Fabrication of Cu2ZnSnS4 (CZTS) absorber films based on different compound targets

Accepted Manuscript Fabrication of Cu2ZnSnS4 (CZTS) absorber films based on different compound targets Fan Yang, Ruixin Ma, Weishuang Zhao, Xiaoyong Zhang, Xiang Li PII:

S0925-8388(16)32420-3

DOI:

10.1016/j.jallcom.2016.08.053

Reference:

JALCOM 38554

To appear in:

Journal of Alloys and Compounds

Received Date: 27 June 2016 Revised Date:

4 August 2016

Accepted Date: 6 August 2016

Please cite this article as: F. Yang, R. Ma, W. Zhao, X. Zhang, X. Li, Fabrication of Cu2ZnSnS4 (CZTS) absorber films based on different compound targets, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.053. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Fabrication of Cu2ZnSnS4 (CZTS) absorber films based on different compound targets.

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Fan Yanga, Ruixin Maa,b,*, Weishuang Zhaoa, Xiaoyong Zhanga, Xiang Lia a School of Metallurgy and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083 P.R.China b Beijing Key Laboratory of Special Melting and Preparation of High-end Metal Materials, Beijing, 100083 P.R.China

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Abstract

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CZTS has been considered to be a perfect suitable replacement for CuInGaSe2 (CIGS) which contains the rare elements. This work developed a novel route to fabricate CZTS absorber films based on the integration of non-vacuum and vacuum techniques. This was illustrated by first synthesizing CZTS powder with the method of solid-phase synthesis. Then three different compound targets made of CZTS precursors and pure CZTS powder were utilized to fabricate CZTS absorber films using Radio Frequency (RF) sputtering, followed by a sulfurization process. All synthesized samples were analyzed with respect to their crystal structure, vibrational property, morphology and chemical composition. The photoelectrical properties investigation of the sulfurized absorber films by UV-VIS spectrophotometer and Hall effect measurement can well meet the requirements for potential application in thin film solar cells, demonstrating this approach had great potential for CZTS solar cell production. Key words: CZTS; powder; compound targets; absorber films

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1. Introduction

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The development of low-cost and high-efficiency thin film solar cells has opened up new solutions to address the challenge of clean and renewable energy. The high-profile CIGS [1-2] thin film solar cell has demonstrated a photoelectric conversion efficiency (PCE) as high as 21.7% [3]. However, the potential problems including the rarity of constituent elements (In, Ga) and environmental issues limit its commercial applications. The quaternary CZTS [4-7], composed of earth-abundance and non-toxicity of the constituent elements, has been considered to be a perfect suitable replacement for CIGS. CZTS is a p-type semiconductor with a near-optimum direct band gap of approximately 1.5eV and absorption coefficient of 104cm-1 [4] in visible light region. Shockley-Queisser photon balance calculations show that the PCE for CZTS device is 32.4% with JSC = 29.6mA/cm2, VOC = 1.21V and FF = 89.9% [8]. Many different techniques have been utilized to fabricate CZTS absorber films based on vacuum methods [9-11] and non-vacuum methods [12-14]. The potentially high uniformity deposition of vacuum method is sputtering from targets of the quaternary materials [15]. Usually, the quaternary CZTS target was prepared by sintering at a high temperature, using solid-state reaction of Cu2S (perhaps CuS), ZnS, SnS2 (perhaps SnS) [16]. The most successful sputtering route to CZTS has been developed by the group of Feng. et al. [17], demonstrating a record CZTS solar cell with a PCE of 8.58%. Meanwhile, progress in colloidal synthesis of high quality nano-crystalline have paved the way for non-vacuum methods. In one approach, the CZTS nano-crystalline can be used in the form of a nano-crystalline ink which is coated on substrates by blade [12] or spin [13] coating process. A 4.92% efficient solar cell was demonstrated based on nano-crystalline ink coating followed by a sulfurization process [18]. In this paper, the fabrication of CZTS absorber films involves a combination of non-vacuum and vacuum methods. This was demonstrated by first synthesizing CZTS powder based on solid-phase synthesis. Then the powder was pressed into a target for RF sputtering by cold isostatic pressing technique rather than coated on Mo-coated substrates in the form of nano-crystalline ink. According to the published data [19], annealing in sulfur atmosphere improves both the grain growth and electrical property. A list of characterizations were performed on the reaction products from three key points in the process of solid-phase synthesis including the reaction product which was just grounded and filtered, the precursor and the annealed product. Finally, three different targets were prepared using powders from the three key points. Comparative studies on the properties of these three CZTS targets and the corresponding CZTS absorber films have been investigated.

