Copper-Zinc-Tin-Sulfur Thin Film Using Spin-Coating ... - MDPI

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Copper-Zinc-Tin-Sulfur Thin Film Using Spin-Coating Technology Min-Yen Yeh 1 , Po-Hsun Lei 2, *, Shao-Hsein Lin 2 and Chyi-Da Yang 1 1 2

*

Department of Microelectronic Engineering, National Kaohsiung Marine University, Kaohsiung 811, Taiwan; [email protected] (M.-Y.Y.); [email protected] (C.-D.Y.) Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wen-Hwa Rd, Hu-Wei, Yun-Lin 632, Taiwan; [email protected] Correspondence: [email protected]; Tel.:+886-5-6315668; Fax: +886-5-6329257

Academic Editor: Dirk Poelman Received: 22 May 2016; Accepted: 27 June 2016; Published: 29 June 2016

Abstract: Cu2 ZnSnS4 (CZTS) thin films were deposited on glass substrates by using spin-coating and an annealing process, which can improve the crystallinity and morphology of the thin films. The grain size, optical gap, and atomic contents of copper (Cu), zinc (Zn), tin (Sn), and sulfur (S) in a CZTS thin film absorber relate to the concentrations of aqueous precursor solutions containing copper chloride (CuCl2 ), zinc chloride (ZnCl2 ), tin chloride (SnCl2 ), and thiourea (SC(NH2 )2 ), whereas the electrical properties of CZTS thin films depend on the annealing temperature and the atomic content ratios of Cu/(Zn + Sn) and Zn/Sn. All of the CZTS films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS), Raman spectroscopy, and Hall measurements. Furthermore, CZTS thin film was deposited on an n-type silicon substrate by using spin-coating to form an Mo/p-CZTS/n-Si/Al heterostructured solar cell. The p-CZTS/n-Si heterostructured solar cell showed a conversion efficiency of 1.13% with V oc = 520 mV, Jsc = 3.28 mA/cm2 , and fill-factor (FF) = 66%. Keywords: CZTS thin film; solar cell; optical band gap; conversion efficiency

1. Introduction Semiconductor solar cells incorporating thin-film absorbers are an attractive option for replacing classical silicon-based solar cells. In recent years, several thin-film absorbers such as III–V compound material (GaInP/GaAs) [1–3], I–III–VI compound material (Cu(In, Ga)Se2 , CIGS) [4–6], and I–II–VI compound material (Cu2 ZnSnS4 , CZTS) [7–9] have been investigated for semiconductor thin-film solar cells. Among these studied materials, direct band gap chalcopyrite-structured CIGS and kesterite-structured CZTS are potential absorber materials for the next generation of thin-film solar cells because of their large optical absorption coefficients over the visual light range. A record conversion efficiency of 20.3% was reported for a CIGS thin-film solar cell [4]. However, use of toxic Se and scarce or expensive elements, such as gallium (Ga) and indium (In), may limit the production development of CIGS thin-film solar cells. Researchers strongly desire to identify a nontoxic inexpensive thin-film absorber with high conversion efficiency to replace CIGS. CZTS has been considered as an alternative to CIGS because of its majority carrier type (p-type), direct band gap of 1.4–1.5 eV, and large optical absorption coefficient (>104 cm´1 ) over the visible light range [10]. In addition, CZTS thin films are composed of elements that are nontoxic, abundant, and inexpensive, namely copper, zinc, tin, and sulfur. CZTS thin films are generally fabricated using vacuum-based techniques such as thermal evaporation [11], sputtering [12], and pulsed laser deposition [13]. These methods can deposit high-quality CZTS thin films on molybdenum (Mo)-coated glass substrates. However, these techniques Materials 2016, 9, 526; doi:10.3390/ma9070526

