. Further details are given in Refs. [10,11]. The crystallinities of the CZTS thin films were analyzed by X-ray diffraction (XRD; Rigaku, RAD3). Their chemical compositions were determined by electron-probe microanalysis (Shimadzu, EPMA1600). Their surface morphologies and thicknesses were investigated by scanning electron microscopy (SEM; Hitachi, S4700). The sample surface was observed without any surface treatment. Transmittance and reflectance spectra of the CZTS thin films were obtained using an optical spectrometer (Jasco, V570). The J–V characteristics of the CZTS solar cells under illumination were determined by the four-probe method using a solar simulator with AM 1.5 and a power density of 100 mW/cm2.
compositions. The peaks were attributed to the (1 1 2), (2 0 0), (2 2 0), and (3 1 2) planes of CZTS and to Mo. No other prominent peaks were observed, which suggests that no secondary phases were present. There is no observable difference in the XRD patterns obtained from films prepared with sol–gel solutions having different chemical compositions. Table 1 shows the chemical compositions of the CZTS films. Fig. 2 shows the measured Cu/(Zn +Sn) and Zn/Sn values for the samples made from the sol–gel solution with Zn/Sn¼1.15. As Table 1 and Fig. 2 show, the CZTS films had different chemical compositions from those of the sol–gel solutions. However, the chemical composition ratio Cu/(Zn+ Sn) of the CZTS film was approximately proportional to that of the sol–gel solution. Zn/Sn of the CZTS thin films was larger than unity, just as in the sol–gel solution. Therefore, Cu-poor and Zn-rich CZTS thin films could be deposited by sol–gel sulfurization. Furthermore, the chemical composition of CZTS thin films deposited by sol–gel sulfurization can be approximately controlled by varying the chemical composition of the sol–gel solution. Cu/(Zn+ Sn) of the CZTS thin films decreased more gradually than that of the sol–gel solutions. This reveals that the amount of Zn or Sn (or Zn and Sn) in the CZTS films decreased with increasing Cu ratio in the sol–gel solution. The reductions in the amounts of Zn and Sn are considered to be due to evaporation, since the CZTS thin films did not contain a second phase (see Fig. 1).
CZTS073 Mo Mo
CZTS080
Diffraction Intensity (a.u.)
2. Experimental
839
CZTS084
CZTS087
CZTS100
3. Results and discussion
CZTS-St
3.1. Properties of CZTS thin films
20
Fig. 1 shows XRD patterns of CZTS films deposited on Mo/SLG substrates using sol–gel solutions with different chemical
30
40 50 60 70 Diffraction Angle, 2 (deg)
80
90
Fig. 1. Dependence of XRD profiles on chemical composition of sol–gel solutions.
Table 1 Chemical compositions of sol–gel solutions and CZTS thin films. Sample name
CZTS073 CZTS080 CZTS084 CZTS087 CZTS100 CZTS-St
Sol–gel solution
CZTS thin film
Cu/(Zn +Sn)
Zn/Sn
Cu/(Zn+ Sn)
Zn/Sn
Cu (at%)
Zn (at%)
Sn (at%)
S (at%)
0.73 0.80 0.84 0.87 1.00 1.00
1.15 1.15 1.15 1.15 1.15 1.00
0.91 0.92 0.95 0.97 0.99 1.03
1.23 1.17 1.18 1.10 1.13 1.03
22.9 22.5 22.7 23.5 23.3 22.7
13.8 13.2 12.9 12.7 12.4 11.1
11.2 11.3 11.1 11.5 11.1 10.8
52.2 52.9 53.4 52.3 53.3 55.3
840
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
Fig. 3 shows surface SEM images of the CZTS thin films. It reveals that samples CZTS-St, CZTS100, and CZTS087 consist of 100 nm grains. The grain size of the CZTS films increased as the Cu/(Zn+ Sn) ratio of the sol–gel solution decreased. The grains in samples CZTS080 and CZTS073 were larger than 1 mm. Table 2 lists the grain sizes of the CZTS films. CuSe functions as a flux for CIGS and CIS thin films since it has a low melting point, which increases the grain size [13,14]. Therefore, to increase their grain sizes, CIGS and CIS films were deposited under Cu-rich conditions and excess CuSe was then removed by KCN etching. In contrast, Cu-poor conditions increase the grain size in CZTS films deposited by sol–gel sulfurization. As mentioned above, the amounts of Zn and Sn decrease during the sulfurization
