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Effects of sulfurization temperature on CZTS thin film solar cell performances
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 98 (2013) 335–340 www.elsevier.com/locate/solener
Effects of sulfurization temperature on CZTS thin film solar cell performances Amin Emrani ⇑, Parag Vasekar, Charles R. Westgate Center for Autonomous Solar Power, State University of New York at Binghamton, Binghamton, NY 13902, USA Received 2 May 2013; received in revised form 3 September 2013; accepted 13 September 2013 Available online 7 November 2013 Communicated by: Associate Editor Hari Mohan Upadhyaya
Abstract Synthesis of Cu2ZnSnS4 thin film solar cells by sulfurization of sputtered Sn/Zn/Cu precursors is studied. The sulfurization temperatures were varied, and the morphology, cross section, and composition of the CZTS were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. To further explore the CZTS layer, the following additional layers were deposited to complete the solar cells: CdS with chemical bath deposition; ZnO and AZO with RF magnetron deposition; and, finally, silver fingers as the front contact. The efficiency and characteristics of the thin film solar cells were measured and a detailed comparison is reported. Sulfurization at 550 °C yields a maximum efficiency of 5.75% without any anti-reflective layers. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: CZTS; Sputtering; Earth-abundant; Solar
1. Introduction Current research trends in photovoltaics include lowtoxic and earth-abundant materials. Copper–indium–gallium–selenide (CIGS) and cadmium telluride (CdTe) are two leading thin film solar technologies in commercial production. For CdTe (Wu, 2004), cadmium is toxic and tellurium is rare. For CIGS, selenium is a toxic material and indium and gallium are not abundant. (Jimbo et al., 2007; Tanaka et al., 2009). However, the elemental constituents of CZTS are not only earth-abundant, but also nontoxic, which makes CZTS a very attractive candidate for the fabrication of solar cells on a large scale. In addition CZTS has a band gap of 1.5 eV and an absorption coefficient of about 104 cm 1 making it one of the most promising candidates for thin film solar cells (Schubert et al., 2011; Ennaoui et al., 2009). The photovoltaic effect in ⇑ Corresponding author.
E-mail address: [email protected] (A. Emrani). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.09.020
CZTS was first reported in 1988 (Ito and Nakazawa, 1988) and followed by subsequent reports using various synthesis techniques (Araki et al., 2008; Kamoun et al., 2007; Moholkar et al., 2011). The current record efficiency for copper zinc tin sulfide– selenide (CZTSeS) thin film solar cells using a solution technique is 11.1% (Todorov et al., 2013), while CZTS solar cells fabricated using vacuum techniques approach an efficiency of 9.3% (Chawla and Clemens, 2012). We report a two-step process with sputtering of elemental precursors followed by sulfurization in dilute H2S (4–8%). We have identified the Sn/Zn/Cu sequence where tin as the first sputtered layer, as the optimum sequence for sputtering. We studied structural and optical properties of CZTS thin films at various sulfurization temperatures between 500 °C and 575 °C. It is observed that CZTS films exhibit compact grain structure and best morphology at a sulfurization temperature of 550 °C. The films formed at this temperature also have a near-ideal bandgap of 1.5 eV and exhibit the highest efficiency of 5.75%.
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A. Emrani et al. / Solar Energy 98 (2013) 335–340
2. Experimental The layers of Sn, Zn and Cu were sequentially deposited on Mo-coated soda lime glass substrate at room temperature. Tin is usually deposited as one of the bottom layers to avoid tin loss during sulfurization because tin has a low partial pressure. The operating pressure in the sputtering chamber for Sn, Zn and Cu was of 10 10 3 Torr, 5 10 3 Torr and 5 10 3 Torr respectively. Sn was deposited by a DC magnetron sputtering with 40 W power. Zn and Cu were deposited by an RF magnetron with sputtering powers of 40 W and 60 W, respectively. The thicknesses of Sn/Zn/Cu layers were 300/220/260 nm respectively. Sulfurization was carried out in a Lindberg tube furnace using dilute H2S (4–8%). The schematic of the sulfurization process is shown in Fig. 1. A slow rate of increase to the desired range of 500–575 °C as well as slow cooling, and a relatively long dwell time (3–4 h) ensure CZTS growth with fewer secondary phases. The H2S flow was stopped when the samples cooled to 100 °C. Subsequently cadmium sulfide (CdS) was deposited using a chemical bath deposition with cadmium sulfate and thiouria as Cd and S sources respectively. An etching treatment using dilute HCl was carried out before CdS deposition. This ensures that any zinc-rich secondary phases, which might otherwise cause degradation in the photovoltaic properties, are removed (Fairbrother et al., 2012). Cells were completed with sputtering of a 50 nm layer of undoped zinc oxide, 600 nm layer of aluminum doped zinc oxide and sputtering of chromium–silver contact fingers. The films were characterized using a Supra 55 VP-Zeiss field emission scanning electron microscope (SEM) and an energy dispersive spectroscopy (EDAX); while a Genesis silicon drift detector was used for composition analysis. X-ray diffraction (XRD) data were taken using copper kalpha radiation with a PANalytical XRD system. The I– V characteristics were measured using a Photo Emission Tech solar simulator under AM 1.5 conditions. Transmittance–reflection data for band-gap measurement was ana-
500
Tempreture ( °C)
400
2 hours
3 hours
2 hours
300
200
30 min 100
0
Time Fig. 1. Schematic of the sulfurization process.