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Materials 2016, 9, 526

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require complicated equipment to maintain a vacuum and high process temperatures. In addition to high conversion efficiency, film content and composition as well as large-scale rather than small-area cell structural uniformity are challenges in developing CZTS thin-film solar cells for industrial use. Many nonvacuum methods of reducing production costs and satisfying the large-area requirements have been studied, including electrodeposition [14], spray pyrolysis techniques [15], sol-gel processing [16], spin-coating methods [17], and chemical bath methods [18]. Among these, the spin-coating method has the advantages of simple construction, low cost, ecological safety, and the large-scale deposition of different semiconductor thin films. However, few studies have reported the preparation of spin-coated CZTS thin films without toxic precursors such as hydrazine [19] or ethanolamine (MEA) [17]. Researchers do not completely and clearly understand the relationships of the deposition parameters to the atomic contents of Cu, Zn, Sn, and S, surface morphology, and electrical properties of spin-coated CZTS thin films. In this study, we developed spin-coating Cu-poor and Zn-rich CZTS thin films with optimal atomic contents of Cu, Zn, Sn, and S onto glass and silicon (Si) substrates. The atomic contents of Cu, Zn, Sn, and S, crystalline morphology, and surface morphology of these CZTS thin films were characterized using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) patterns, field-emission scanning electron microscopy (FE-SEM) images, and Raman measurements. The optical band gap and electrical properties were measured using absorption spectra and Hall measurements. Finally, an optimal CZTS thin was spun on an n-type Si substrate to form a Mo/p-CZTS/n-Si/Al solar cell. 2. Results and Discussion The crystallinity, morphology, electrical properties, and band gap energy of CZTS thin films show a strong dependence on the stoichiometry of Cu, Zn, Sn, and S in those CZTS thin films [9,16,20–22]. Well-controlled atomic contents of Cu, Zn, Sn, and S in CZTS thin films has become a crucial issue for fabricating high-performance kesterite CZTS solar cells. To determine the optimal atomic contents of Cu, Zn, Sn, and S in CZTS thin films, the chemical concentrations of CuCl2 , ZnCl2 , SnCl2 , and thiourea aqueous precursor solution should be analyzed. Figure 1a–d show the atomic content ratios of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for CZTS thin films as functions of the concentrations of CuCl2 , ZnCl2 , SnCl2 , and thiourea aqueous precursor solution, respectively, for an annealing temperature of 350 ˝ C. As shown in Figure 1a–c, the atomic contents of Cu, Zn, and Sn increase with raising the concentration of CuCl2 , ZnCl2 , and SnCl2 aqueous precursor solution, respectively, implying that the atomic contents of Cu, Zn, Sn, and S in CZTS thin films can be modulated by changing the concentrations of aqueous precursor solution. Cu-rich CZTS thin films can facilitate the formation of passivated defect clusters such as CuZn + SnZn and 2CuZn + SnZn . The former produces a deep donor level in the band gap of CZTS, whereas the latter can significantly reduce the band gap of CZTS [20]. However, a high Zn/Sn ratio or high Zn atomic content is necessary to form the ZnSn acceptors and reduce the SnZn donors to increase the conductivity of the CZTS thin film [23]. However, high Zn atomic content, which results in a low Cu/Zn ratio, can bring about a p/n conduction type [22]. To avoid defect clusters and increase the conductivity of p-type CZTS thin films, the Cu/(Zn + Sn) atomic content ratio should be less than 1, the Zn/Sn atomic content ratio should be more than 1, and the Cu/Zn atomic content ratio should not be too low. In Figure 1a, the atomic content ratio of Cu/(Zn + Sn) is lower than 1 for the CuCl2 concentration below 0.14 M and approaches 1 at the CuCl2 concentration of 0.16 M. When the ZnCl2 , SnCl2 , and thiourea concentrations increase with a CuCl2 concentration of 0.14 M, the Cu/(Zn + Sn) atomic content ratio remains below 1, as seen in Figure 1b–d. These results indicate that Cu-poor and Zn-rich CZTS thin films can be produced from properly controlled concentrations of aqueous precursor solutions. The atomic content ratio of S/Cu + Zn + Sn in CZTS thin films can approach 1 for the concentrations of CuCl2 , ZnCl2 , and SnCl2 , as shown in Figure 1a–c, respectively. In addition, in Figure 1d, the atomic content of S increases


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slightly as the thiourea concentration rises from 0.6 to 0.8 M, and saturates at a thiourea concentration above 0.8 M,9,possibly because of the solubility of S in the CZT thin film. Materials 2016, 526 3 of 12 3.0

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Figure 1. Atomic content ratios of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for copper-zincFigure 1. Atomic content ratios of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for tin-sulfur (CZTS) thin films with (a) CuCl2; (b) ZnCl2; (c) SnCl2; and (d) thiourea concentrations, copper-zinc-tin-sulfur (CZTS) thin films with (a) CuCl2 ; (b) ZnCl2 ; (c) SnCl2 ; and (d) thiourea respectively. concentrations, respectively.