1.4
Chemical composition of Cu/(Zn+Sn) and Zn/Sn in CZTS thin films
1.2
1.0
0.8 sol-gel solution of Zn/Sn = 1.15 0.6
CZTS-St
process. Zn or Sn could be present in the CZTS precursors as hydrooxides, oxides, or other complexes. These Zn or Sn complexes could melt during sulfurization so that the melting complexes could function as fluxes, increasing the grain size. These melted Zn or Sn complexes would subsequently evaporate. Further study is necessary to clarify the grain growth mechanism during the sulfurization process. The properties of the CZTS thin films should be influenced by the chemical composition of the CZTS thin films. From the Fig. 1 and Table 2, the CZTS thin films can be classified into three groups: those with large grains and Cu-poor compositions, those with small grains and slightly Cu-poor compositions, and stoichiometric CZTS thin films. Fig. 4 shows a plot of ðahnÞ2 against the photon energy for the Cu-poor sample of CZTS080, the slightly Cu-poor CZTS087 sample, and the stoichiometric sample of CZTS-St, where a ðcm1 Þ is the absorption coefficient and hn (eV) is the photon energy. The band gap energy is estimated by extrapolating the linear region in the ðahnÞ2 plot and determining the intercept with the photon energy axis. As shown in Fig. 4, the band gap energy of the CZTS thin films shifted to higher energies as Cu/(Zn+ Sn) of the CZTS thin films decreased. The CZTS080, CZTS087, and CZTS-St samples had band gap energies of 1.62, 1.58 and 1.40 eV. Babu et al. investigated Cu2ZnSnSe4 (CZTSe), which is analogous to CZTS, as a function of the chemical composition of Cu/(Zn+ Sn). They found that the band gap energy of CZTSe shifts to higher energies as Cu/(Zn +Sn) decreases [15]. They attributed this to changes in the degree of p–d hybridization between the Cu d-levels and Se p-levels. The valence band maximum (VBM) of CZTSe is due
0.4 Cu/(Zn+Sn) 0.2
Table 2 Grain sizes of CZTS thin films.
Zn/Sn
0.0 0.7
0.8 0.9 1.0 Chemical composition of Cu/(Zn+Sn) in sol-gel solutions
Fig. 2. Cu/(Zn+Sn)and Zn/Sn in CZTS thin films as a function of Cu/(Zn+Sn) in sol–gel solution.
Cu/(Zn+Sn)=1.00 Zn/Sn=1.00
Cu/(Zn+Sn)=0.835 Zn/Sn=1.15
Sample name
Grain size
CZTS073 CZTS080 CZTS084 CZTS087 CZTS100 CZTS-St
1 mm 1 mm 200 nm 100 nm 100 nm 50 nm
Cu/(Zn+Sn)=1.00 Zn/Sn=1.15
Cu/(Zn+Sn)=0.80 Zn/Sn=1.15
Cu/(Zn+Sn)=0.87 Zn/Sn=1.15
Cu/(Zn+Sn)=0.73 Zn/Sn=1.15
Fig. 3. Surface SEM images of thin films produced from sol–gel solutions with different chemical compositions.
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
Wavelength (nm) [×109]
10
1000
2.0
841
500
CZTS080 CZTS087
Current density (mA/cm2)
( )2 (eV cm-1)2
8
1.0
CZTS080 CZTS087
CZTS-St 6
4
2
CZTS-St
0.0 1.0
1.5 2.0 Photon energy (eV)
2.5
Fig. 4. ðahnÞ2 as a function of photon energy for Cu-poor, slightly Cu-poor, and stoichiometric CZTS.
to antibonding of Cu 3d and Se 4p orbitals [16] and the VBM of CZTS is due to antibonding of Cu 3d and S 3p orbitals [17]. Therefore, the band gap energy shifts of CZTS may be also attributed to changes in the p–d hybridization between the Cu d-levels and S p-levels. A possible reason for the band gap shift is the Moss–Burstein shift [18,19]. The excess holes reduce the VBM so that the band gap is blue shifted. Chen et al. reported that Cu vacancies are the main origin of acceptors in CZTS [12]. Therefore, Cu-poor and Zn-rich samples have many excess holes and the band gaps are blue shifted. Persson reported that for CuInSe2, in which the VBM consists of Cu d-orbitals and Se p-orbitals, Cu vacancies reduce the VBM [20].