lyzed using an Angstrom Sun Technologies’ spectrophotometer. The quantum efficiency measurement was performed using a PV measurements’ system equipped with a xenon light source and a monochromator with a chopper. Raman measurements were performed using Renishaw InVia Confocal Raman microscope in reflection mode. For excitation, an Ar+ laser with 488 nm wavelength was used. The penetration depth was estimated to be 150 nm in the CZTS absorber. The focused spot size is estimated to be 0.5 lm with an excitation power of 5 mW. 3. Results and discussion Fig. 2 shows the surface morphology of the CZTS thin films annealed at temperatures ranging from 500 °C to 575 °C. It is observed that at 500 °C, the grains are smaller and some voids are also present. The grains start coalescing as the temperature increases and compact and larger grains are observed at 525 °C and 550 °C. At 550 °C, the grain size is approximately 1 lm with a dense morphology. However, beyond 550 °C, it can be seen that there is some grain coarsening taking place which appears to degrade the photovoltaic performance. Thus 550 °C is found to be the optimum condition to get the best photovoltaic performance. Increasing the annealing temperature improves the crystallinity of CZTS thin films, and reduces the number of secondary phases. It was confirmed by EDAX analysis, that the film is zinc rich and copper poor with a Zn/Sn ratio of 1.2 and Cu/(Zn + Sn) ratio of 0.89 (Table 1). So far, higher efficiencies have been obtained with zinc rich and copper poor compositions which is consistent with previous reports (Todorov et al., 2010; Katagiri et al., 2009). Cross-sections of the sulfurized films for different temperatures (a: 500, b: 525, c: 550, and d: 575 °C) are shown in Fig. 3. With an increase in the annealing temperature, larger grains as well as compact and void-free structure is observed. With larger grains and grain boundaries oriented along the direction of the flow of carriers, there are fewer chances of recombination and subsequent losses in the performance. Although there are different schools of thoughts regarding the role of grain boundaries in thin film solar cells, it is generally accepted that grain boundaries act as recombination centers (Hegedus and Shafarman, 2004). Due to grain coarsening at 575 °C, the surface is rougher compared to films annealed at lower temperatures. Fig. 4 shows XRD patterns for the films sulfurized at the four different temperatures. From the XRD spectra, the 112-oriented Kesterite phase is confirmed according to ICDD 01-075-4122. The other peaks of the Kesterite phase have also been observed and indexed. Bragg peaks of secondary phases (indicated by ) are also observed at lower temperatures; however, their intensity diminishes at higher temperatures indicating mostly single phase CZTS phase formation. The secondary phases are attributed primarily to Cu2S and ZnS. Fig. 5 shows the Raman spectra of these films taken using the excitation laser wavelength of 488 nm. The two
A. Emrani et al. / Solar Energy 98 (2013) 335–340
337
a
b
1µm
1µm
d
c
1µm
1µm
Fig. 2. Surface morphology at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
Table 1 EDAX composition for the four different temperatures. Elements
Atomic percentages for different temperatures (°C) 500 °C
525 °C
550 °C
575 °C
Sn Cu Zn Zn/Sn Cu/(Zn + Sn)
24.1 46.6 29.3 1.22 0.87
24.5 46.8 28.7 1.17 0.88
23.9 47.7 28.4 1.19 0.91
23.7 47.6 28.7 1.21 0.91
1µm
a
1µm
b
1µm
c
1µm
d
Fig. 3. Cross sections of the films at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
strongest peaks (289 and 339 cm 1) are attributed to CZTS. Based on the absorption coefficient, the penetration depth is calculated around 150 nm. The secondary phase of Cu2S observed in the XRD pattern (Fig. 4) is not observed in Raman spectra, evident since there are no peaks at 475 cm 1. This means that Cu2 xS only exists on the surface of the film. The right shoulder on the larger CZTS peak (339 cm 1) is due to the broad peaks of CZTS at