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2. Experiments 2.1 Targets fabrication

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The CZTS powder was prepared by grinding CuCl (AR), Zn(CH3COO)2 (AR), SnCl4·5H2O (AR), Na2S·9H2O (AR) and C6H8O6 (AR) in the mole ratio 2:1:1:4.8:2 under an air atmosphere. All above chemicals were analytical reagent (AR) and purchased from Aladdin. C6H8O6 (AR) was required to prevent CuCl (AR) from oxidation during the grinding process. In order to remove the foreign ions from CZTS system, the dark brown products were washed by absolute ethanol and deionized water. And then the filtered material (Powder 1) was kept in an oven at the temperature of 180℃ in air for 3 hours (Powder 2), followed by an annealing process in an Ar atmosphere in a tube furnace with quartz tube at 450℃ (Powder 3). The sputtering target was made of 15g CZTS powder by cold isostatic pressing technology with Φ5mm in diameter, 3mm thickness attached to a Mo plate. Target 1 (T1), Target 2 (T2) and Target 3 (T3) correspond to Powder 1 (P1), Powder 2 (P2) and Powder 3 (P3), respectively. 2.2 CZTS absorber films fabrication

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The CZTS absorber films in this work have been deposited on approximate 1-µm-thick Mo-coated substrates using T1, T2 and T3. The sputtering was carried out at room temperature to minimize the possibility of forming MoS2 at the CZTS/Mo interface using high pure Ar as the working gas. The chamber pressure before deposition and during deposition was maintained at 2×10-4Pa and 0.45Pa, respectively. During the growth of the CZTS absorber films, the flow rate of Ar and the distance of the target to substrate were maintained at 30SCCM and 60mm, respectively. The sputtering power for each of the targets was 50W and the film thickness was controlled by a quartz crystal monitor in combination with a baffle. The as-deposited CZTS absorber precursors have been sulfurized in a home-made graphite box in the presence of 100mg sulfur per sample. The graphite box was placed in Ar environment at atmosphere pressure in a tube furnace with quartz tube. The CZTS absorber precursors were heated to 550℃ at a rate of 20℃/min and kept at the annealing temperature for 30mins followed by cooling down naturally. The Film 1 (F1), Film 2 (F2) and Film 3 (F3) correspond to T1, T2 and T3, respectively. 2.3 Characterization X-ray diffraction (XRD) patterns of CZTS powders and sulfurized absorber films coated on bare sold lime glass substrates were collected with a SmartLab from Rigaku at 40Kv and 150mA by using Cu-Kα radiation (λ=0.15405nm). Raman spectrums were measured by a LabRAM HR Evolution using the 532nm line of an Ar laser as

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the excitation source. The field emission scanning electron microscope (FESEM) images were obtained on a ZEISS SUPRA55, and localized compositional analysis was performed on the attachment of energy dispersive spectrometry (EDS, Thermo-NS7) using a 20kV acceleration voltage, calibrated using elemental standards for Cu, Zn, Sn and S. Carrier concentration and resistivity of the films were analyzed by Hall effect measurement with van der Pauw method. Four dots of Ag electrode acted as ohmic contacts were placed on the middle of each edge of the quaternary films. Hall Effect measurements were conducted with a magnetic field intensity of 0.5T and current 0.1mA under dark testing environment. The optical property of the films was measured using a UV-2550 from Shimadzu Corporation.

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3. Results and discussion

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Fig.1 depicts the comparison of XRD patterns as obtained from T1, T2 and T3. The vertical lines with different colors represent the peak position of different phases according to the standard patterns in diffraction database. All the XRD patterns yield strong diffraction patterns especially for T3 which was annealed at the temperature of 450℃. Taking a closer look at the targets photos (see Fig.1.a), you will notice the slight color variation of the targets. Namely, colors of T1, T2 and T3 are brown, black brown and pure black, respectively. From T1 to T3, the color darkens to pure black since the CZTS phase which has a high optical absorption coefficient gradually increased. Some binary and ternary phases including CuS, ZnS, SnS, SnS2, Cu2SnS3 and Cu3SnS4 are expected to occur in CZTS targets especially for T1 and T2 as they do not experience the annealing process. Cubic-ZnS and tetragonal-Cu2SnS3 both have similar lattice constants and crystal structures as CZTS, so their XRD patterns are very similar to that of the CZTS phase. Besides, orthorhombic-Cu3SnS4 (JCPDS NO. 36-217) also have strong diffraction peaks at 28.42° overlapping with (112) crystal plane of CZTS. Table 1 depicts the main XRD peak positions and crystal planes for tetragonal-CZTS (JCPDS NO. 26-0575), tetragonal-Cu2SnS3 (JCPDS NO. 089-4714), tetragonal-Cu3SnS4 (JCPDS NO. 33-0501) and cubic-ZnS (JCPDS NO. 05-0566). Estimation of the peak deviation, ∆, of each phase in comparison to the CZTS peak is also listed. For the limits of XRD measurement, an approximately 0.1° movement of the XRD peak would not be convincingly significant to clearly tell one phase from another as deviations in this tiny movement could also be related to strain effects or to the test error. Fig.1.a also gives the larger view of XRD diagrams around 28.4° circled in dashed lines. The obvious “peak splitting” appeared for T1 and T2, and the reason for this phenomenon is that CZTS phase comprises some binary and ternary impurity phases (as discussed above). It is obviously noted that the measured sharp and distinct peak at 2θ=28.44° corresponds well with the (112) peak of tetragonal-CZTS for T3. Besides, the other characteristic peaks correspond with the tetragonal-CZTS structure, such as those at 32.93°, 47.33° and 56.09° are also easily founded. The strong peaks of the ZnS, Cu2SnS3, Cu3SnS4, and CZTS phases are very close, and the weak peaks do not sufficiently prove the presence of the respective phase. Only covellite-CuS (JCPDS NO. 78-2122) and cubic-SnS (JCPDS NO. 89-2755) whose XRD patterns have distinguishable difference compared with CZTS are marked. Fig.1.b shows the XRD patterns from 2θ=45° to 2θ=50°, along with the fitting peaks with lorentzian curves for T1. The XRD pattern of T1 can be divided into three intense peaks at 2θ=47.15°, 47.91° and 48.83°, where 47.15° peak is recognized as (220) crystal plane of SnS. Peaks at 2θ=47.91° and 48.83° can be assigned as characteristic peaks of CuS and Cu2S, respectively. The appearance of CuS, Cu2S and SnS in T1 is acceptable because T1 is not processed at high temperature. The diffraction patterns of T2 and T3 are more proximate to single peak, especially for T3 which has a narrower full width at half maximum (FWHM). The peak center of T2 is