3.2. Properties of CZTS thin-film solar cells CZTS thin film solar cells were prepared from Cu-poor CZTS080, slightly Cu-poor CZTS087, and stoichiometric CZTS-St. Fig. 5 shows the J–V characteristics of these CZTS solar cells under light irradiation. Table 3 shows the photovoltaic properties of the CZTS solar cells, where Voc is the open circuit voltage, Jsc is the short current density, FF is the fill factor, and Z is the conversion efficiency. As shown in Table 3, CZTS080 has the highest efficiency of 2.03%; this is considered to be due to its high Jsc and Voc. Its high Jsc is attributed to its relatively large grains (see Fig. 3). The grain boundary area decreases as the grain size increases. Electrons and holes recombine at grain boundaries, reducing the current density. Consequently, CZTS080 exhibits the highest Jsc. Chen reported that acceptors in CZTS are due to Cu vacancies (VCu) and substitution of Cu in Zn site (CuZn). They calculated the acceptor transition energy levels for CuZn and VCu to be 0.10 and 0.02 eV, respectively, which are both above the VBM [12]. Since CuZn is a relatively deep acceptor, the shallow acceptor of VCu is effective for improving the efficiency of CZTS solar cells. The Cu-poor composition suppresses CuZn formation and enhances VCu formation. Consequently, Jsc of the Cu-poor CZTS solar cell is higher than that of the slightly Cu-poor and stoichiometric CZTS solar cells. Voc increases as Jsc increases because Voc is proportional to (AkT/q)ln(Jsc/J00), where A is the ideality factor, kT/q is the thermal voltage, and J00 is a weakly temperaturedependent prefactor [21].
0 0
200
400
600
Voltage (mV) Fig. 5. J–V curves of CZTS thin-film solar cells under light irradiation.
Table 3 Photovoltaic properties of CZTS solar cells. Sample name
Z ð%Þ
Voc (mV)
Isc (mA/cm2)
FF (%)
CZTS080 CZTS087 CZTS-St
2.03 0.872 0.612
575 442 410
9.69 5.39 3.83
36.4 36.6 39.1
4. Conclusion The dependence of the properties of CZTS thin films deposited by sol–gel sulfurization on the chemical composition of sol–gel solutions was investigated. The chemical composition of the CZTS thin films could be approximately controlled by varying the chemical composition of the sol–gel solution. As Cu/(Zn +Sn) of the sol–gel solution decreased, the grains in the CZTS thin films became larger. The optical band gap of the CZTS thin films shifted to higher energies as Cu/(Zn+Sn) of the CZTS thin film decreased. The CZTS thin film solar cell prepared from the CZTS thin film deposited using the sol–gel solution with Cu/(Zn+Sn) ¼0.80 exhibited the highest efficiency of 2.03% due to the large grains in its CZTS absorber layer.
Acknowledgement We would like to thank H. Katagiri of Nagaoka National College for the loan of the solar simulator. References [1] H. Katagiri, N. Sasaguchi, S. Hando, S. Hoshino, J. Ohashi, T. Yokota, Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E–B evaporated precursors, Sol. Energy Mater. Sol. Cells 49 (1997) 407–414. [2] T. Kobayashi, K. Jimbo, K. Tsuchida, S. Shinoda, T. Oyanagi, H. Katagiri, Investigation of Cu2ZnSnS4-based thin film solar cells using abundant materials, Jpn. J. Appl. Phys. 44 (2005) 783–787. [3] H. Katagiri, Cu2ZnSnS4 thin film solar cells, Thin Solid Films 480–481 (2005) 426–432. [4] H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W.S. Maw, T. Fukano, T. Ito, T. Motohiro, Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film
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K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
solar cells by using preferential etching technique, Appl. Phys. Exp. 1 (2008) 041201-1–141201-2. H. Katagiri, K. Jimbo, W.S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Development of CZTS-based thin film solar cells, Thin Solid Films 517 (2009) 2455–2460. H. Araki, Y. Kubo, A. Mikaduki, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, K. Oishi, A. Takeuchi, Preparation of Cu2ZnSnS4 thin films by sulfurizing electroplated precursors, Sol. Energy Mater. Sol. Cells 93 (2009) 996–999. H. Araki, Y. Kubo, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, K. Oishi, A. Takeuch, Preparation of Cu2ZnSnS4 thin films by sulfurization of coelectroplated Cu–Zn–Sn precursors, Phys. Status Solidi C 6 (2009) 1266–1268. A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kotschau, H.-W. Schock, R. Schurr, A. Holzing, S. Jost, R. Hock, T. Vos, J. Schulze, A. Kirbs, Cu2ZnSnS4 thin film solar cells from electroplated precursors: novel low-cost perspective, Thin Solid Films 517 (2009) 2511–2514. K. Tanaka, N. Moritake, H. Uchiki, Preparation of Cu2ZnSnS4 thin films by sulfurizing sol–gel deposited precursors, Sol. Energy Mater. Sol. Cells 91 (2007) 1199–1201. K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing, Sol. Energy Mater. Sol. Cells 93 (2009) 583–587. N. Moritake, Y. Fukui, M. Oonuki, K. Tanaka, H. Uchiki, Preparation of Cu2RZnSnS4 thin film solar cells under non-vacuum condition, Phys. Status Solidi C 6 (2009) 1233–1236. S. Chen, X.G. Gong, A. Walsh, S. Wei, Defect physics of the kesterite thin-film solar cell absorber Cu2ZnSnS4, Appl. Phys. Lett. 96 (2010) 021902 (3 pp).