350 and 368 cm 1 and the sharp peak of ZnS at 352 cm 1. A shoulder between peaks at 289 and 339 cm 1 can be assigned to the presence of Cu2SnS3 which cannot be distinguished from CZTS in the XRD pattern (Fernandes et al., 2010; Seol et al., 2003; Yoo and Kim, 2010). To further investigate these films, the spectral variation of absorption coefficients (a) were determined (Pawar et al., 2010) and (ahm)2 vs. hv are plotted to calculate the band
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400
350
350
o
T= 500 C
0
(312/116)
∗
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ
o
(112)
350
CZTS
350
o
T= 550 C
o
(112)
400
400
CZTS o
T= 575 C
300
50
(103) (200) (211) Mo (213/105)
100
(110)
150
(400/008) Mo (332)
200
(312/116)
(204/220)
250
(002) (101)
(103) (200) (211) Mo (213/105)
(110)
(002) (101)
150
Intensity (a.u.)
200
(400/008) Mo (332)
(204/220)
250
(312/116)
300
0
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ
o
2θ
o
Fig. 4. XRD patterns of the films sulfurized at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
525°C
Intensity (a.u.)
500°C
200
250
300
350
400
450
500
200
250
300
350
400
450
500
575°C
550°C
Intensity (a.u.)
Intensity (a.u.)
∗
(400/008) Mo (332)
50
(103) (200) (211) Mo (213/105)
100
(110)
150
0
2θ
50
(204/220)
200
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
100
o
T= 525 C
250
(002) (101)
Intensity (a.u.) Mo (332)
∗
∗
Mo (213/105)
(103) (200)
150
(400/008)
(204/220)
200
(312/116)
(112)
250
50
CZTS
300
(002) (101) (110)
Intensity (a.u.)
300
100
(112)
CZTS
200
250
300
350
400
450
500
200
250
300
350
400
450
Raman Shift (Cm-1) Fig. 5. Raman spectra at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
500
A. Emrani et al. / Solar Energy 98 (2013) 335–340
339
550°C
500°C
(α hυ ) 2
a.u.
1.4
1.5
1.6
1.7
1.8
1.4
525°C
1.5
1.6
1.7
575°C
(α hυ ) 2
a.u.
1.4
1.5
1.6
1.7
1.8
1.3
1.4
hυ (eV )
1.5
1.6
1.7
1.8
hυ (eV )
Fig. 6. Bandgap measurements at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
Table 2 Bandgaps at four different temperatures. Temperature (°C) Band gap (eV)
20
500 1.6
525 1.59
550 1.5
575 1.47
ZnO/AZO
2
Current Density (mA/Cm )
CdS CZTS
10
MoS2 Mo
0
-10
VOC =0.59V, JSC=20.5mA/cm2 =5.75%, FF=48% , Area=0.45 Cm2.
-20
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V) Fig. 7. Current–voltage characteristics of the highest efficiency cell.
gaps (Fig. 6). Table 2 shows the band gaps obtained for the films sulfurized at the four different temperatures. It can be observed that the bandgap is between 1.45 eV and 1.6 eV for all the films and at 550 °C, a near-optimum bandgap of 1.5 eV is obtained. Fig. 7 shows dark and light I–V curves of the highest efficiency film obtained at 550 °C. The highest efficiency is 5.75%, with an open circuit voltage of 589 mV and short circuit current density of 20.5 mA/cm2. The I–V data for all the films has been summarized in Table 3. The normalized quantum efficiencies for these films are as shown in Fig. 8. It can be seen that for the films sulfurized at 500 °C, 525 °C and 575 °C, there are losses in the red region indicating that the CZTS absorber quality is not optimized. The film at 550 °C shows better red response indicating that CZTS absorber quality is optimum and reduced secondary phases. However, it is also observed that films sulfurized at 500 °C and 525 °C show better blue response compared to their higher temperature counterparts. This means that the junction properties degrade as the sulfurization temperature increases, probably because the film roughness increases at higher temperature leading to the formation of a poor PN junction.