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very close to the location of CuS, and the peak at 2θ=47.34°, correlating well with the (220) crystal plane of CZTS for T3. Distinction between the main binary and ternary phases expected in CZTS is compromised by the strong similarity in the positions of the main diffraction peaks in a single XRD characterization. Therefore, Raman scattering was employed for accurate and detailed phase analysis since the Raman spectroscopic characteristics of each phase differed largely from the others in a CZTS system. Fig.2.a shows the spectrums of all three targets for 532nm excitation wavelengths. The measured signal represents a convolution of the individual signals coming from each phase, and each of these individual signals can have different strengths depending on their contents. All major as well as minor signals are labeled in vertical lines with different colors. The spectrums of T1 and T2 indicate a multiple phase compositions, and almost all common impurity phases can be observed. Contributions due to CuS at 475cm-1 [12] are observed for both targets, but the strength of T2 is much stronger than that of T1. The stronger strength of the band at 475 cm-1, the higher content of CuS, and that is why the diffraction peak around 48° for T2 was identified to be related to CuS (see Fig.1.b). The reason for the above status is that the filtered powder was kept in an oven at the temperature of 180℃, 3h, air atmosphere before it was pressed into T2. It was then the air flew into the powder to oxidize the monovalent copper into divalent. The Raman spectrum of T3 exhibits significant changes compared to those of T1 and T2 because the powder was annealed in an Ar atmosphere in a tube furnace with quartz tube at 450℃. Two sharp Raman peaks assumed as the vibrational modes of kesterite CZTS at 287.6 cm-1 and 337.5 cm-1 [20] are dominant for T3, at the same time the visibly minor peaks attributed to impurity phases are also observed. Besides, contributions due to CuS at 475cm-1 are much reduced. Fig.2.b depicts the Raman spectrums with peak fitting for all three targets from 310 cm-1 to 345 cm-1. The Raman peaks at 321.4 cm-1 and 323.1 cm-1 [20] are assigned as characteristic Raman peaks of Cu3SnS4 for T1 and T2. Generally, the CZTS phase in a state of fine crystalline reveals a dominating A mode peak at 338 cm-1 [4, 15]. However, the disorder of Cu/Zn sublattices can initiate a change of the crystal symmetry from kesterite-type (I-4) to (I-42m) space group [21]. The presence of a disordered phase can be reflected in the appearance of an additional broadened peak around 332cm-1. The A mode peaks attributed to CZTS phase in T1, T2 and T3 are located at 334.0 cm-1, 336.4 cm-1 and 337.5 cm-1, respectively. Besides, a small peak at 331.7 cm-1 is also observed for T3. These results demonstrate that the Cu/Zn disorder exists for all three targets but gradually decreases from T1 to T3. This could happen for two main reasons after an exhaustive literature research. Firstly, Valakh et al. [22] found an additional peak at 331cm-1 corresponding to Cu-poor conditions where all Zn-atoms and half of the Cu-atoms were statistically distributed on both 2d- and 2c-sites. But in Cu-rich CZTS samples all Zn-atoms were fixed in the appropriate 2d-sites of kesterite symmetry. Table 2 contains composition summaries of all samples of this study. The content of copper gradually increases from T1 to T3, correlating well with the literature [22]. Secondly, near-resonant Raman scattering was conducted to track Cu/Zn disorder in CZTS films in Ref. [21]. It is found that the disordered phase can