Figure 2a–c represent the XRD patterns for films with various concentrations of CuCl2, ZnCl2, Figure 2a–c represent the XRD patterns for films with various concentrations of CuCl2 , ZnCl2 , and SnCl 2 aqueous precursor solutions that were annealed at a temperature of 350 °C. The XRD peaks ˝ C. The XRD and SnCl aqueous solutions that annealed at a temperature 350CZTS 2 were indexed using precursor JADE software (Jade 5.0,were Christchurch, New Zealand) asofthe kesterite peaks were indexed using JADE software (Jade 5.0, Christchurch, New Zealand) as the CZTS kesterite structure; and they closely matched the standard data (PDF#26-5075). All of the spin-coated CZTS structure; they closely matched the standard data (PDF#26-5075). All broad of the spin-coated thin thin films and exhibited a polycrystalline kesterite crystal structure with six peaks alongCZTS the (101), films exhibited a polycrystalline kesterite crystal structure with six broad peaks along the (101), (112), (112), (200), (220), (312) and (224) planes as shown in Figure 2a–c. The secondary phases including (200), (220), (312) and (224) planes as shown in Figure 2a–c. The secondary phases including Cu2 SnS Cu 2SnS3, CuS, SnS and ZnS would form during growth of CZTS thin films. They will degrade the3 , CuS, SnS andof ZnS would form during growth of to CZTS thin They will degrade the performance performance CZTS solar cells. It is important avoid thefilms. formation of secondary phases in CZTS of CZTS solar cells. It is important to avoid the formation of secondary phases in CZTS thincubicfilms. thin films. Among these secondary phases, tetragonal-structured Cu2SnS3 (PDF#33-0501) and Among these phases, tetragonal-structured Cu2 SnSto and cubic-structured 3 (PDF#33-0501) structured ZnSsecondary (PDF#05-0566) show the similar XRD patterns kesterite-structured CZTS between ZnS (PDF#05-0566) show the similar XRD patterns to kesterite-structured CZTS between diffraction angles of 20 and 80 degrees because of the same lattice constants [15]. As a diffraction result, the angles of 20 and 80 degrees because of the same lattice constants [15]. As a result, the measured measured XRD patterns of CZTS thin film might be related to either CZTS or Cu2SnS3 or ZnS pahse. XRDweak patterns of CZTS thin film might related to either CZTS or Cu2 SnS3 or ZnS pahse. The peak at 18.2° attributed to be (101) plane indicates a kesterite-structured CZTS The thinweak film ˝ attributed to (101) plane indicates a kesterite-structured CZTS thin film because it is not peak at 18.2 because it is not found in the XRD patterns of tetragonal-structured Cu2SnS3 and cubic-structured foundConsequently, in the XRD patterns of tetragonal-structured cubic-structured ZnS. 2 SnS3 and ZnS. a CZTS thin film without Cu secondary phase, which leads toConsequently, a degraded a CZTS thin film without which leads to a degraded performance of CZTS solar performance of CZTS solarsecondary cells, can bephase, obtained using properly controlled concentrations of aqueous cells, can be obtained using properly controlled concentrations of aqueous precursor solutions and precursor solutions and a proper annealing process. To further confirm the XRD characteristics ofa proper annealingCZTS process. further confirm XRD characteristics of this spin-coated this spin-coated thinTo film, which differ the from similar XRD patterns such as those ofCZTS ZnS thin and film, which differ from similar XRD patterns such as those of ZnS and Cu SnS , Raman spectroscopy 2 3 Cu2SnS3, Raman spectroscopy was performed. Figure 2e shows the Raman spectrum for a spin-coated was performed. Figure 2e shows Raman spectrum for a spin-coated CZTS thin film prepared CZTS thin film prepared from 0.14 the M CuCl 2, 0.07 M ZnCl2, 0.07 M SnCl2, and 0.8 M thiourea aqueous from 0.14 M CuCl2 , 0.07 ZnCl2 , at 0.07 M°C. SnCl 0.8 M thiourea precursor and 2 , and −1, precursor solutions and M annealed 350 The characteristic CZTSaqueous peak in Figure 2e issolutions at 336.2 cm ˝ C. The characteristic CZTS peak in Figure 2e is at 336.2 cm´1 , which is different from annealed at 350 which is different from the Raman peaks of Cu2–xS at 475 cm−1, ZnS at 273 and 351 cm−1, Cu2SnS3 at 290 and 352 cm−1, and of Sn2S3 at 234 and 307 cm−1, indicating a spin-coated CZTS thin film deposited on glass substrate [24,25].


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the Raman peaks of Cu2–x S at 475 cm´1 , ZnS at 273 and 351 cm´1 , Cu2 SnS3 at 290 and 352 cm´1 , and of ´1 Sn2 S3 at2016, 234 9,and Materials 526 307 cm , indicating a spin-coated CZTS thin film deposited on glass substrate [24,25]. 4 of 12 CZTS (112)

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Figure 2. X-ray diffraction (XRD) patterns for CZTS thin film with varied concentration of (a) CuCl2; Figure 2. X-ray diffraction (XRD) patterns for CZTS thin film with varied concentration of (a) CuCl2 ; (b) ZnCl2; and (c) SnCl2 aqueous precursor solution under an annealing temperature of 350 °C. The (b) ZnCl2 ; and (c) SnCl2 aqueous precursor solution under an annealing temperature of 350 ˝ C. 2, ZnCl2, SnCl2, and thiourea aqueous precursor Raman spectrum for for a CZTS thin film with The Raman spectrum a CZTS thin film withCuCl CuCl 2 , ZnCl2 , SnCl2 , and thiourea aqueous precursor solution concentration of 0.14, 0.07, 0.07, and 0.8 M, after after annealing at 350 at °C350 is shown ˝ C is solution concentration of 0.14, 0.07, 0.07, and 0.8 respectively, M, respectively, annealing in (d). shown in (d).

The mean crystallite grain size (D) of CZTS thin film can be estimated by the Scherrer formula [26]: The mean crystallite grain size (D) of CZTS thin film can be estimated by the Scherrer formula [26]: 0.9λ = (1) 0.9λ β cos θ D“c (1) βcosθ where β, λ, and θB are the line width at full width at half the maximum of the film diffraction peak at 2è, X-ray wavelength nm), and Bragg diffraction angle, respectively. Figure 3a–c show where β, λ, and θB are(0.15406 the line width at full width at half the maximum of the film diffraction peakthe at full widthwavelength at half maximum (FWHM) of (112) peak and calculated mean crystallite grain sizeshow of CZTS 2θ, X-ray (0.15406 nm), and Bragg diffraction angle, respectively. Figure 3a–c the thin films at as half a function of the variedofconcentration of calculated CuCl2, ZnCl 2, and SnCl2 aqueous precursor full width maximum (FWHM) (112) peak and mean crystallite grain size of CZTS solutions annealed at varied a temperature of 350 °C. The grain size enlarges with increasing CuCl2 thin filmsthat as awere function of the concentration of CuCl , ZnCl , and SnCl aqueous precursor 2 2 2 concentration, as shown in Figure 3a becauseof of350 the˝increase of Cu atomic content the CZTS thin2 solutions that were annealed at a temperature C. The grain size enlarges with in increasing CuCl films [27,28]. This increaseininFigure the grain size might be due to agglomeration grains.inInthe addition, the concentration, as shown 3a because of the increase of Cu atomicof content CZTS thin grain size of CZTS thin films represents a slight 2, and SnClof 2 concentrations increase films [27,28]. This increase in the grain size mightchange be dueas toZnCl agglomeration grains. In addition, the or decrease while CuCl 2 concentration is maintained at 0.14 M, as shown in Figure 3b,c. This is grain size of CZTS thin films represents a slight change as ZnCl2 , and SnCl2 concentrations increase or attributed to theCuCl fairly constant Cu atomic contentatin0.14 these thinin films. Consequently, the grain decrease while is maintained M, CZTS as shown Figure 3b,c. This is attributed 2 concentration size offairly CZTSconstant thin films the content same annealing temperature is determined by the atomic to the Cuwith atomic in these CZTS thin films. Consequently, theCu grain size content. of CZTS thin films with the same annealing temperature is determined by the Cu atomic content.