[13] A.M. Gabor, J.R. Tuttle, S.S. Albin, M.A. Contrears, R. Noufi, High-efficiency CuInxGa1 xSe2 solar cells made from (Inx, Ga1 x)2Se3 precursor films, Appl. Phys. Lett. 65 (1994) 198–200. [14] R.A. Michkeksen, W.S. Chen, Y.R. Hsiao, V.E. Lowe, Polycrystalline thin-film CuInSe2/CdZnS solar cells, IEEE Trans. Electron Devices 31 (1984) 542–546. [15] G. Suresh Babu, Y.B. Kishore Kumar, P. Uday Bhaskar, V. Sundara Raja, Effect of Cu/(Zn+ Sn) ratio on the properties of co-evaporated Cu2ZnSnSe4 thin films, Sol. Energy Mater. Sol. Cells 94 (2010) 221–226. [16] S. Nakamura, T. Maeda, T. Wada, Electronic structure of stannite-type Cu2ZnSnSe4 by first principles calculations, Phys. Status Solidi C 6 (2009) 1261–1265. [17] J. Paier, R. Asahi, A. Nagoya, G. Kresse, Cu2ZnSnS4 as a potential photovoltaic material: a hybrid Hartree–Fock density functional theory study, Phys. Rev. B 79 (2009) 115126 8pp. [18] E. Burstein, Anomalous optical absorption limit In InSb, Phys. Rev. 93 (1954) 632–633. [19] T.S. Moss, The interpretation of the properties of Indium antimonide, Proc. Phys. Soc. London, Section B 67 (1954) 775–782. [20] C. Persson, A. Zunger, Anomalous grain boundary physics in polycrystalline CuInSe2: the existence of a hole barrier, Phys. Rev. Lett. 91 (2003) 266401 (4 pp). [21] K. Tanaka, T. Minemoto, H. Takakura, Analysis of heterointerface recombination by Zn1 xMgxO for window layer of Cu(In,Ga)Se2 solar cells, Sol. Energy 83 (2009) 477–479.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol–gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency Kunihiko Tanaka , Yuki Fukui, Noriko Moritake, Hisao Uchiki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka, Niigata 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2010 Received in revised form 25 October 2010 Accepted 31 October 2010 Available online 20 November 2010
The properties of Cu2ZnSnS4 (CZTS) thin films deposited by sol–gel sulfurization were investigated as a function of the chemical composition of the sol–gel solutions used. The chemical composition ratio Cu/(Zn+ Sn) of the sol–gel solution was varied from 0.73 to 1.00, while the ratio Zn/Sn was kept constant at 1.15. CZTS films deposited using sol–gel solutions with Cu=ðZn þ SnÞ o 0:80 exhibited large grains. In addition, the band gaps of these Cu-poor CZTS thin films were blue shifted. Solar cells with the structure Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass were fabricated under non-vacuum conditions. The solar cell with the CZTS layer deposited using the sol–gel solution with Cu/(Zn +Sn) ¼ 0.80 exhibited the highest conversion efficiency of 2.03%. & 2010 Elsevier B.V. All rights reserved.