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transform into ordered phase when the processing temperature is above 260℃, so the amount of Cu/Zn disorders remaining at room temperature (T1) and 180℃ (T2) is likely to be relatively large. Fig.3.a, d and e shows the plane-view FESEM images of CZTS powders for preparing T1, T2 and T3, respectively. Fig.3.b and c are views of partial enlargement of Fig.3.a. Fig.3.a depicts two different morphologies namely nanostructured flowers (see Fig.3.b) and cotton-like cumulus (see Fig.3.c). Qualitative EDS analyses (not shown) of the two different structures give indications for the nanostructured flowers to be Cu3SnS4 since its content of Zn is very low compared to the cotton-like cumulus. The flower-like Cu3SnS4, which was similar to ours, was also synthesized by Tipcompor et al. [23] using microwave-refluxing method. And the Raman peak at 321.4cm-1 also proves the existence of Cu3SnS4 in T1. The cotton-like cumulus in Fig.3.c is due to the effect of small grain size and poor crystallinity because the powder is not subjected to a heat treatment. The EDS results indicate the formation of a near stoichiometric CZTS for the cotton-like cumulus. Combined with previous phase composition analysis, it is considered as the coalition result of CZTS as well as impurity (binary and ternary) phases. In Fig.3.d, the petal-shaped portions evenly distribute on small particles with high tap density. The annealed powder as depicted in Fig.3.e has good crystallinity with euhedral and variably-sized blocky shape, meanwhile, the nanostructured flowers completely disappear. Besides, the characteristic peaks corresponding to Cu3SnS4 also vanish in both XRD and Raman spectrums of T3. All these results indicate that the heat-treatment is a quite effective means to eliminate Cu3SnS4 and other impurity phases. While affirming the heat-treatment, we should also notice that it induced notable agglomeration of the powder. Powders for preparing T1, T2 and T3 revealed much different morphologies, and different morphologies may have an impact on sputtering rate and even the properties of the final absorber films. The obtained values in Table 2 are averages from multiple EDS measurements. The acceleration voltage used in the EDS experiments is of 20kV, meaning that the X-ray generation range is very large and gives the chemical composition of samples comprehensively. Due to their high vapor pressure, losses of tin and sulfur, in the form of SnS and/or SnS2, are expected in T2 and T3 which were heat treated. Composition ratios of Zn/Sn and Cu/(Zn+Sn) would have a significant effect on the conversion efficiency since improved solar cells has been attributed to a Cu-poor and Zn-rich composition [24]. For T1, a proper composition with atomic ratios of Zn/Sn=1.14 and Cu/(Zn+Sn)=0.70 were obtained. After a heat treatment process, the Zn/Sn ratios increased to approximately 1.3 and the Cu/(Zn+Sn) of approximately 1.1 was achieved for T2 and T3. EDS results show that all three targets have a Zn-rich composition, but many impurity phases related to Sn-rich composition were detected in the corresponding Raman spectrums. This is because the energy of the laser is smaller than the band gap of ZnS in the case of 532 nm, the majority of the laser passes through the transparent ZnS while only a small part can be scattered. Fig.4 gives the phase identification of the CZTS absorber films after the sulfurization process (a: XRD patterns, b: Raman scattering spectrums). In order to