Materials 2016, 9, 526 Materials 2016, 9, 526

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(c) Figure Figure 3. 3. Full Full width width at at half half maximum maximum (FWHM) (FWHM) of of (112) (112) peak peak and and calculated calculated grain grain size size for for CZTS CZTS thin thin films with varied concentration of (a) CuCl 2; (b) ZnCl2; and (c) SnCl2 aqueous precursor solution films with varied concentration of (a) CuCl2 ; (b) ZnCl2 ; and (c) SnCl2 aqueous precursor solution under under an annealing temperature an annealing temperature of 350 ˝ofC.350 °C.

The conversion efficiency of a CZTS thin-film solar cell is strongly associated with the grain size The conversion efficiency of a CZTS thin-film solar cell is strongly associated with the grain size because an absorber layer with a large grain size maximizes the minority carrier diffusion length and because an absorber layer with a large grain size maximizes the minority carrier diffusion length and the inherent potential of a polycrystalline solar cell [16,24]. Figure 4 shows the FE–SEM images of the the inherent potential of a polycrystalline solar cell [16,24]. Figure 4 shows the FE–SEM images of the CZTS thin film. Figure 4a–c show plane views of CZTS thin films with CuCl2 concentrations of 0.1, CZTS thin film. Figure 4a–c show plane views of CZTS thin films with CuCl concentrations of 0.1, 0.14, 0.14, and 0.16 M with concentrations of ZnCl2, SnCl2, and thiourea 2at 0.07, 0.07, and 0.8 M, and 0.16 M with concentrations of ZnCl , SnCl , and thiourea at 0.07, 0.07, and 0.8 M, respectively. The respectively. The thickness of all the 2CZTS 2thin films was approximately 2.4 ìm, and Figure 4d thickness of all the CZTS thin films was approximately 2.4 µm, and Figure 4d represents the FE-SEM represents the FE-SEM cross section image of CZTS thin film with concentrations of CuCl2, ZnCl2, cross section image of CZTS thin film with concentrations of CuCl2 , ZnCl2 , SnCl2 , and thiourea at SnCl2, and thiourea at 0.14, 0.07, 0.07, and 0.8 M. As shown in Figure 4a–c, large numbers of voids 0.14, 0.07, 0.07, and 0.8 M. As shown in Figure 4a–c, large numbers of voids were observed in the were observed in the Cu-rich CZTS thin films. Voids of Cu-rich CZTS absorbers in thin-film solar Cu-rich CZTS thin films. Voids of Cu-rich CZTS absorbers in thin-film solar cells cause low conversion cells cause low conversion efficiencies for photovoltaic applications [16,29]. Well-controlled Cu efficiencies for photovoltaic applications [16,29]. Well-controlled Cu atomic content is important to atomic content is important to obtain a large grain size and less number of voids in CZTS thin films. obtain a large grain size and less number of voids in CZTS thin films. Figure 4e shows the plane Figure 4e shows the plane view of a CZTS thin film with a high ZnCl2 concentration of 0.1 M and view of a CZTS thin film with a high ZnCl2 concentration of 0.1 M and CuCl2 , SnCl2 , and thiourea CuCl2, SnCl2, and thiourea concentrations of 0.14, 0.07, and 0.8 M. The surface of Zn-rich CZTS thin concentrations of 0.14, 0.07, and 0.8 M. The surface of Zn-rich CZTS thin film is smoother than that of film is smoother than that of Zn-poor CZTS thin films because of the large columnar grains at the Zn-poor CZTS thin films because of the large columnar grains at the bottom [30]. bottom [30].

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(e) Figure 4. Plane-view field-emission scanning electron microscopy (FE–SEM) images of CZTS thin Figure 4. Plane-view field-emission scanning electron microscopy (FE–SEM) images of CZTS thin films films deposited with ZnCl 2, SnCl2, and thiourea concentrations of 0.07, 0.07, and 0.8 M, respectively, deposited with ZnCl 2 , SnCl2 , and thiourea concentrations of 0.07, 0.07, and 0.8 M, respectively, and 2 concentrations of (a) 0.1 M; (b) 0.14 M; and (c) 0.16 M, respectively. The cross section image and CuCl CuCl 2 concentrations of (a) 0.1 M; (b) 0.14 M; and (c) 0.16 M, respectively. The cross section image of a 2, ZnCl2, SnCl2, and thiourea concentrations of 0.14, 0.07, 0.07, and 0.8 of a CZTS film with CZTS thin thin film with CuClCuCl 2 , ZnCl2 , SnCl2 , and thiourea concentrations of 0.14, 0.07, 0.07, and 0.8 M is M is represented in (d). A CZTS thin film with fabricated 2, SnCl2, CuCl2, and thiourea represented in (d). A CZTS thin film fabricated ZnCl2 , with SnCl2 ZnCl , CuCl 2 , and thiourea concentrations concentrations of 0.1, 0.07, 0.14, and 0.8 M, respectively, is shown in (e). of 0.1, 0.07, 0.14, and 0.8 M, respectively, is shown in (e).