Table 3 I–V data for films at four different temperatures. Sample
Efficiency (%)
Open circuit voltage (mV)
Short circuit density (mA/cm2)
Series resistance (X)
Shunt resistance (X)
Fill factor (%)
500 525 550 575
1.1 2.4 5.75 4.5
322 412 593 471
12 15 20.5 18.55
48 26 19 14.5
270 340 620 580
28 39 48 52
340
A. Emrani et al. / Solar Energy 98 (2013) 335–340 100
500°C 525°C 550°C 575°C
Normilized EQE
80
60
40
20
0 400
600
800
1000
Wavelength (nm) Fig. 8. Normalized quantum efficiencies at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
4. Conclusions Elemental precursors of Sn/Zn/Cu were sputtered and sulfurized to obtain the highest efficiency of 5.75% at 550 °C. Efficiencies are lower at lower temperatures, which can be attributed to the CZTS absorber quality and higher series resistance; mostly imparted due to molybdenum sulfide formation and secondary phases such as zinc sulfide. Films at 550 °C exhibit compact grains with dense morphology and a near optimum bandgap of 1.5 eV. The Kesterite structure has been observed in XRD for all the films and some presence of secondary phases has been detected for films sulfurized at lower temperatures, with both XRD and Raman. Raman spectroscopy also confirms CZTS phase formation. Possible mechanisms for losses have been discussed using quantum efficiency analysis. Acknowledgements This work was supported in part by Defense Advanced Research Projects Agency (DARPA) award number HR0011101002. Authors also acknowledge support from the Analytical and Diagnostic Lab (ADL) at the Binghamton University and the Cornell center for Materials Research (CCMR) at the Cornell University, for analytical measurements. References Araki, H., Mikaduki, A., Kubo, Y., Sato, T., Jimbo, K., Maw, W.S., Katagiri, H., Yamazaki, M., Oishi, K., Takeuchi, A., 2008. Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers. Thin Solid Films 517, 1457–1460.
Chawla, V., Clemens, B., 2012. Effect of composition on high efficiency CZTSSe devices fabricated using co-sputtering of compound targets. In: 38th IEEE Photovoltaic Specialists Conference (PVSC). p. 002990– 002992. Ennaoui, A., Lux-Steiner, M., Weber, A., Abou-Ras, D., Ko¨tschau, I., Schock, H.-W., Schurr, R., Ho¨lzing, A., Jost, S., Hock, R., Voß, T., Schulze, J., Kirbs, A., 2009. Cu2ZnSnS4 thin film solar cells from electroplated precursors: novel low-cost perspective. Thin Solid Films 517, 2511–2514. Fairbrother, A., Garcı´a-Hemme, E., Izquierdo-Roca, V., Fontane´, X., Pulgarı´n-Agudelo, F.A., Vigil-Gala´n, O., Pe´rez-Rodrı´guez, A., Saucedo, E., 2012. Development of a selective chemical etch to improve the conversion efficiency of Zn-rich Cu2ZnSnS4 solar cells. Journal of the American Chemical Society 134 (19), 8018–8021. Fernandes, P.A., Salome´, P.M.P., da Cunha, A.F., 2010. A study of ternary Cu2SnS3 and Cu3 SnS4 thin films prepared by sulfurizing stacked metal precursors. Journal of Physics D 43, 215403. Hegedus, S.S., Shafarman, W.N., 2004. Thin-film solar cells: device measurements and analysis. Progress in Photovoltaics: Research and Applications 12, 155–176. Ito, K., Nakazawa, T., 1988. Electrical and optical properties of stannitetype quaternary semiconductor thin films. Japanese Journal of Applied Physics 27, 2094–2097. Jimbo, K., Kimura, R., Kamimura, T., Yamada, S., Maw, W.S., Araki, H., Oishi, K., Katagiri, H., 2007. Cu2ZnSnS4-type thin film solar cells using abundant materials. Thin Solid Films 515, 5997–5999. Kamoun, N., Bouzouita, H., Rezig, B., 2007. Fabrication and characterization of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique. Thin Solid Films 515, 5949–5952. Katagiri, H., Jimbo, K., Maw, W.S., Oishi, K., Yamazaki, M., Araki, H., Takeuchi, A., 2009. Development of CZTS-based thin film solar cells. Thin Solid Films 517, 