Keywords: Cu2ZnSnS4 Solar cell Sol–gel sulfurizing Non-vacuum process
1. Introduction Cu2ZnSnS4 (CZTS) is a promising material for the absorber layer of thin-film solar cells. CZTS has a direct band gap energy of about 1.5 eV and an absorption coefficient that could potentially be greater than 104 cm 1. Furthermore, CZTS does not contain any toxic or low abundance elements. Because of their superior optoelectronic characteristics, CZTS thin films have been investigated as the absorber layer of thin-film solar cells. Katagiri et al. fabricated CZTS-based solar cells with the structure of Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass (SLG) [1–5]. The CZTS layers were prepared by sulfurizing the precursors deposited by electron-beam evaporation or radio-frequency magnetron co-sputtering. The solar cells had efficiencies of 1.08%, 3.93%, and 6.77% for CZTS chemical compositions of Cu/(Zn+Sn)¼0.99 and Zn/Sn¼1.01, Cu/(Zn+Sn)¼0.73 and Zn/Sn¼1.70, and Cu/(Zn+Sn)¼0.87 and Zn/Sn¼1.15, respectively. Thus, higher efficiencies were obtained using Cu-poor, Zn-rich CZTS absorber layers. These CZTS absorber layers were prepared in a vacuum, which entails high costs and complex processes. Several research groups have prepared CZTS absorber layers under non-vacuum conditions. Araki et al. prepared CZTS absorber layers by sulfurizing electroplated precursors [6,7]. The most efficient solar cell exhibited an
Corresponding author.
E-mail address: [email protected] (K. Tanaka). 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.10.031
efficiency ðZÞ of 3.16%, an open-circuit voltage (Voc) of 540 mV, a short-circuit current density (Jsc) of 12.6 mA/cm2, and a fill factor (FF) of 46.4%. This solar cell was Cu poor and Zn rich. Ennaoui et al. prepared CZTS layers by sulfurization of Cu–Zn–Sn precursors deposited by one-step electrodeposition [8]; the most efficient cell had Z ¼ 3:4%, Voc ¼563 mV, Jsc ¼14.8 mA/cm2, and FF¼41.0%. Its CZTS chemical composition was Cu/(Zn+Sn)¼0.98 and Zn/Sn¼1.00. All the semiconductor layers of the CZTS solar cells fabricated in our previous studies were deposited under non-vacuum conditions [9–11]. CZTS absorber, CdS buffer, and ZnO:Al window layers were deposited by sol–gel sulfurization, chemical bath deposition, and the sol–gel method, respectively. The CZTS absorber layer that exhibited the highest efficiency of 1.6% was Cu poor and Zn rich (Cu/(Zn+Sn) ¼ 0.96 and Zn/Sn¼1.17) [11]. Chen et al. found that Cu-poor and Zn-rich conditions improved the efficiency of CZTS solar cells because Cu-poor conditions enhance the formation of Cu vacancies, which give rise to shallow acceptors in CZTS, while Zn-rich conditions suppress the substitution of Cu at Zn sites, which give rise to relatively deep acceptors [12]. The above-mentioned results indicate that using Cu-poor and Zn-rich CZTS layers results in high-efficiency solar cells. Therefore, it is critical to control the chemical composition of the CZTS absorber layer to produce high-efficiency CZTS solar cells. In this report, we investigated whether the chemical composition of CZTS thin films deposited by sol–gel sulfurization can be controlled by varying the chemical composition of the sol–gel solution. We also investigated the influence of the chemical composition of the sol–gel solutions on several properties of CZTS thin films.