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eliminate the masking effect of Mo back contact, all films were sputtered and examined (XRD, Raman and EDS) on bare soda lime glass substrates. All the XRD characterizations yield strong diffraction patterns (see Fig.4.a). The main peaks of the sulfurized CZTS absorber films at around 2θ=23.09°, 28.54°, 33.03°, 37.13°, 38.01°, 47.47°, 56.25°, 58.99°, 69.42° and 76.64° match very well with the diffraction of the (110), (112), (200), (202), (211), (220), (312), (224), (008) and (332) crystal planes of the tetragonal-CZTS structure with a preferred grain orientation along the (112) direction for all three films. The XRD patterns of sulfurized CZTS absorber films also include several weak diffraction peaks at 2θ=31.77°, 32.36°, 45.53° and 46.30°, matching the peak locations of herzenbergite-SnS (JCPDS NO. 75-2155) and/or berndtite-SnS2 (JCPDS NO. 89-2358) (see insets in Fig.4.a). The diffraction intensities of SnS and SnS2 are much smaller in F1 compared with those in F2 and F3, so these impurity phases do not constitute a significant fraction of the phase composition of F1 however as their minor diffraction peaks are also absent. As for F2 and F3, minor but distinct peaks correspond with SnS, SnS2 and even Cu3SnS4 (JCPDS NO. 36-0217) appear. Indeed, peaks corresponding to SnS can be observed for all three films in the Raman scattering spectrums (see Fig.4.b), and the peak at approximately 313cm-1 should be attributed to SnS2 for F3. The narrow phase stability can promote the growth of the secondary phase in the CZTS system especially for Cu-poor conditions [25]. The results in Table 2 show that the value of Cu/(Zn+Sn) is less than 1.0 for all three films. Besides, the diffusion of S during the sulfurization process further creates favorable conditions for the growth of the secondary phase. The CZTS absorber films were fabricated by Xie et al. [26] and Bras et al. [27] based on the sputtering method, and the impurity phases of SnS and SnS2 were also hard to eliminate thoroughly. Aside from the contributions of SnS and SnS2, the other visibly intense peaks can be wholly assumed as the vibrational modes of kesterite-CZTS. The spectrums depict two intense bands at 287.1cm-1 and 337.8cm-1, where 337.8cm-1 [4, 15] band is assigned as the A mode of kesterite-CZTS. However, people have not come to consensus on the type of vibrational modes resulting from the band at 287.1cm-1. Khare et al. [28] reported a band at 290cm-1 (E(LO)) while Gurel et al. [29] reported a band at 289.8cm-1 (B(LO)). Interestingly, Fontane et al. [20] identified this band as another A symmetry mode. Finally the band at 287.1cm-1 was labeled as E/B/A mode. In addition, there is also a shoulder peak at 373.4cm-1 which is assigned as the LO component of the polar B symmetry modes [29]. On the basis of the previous XRD analysis, it concludes that F1 has a higher phase purity than F2 and F3. Fig.5 shows plane view (a, b & c) and cross-sectional view (d, e & f) FESEM images of CZTS absorber films (F1, a & d, F2, b & e, F3, c & f) on Mo-coated substrates after the sulfurization process. All films are dense and compact and there are no obvious cracks or voids which can lead to the generation of leakage current. The grains in F1 tightly arranged with an average size of 100nm. Some large size patty structures lie on the top of the small grains. We hypothesize that a small part of grains were pushed outside of the film because of the grain growth during the sulfurization process, and the overflowed grains were not compressed by surroundings and grew large gradually. It is noteworthy that this patty structures is

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clearly marked off from the flower-like Cu3SnS4. As previously described in Fig.3.b, the obtained Cu3SnS4 is of the 2D sheet-like morphology while the patty structures have some thickness in the third dimension. An enhanced grain growth can be observed in F2, and the overflowed grains lying above the surface of the film are larger compared with those in grain arrays as well. Size of the initial grains in targets may have an impact on the final morphologies of CZTS absorber films. From T1 to T3, the grains get larger gradually as the treatment temperature increased. So the reason for grain growth in F2 is possibly because the initial grains in T2 are larger than those in T1. On the basis of this speculation, the grains in F3 should be the largest among all films. However, the size distribution among grains in F3 is rather non-uniform. Fig.3.e shows notable agglomeration of the grains when the heat treatment temperature rose up to 450℃. There is a possibility that some large particles were directly sputtered on Mo-coated substrates and acted as the growth centers. Then small particles grew around the growth centers for the formation of F3. EDS analysis was measured on bare soda lime glass substrates since the S Kа peak overlapped with the Mo Lа peak. Comparing the composition of targets with the corresponding films, the most obvious change is the content of Sn element (see Table 2). Sn-loss during the heat treatment is suppressed by including sulfur in the as-deposited CZTS precursors which ultimately results in a Sn-rich composition. Usually Sn-excess in CZTS devices is detrimental for efficiency due to segregation of conductive phases such as SnSx or CuxSnSy. Besides, the thickness of these absorber films is between 0.8µm to 1.1µm with a very thin MoS2 layer. The MoS2 layer, which can be formed by the reaction with sulfur supplied by the CZTS precursor films and the environment containing sulfur vapor, is expected to exist between the Mo layer and the CZTS absorber film. The MoS2 layer plays a role of back contact blocking barrier which blocks carrier transport across the CZTS/Mo interface to the Mo layer. So the thin MoS2 layer is benefit to improve the photoelectric performance in a complete CZTS solar cell. The electrical properties investigation by Hall effect measurement reveals that all sulfurized CZTS absorber films are p-type with a carrier density, Hall mobility and resistivity as shown in Table 3. As discussed earlier, size of the sputtered particles from T1 and T2 should be relatively small. The small sputtered particles allow good mobility on Mo layer and result in a uniform film with very few defects. Thus, a high carrier concentration with a small Hall mobility value is obtained for both F1 and F2. Besides, large grains can lead to significant improvement on device performance due to less probability for recombination of carrier at the grain boundaries which makes F2 has a carrier concentration as high as 5.77×1018cm-3. Phase analysis indicates that SnSx which has a better electrical conductivity exists in F3. Finally, F3 reveals the highest hall mobility and the lowest value of film resistivity. The results show that the electrical properties of all sputtered CZTS absorber films can well meet the requirements for potential application in thin film photovoltaic cells. The band gap energy is deduced by extrapolating the linear portion of (αhν)2 versus the photo energy to zero as depicted in Fig.6, where α is the absorption coefficient. The band gaps of the films are determined to be 1.50eV and 1.48eV which consists with the data reported for F1 and F2, respectively. However, F3 depicts an optical

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band gap of around 1.41eV. This value is kind of small compared with the bulk literature value of 1.5eV [4]. Actually, Zhang et al. [30] also fabricated CZTS absorber films with a direct band gap of 1.42eV by the sol-gel sulfurization. Another point to bring up here is that the existence of SnS in F3 can lower its band gap to some extent since SnS is also a p-type semiconductor with a direct band gap of 1.24eV [31].