Figure 5 shows the graph of optical band gap energy as a function of the Cu/(Zn + Sn) atomic content ratio. The optical band gaps of these CZTS thin films range from 1.39 to 1.69, whereas the Cu/(Zn + Sn) atomic content ratios range from 0.58 to 0.76. This shift is attributed to the change in the p-d hybridization between Cu d-levels and S p-levels [16]. In addition, the relation of the extended


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1 0 1 0 e V 2 c m -2 )

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Optical Band gap (eV)

Figure 5 shows the graph of optical band gap energy as a function of the Cu/(Zn + Sn) atomic content ratio. The optical band gaps of these CZTS thin films range from 1.39 to 1.69, whereas the Cu/(Zn + Sn) content ratios range from 0.58 to 0.76. This shift is attributed to the change in the Materials 2016, 9, atomic 526 7 of 12 p-d hybridization between Cu d-levels and S p-levels [16]. In addition, the relation of the extended optical optical band band gap to to the the decrease decrease in in grain grain size size is is attributed attributed to the fact fact that that the the band band gap gap energy energy of of aa material material is dependent dependent on the particle size of that material according to the quantum size effect [15]. As shown in As shown in Figure Figure 3a–c, 3a–c, the the grain grain size size of of aa Cu-poor Cu-poor CZTS CZTS thin thin film film is is smaller smaller than than that that of of aa Cu-rich Cu-rich CZTS CZTS thin thin film, film, resulting resulting in in aa wide wide optical optical band band gap. gap. The The inset inset of of Figure Figure 55 shows shows aa plot plot of of(αhν) (αhν)22 against 1.12) CZTS thin film where α against the the photon photon energy energy of of aa Cu-poor Cu-poor and and Zn-rich Zn-rich (Cu/Zn (Cu/Zn == 1.12) α is the the absorption coefficient and hν is the photon energy. The optical band gap energy is estimated absorption hν is the photon energy. estimated by 2 2 extrapolating in the (αhν) plotplot and and determining the intercept with the photon energy extrapolatingthe thelinear linearregion region in the (αhν) determining the intercept with the photon axis. The optical gap for gap an optimal CZTS absorber is approximately 1.5 eV. 1.5 eV. energy axis. The band optical band for an optimal CZTS absorber is approximately

o Annealing Temp. = 350 C [CuCl2] = 0.14 M [ZnCl2] = 0.07 M [SnCl2] = 0.07 M [Thiourea] = 0.8 M

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Cu/(Zn+Sn) Ratio Figure 5. 5. Optical Optical band band gap gap as as aa function function of of the theCu/(Zn Cu/(Zn + + Sn) ratio, and and the the inset inset shows shows the the plot plot of of Figure Sn) ratio, (αhν)22 against energy of of aa Cu-poor Cu-poor and and Zn-rich Zn-rich (Cu/Zn (Cu/Zn ==1.12) (αhν) against the the photon photon energy 1.12)CZTS CZTSthin thinfilm. film.

Figure 6a,b show the carrier concentration and resistivity of CZTS thin films as a function of Figure 6a,b show the carrier concentration and resistivity of CZTS thin films as a function CuCl2 and ZnCl2 concentration, respectively. The carrier concentration shown in Figure 6a increases of CuCl2 and ZnCl2 concentration, respectively. The carrier concentration shown in Figure 6a with CuCl2 concentration. The formation of a CuZn + SnZn deep level depends on the Cu/(Zn + Sn) and increases with CuCl2 concentration. The formation of a CuZn + SnZn deep level depends on the Zn/Sn atomic content ratio, and an optimal condition occurs at Cu/(Zn + Sn) ≈ 0.8 and Zn/Sn ≈ 1.2 Cu/(Zn + Sn) and Zn/Sn atomic content ratio, and an optimal condition occurs at Cu/(Zn + Sn) « 0.8 [20]. As the CuCl2 concentration increases from 0.1 to 0.14 M, the Cu/(Zn + Sn) atomic content ratio and Zn/Sn « 1.2 [20]. As the CuCl2 concentration increases from 0.1 to 0.14 M, the Cu/(Zn + Sn) approaches the optimal value because of the suppression of the formation of a CuZn + SnZn deep donor atomic content ratio approaches the optimal value because of the suppression of the formation of level in the CZTS band gap, leading to a significant increase in hole concentration. Nevertheless, the a CuZn + SnZn deep donor level in the CZTS band gap, leading to a significant increase in hole Cu/(Zn + Sn) atomic content ratio increases with CuCl2 concentration to 1.6 M, causing the generation concentration. Nevertheless, the Cu/(Zn + Sn) atomic content ratio increases with CuCl2 concentration of a CuZn + SnZn deep donor defect and a reduction of the CuZn antisite acceptor defects, resulting in to 1.6 M, causing the generation of a CuZn + SnZn deep donor defect and a reduction of the CuZn antisite a decrease of carrier concentration. Under the optimal Cu/(Zn + Sn) atomic content ratio of 0.714, the acceptor defects, resulting in a decrease of carrier concentration. Under the optimal Cu/(Zn + Sn) CZTS thin film has a high carrier concentration of 1.07 × 1019 cm−3 and a resistivity of 1.19 Ωcm. The atomic content ratio of 0.714, the CZTS thin film has a high carrier concentration of 1.07 ˆ 1019 cm´3 atomic content ratio of Zn/Sn is also a crucial factor determining the electrical properties of a CZTS and a resistivity of 1.19 Ωcm. The atomic content ratio of Zn/Sn is also a crucial factor determining the thin film. The ZnSn antisite defect with an inherent p-type conduction type in a CZTS thin film can electrical properties of a CZTS thin film. The ZnSn antisite defect with an inherent p-type conduction easily form under a high Zn/Sn atomic content ratio. However, further increases in Zn/Sn atomic type in a CZTS thin film can easily form under a high Zn/Sn atomic content ratio. However, further content ratio would generate a large population of intrinsic defects, causing a detrimental influence increases in Zn/Sn atomic content ratio would generate a large population of intrinsic defects, causing relative to a CZTS thin film with a atomic content ratio of Cu/(Zn + Sn) = 1 and Zn/Sn = 1. In Figure a detrimental influence relative to a CZTS thin film with a atomic content ratio of Cu/(Zn + Sn) = 1 and 6b, the hole concentration decreases as the ZnCl2 concentration rises from 0.7 to 0.9 M because of the Zn/Sn = 1. In Figure 6b, the hole concentration decreases as the ZnCl2 concentration rises from 0.7 to population of donor defect clusters [20]. Further increases in ZnCl2 concentration would move the 0.9 M because of the population of donor defect clusters [20]. Further increases in ZnCl2 concentration Cu/(Zn + Sn) atomic content ratio away from 0.8 (see Figure 1b), leading to a translation of conduction types.