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
(312)
Mo
(220)
(200)
(112)
In this study, CZTS thin films were prepared under non-vacuum conditions by sol–gel sulfurization [9]. Mo-coated SLG (Mo/SLG) and SLG were used as substrates for these thin films. The SLG substrate was used for observing the optical properties of the thin films. The metal sources of the sol–gel solutions were copper (II) acetate monohydrate, zinc (II) acetate dihydrate, and tin (II) chloride dihydrate. The sol–gel solutions were Cu poor and Zn rich with chemical compositions of Cu/(Zn+Sn)¼0.73–1.00 and Zn/Sn¼1.15. In addition, a stoichiometric sol–gel solution of Cu/(Zn+Sn)¼1.00 and Zn/Sn ¼ 1.00. Table 1 lists the names and the chemical compositions of the sol–gel solutions employed. Sol–gel solutions with two metal ion concentrations (0.35 and 1.75 M) were prepared. The metal sources were dissolved in 2-methoxyethanol. Deionized water and ammonium acetate were added as a stabilizer for the 0.35 M sol–gel solution and monoethanolamine was added as a stabilizer for the 1.75 M sol–gel solution. Because high-concentration sol–gel solutions erode Mo, the Mo/SLG substrates were precoated with the low-concentration (0.35 M) sol–gel solution before being coated with the 1.75 M sol–gel solution [10]. The same coating procedure was applied to SLG substrates, which were used to characterize the optical properties of the thin films. The sol–gel solution coated substrates, which were precursors, were then annealed in an atmosphere of N2 and H2S (5%) at 500 1C for 1 h. Further details are given in Refs. [10,11]. Solar cells with the structure of Al/ZnO:Al/CdS/CZTS/Mo/SLG and a cell area of 0.12 cm2 were prepared in the present study. All the semiconductor layers were deposited under non-vacuum conditions. The CdS buffer layer was grown by chemical bath deposition method from an aqueous solution of CdI2, thiourea, and ammonia. The CdS buffer layer was a few tens of nanometers thick. The ZnO:Al window layer was deposited by the sol–gel method. Zinc (II) acetate dihydrate, aluminum chloride hexahydrate, 2-methoxyethanol, and monoethanolamine were used in the sol–gel solution. The window layer was 150 nm thick. Further details are given in Refs. [10,11]. The crystallinities of the CZTS thin films were analyzed by X-ray diffraction (XRD; Rigaku, RAD3). Their chemical compositions were determined by electron-probe microanalysis (Shimadzu, EPMA1600). Their surface morphologies and thicknesses were investigated by scanning electron microscopy (SEM; Hitachi, S4700). The sample surface was observed without any surface treatment. Transmittance and reflectance spectra of the CZTS thin films were obtained using an optical spectrometer (Jasco, V570). The J–V characteristics of the CZTS solar cells under illumination were determined by the four-probe method using a solar simulator with AM 1.5 and a power density of 100 mW/cm2.
compositions. The peaks were attributed to the (1 1 2), (2 0 0), (2 2 0), and (3 1 2) planes of CZTS and to Mo. No other prominent peaks were observed, which suggests that no secondary phases were present. There is no observable difference in the XRD patterns obtained from films prepared with sol–gel solutions having different chemical compositions. Table 1 shows the chemical compositions of the CZTS films. Fig. 2 shows the measured Cu/(Zn +Sn) and Zn/Sn values for the samples made from the sol–gel solution with Zn/Sn¼1.15. As Table 1 and Fig. 2 show, the CZTS films had different chemical compositions from those of the sol–gel solutions. However, the chemical composition ratio Cu/(Zn+ Sn) of the CZTS film was approximately proportional to that of the sol–gel solution. Zn/Sn of the CZTS thin films was larger than unity, just as in the sol–gel solution. Therefore, Cu-poor and Zn-rich CZTS thin films could be deposited by sol–gel sulfurization. Furthermore, the chemical composition of CZTS thin films deposited by sol–gel sulfurization can be approximately controlled by varying the chemical composition of the sol–gel solution. Cu/(Zn+ Sn) of the CZTS thin films decreased more gradually than that of the sol–gel solutions. This reveals that the amount of Zn or Sn (or Zn and Sn) in the CZTS films decreased with increasing Cu ratio in the sol–gel solution. The reductions in the amounts of Zn and Sn are considered to be due to evaporation, since the CZTS thin films did not contain a second phase (see Fig. 1).
CZTS073 Mo Mo
CZTS080
Diffraction Intensity (a.u.)
2. Experimental
839
CZTS084
CZTS087
CZTS100
3. Results and discussion
CZTS-St
3.1. Properties of CZTS thin films
20
Fig. 1 shows XRD patterns of CZTS films deposited on Mo/SLG substrates using sol–gel solutions with different chemical
30
40 50 60 70 Diffraction Angle, 2 (deg)
80
90
Fig. 1. Dependence of XRD profiles on chemical composition of sol–gel solutions.