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4. Conclusion

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In conclusion, relatively pure CZTS powder has been prepared with the simple method of solid-phase synthesis. Some knowledge about the reaction mechanism was acquired based on the study of the reaction products from three key points in the reaction process. The cold-formed targets were prepared for the first time and all three of them displayed the availability to fabricate CZTS absorber films especially for T1. The sulfurized films demonstrated suitable properties for utilization in thin film solar cells. And yet minor cracks can be observed for all three targets after a dozen times of repeated use. This work broadened the scopes of raw materials and fabrication technology of targets. Optimization to enhance film crystallinity, smooth the film surface and adjust the chemical composition to a Zn-rich state is expected to achieve further improvement of film quality, making this process one of the most promising approaches to fabricating low-cost and high-efficiency CZTS solar cells.

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Acknowledgements

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This work was supported by Fundamental Research Funds for the Central Universities (FRF-BD-15-004A) in 2016. I am gratefully appreciated Prof. Jianping He for helpful conversations.

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Reference

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[1] T.-J. Hsueh, J.-M. Shieh, Y.-M. Yeh, Hybrid Cd-free CIGS solar cell/TEG device with ZnO nanowires, Progress in Photovoltaics: Research and Applications, 23 (2015) 507-512. [2] H. Young Park, D. Gwon Moon, J. Ho Yun, S.K. Ahn, K.H. Yoon, S. Ahn, Efficiency limiting factors in Cu(In,Ga)Se2 thin film solar cells prepared by Se-free rapid thermal annealing of sputter-deposited Cu-In-Ga-Se precursors, Applied Physics Letters, 103 (2013) 263903. [3] P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T.M. Friedlmeier, M. Powalla, Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%, physica status solidi (RRL) – Rapid Research Letters, 9 (2015) 28-31. [4] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a high-performance solution-processed kesterite solar cell, Solar Energy Materials and Solar Cells, 95 (2011) 1421-1436. [5] S. Chen, A. Walsh, X.-G. Gong, S.-H. Wei, Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers, Advanced Materials, 25 (2013) 1522-1539. [6] A. Walsh, S. Chen, S.-H. Wei, X.-G. Gong, Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4, Advanced Energy Materials, 2 (2012) 400-409. [7] L.E. Valle Rios, K. Neldner, G. Gurieva, S. Schorr, Existence of off-stoichiometric single phase kesterite, Journal of Alloys and Compounds, 657 (2016) 408-413. [8] Q. Guo, H.W. Hillhouse, R. Agrawal, Synthesis of Cu2ZnSnS4 Nanocrystal Ink and Its Use for Solar Cells, Journal of the American Chemical Society, 131 (2009) 11672-11673. [9] Y.-P. Lin, Y.-F. Chi, T.-E. Hsieh, Y.-C. Chen, K.-P. Huang, Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells, Journal of Alloys and Compounds, 654 (2016) 498-508. [10] G. Gordillo, C. Calderón, P. Bartolo-Pérez, XPS analysis and structural and morphological characterization of Cu2ZnSnS4 thin films grown by sequential evaporation, Applied Surface Science, 305 (2014) 506-514. [11] N. Song, M. Young, F. Liu, P. Erslev, S. Wilson, S.P. Harvey, G. Teeter, Y. Huang, X. Hao, M.A. Green, Epitaxial Cu2ZnSnS4 thin film on Si (111) 4° substrate, Applied Physics Letters, 106 (2015) 252102. [12] R. Ma, F. Yang, S. Li, X. Zhang, X. Li, S. Cheng, Z. Liu, Fabrication of Cu2ZnSn(S,Se)4 (CZTSSe) absorber films based on solid-phase synthesis and blade coating processes, Applied Surface Science, 368 (2016) 8-15. [13] J. Wang, P. Zhang, X. Song, L. Gao, Cu2ZnSnS4 thin films: spin coating synthesis and photoelectrochemistry, RSC Advances, 4 (2014) 21318-21324. [14] X. He, H. Shen, W. Wang, J. Pi, Y. Hao, X. Shi, Synthesis of Cu2ZnSnS4 films from co-electrodeposited Cu–Zn–Sn precursors and their microstructural and optical