2455–2460. Moholkar, A.V., Shinde, S.S., Babar, A.R., Sim, K., Kwon, Y., Rajpure, K.Y., Patil, P.S., Bhosale, C.H., Kim, J.H., 2011. Development of CZTS thin films solar cells by pulsed laser deposition: influence of pulse repetition rate. Solar Energy 85, 1354–1363. Pawar, S.M., Moholkar, A.V., Kim, I.K., Shin, S.W., Moon, J.H., Rhee, J.I., Kim, J.H., 2010. Effect of laser incident energy on the structural, morphological and optical properties of Cu2ZnSnS4 (CZTS) thin films. Current Applied Physics 10, 565–569. Schubert, B., Marsen, B., Cinque, S., Unold, T., Klenk, R., Schorr, S., Schock, H., 2011. Cu2ZnSnS4 thin film solar cells by fast coevaporation. Progress in Photovoltaics: Research and Applications 19, 93–96. Seol, J., Lee, S., Lee, J., Nam, H., Kim, K., 2003. Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process. Solar Energy Materials Solar Cells 75, 155–162. Tanaka, K., Oonuki, M., Moritake, N., Uchiki, H., 2009. Thin film solar cells prepared by non-vacuum processing. Solar Energy Materials Solar Cells 93, 583–587. Todorov, T.K., Reuter, K.B., Mitzi, D.B., 2010. High-efficiency solar cell with earth-abundant liquid-processed absorber. Advanced Materials 22 (20), E156–E159. Todorov, T.K., Tang, J., Bag, S., Gunawan, O., Gokmen, T., Zhu, Y., Mitzi, D.B., 2013. Beyond 11% efficiency: characteristics of state-ofthe-art Cu2ZnSn(S, Se)4 solar cells. Advanced Energy Materials 3, 34– 38. Wu, X., 2004. High-efficiency polycrystalline CdTe thin-film solar cells. Solar Energy 77, 803–814. Yoo, H., Kim, J., 2010. Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films. Thin Solid Films 518, 6567– 6572.
ScienceDirect Solar Energy 98 (2013) 335–340 www.elsevier.com/locate/solener
Effects of sulfurization temperature on CZTS thin film solar cell performances Amin Emrani ⇑, Parag Vasekar, Charles R. Westgate Center for Autonomous Solar Power, State University of New York at Binghamton, Binghamton, NY 13902, USA Received 2 May 2013; received in revised form 3 September 2013; accepted 13 September 2013 Available online 7 November 2013 Communicated by: Associate Editor Hari Mohan Upadhyaya
Abstract Synthesis of Cu2ZnSnS4 thin film solar cells by sulfurization of sputtered Sn/Zn/Cu precursors is studied. The sulfurization temperatures were varied, and the morphology, cross section, and composition of the CZTS were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. To further explore the CZTS layer, the following additional layers were deposited to complete the solar cells: CdS with chemical bath deposition; ZnO and AZO with RF magnetron deposition; and, finally, silver fingers as the front contact. The efficiency and characteristics of the thin film solar cells were measured and a detailed comparison is reported. Sulfurization at 550 °C yields a maximum efficiency of 5.75% without any anti-reflective layers. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: CZTS; Sputtering; Earth-abundant; Solar
1. Introduction Current research trends in photovoltaics include lowtoxic and earth-abundant materials. Copper–indium–gallium–selenide (CIGS) and cadmium telluride (CdTe) are two leading thin film solar technologies in commercial production. For CdTe (Wu, 2004), cadmium is toxic and tellurium is rare. For CIGS, selenium is a toxic material and indium and gallium are not abundant. (Jimbo et al., 2007; Tanaka et al., 2009). However, the elemental constituents of CZTS are not only earth-abundant, but also nontoxic, which makes CZTS a very attractive candidate for the fabrication of solar cells on a large scale. In addition CZTS has a band gap of 1.5 eV and an absorption coefficient of about 104 cm 1 making it one of the most promising candidates for thin film solar cells (Schubert et al., 2011; Ennaoui et al., 2009). The photovoltaic effect in ⇑ Corresponding author.