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would move the Cu/(Zn + Sn) atomic content ratio away from 0.8 (see Figure 1b), leading to a translation conduction types. Materials 2016,of 9, 526 8 of 12

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Figure 6. Carrier concentration and resistivity of the CZTS thin film as a function of the (a) CuCl2; and Figure 6. Carrier concentration and resistivity of the CZTS thin film as a function of the (a) CuCl2 ; (b) ZnCl2 concentrations. and (b) ZnCl2 concentrations.

The sulfer vacancy (VS) is also a dominant donor-like defect in a CZTS thin film because of its The sulfer vacancy also a dominant defectfrom in a CZTS because low S ) isconduction low formation energy. (V The type donor-like would change p-typethin to film n-type and of theitshole formation energy. The conduction type would change from p-type to n-type and the hole concentration concentration might decrease in an S-poor CZTS thin film due to a high concentration of VS. High S might decrease S-poor CZTS film due to a high S content or S-rich S .S.High content or S-richininanCZTS thin filmthin is crucial to reduce theconcentration concentrationofofVV Kosyak et al. [31] have in CZTS thin film is crucial to reduce the concentration of V . Kosyak et al. [31] have calculated S calculated the concentration of ZnSn antisite and VS in CZTS thin films with different Cu, Zn, andthe Sn concentration of Zn antisite and V in CZTS thin films with different Cu, Zn, and Sn content, and Sn S content, and represented that the concentration of acceptor-like ZnSn antisite is higher than that of representedVthat the concentration of acceptor-like Zn Based antisite is higher that of donor-like VS donor-like S in a Zn-rich and Sn-poor CZTS thin film. Sn on this result,than we measure the electrical in a Zn-rich and Sn-poor CZTS thin film. Based on this result, we measure the electrical properties properties of CZTS thin films with various thiourea concentration under optimal Cu/(Zn + Sn) atomic of CZTSratio thin of films with various thiourea concentration optimal + Sn) atomicratio content content 0.714. Table 1 lists the measured results under of S/(Cu + Zn +Cu/(Zn Sn) atomic content and ratio of 0.714. Table 1 lists the measured results of S/(Cu + Zn + Sn) atomic content ratio and electrical electrical characteristics of CZTS thin films depending on various thiourea concentration. The S/(Cu of CZTS thin films depending various thiourea concentration. The S/(Cu Zna +high Sn) +characteristics Zn + Sn) atomic content ratio increases withonrising thiourea concentration, implying that + the atomic content ratioCZTS increases withcan rising thiourea concentration, implying that thethiourea a high Saqueous content S content or S-rich thin film be obtained by changing the concentration or S-rich CZTS thin film can be obtained by changing the concentration thiourea aqueous solution. solution. As thiourea concentration rises from 0.6 to 0.8 M, the hole concentration and conductivity As thiourea rises from 0.6 to 0.8 of M,reduction the hole concentration and conductivity of the of the p-typeconcentration CZTS thin film increase because of VS concentration. However, further p-type thin film increase because of reduction VS concentration. further increase increaseCZTS in thiourea concentration would reduce theofhole concentration However, and conductivity possible in thiourea concentration would reduce the hole concentration and conductivity possible due to the due to the formation of ZnS [32]. formation of ZnS [32]. Table 1. The S/(Cu + Zn + Sn) atomic content ratio and electrical characteristics of copper-zinc-tinTable 1. The S/(Cu + Zn + Sn) atomic content ratio and electrical characteristics of copper-zinc-tin-sulfur sulfur (CZTS) thin films with different thiourea concentration. (CZTS) thin films with different thiourea concentration. Mobility Resistivity Thiourea S/(Cu + Conduction Carrier Concentration −3) 2/Vs) (cm (Ùcm) Concentration Zn + Sn) Type (cmConcentration Thiourea (M) Conduction Carrier Mobility Resistivity S/(Cu + Zn + Sn) ´3 ) Concentration (M) (cm (Ùcm) 0.6 1.04 p/n Type 9.13 ×(cm 1017 0.342 /Vs) 20.7 18 17 0.7 1.07 p/n 1.32 × 10 0.22 19.2 0.6 1.04 p/n 0.34 20.7 9.13 ˆ 10 0.49 1.19 0.8 1.091.07 p p/n 1.071.32 × 10ˆ191018 0.7 0.22 19.2 0.9 1.191.09 p 8.871.07 × 10ˆ181019 0.55 1.23 0.8 p 0.49 1.19 0.9 1.19 p 0.55 1.23 8.87 ˆ 1018