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properties, Applied Surface Science, 282 (2013) 765-769. [15] B.-T. Jheng, P.-T. Liu, M.-C. Wu, A promising sputtering route for dense Cu2ZnSnS4 absorber films and their photovoltaic performance, Solar Energy Materials and Solar Cells, 128 (2014) 275-282. [16] J. Wang, S. Li, J. Cai, B. Shen, Y. Ren, G. Qin, Cu2ZnSnS4 thin films: Facile and cost-effective preparation by RF-magnetron sputtering and texture control, Journal of Alloys and Compounds, 552 (2013) 418-422. [17] Y. Feng, T.-K. Lau, G. Cheng, L. Yin, Z. Li, H. Luo, Z. Liu, X. Lu, C. Yang, X. Xiao, A low-temperature formation path toward highly efficient Se-free Cu2ZnSnS4 solar cells fabricated through sputtering and sulfurization, CrystEngComm, 18 (2016) 1070-1077. [18] W. Wang, H. Shen, L.H. Wong, Z. Su, H. Yao, Y. Li, A 4.92% efficiency Cu2ZnSnS4 solar cell from nanoparticle ink and molecular solution, RSC Advances, 6 (2016) 54049-54053. [19] S. Kermadi, S. Sali, F. Ait Ameur, L. Zougar, M. Boumaour, A. Toumiat, N.N. Melnik, D.W. Hewak, A. Duta, Effect of copper content and sulfurization process on optical, structural and electrical properties of ultrasonic spray pyrolysed Cu2ZnSnS4 thin films, Materials Chemistry and Physics, 169 (2016) 96-104. [20] X. Fontané, V. Izquierdo-Roca, E. Saucedo, S. Schorr, V.O. Yukhymchuk, M.Y. Valakh, A. Pérez-Rodríguez, J.R. Morante, Vibrational properties of stannite and kesterite type compounds: Raman scattering analysis of Cu2(Fe,Zn)SnS4, Journal of Alloys and Compounds, 539 (2012) 190-194. [21] J.J.S. Scragg, L. Choubrac, A. Lafond, T. Ericson, C. Platzer-Björkman, A low-temperature order-disorder transition in Cu2ZnSnS4 thin films, Applied Physics Letters, 104 (2014) 041911. [22] M.Y. Valakh, O.F. Kolomys, S.S. Ponomaryov, V.O. Yukhymchuk, I.S. Babichuk, V. Izquierdo-Roca, E. Saucedo, A. Perez-Rodriguez, J.R. Morante, S. Schorr, I.V. Bodnar, Raman scattering and disorder effect in Cu2ZnSnS4, physica status solidi (RRL) – Rapid Research Letters, 7 (2013) 258-261. [23] T. Narongrit, T. Somchai, T. Titipun, Characterization of Cu3SnS4 Nanoparticles and Nanostructured Flowers Synthesized by a Microwave-Refluxing Method, Japanese Journal of Applied Physics, 52 (2013) 111201. [24] W. Dang, X. Ren, W. Zi, L. Jia, S. Liu, Composition controlled preparation of Cu–Zn–Sn precursor films for Cu2ZnSnS4 solar cells using pulsed electrodeposition, Journal of Alloys and Compounds, 650 (2015) 1-7. [25] M. Kumar, A. Dubey, N. Adhikari, S. Venkatesan, Q. Qiao, Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS-Se solar cells, Energy & Environmental Science, 8 (2015) 3134-3159. [26] M. Xie, D. Zhuang, M. Zhao, B. Li, M. Cao, J. Song, Fabrication of Cu2ZnSnS4 thin films using a ceramic quaternary target, Vacuum, 101 (2014) 146-150. [27] P. Bras, J. Sterner, C. Platzer-Björkman, Influence of hydrogen sulfide annealing on copper–zinc–tin–sulfide solar cells sputtered from a quaternary compound target, Thin Solid Films, 582 (2015) 233-238. [28] A. Khare, B. Himmetoglu, M. Johnson, D.J. Norris, M. Cococcioni, E.S. Aydil,

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Calculation of the lattice dynamics and Raman spectra of copper zinc tin chalcogenides and comparison to experiments, Journal of Applied Physics, 111 (2012) 083707. [29] T. Gürel, C. Sevik, T. Çağın, Characterization of vibrational and mechanical properties of quaternary compounds Cu2ZnSnS4 and Cu2ZnSnSe4 in kesterite and stannite structures, Physical Review B, 84 (2011) 205201. [30] K. Zhang, Z. Su, L. Zhao, C. Yan, F. Liu, H. Cui, X. Hao, Y. Liu, Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization, Applied Physics Letters, 104 (2014) 141101. [31] B.Y.J. Liang, Y.M. Shen, S.C. Wang, J.L. Huang, The influence of reaction temperatures and volume of oleic acid to synthesis SnS nanocrystals by using thermal decomposition method, Thin Solid Films, 549 (2013) 159-164.