Table 1 Chemical compositions of sol–gel solutions and CZTS thin films. Sample name
CZTS073 CZTS080 CZTS084 CZTS087 CZTS100 CZTS-St
Sol–gel solution
CZTS thin film
Cu/(Zn +Sn)
Zn/Sn
Cu/(Zn+ Sn)
Zn/Sn
Cu (at%)
Zn (at%)
Sn (at%)
S (at%)
0.73 0.80 0.84 0.87 1.00 1.00
1.15 1.15 1.15 1.15 1.15 1.00
0.91 0.92 0.95 0.97 0.99 1.03
1.23 1.17 1.18 1.10 1.13 1.03
22.9 22.5 22.7 23.5 23.3 22.7
13.8 13.2 12.9 12.7 12.4 11.1
11.2 11.3 11.1 11.5 11.1 10.8
52.2 52.9 53.4 52.3 53.3 55.3
840
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
Fig. 3 shows surface SEM images of the CZTS thin films. It reveals that samples CZTS-St, CZTS100, and CZTS087 consist of 100 nm grains. The grain size of the CZTS films increased as the Cu/(Zn+ Sn) ratio of the sol–gel solution decreased. The grains in samples CZTS080 and CZTS073 were larger than 1 mm. Table 2 lists the grain sizes of the CZTS films. CuSe functions as a flux for CIGS and CIS thin films since it has a low melting point, which increases the grain size [13,14]. Therefore, to increase their grain sizes, CIGS and CIS films were deposited under Cu-rich conditions and excess CuSe was then removed by KCN etching. In contrast, Cu-poor conditions increase the grain size in CZTS films deposited by sol–gel sulfurization. As mentioned above, the amounts of Zn and Sn decrease during the sulfurization
1.4
Chemical composition of Cu/(Zn+Sn) and Zn/Sn in CZTS thin films
1.2
1.0
0.8 sol-gel solution of Zn/Sn = 1.15 0.6
CZTS-St
process. Zn or Sn could be present in the CZTS precursors as hydrooxides, oxides, or other complexes. These Zn or Sn complexes could melt during sulfurization so that the melting complexes could function as fluxes, increasing the grain size. These melted Zn or Sn complexes would subsequently evaporate. Further study is necessary to clarify the grain growth mechanism during the sulfurization process. The properties of the CZTS thin films should be influenced by the chemical composition of the CZTS thin films. From the Fig. 1 and Table 2, the CZTS thin films can be classified into three groups: those with large grains and Cu-poor compositions, those with small grains and slightly Cu-poor compositions, and stoichiometric CZTS thin films. Fig. 4 shows a plot of ðahnÞ2 against the photon energy for the Cu-poor sample of CZTS080, the slightly Cu-poor CZTS087 sample, and the stoichiometric sample of CZTS-St, where a ðcm1 Þ is the absorption coefficient and hn (eV) is the photon energy. The band gap energy is estimated by extrapolating the linear region in the ðahnÞ2 plot and determining the intercept with the photon energy axis. As shown in Fig. 4, the band gap energy of the CZTS thin films shifted to higher energies as Cu/(Zn+ Sn) of the CZTS thin films decreased. The CZTS080, CZTS087, and CZTS-St samples had band gap energies of 1.62, 1.58 and 1.40 eV. Babu et al. investigated Cu2ZnSnSe4 (CZTSe), which is analogous to CZTS, as a function of the chemical composition of Cu/(Zn+ Sn). They found that the band gap energy of CZTSe shifts to higher energies as Cu/(Zn +Sn) decreases [15]. They attributed this to changes in the degree of p–d hybridization between the Cu d-levels and Se p-levels. The valence band maximum (VBM) of CZTSe is due
0.4 Cu/(Zn+Sn) 0.2
Table 2 Grain sizes of CZTS thin films.
Zn/Sn
0.0 0.7
0.8 0.9 1.0 Chemical composition of Cu/(Zn+Sn) in sol-gel solutions
Fig. 2. Cu/(Zn+Sn)and Zn/Sn in CZTS thin films as a function of Cu/(Zn+Sn) in sol–gel solution.
Cu/(Zn+Sn)=1.00 Zn/Sn=1.00
Cu/(Zn+Sn)=0.835 Zn/Sn=1.15
Sample name
Grain size
CZTS073 CZTS080 CZTS084 CZTS087 CZTS100 CZTS-St
1 mm 1 mm 200 nm 100 nm 100 nm 50 nm
Cu/(Zn+Sn)=1.00 Zn/Sn=1.15
Cu/(Zn+Sn)=0.80 Zn/Sn=1.15
Cu/(Zn+Sn)=0.87 Zn/Sn=1.15
Cu/(Zn+Sn)=0.73 Zn/Sn=1.15
Fig. 3. Surface SEM images of thin films produced from sol–gel solutions with different chemical compositions.
