Phase-selective and photoactivity investigation of solvothermal synthesized Cu2ZnSnS4 nanoparticles

    Phase-selective and photoactivity investigation of solvothermal synthesized Cu2 ZnSnS4 nanoparticles Qing Zhang, Meng Cao, Wang Sheng Gao, Jin Yang, Jie Sheng Shen, Jian Huang, Yan Sun, Lin Jun Wang, Yue Shen PII: DOI: Reference:

S0264-1275(15)30815-7 doi: 10.1016/j.matdes.2015.11.074 JMADE 977

To appear in: Received date: Revised date: Accepted date:

17 June 2015 16 November 2015 20 November 2015

Please cite this article as: Qing Zhang, Meng Cao, Wang Sheng Gao, Jin Yang, Jie Sheng Shen, Jian Huang, Yan Sun, Lin Jun Wang, Yue Shen, Phase-selective and photoactivity investigation of solvothermal synthesized Cu2 ZnSnS4 nanoparticles, (2015), doi: 10.1016/j.matdes.2015.11.074

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ACCEPTED MANUSCRIPT Phase-selective and photoactivity investigation of solvothermal synthesized Cu2ZnSnS4 nanoparticles

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Qing Zhanga, Meng Caoa*, Wang Sheng Gaoa, Jin Yanga, Jie Sheng Shena, Jian

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Huanga, Yan Sunb, Lin Jun Wanga, Yue Shena a

School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China

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National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese

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Academy of Sciences, Shanghai 200083, China

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Abstract: Cu2ZnSnS4 (CZTS) nanoparticles with bandgap of about 1.5 eV were synthesized by a simple solvothermal method with oleylamine (OAm) and ethanediamine

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(En) as the reaction solvents. Raman spectra and X-ray photoelectron spectroscopy

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examined the phase purities of CZTS nanoparticles. Kesterite structured CZTS were

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gradually changed into the wurtzite structured CZTS by varying the volume ratios of OAm and En, which was confirmed by X-ray diffraction measurements. Time-dependent

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experiments were performed to study the mechanism of the phase selection of CZTS, which indicated that En played an important role in the formation of wurtzite structured CZTS. Annealing process improved the crystallinities of CZTS nanoparticle thin films, but wurtzite structured CZTS was changed to more stable kesterite phase. Photo-electrochemical measurement indicated that wurtzite structured CZTS nanoparticle thin films had better photoelectric properties.

Keywords: Cu2ZnSnS4, Nanoparticles, Wurtzite, Solvothermal, Solar cells * To whom correspondence should be addressed: 1

ACCEPTED MANUSCRIPT Corresponding author: Meng Cao *E-mail address: [email protected]

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Fax: 86-21-56332475

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Tel:86-21-66137128

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ACCEPTED MANUSCRIPT 1. Introduction Cu2ZnSnS4 (CZTS) has emerged as a promising candidate for absorber materials of

) of about 1.50 eV and a high absorption coefficient(>104 cm-1) [1,2]. The

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band gap (

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thin film solar cells because of its less toxicity, earth abundance, nearly optimum direct

highest energy conversion efficiency of CZTS thin film solar cells has reached 12.6% [3]. According to Shockley-Queisser photon balance calculations, its theoretical limit of the

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power conversion efficiency is as high as 32.2% [4]. Up to date, many methods about the

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preparation of CZTS have been reported, such as sputtering [5], evaporation [6], electrodeposition [7], chemical bath deposition [8], hot-injection [9] and solvothermal

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method [10]. Even though many CZTS thin film solar cells with high-efficiencies have

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been obtained by vapor deposition, they impose a substantial cost of materials.