Table 2 shows the relation between annealing temperature and conduction type, carrier concentration, mobility, resistivity of CZTS thin films with concentrations of CuCl2, type, ZnCl2,carrier SnCl2, Table 2 shows theand relation between annealing temperature and conduction and thiourea aqueous precursor solution at 0.14, 0.07, 0.07, and 0.8 M, respectively. The hole, concentration, mobility, and resistivity of CZTS thin films with concentrations of CuCl2 , ZnCl 2 14 concentration for annealing of 300 °C, 350 0.07, °C, 400 °C,and and0.8 450M,°Crespectively. are 2.02 × 10The , 1.07 × SnCl2 , and thiourea aqueoustemperatures precursor solution at 0.14, 0.07, hole 19, 2.80 × 1015, and 1.24 × 1016 cm−3. The varied ˝carrier ˝ 10 concentrations of CZTS thin films with ˝ ˝ 14 concentration for annealing temperatures of 300 C, 350 C, 400 C, and 450 C are 2.02 ˆ 10 , increasing temperatures is 16 attributed to varied the changes Cu/(Zn + Sn) and Zn/Sn content 1.07 ˆ 1019 ,annealing 2.80 ˆ 1015 , and 1.24 ˆ 10 cm´3 . The carrierofconcentrations of CZTS thin films ratios, which cause the formation of CuZn + SnZn deep donor defects by reducing the CuZn antisite acceptor defects [20,25]. The mobility of CZTS thin films shows a counterclockwise dependence on the carrier concentration because of the impurity scattering. In conclusion, with an annealing temperature of 350 °C, CZTS thin films have a stable p-type conduction type with a high hole concentration of 1.07 × 1019 cm−3 and a low resistivity of 1.19 Ωcm. To realize the optimal CZTS


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with increasing annealing temperatures is attributed to the changes of Cu/(Zn + Sn) and Zn/Sn content ratios, which cause the formation of CuZn + SnZn deep donor defects by reducing the CuZn antisite acceptor defects [20,25]. The mobility of CZTS thin films shows a counterclockwise dependence on the carrier concentration because of the impurity scattering. In conclusion, with an annealing temperature of 350 ˝ C, CZTS thin films have a stable p-type conduction type with a high hole concentration Materials 2016, 9, 526 9 of 12 of 1.07 ˆ 1019 cm´3 and a low resistivity of 1.19 Ωcm. To realize the optimal CZTS absorber for photovoltaic solar cell applications, a CZTS thin film wasthin spin-coated onto an Si substrate to substrate fabricate absorber for photovoltaic solar cell applications, a CZTS film was spin-coated onto an Si atoMo/p-CZTS/n-Si/Al heterostructured solar cell. solar cell. fabricate a Mo/p-CZTS/n-Si/Al heterostructured Table 2. Temperature dependence of conduction type, carrier concentration, mobility, and resistivity of Table 2. Temperature dependence of conduction type, carrier concentration, mobility, and resistivity the CZTS thin film measured using Hall measurements. of the CZTS thin film measured using Hall measurements. Annealing Conduction Annealing Conduction ˝ C) Temperature ( Type Temperature (°C) Type p/n 300 300 p/n 350 350 pp 400 p/n 400 p/n 450 p/n 450 p/n

Carrier Carrier Concentration ´3 ) (cm Concentration (cm−3) 2.02 2.02ˆ× 10 101414 19 1.07 1.07ˆ× 10 101519 2.80 ˆ 10 15 2.80 × 10 1.24 ˆ 1016

Mobility Mobility Resistivity Resistivity 2 /Vs) (cm (Ùcm) 2 (cm /Vs) (Ùcm) 126126 0.49 0.49 7.95 7.95 0.094

1.24 × 1016

0.094

245 1.19 280 5310

245 1.19 280 5310

Figure solar cell cell under Figure 77 shows showsthe the(current-voltage) (current-voltage)I–V I–Vcharacteristics characteristicsofofthis thisMo/CZTS/Si/Al Mo/CZTS/Si/Al solar under (air mass) AM 1.5 G. The open circuit voltage, short current density, fill factor, and efficiency (air mass) AM 1.5 G. The open circuit voltage, short current density, fill factor, and efficiency were were 2 , 66%, and 1.13%, respectively. The Mo/CZTS/Si/Al heterostructured solar 520 mV, 3.28 mA/cm 2 520 mV, 3.28 mA/cm , 66%, and 1.13%, respectively. The Mo/CZTS/Si/Al heterostructured solar cell cell prepared in work this work was simple and the characteristics of thethin CZTS film used as the prepared in this was simple and the characteristics of the CZTS filmthin used as the absorber absorber in this solar cell should be investigated as soon as possible. Compared withthe thetechniques techniques of in this solar cell should be investigated as soon as possible. Compared with of preparing CZTS thin films reported by other authors, our CZTS thin film preparation method of using preparing CZTS thin films reported by other authors, our CZTS thin film preparation method of spin-coating exhibits superior performance for photovoltaic solar cell applications. using spin-coating exhibits superior performance for photovoltaic solar cell applications.