E-mail address: [email protected] (A. Emrani). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.09.020
CZTS was first reported in 1988 (Ito and Nakazawa, 1988) and followed by subsequent reports using various synthesis techniques (Araki et al., 2008; Kamoun et al., 2007; Moholkar et al., 2011). The current record efficiency for copper zinc tin sulfide– selenide (CZTSeS) thin film solar cells using a solution technique is 11.1% (Todorov et al., 2013), while CZTS solar cells fabricated using vacuum techniques approach an efficiency of 9.3% (Chawla and Clemens, 2012). We report a two-step process with sputtering of elemental precursors followed by sulfurization in dilute H2S (4–8%). We have identified the Sn/Zn/Cu sequence where tin as the first sputtered layer, as the optimum sequence for sputtering. We studied structural and optical properties of CZTS thin films at various sulfurization temperatures between 500 °C and 575 °C. It is observed that CZTS films exhibit compact grain structure and best morphology at a sulfurization temperature of 550 °C. The films formed at this temperature also have a near-ideal bandgap of 1.5 eV and exhibit the highest efficiency of 5.75%.
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A. Emrani et al. / Solar Energy 98 (2013) 335–340
2. Experimental The layers of Sn, Zn and Cu were sequentially deposited on Mo-coated soda lime glass substrate at room temperature. Tin is usually deposited as one of the bottom layers to avoid tin loss during sulfurization because tin has a low partial pressure. The operating pressure in the sputtering chamber for Sn, Zn and Cu was of 10 10 3 Torr, 5 10 3 Torr and 5 10 3 Torr respectively. Sn was deposited by a DC magnetron sputtering with 40 W power. Zn and Cu were deposited by an RF magnetron with sputtering powers of 40 W and 60 W, respectively. The thicknesses of Sn/Zn/Cu layers were 300/220/260 nm respectively. Sulfurization was carried out in a Lindberg tube furnace using dilute H2S (4–8%). The schematic of the sulfurization process is shown in Fig. 1. A slow rate of increase to the desired range of 500–575 °C as well as slow cooling, and a relatively long dwell time (3–4 h) ensure CZTS growth with fewer secondary phases. The H2S flow was stopped when the samples cooled to 100 °C. Subsequently cadmium sulfide (CdS) was deposited using a chemical bath deposition with cadmium sulfate and thiouria as Cd and S sources respectively. An etching treatment using dilute HCl was carried out before CdS deposition. This ensures that any zinc-rich secondary phases, which might otherwise cause degradation in the photovoltaic properties, are removed (Fairbrother et al., 2012). Cells were completed with sputtering of a 50 nm layer of undoped zinc oxide, 600 nm layer of aluminum doped zinc oxide and sputtering of chromium–silver contact fingers. The films were characterized using a Supra 55 VP-Zeiss field emission scanning electron microscope (SEM) and an energy dispersive spectroscopy (EDAX); while a Genesis silicon drift detector was used for composition analysis. X-ray diffraction (XRD) data were taken using copper kalpha radiation with a PANalytical XRD system. The I– V characteristics were measured using a Photo Emission Tech solar simulator under AM 1.5 conditions. Transmittance–reflection data for band-gap measurement was ana-
500
Tempreture ( °C)
400
2 hours
3 hours
2 hours
300
200
30 min 100
0
Time Fig. 1. Schematic of the sulfurization process.