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Alternative processing strategy with reasonable success is colloidal synthesis of high quality CZTS nanoparticles to fabricate high efficient solar cells with printing, spraying

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or dip coating methods. The highest power conversion efficiency of CZTS nanoscrystal thin film solar cells has reached 9.6% [11]. The crystal structures of the as-obtained CZTS nanoparticles have been identified as kesterite, stannite and wurtzite phase. Different from CZTS in the kesterite or stannite phase with a tetragonal crystal cell, wurtzite phase CZTS has a hexagonal crystal cell. In this structure, sulfur atoms stay in the six-party compact accumulation. At the same time, Cu (Ⅰ), zinc (Ⅱ) and Sn (Ⅳ) cations replace half lattice positions and arrange disorderly [12]. CZTS with wurtzite structure was reported to exhibit a much higher carrier concentration and lower resistivity than that of kesterite and stannite structured CZTS 3

ACCEPTED MANUSCRIPT [13]. Lu et al reported firstly the synthesis of wurtzite CZTS nanocrystal with hot-injection method by using 1-dodecanethiol (1-DDT) as the sulfur source [14]. Wang

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et al synthesized CZTS nanoparticles with hydrothermal method by using different

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sulphur precursors, but failed to get wurtzite CZTS with pure phase [15]. And the mechanism of the phase selection was not well understood. Wu et al synthesized wurtzite CZTS nanoparticles through solvothermal method by using 1-DDT and OAm as the

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solvents [16]. But the metal precursors, such as Cu(acac)2, Zn(acac)2, and Sn(acac)4, were

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complicated to prepare relatively. And the Raman peak of as-synthesized wurtzite CZTS was reported to be at 325 cm-1, which was different from the other studies. However, it

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indicated obviously that the kind of reaction solvent could influence the structures of

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CZTS nanoparticles and further studies were needed to understand the mechanism of the

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phase selection. Moreover, the optoelectronic properties of as-synthesized CZTS nanoparticles influenced by the kind of reaction solvent were also worthy to be well

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studied.

In this paper, a low-cost and more convenient one-pot solvothermal method was described to prepare pure phase CZTS nanoparticles. By adjusting the volume ratios of OAm and En, pure phase metastable wurtzite and kesterite CZTS nanoparticles were obtained. The structural, morphological, the mechanism of the phase selection and optoelectronic properties of CZTS nanoparticles influenced by the reaction solvents were studied, which were contributive to the development of high efficient, yet low cost solar cells.

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ACCEPTED MANUSCRIPT 2. Experimental details 2.1. Materials

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Ethanediamine (En), copper (II) chloride dihydrate (CuCl2·2H2O), Zinc (II) chloride,

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tin (IV) chloride pentahydrate (SnCl4·5H2O) and thioacetamide (99.999%) were all of analytical grade and purchased from Sinopharm chemical LTD (Shanghai). Oleylamine

2.2. Synthesis of CZTS nanoparticles

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(OAm) was purchased from J&K scientific LTD (Shanghai).

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In a typical process, 0.8 mmol CuCl2·2H2O, 0.48～0.52 mmol ZnCl2, 0.48～0.5 mmol SnCl4·5H2O, 3.48～3.54 mmol thioacetamide were dissolved in 18 mL of OAm or

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En under magnetic stirring, as shown in Table 1. Then the mixture was loaded into a

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Teflon-lined stainless-steel autoclave of 50 ml capacity. After the mixture was stirred for

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3mins, the autoclave was sealed and transferred to an oven set at 180 °C for 24 h. Finally, the solution was naturally cooled to room temperature. The precipitates were washed with

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absolute ethanol and toluene to remove excess solvent and unreacted reagents. Then, the particles were redispersed in toluene for further analysis. 2.3. Preparation of CZTS thin films The Mo/glass substrates were cleaned in an ultrasonic acetone bath for 10 min, and then rinsed with deionized water. Finally they were dried under N2 gas flow. The spin-coating solution was prepared by dispersing 0.1g CZTS nanoparticles in absolute ethyl alcohol (10ml). Then they were stirred in a sealed vessel for about 40 mins to form uniform CZTS nanoparticles solution. Once a time, 2ml of the solution was dripped on Mo/glass substrates and spun at 1500 rpm for 30 s. After the spin coating was repeated 5