ACCEPTED MANUSCRIPT Table 1. The main XRD peak positions and crystal planes for tetragonal-CZTS (JCPDS NO. 26-0575), tetragonal-Cu2SnS3 (JCPDS NO. 089-4714), tetragonal -Cu3SnS4 (JCPDS NO. 33-0501) and cubic-ZnS (JCPDS NO. 05-0566). Estimation of the peak deviation, ∆, of each phase in comparison to the CZTS peak. Tetragonal-Cu2SnS3

2θ(°)

2θ(°)

hkl

∆(°)

2θ(°)

28.49 112 0.05

28.44 112

28.54 112

0.10

32.93

200

33.07

200

0.14

33.02

400

47.33

220

47.47

204

0.14

56.09

312

56.20 116

56.32

312

0.12

76.41

76.68

316

0.27

∆(°)

47.46

220 0.13

56.03

132 0.06

76.52

2θ(°)

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hkl

28.50 111

332 0.11

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hkl

Cubic-ZnS ∆(°) 0.06

33.03

200

0.10

47.40

220

0.07

56.24

311

0.06

76.56

331

0.15

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Tetragonal -Cu3SnS4

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ACCEPTED MANUSCRIPT Table 2. The chemical composition of samples. Cu/at%

Zn/at%

Sn/at%

S/at%

Zn/Sn

Cu/(Zn+Sn)

Metal/S

T1

19.07

14.44

12.67

53.82

1.14

0.70

0.86

T2

26.61

13.82

10.58

48.99

1.31

1.09

1.04

T3

26.95

14.34

10.70

48.01

1.34

1.08

1.08

F1

23.26

12.83

13.65

50.26

0.94

0.88

0.99

F2

25.33

12.58

13.42

48.67

0.94

0.97

1.05

F3

23.51

11.70

14.29

50.51

0.82

0.90

0.98

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Sample

ACCEPTED MANUSCRIPT Table 3. Results of Hall effect measurement of CZTS absorber films. Sample F1 F2

P P P

Carrier density (/cm3)

Mobility (cm2/Vs)

Resistivity (Ω·cm)

3.85×10

18

2.56

0.63

5.77×10

18

1.73

0.63

2.79×10

18

3.83

0.58

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F3

Conductivity type

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Fig.1. (a) Comparison of XRD patterns as obtained from T1, T2 and T3. (b) Enlarged XRD patterns from 2θ=45° to 2θ=50° to show the differing peak positions of (220) crystal plane of the kesterite-CZTS, the (220) crystal plane of the cubic-SnS, the (110) crystal plane of the covellite-CuS, the (106) crystal plane of monoclinic-Cu2S. The vertical lines with different colors in (a) and (b) represent the peak position of different phases according to the standard patterns in diffraction database. (tetragonal-CZTS: JCPDS NO. 26-0575; cubic-ZnS: JCPDS NO. 05-0566; covellite-CuS: JCPDS NO. 78-2122; cubic-SnS JCPDS NO. 89-2755; monoclinic-Cu2S JCPDS NO.33-490).

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Fig.2. (a) Comparison of Raman scattering spectrums as obtained from T1, T2 and T3 in the case of 532nm excitation wavelengths. (b) Enlarged Raman scattering spectrums from shift=310cm-1 to shift=345cm-1 to show the differing vibrational properties of CZTS and Cu3SnS4. All major as well as minor signals are labeled in vertical lines with different colors in (a) and (b).

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Fig.3. Plane-view FESEM images of CZTS powders for preparing (a) T1, (d) T2 and (e) T3. (b) and (c) are views of partial enlargement of (a).

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Fig.4. (a) XRD patterns of CZTS absorber films, identified as the kesterite-CZTS, on bare soda lime glass substrate. Peaks arising from herzenbergite-SnS (JCPDS NO. 75-2155), berndtite-SnS2 (JCPDS NO. 89-2358) and orthorhombic-Cu3SnS4 (JCPDS NO. 36-217) are also clearly marked. (b) The corresponding Raman scattering spectrums.

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Fig.5. Plane-view (a, b & c) and cross-sectional view (d, e & f) FESEM images of CZTS absorber films on Mo-coated substrates after the sulfidation process. The film types F1 (a & d), F2 (b & e) and F3 (c & f) were sputtered using Target 1, Target 2 and Target 3, respectively.

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Fig.6. The (αhν)2 vs eV plots of CZTS absorber films.

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Highlights: 1. Pure CZTS nano-crystalline has been prepared with the method of the simple solid-phase synthesis. 2. This work broadened the scopes of raw materials and fabrication technology of CZTS targets. 3. Comparative studies on the properties of different CZTS targets and the corresponding CZTS absorber films have been investigated. 4. A novel route based on the integration of non-vacuum and vacuum technique was developed to fabricate CZTS absorber films.