lyzed using an Angstrom Sun Technologies’ spectrophotometer. The quantum efficiency measurement was performed using a PV measurements’ system equipped with a xenon light source and a monochromator with a chopper. Raman measurements were performed using Renishaw InVia Confocal Raman microscope in reflection mode. For excitation, an Ar+ laser with 488 nm wavelength was used. The penetration depth was estimated to be 150 nm in the CZTS absorber. The focused spot size is estimated to be 0.5 lm with an excitation power of 5 mW. 3. Results and discussion Fig. 2 shows the surface morphology of the CZTS thin films annealed at temperatures ranging from 500 °C to 575 °C. It is observed that at 500 °C, the grains are smaller and some voids are also present. The grains start coalescing as the temperature increases and compact and larger grains are observed at 525 °C and 550 °C. At 550 °C, the grain size is approximately 1 lm with a dense morphology. However, beyond 550 °C, it can be seen that there is some grain coarsening taking place which appears to degrade the photovoltaic performance. Thus 550 °C is found to be the optimum condition to get the best photovoltaic performance. Increasing the annealing temperature improves the crystallinity of CZTS thin films, and reduces the number of secondary phases. It was confirmed by EDAX analysis, that the film is zinc rich and copper poor with a Zn/Sn ratio of 1.2 and Cu/(Zn + Sn) ratio of 0.89 (Table 1). So far, higher efficiencies have been obtained with zinc rich and copper poor compositions which is consistent with previous reports (Todorov et al., 2010; Katagiri et al., 2009). Cross-sections of the sulfurized films for different temperatures (a: 500, b: 525, c: 550, and d: 575 °C) are shown in Fig. 3. With an increase in the annealing temperature, larger grains as well as compact and void-free structure is observed. With larger grains and grain boundaries oriented along the direction of the flow of carriers, there are fewer chances of recombination and subsequent losses in the performance. Although there are different schools of thoughts regarding the role of grain boundaries in thin film solar cells, it is generally accepted that grain boundaries act as recombination centers (Hegedus and Shafarman, 2004). Due to grain coarsening at 575 °C, the surface is rougher compared to films annealed at lower temperatures. Fig. 4 shows XRD patterns for the films sulfurized at the four different temperatures. From the XRD spectra, the 112-oriented Kesterite phase is confirmed according to ICDD 01-075-4122. The other peaks of the Kesterite phase have also been observed and indexed. Bragg peaks of secondary phases (indicated by ) are also observed at lower temperatures; however, their intensity diminishes at higher temperatures indicating mostly single phase CZTS phase formation. The secondary phases are attributed primarily to Cu2S and ZnS. Fig. 5 shows the Raman spectra of these films taken using the excitation laser wavelength of 488 nm. The two
A. Emrani et al. / Solar Energy 98 (2013) 335–340
337
a
b
1µm
1µm
d
c
1µm
1µm
Fig. 2. Surface morphology at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
Table 1 EDAX composition for the four different temperatures. Elements
Atomic percentages for different temperatures (°C) 500 °C
525 °C
550 °C
575 °C
Sn Cu Zn Zn/Sn Cu/(Zn + Sn)
24.1 46.6 29.3 1.22 0.87
24.5 46.8 28.7 1.17 0.88
23.9 47.7 28.4 1.19 0.91
23.7 47.6 28.7 1.21 0.91
1µm
a
1µm
b
1µm
c
1µm
d
Fig. 3. Cross sections of the films at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
strongest peaks (289 and 339 cm 1) are attributed to CZTS. Based on the absorption coefficient, the penetration depth is calculated around 150 nm. The secondary phase of Cu2S observed in the XRD pattern (Fig. 4) is not observed in Raman spectra, evident since there are no peaks at 475 cm 1. This means that Cu2 xS only exists on the surface of the film. The right shoulder on the larger CZTS peak (339 cm 1) is due to the broad peaks of CZTS at
350 and 368 cm 1 and the sharp peak of ZnS at 352 cm 1. A shoulder between peaks at 289 and 339 cm 1 can be assigned to the presence of Cu2SnS3 which cannot be distinguished from CZTS in the XRD pattern (Fernandes et al., 2010; Seol et al., 2003; Yoo and Kim, 2010). To further investigate these films, the spectral variation of absorption coefficients (a) were determined (Pawar et al., 2010) and (ahm)2 vs. hv are plotted to calculate the band
338
A. Emrani et al. / Solar Energy 98 (2013) 335–340 400
400
350
350
o
T= 500 C
0
(312/116)
∗
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ
o
(112)
350
CZTS
350
o
T= 550 C
o
(112)
400
400
CZTS o
T= 575 C
300
50
(103) (200) (211) Mo (213/105)
100
(110)
150
(400/008) Mo (332)
200
(312/116)
(204/220)
250
(002) (101)
(103) (200) (211) Mo (213/105)
(110)
(002) (101)
150
Intensity (a.u.)
200
(400/008) Mo (332)
(204/220)
250
(312/116)
300
0
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ
o
2θ
o
Fig. 4. XRD patterns of the films sulfurized at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
525°C
Intensity (a.u.)
500°C
200
250
300
350
400
450
500
200
250
300
350
400
450
500
575°C
550°C
Intensity (a.u.)
Intensity (a.u.)
∗
(400/008) Mo (332)
50
(103) (200) (211) Mo (213/105)
100
(110)
150
0
2θ
50
(204/220)
200
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
100
o
T= 525 C
250
(002) (101)
Intensity (a.u.) Mo (332)
∗
∗
Mo (213/105)
(103) (200)
150
(400/008)
(204/220)
200
(312/116)
(112)
250
50
CZTS
300
(002) (101) (110)
Intensity (a.u.)