ACCEPTED MANUSCRIPT for 5 times, the CZTS thin film was heated on a hot plate at 90 °C for 30 mins. 2.4. Characterization of CZTS nanoparticles

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The crystallographic structures of CZTS nanoparticles were identified by X-ray

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diffractometer (XRD, Rigaku D/max 220 kV, Cu Ka: k = 0.154 nm). Raman spectra (JY, H800UV) were measured to analyze the phase purities of CZTS nanoparticles. The morphologies of CZTS nanoparticles were characterized by scanning electronic

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microscope (SEM, FEI Sirion 200) and transmission electron microscopy (TEM, JEOL

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2010F). Chemical compositions of CZTS nanoparticles were determined by Energy Dispersive Spectrometry (EDS). The oxidation states of all the elements in the CZTS were

confirmed

by

X-ray

photoelectron

spectroscopy

(XPS,

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nanoparticles

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ESCALAB250Xi) analysis. To measure the absorption spectra, CZTS nanoparticles were

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dispersed in toluene by ultrasound for several minutes, then the final solution was transferred to the cuvette and measured with UV-Vis spectrophotometer (Jasco UV-570),

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respectively. The photocurrent-time responses of CZTS thin films on a Mo/glass substrates were tested in 0.5 mol/L H2SO4 solution by using a CHI660B electrochemical workstation under simulated AM 1.5 G spectrum at 100 mW/ cm2 (1 sun) illumination. The current density-voltage (J-V) characteristics of CZTS thin films were measured by using a Keithley 4200 semiconductor characterization system.

3. Results and discussion 3.1. Structural and morphological analysis of CZTS nanoparticles The atomic percentages of as-synthesized CZTS nanoparticles were investigated 6

ACCEPTED MANUSCRIPT according to EDS listed in Table 2. It is easy to find that the ratios of Cu/Zn/Sn/S are basically matched to the stoichiometric ratio of CZTS (2:1:1:4). The atomic ratios of

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Zn/Sn can be readily tuned by properly adjusting the ratios of metal precursors. When

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OAm is used, more ZnCl2 is needed to satisfy the proper composition ratio. The CZTS nanoparticles with Cu-poor/Zn-rich atomic compositions are suitable to achieve high-efficient solar cells [17]. Through EDS elemental mapping images in Fig. 1, Cu, Zn,

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Sn and S elements distribute uniformly in the selected area of CZTS nanoparticles

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without noticeable compositional variation. The several different selected areas of CZTS-A, CZTS-C and CZTS-E exhibit the similar results.

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Fig. 2a shows the XRD patterns of as-synthesized CZTS nanoparticles. When

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OAm is used as the solvent (CZTS-A), the diffraction pattern shows peaks at

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2=28.38°, 33.68°, 47.38° and 56.14°, which corresponds well with (112),(200), (220) and (312) planes of kesterite structured CZTS (JCPDS 26-0575). No other

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characteristic peaks are detected for impurities. The lattice parameters of CZTS-A (a = b = 0.5441 nm, c = 1.0787 nm) were calculated according to equation (1) (1)

The calculated results agree well with the cell parameters (a=b = 0.5427 nm, c= 1.0848 nm) of tetragonal kesterite structured CZTS [18]. With decreasing the amount of OAm and increasing the amount of En, wurtzite structure appears in the XRD pattern of as-synthesized CZTS nanoparticles. And the weak diffraction peak at around 32.80°, which corresponds to (200) plane of kesterite CZTS, disappears gradually. The X-ray diffraction peaks at around 28.30°, 47.28° and 56.04° correspond to (112), (220), (312) 7

ACCEPTED MANUSCRIPT planes of kesterite structured CZTS and (002), (110), (112) planes of wurtzite structured CZTS [19]. When only En is used, the major diffraction peaks are observed at 2θ values

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of 26.82°,30.32°, 39.37° and 51.67°, which correspond to (210), (211), (212) and (213)

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planes of wurtzite structured CZTS.