K. Tanaka et al. / Solar Energy Materials & Solar Cells 95 (2011) 838–842
Wavelength (nm) [×109]
10
1000
2.0
841
500
CZTS080 CZTS087
Current density (mA/cm2)
( )2 (eV cm-1)2
8
1.0
CZTS080 CZTS087
CZTS-St 6
4
2
CZTS-St
0.0 1.0
1.5 2.0 Photon energy (eV)
2.5
Fig. 4. ðahnÞ2 as a function of photon energy for Cu-poor, slightly Cu-poor, and stoichiometric CZTS.
to antibonding of Cu 3d and Se 4p orbitals [16] and the VBM of CZTS is due to antibonding of Cu 3d and S 3p orbitals [17]. Therefore, the band gap energy shifts of CZTS may be also attributed to changes in the p–d hybridization between the Cu d-levels and S p-levels. A possible reason for the band gap shift is the Moss–Burstein shift [18,19]. The excess holes reduce the VBM so that the band gap is blue shifted. Chen et al. reported that Cu vacancies are the main origin of acceptors in CZTS [12]. Therefore, Cu-poor and Zn-rich samples have many excess holes and the band gaps are blue shifted. Persson reported that for CuInSe2, in which the VBM consists of Cu d-orbitals and Se p-orbitals, Cu vacancies reduce the VBM [20].
3.2. Properties of CZTS thin-film solar cells CZTS thin film solar cells were prepared from Cu-poor CZTS080, slightly Cu-poor CZTS087, and stoichiometric CZTS-St. Fig. 5 shows the J–V characteristics of these CZTS solar cells under light irradiation. Table 3 shows the photovoltaic properties of the CZTS solar cells, where Voc is the open circuit voltage, Jsc is the short current density, FF is the fill factor, and Z is the conversion efficiency. As shown in Table 3, CZTS080 has the highest efficiency of 2.03%; this is considered to be due to its high Jsc and Voc. Its high Jsc is attributed to its relatively large grains (see Fig. 3). The grain boundary area decreases as the grain size increases. Electrons and holes recombine at grain boundaries, reducing the current density. Consequently, CZTS080 exhibits the highest Jsc. Chen reported that acceptors in CZTS are due to Cu vacancies (VCu) and substitution of Cu in Zn site (CuZn). They calculated the acceptor transition energy levels for CuZn and VCu to be 0.10 and 0.02 eV, respectively, which are both above the VBM [12]. Since CuZn is a relatively deep acceptor, the shallow acceptor of VCu is effective for improving the efficiency of CZTS solar cells. The Cu-poor composition suppresses CuZn formation and enhances VCu formation. Consequently, Jsc of the Cu-poor CZTS solar cell is higher than that of the slightly Cu-poor and stoichiometric CZTS solar cells. Voc increases as Jsc increases because Voc is proportional to (AkT/q)ln(Jsc/J00), where A is the ideality factor, kT/q is the thermal voltage, and J00 is a weakly temperaturedependent prefactor [21].
0 0
200
400
600
Voltage (mV) Fig. 5. J–V curves of CZTS thin-film solar cells under light irradiation.
Table 3 Photovoltaic properties of CZTS solar cells. Sample name
Z ð%Þ
Voc (mV)
Isc (mA/cm2)
FF (%)
CZTS080 CZTS087 CZTS-St
2.03 0.872 0.612
575 442 410
9.69 5.39 3.83
36.4 36.6 39.1
4. Conclusion The dependence of the properties of CZTS thin films deposited by sol–gel sulfurization on the chemical composition of sol–gel solutions was investigated. The chemical composition of the CZTS thin films could be approximately controlled by varying the chemical composition of the sol–gel solution. As Cu/(Zn +Sn) of the sol–gel solution decreased, the grains in the CZTS thin films became larger. The optical band gap of the CZTS thin films shifted to higher energies as Cu/(Zn+Sn) of the CZTS thin film decreased. The CZTS thin film solar cell prepared from the CZTS thin film deposited using the sol–gel solution with Cu/(Zn+Sn) ¼0.80 exhibited the highest efficiency of 2.03% due to the large grains in its CZTS absorber layer.
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