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Voltage (V) 7. Current-voltage Current-voltage(I–V) (I–V) characteristics characteristicsof ofaaMo/CZTS/Si/Al Mo/CZTS/Si/Al heterostructured Figure 7. heterostructured solar cell under (air (air mass) mass) AM AM 1.5 1.5 G. G.

3. Experimental ExperimentalDetails Details 3.1. Preparation of CZTS Thin Film Because and thiourea thiourea powder in deionized (DI) water Because the the solubility solubility levels levelsof of CuCl CuCl22,, ZnCl ZnCl22, SnCl22,, and differ, these powders were dissolved in DI water individually differ, these powders were dissolved in DI water individually to to form form aqueous aqueous precursor precursor solutions solutions with with varied varied molar molar concentrations. concentrations. If the aqueous precursor solutions do not contain toxic organic organic materials, such as MEA, spin-coating them uniformly onto glass substrates can be difficult. To spread precursor solutions uniformly without using toxic materials, the surfaces of the glass substrates were modified by using plasma cleaning to generate hydrophilic surfaces. To increase the S content of the CZTS thin film and prevent the mixed aqueous solution from becoming turbid, two separate spincoating steps were applied to deposit CZTS thin films. For the first step, CuCl2, ZnCl2, and SnCl2


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materials, such as MEA, spin-coating them uniformly onto glass substrates can be difficult. To spread precursor solutions uniformly without using toxic materials, the surfaces of the glass substrates were modified by using plasma cleaning to generate hydrophilic surfaces. To increase the S content of the CZTS thin film and prevent the mixed aqueous solution from becoming turbid, two separate spin-coating steps were applied to deposit CZTS thin films. For the first step, CuCl2 , ZnCl2 , and SnCl2 aqueous precursor solutions were mixed and stirred uniformly to form a metallic aqueous solution that was spin-coated onto soda lime glass substrates. The glass substrates, coated with uniformly distributed metallic aqueous solution, were treated using thermal processes, namely baking on a hot plate at 100 ˝ C for 3 min to dewater and annealing in a furnace at 350 ˝ C for 5 min to tailor the crystallinity and morphology of the CZT layer. In the second step, aqueous thiourea solution was spun onto the CZT layers, and then the samples were treated by being baked on a hot plate for 3 min at 100 ˝ C. Then, thermal annealed in a furnace for 180 min at 350 ˝ C to form CZTS thin films. The surface morphologies of the CZTS thin films were characterized using FE-SEM and XRD patterns by using a Bruker D8 advanced diffractometer (XRD: Bruker AXS-DS Discov, Taichung Taiwan) equipped with a CuKá (λ = 0.154 nm). The atomic contents of Cu, Zn, Sn, and S in these CZTS thin films were measured using EDX with the same diffractometer. The absorption of CZTS thin films in the visual range was measured using a UV–Vis–NIR spectrophotometer (UVD-350, Yunlin Taiwan). The electrical properties of CZTS thin films, including carrier concentration, carrier mobility, and resistivity, were measured using Hall measurements (HMS-5000) at room temperature. 3.2. Fabrication of the Mo/CZTS/n-Si/Al Heterostructured Solar Cell The preparation of the Mo/p-CZTS/n-Si/Al heterostructured solar cell can be described as follows. First, aluminum (Al) metal was deposited on the back of an n–Si substrate using a thermal evaporation process and then a thermal annealing treatment was applied to the sample to form an ohmic contact between Si and Al. Second, the optimal CZTS absorber layer was deposited on the surface of the n-Si substrate through spin-coating, followed by a thermal annealing procedure. Third, the Mo metal was sputtered onto the CZTS absorber contact region to serve as ohmic contact metal. The sample was then annealed in N2 atmosphere for 5 min at 400 ˝ C. 4. Conclusions In this study, we spin-coated CZTS thin films onto glass substrates and annealed them to improve the crystallinity and morphology of the thin films. The amounts of Cu, Zn, Sn, and S in these CZTS thin films depend on the CuCl2 , ZnCl2 , SnCl2 , and SC(NH2 )2 concentrations of the precursor solutions. The surface morphology and optical band gap properties of these CZTS thin films relate to the Cu content and the electrical properties of these CZTS thin films depend on the annealing temperature and the Cu/(Zn + Sn) and Zn/Sn ratios. All of the CZTS thin films exhibited a polycrystalline kesterite crystal structure with three major broad peaks along the (112), (220), and (312) planes in XRD patterns. The optimal CZTS thin film can be deposited on a glass and Si substrate with proportions of [CuCl2 ] = 0.14 M, [ZnCl2 ] = 0.07 M, [SnCl2 ] = 0.07 M, and [SC(NH2 )2 ] = 0.8 M and an annealing temperature of 350 ˝ C. Finally, an optimal CZTS thin film was deposited on an n-type silicon substrate to produce a Mo/p-CZTS/n-Si/Al heterostructured solar cell with a conversion efficiency of 1.13% and with V oc = 520 mV, Jsc = 3.28 mA/cm2 , and fill-factor (FF) = 66%. Acknowledgments: The authors gratefully acknowledge the financial support of the Ministry of Science and Technology, R.O.C. (101-2622-E-150-013-CC3). Author Contributions: Min-Yen Yeh and Po-Hsu Lei organized and designed the experiment procedure, and wrote the paper. Shao-Hsein Lin execute the deposition of CZTS thin films. Chyi-Da Yang performed the measurement of CZTS thin films. All of the authors read and approved the final version of the manuscript to be submitted. Conflicts of Interest: The authors declare that there is no conflict of interests.


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