(2)

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Calculated by equation (2), the lattice parameters (a = b = 0.3849 nm; c = 0.6561 nm) of

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CZTS-E match well with the reported data of hexagonal wurtzite structured CZTS ( a = b = 0.38387 nm and c = 0.63388 nm) [16]. Therefore, with increasing contents of En from

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CZTS-A to CZTS-E, the kesterite phase gradually converts into the wurtzite phase. Due

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to similar X-ray diffraction patterns of kesterite CZTS with that of tetragonal Cu2SnS3

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(PCPDF no. 98-005-8919) and hexagonal ZnS (PCPDF no. 98-001-7525), the Raman spectra of the five samples were measured to further determine the purities of

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as-synthesized CZTS nanoparticles, as shown in Fig. 2b. Both of kesterite and wurtzite structured CZTS nanoparticles show the single Raman peaks at about 333 cm-1, which agrees well with other reported values [20,21]. In fact, most kesterite structured CZTS shows Raman peaks at 331~338 cm-1 and wurtzite structured CZTS shows Raman peaks at 331~333 cm-1. The little difference in the characteristic peaks between them may be caused by the different arrangements of the surrounding atoms inside their crystal cells. A1 vibration modes of kesterite structured CZTS locate at 338 cm-1, in which the S atom is in motion and Cu, Zn, Sn atoms are at rest. The structure of wurtzite CZTS is originated from wurtzite ZnS structure, which displays the A1 (LO) mode at 351 cm-1 8

ACCEPTED MANUSCRIPT [22]. In Fig. 2b, the broadening of the Raman peak is due to the phonon confinement within the nanoparticles [23]. No additional peaks for other phases such as ZnS (350

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cm−1) [24], SnS(196 cm-1, 217 cm-1, 353 cm-1) [25], CuS (472 cm-1) [26], Cu2S (475 cm-1)

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[27], Cu2SnS3 (336 cm-1, 351 cm-1) [28] and Cu3SnS4 (318 cm-1, 348 cm-1, 295 cm-1) [29,30] are found, which confirms the single phase of the CZTS nanoparticles. Fig. 3a shows the XRD pattern of as-synthesized CZTS nanoparticles annealed at

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500 °C in vacuum for 30mins. After the annealing process, the crystallinities of the CZTS

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nanoparticles thin films are greatly improved and the grain sizes are also increased obviously, as shown in Table 2. However, no trace of wurtzite phase CZTS is found and

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only kesterite phase CZTS is detected in the XRD patterns of all the samples. The single

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Raman peak at about 331 cm-1 also reveals the purity of kesterite phase CZTS in Fig. 3b.

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It indicates that the annealing process has resulted in the phase transformation from unstable wurtzite CZTS to more stable kesterite phase, which agrees well with the other

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reports [31,32].

To further understand the phase selection of CZTS nanoparticles influenced by the reaction solvent, time dependent EDS, XRD and Raman spectra of CZTS-A (a1,a2,a3) and CZTS-C (b1,b2,b3) were measured, as shown in Fig. 4. At the reaction time of 0.5 h, the EDS measurements of CZTS-A indicate that the content of Cu is very high (Fig. 4a1) and the XRD patterns show that there are weak diffraction peaks at around 31.9° and 52.6°, which correspond to (103) and (108) plane of the secondary phase of CuS (PDF#06-0464) (Fig. 4a2). In addition, some weak diffraction peaks are found at around 39.5° and 51.7°, which belong to the secondary phase of Cu3SnS4 (PDF#36-0217). The 9

ACCEPTED MANUSCRIPT strong Raman peaks at 472 cm-1 (Fig. 4a3) also confirm the secondary phase of CuS. With the extension of reaction time, the content of Cu decreases and the secondary phase of

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CuS disappears gradually. At the reaction time of 12 h, the atomic percentages of

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as-synthesized CZTS are close to stoichiometric ratios of 2:1:1:4 and the secondary phase of CuS disappears completely. But the secondary phase of Cu3SnS4 is still found, which can be confirmed by the Raman peaks at 318 cm-1. For CZTS-C, the EDS measurements