300
100
(112)
CZTS
200
250
300
350
400
450
500
200
250
300
350
400
450
Raman Shift (Cm-1) Fig. 5. Raman spectra at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
500
A. Emrani et al. / Solar Energy 98 (2013) 335–340
339
550°C
500°C
(α hυ ) 2
a.u.
1.4
1.5
1.6
1.7
1.8
1.4
525°C
1.5
1.6
1.7
575°C
(α hυ ) 2
a.u.
1.4
1.5
1.6
1.7
1.8
1.3
1.4
hυ (eV )
1.5
1.6
1.7
1.8
hυ (eV )
Fig. 6. Bandgap measurements at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
Table 2 Bandgaps at four different temperatures. Temperature (°C) Band gap (eV)
20
500 1.6
525 1.59
550 1.5
575 1.47
ZnO/AZO
2
Current Density (mA/Cm )
CdS CZTS
10
MoS2 Mo
0
-10
VOC =0.59V, JSC=20.5mA/cm2 =5.75%, FF=48% , Area=0.45 Cm2.
-20
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V) Fig. 7. Current–voltage characteristics of the highest efficiency cell.
gaps (Fig. 6). Table 2 shows the band gaps obtained for the films sulfurized at the four different temperatures. It can be observed that the bandgap is between 1.45 eV and 1.6 eV for all the films and at 550 °C, a near-optimum bandgap of 1.5 eV is obtained. Fig. 7 shows dark and light I–V curves of the highest efficiency film obtained at 550 °C. The highest efficiency is 5.75%, with an open circuit voltage of 589 mV and short circuit current density of 20.5 mA/cm2. The I–V data for all the films has been summarized in Table 3. The normalized quantum efficiencies for these films are as shown in Fig. 8. It can be seen that for the films sulfurized at 500 °C, 525 °C and 575 °C, there are losses in the red region indicating that the CZTS absorber quality is not optimized. The film at 550 °C shows better red response indicating that CZTS absorber quality is optimum and reduced secondary phases. However, it is also observed that films sulfurized at 500 °C and 525 °C show better blue response compared to their higher temperature counterparts. This means that the junction properties degrade as the sulfurization temperature increases, probably because the film roughness increases at higher temperature leading to the formation of a poor PN junction.
Table 3 I–V data for films at four different temperatures. Sample
Efficiency (%)
Open circuit voltage (mV)
Short circuit density (mA/cm2)
Series resistance (X)
Shunt resistance (X)
Fill factor (%)
500 525 550 575
1.1 2.4 5.75 4.5
322 412 593 471
12 15 20.5 18.55
48 26 19 14.5
270 340 620 580
28 39 48 52
340
A. Emrani et al. / Solar Energy 98 (2013) 335–340 100
500°C 525°C 550°C 575°C
Normilized EQE
80
60
40
20
0 400
600
800
1000
Wavelength (nm) Fig. 8. Normalized quantum efficiencies at (a) 500 °C (b) 525 °C (c) 550 °C and (d) 575 °C.
4. Conclusions Elemental precursors of Sn/Zn/Cu were sputtered and sulfurized to obtain the highest efficiency of 5.75% at 550 °C. Efficiencies are lower at lower temperatures, which can be attributed to the CZTS absorber quality and higher series resistance; mostly imparted due to molybdenum sulfide formation and secondary phases such as zinc sulfide. Films at 550 °C exhibit compact grains with dense morphology and a near optimum bandgap of 1.5 eV. The Kesterite structure has been observed in XRD for all the films and some presence of secondary phases has been detected for films sulfurized at lower temperatures, with both XRD and Raman. Raman spectroscopy also confirms CZTS phase formation. Possible mechanisms for losses have been discussed using quantum efficiency analysis. Acknowledgements This work was supported in part by Defense Advanced Research Projects Agency (DARPA) award number HR0011101002. Authors also acknowledge support from the Analytical and Diagnostic Lab (ADL) at the Binghamton University and the Cornell center for Materials Research (CCMR) at the Cornell University, for analytical measurements. References Araki, H., Mikaduki, A., Kubo, Y., Sato, T., Jimbo, K., Maw, W.S., Katagiri, H., Yamazaki, M., Oishi, K., Takeuchi, A., 2008. Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers. Thin Solid Films 517, 1457–1460.
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