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suggest that the contents of Cu, Zn, Sn, and S are close to the stoichiometric ratio of

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2:1:1:4 relatively at the time of 0.5 h (Fig. 4b1). XRD (Fig. 4b2) and Raman spectra (Fig. 4b3) also indicate that wurtzite structured CZTS has formed at 0.5 h and the structures are

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not changed with the extension of reaction time. The reaction process agrees well with

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that when pure En is used [33]. As a strong bidentate ligand, EN binds tightly with M+n

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ions (M+n= Cu+2, Zn+2 and Sn+4) to form a stable [M(EN)2]+n coordination complex. On the other hand, EN acts as a reducing agent for the dissolved thioacetamide to produce

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S−2 ions. The stability of [M(EN)2]+n complex will decrease with increasing the reaction temperature. And [M(EN)2]+n ions reacts with S−2 ions to initiate the nucleation of CZTS when the temperature is above 120 °C (the boiling point of EN is 116~118 °C)[34]. When OAm is used, the reaction process is similar. The major difference is that the boiling point of OAm is 348 °C, the [M(OAm)2]+n complex may be relatively stable even at the reaction temperature of 180 °C. So, the reaction rates between [M(OAm)2]+n and sulphur precursor is relatively low. It has been reported that fast reaction between Zn2+ and sulfur precursors favours the formation of wurtzite CZTS while a slow reaction between them may lead to kesterite phase[15,35]. Our experiments results indicated that 10

ACCEPTED MANUSCRIPT the reaction rate between Zn2+ and sulfur precursors was really decreased when more En was used in the reaction process, which was contributive to form wurtzite CZTS.

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The morphologies of as-synthesized CZTS-A and CZTS-C nanoparticles were

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investigated by TEM, as shown in Fig. 5. When pure OAm is used as the solvent, the products are mainly small irregular particles (Fig. 5a). When 9 ml OAm and 9 ml En are used, some sheet-like particles are obviously mixed in the products of CZTS-C (Fig. 5d).

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The size distributions of both samples are not uniform. Small particles with sizes of about

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10 nm are mixed with large particles with sizes ranging from 40 nm to 100 nm. The high-resolution TEM images (Fig. 5 (b,e)) confirm the single crystalline nature of the

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as-synthesized CZTS-A and CZTS-C nanoparticles with high qualities. The lattice

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fringes of CZTS-A and CZTS-C with interplanar spacing of 0.312 nm and 0.331 nm are

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ascribed to the (112) plane of kesterite CZTS and (100) plane of wurtzite CZTS, respectively. The selected area electron diffraction (SAED) patterns of a random region

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of CZTS nanoparticles confirm that CZTS-A has tetragonal structure (Fig. 5c) and CZTS-C has hexagonal structure (Fig. 5f), which are consistent with the above XRD results.

3.2. Valance states and optical properties of CZTS nanoparticles X-ray photoelectron spectroscopy (XPS) was used to determine the stoichiometries and the oxidation states of the elements of CZTS-A and CZTS-E nanoparticles, as shown in Fig. 6. The peak of Cu 2p splits into 930. 70 (2p3/2) and 950.55 eV (2p1/2) and a peak splitting of 19.85 eV indicates Cu(I) (Fig. 6a) [36].The peaks of Zn 2p at 1044.9 and 1021.8 eV suggest the presence of Zn(II) with a peak splitting of 23.1 eV (Fig. 6b) [37] . 11

ACCEPTED MANUSCRIPT The Sn 3d5/2 and 3d3/2 peaks at 486.5 and 495 eV with a peak splitting of 8.5 eV confirm the Sn(IV) state, as shown in Fig. 6c [38]. The S(2p) peaks are located at 160.57 and

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161.72 eV, which match well with the 160-164 eV range of S in the sulfide phases (Fig.

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6d) [39,40]. XPS results confirm the phase purities of CZTS-A and CZTS-E. The UV-Vis absorption spectra of as-synthesized CZTS nanoparticles were also investigated, as shown in Fig. 7. Fig. 7a indicates that the UV-Vis absorption spectra of

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as-synthesized CZTS nanoparticles exhibit a broad absorption in the visible region and a

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tail extending to longer wavelengths. The energy gaps can be calculated with the following relation: αhν= K(hν−Eg)n (K is a constant, Eg is the energy gap, n=1/2 for

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direct transition, ν means infrared frequency, and h is Planck’s constant). Tauc's plots

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have been shown in the Fig. 7b. The band gaps were obtained by plotting (αhν)2 as a

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function of hν[41]. From the long wavelength extrapolation of the band edge, the band gap values of CZTS-A to CZTS-E were determined to be 1.52,1.51,1.46,1.43 and 1.44 eV,

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respectively. These values match well with the reported 1.4-1.6 eV values for the kesterite and wurtzite CZTS nanoparticles. The reduced bandgaps of CZTS-D and CZTS-E are mainly caused by the structural change of CZTS due to the solvent replacement, which agrees well with other reports[19]. 3.3 Deposition of CZTS nanoparticle thin films CZTS thin films with similar thicknesses were deposited on a Mo/glass substrates to evaluate the photoresponse of CZTS nanoparticles. From the morphologies of the CZTS nanoparticle thin films in a large scale (Fig. 8(a1-c1)), it can be found that there are more prominent big clusters on the surface of the CZTS thin films when OAm is used. With 12

ACCEPTED MANUSCRIPT increasing the amount of En, the prominent big clusters become less and smaller. The cross-sectional view SEM images of the CZTS nanoparticle thin films indicate that the

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film thicknesses are in the range of 3.5 to 4 microns before annealing, as shown in the

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inset of Fig. 8. The films are densely packed and the particles sizes are enhanced relatively after the annealing process at 500 °C in vacuum for 30 mins. 3.4 Photo-electrochemical measurement of CZTS nanoparticle thin films

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Fig. 9a shows the current densities swept from 0 to 400s at -0.4 V (vs. Ag/AgCl

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reference) with a 10 s period of chopped light (10 s light on and 10 s off). When light is on, the current densities increase suddenly and this perform as the curve’s sudden rising,

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while it is opposite to that when light is off. We call this a “high” current under

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illumination and a “low” current in the dark. After current density saltation, it will tend to

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stabilization quickly. Switching the light for many cycles, there is no significant photocurrent decay, which suggests that the switching for the light irradiation on/off is

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very nimble and reversible. This good photo-stability is beneficial to their applications in photovoltaic devices. Moreover, this photoresponse profiles enhance in turn from CZTS-A to CZTS-E. In the darkness, the current density is about 0.117 mA/cm2 and 0.242 mA/cm2 for CZTS-A and CZTS-E, respectively. Under illumination, the numbers are about 0.131 mA/cm2 and 0.364 mA/cm2 for CZTS-A and CZTS-E, respectively. The on/off switching current density ratio of CZTS-A is minimum (about 1.12) and the ratio for CZTS-E is maximum (about 1.50). After annealing process, it is easy to find that the on/off switching ratios of the five samples are close to each other, as indicated from their photoresponse curves in Fig. 9b. The ratios for CZTS-D (about 1.07) and CZTS-E (about 13

ACCEPTED MANUSCRIPT 1.07) are a little bigger than that of CZTS-A (about 1.03), CZTS-B (about 1.03) and CZTS-C (about 1.04). As known, wurtzite structured CZTS was reported to have better

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optoelectronic characteristics than kesterite structured CZTS [42]. Based on the analysis

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above, it is interesting to note that the photoresponse characteristics of CZTS-E are still better than that of CZTS-A after the annealing process, even though they have same kesterite structure. After annealing process, the tiny difference of the photoresponse

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characteristics from CZTS-A to CZTS-E may be caused by the bigger grain sizes of

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CZTS-E, as shown in Table 2, which is contributive to enhance its photoresponse

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Conclusions

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characteristics.

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In summary, CZTS nanoparticles with structural transform from kesterite to wurtzite can be solvothermal synthesized by changing the solvent ratios of OAm and En. The

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results indicated that En was contributive to the formation of wurtzite structured CZTS nanoparticles. XRD and Raman measurements confirmed the structural transitions and phase purities of CZTS nanoparticles. Photo-electrochemical measurement indicated that the CZTS thin films had obviously light response and stability. And CZTS nanoparticle thin films with wurtzite structure had better photoelectric properties.

Acknowledgements This work was supported by the National Key Basic Research Program of China (973 program, Grant No. 2012CB934300), the National Natural Science Foundation of China 14

ACCEPTED MANUSCRIPT (Grants No.61006089). We acknowledge Instrumental Analysis & Research Center of

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Shanghai University for the help of characterization works.

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[13] Y. Zhao , Q. Qiao , W.H. Zhou , X.Y. Cheng, D.X. Kou, Z.J. Zhou, S.X. Wu, Wurtzite Cu2ZnSnS4 nanospindles with enhanced optical and electrical properties, Chem. Phys. Lett. 592 (2014) 144-148. [14] X.T. Lu, Z.B. Zhuang, Q. Peng, Y.D. Li, Wurtzite Cu2ZnSnS4 nanocrystals: a novel quaternary semiconductor, Chem.Commun. 47 (2011) 3141-3143. [15] V.T. Tiong, Y. Zhang, J. Bell, H.X. Wang, Phase-Selective Hydrothermal Synthesis of Cu2ZnSnS4 Nanocrystals: The effect of sulphur precursor, CrystEngComm. 16 (2014) 4306-4313. [16] M. Li, W.H. Zhou, J. Guo, Y.L. Zhou, Z.L. Hou, J. Jiao, Z.J. Zhou, Z.L. Du, S.X. Wu, 17

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[17] S.Y. Chen, X.G. Gong, A. Walsh, S.H. Wei, Defect physics of the kesterite thin-film

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solar cell absorber Cu2ZnSnS4, Appl. Phys. Lett. 96 (2010) 021902 (3). [18] Y.L. Zhou, W.H. Zhou, Y.F. Du, M. Li, S.X. Wu, Sphere-like kesterite Cu2ZnSnS4 nanoparticles synthesized by a facile solvothermal method, Mater. Lett. 65 (2011)

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[19] H.C. Jiang, P.C. Dai, Z.Y. Feng, W.L. Fan, J.H. Zhan, Phase selective synthesis of metastable orthorhombic Cu2ZnSnS4, J. Mater. Chem. 22 (2012) 7502-7506.

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films by raman scattering, J. Alloys Compd. 509 (2011) 7600-7606.

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[22] W. Zhang, L.L. Zhai, N. He, C. Zou, X.Z. Geng, L.J. Cheng,Y.Q. Dong, S.M. Huang, Solution-based synthesis of wurtzite Cu2ZnSnS4 nanoleaves introduced by α-Cu2S nanocrystals as a catalyst, Nanoscale. 5（2013）8114-8121. [23] A. Singh, H. Geaney, F. Laffir, K.M. Ryan, Colloidal Synthesis of Wurtzite Cu2ZnSnS4 Nanorods and Their Perpendicular Assembly, J. Am. Chem. Soc. 134 (2012) 2910-2913. [24] L. Sun, J. He, H. Kong, F.Y.Yue, P.X. Yang, J.H. Chu, Structure,composition and 18

ACCEPTED MANUSCRIPT optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method, Sol. Energy Mater. Sol. Cells. 95 (2011) 2907-2913.

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[25] W. Wang, H.L. Shen , X.C. He, Study on the synthesis and formation mechanism of

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Cu2ZnSnS4 particles by microwave irradiation, Mater. Res. Bull. 48 (2013) 3140-3143. [26] O. Vigil-Galán, M. Espíndola-Rodríguez, MaykelCourel, X. Fontané, D. Sylla b, V. Izquierdo-Roca, A. Fairbrother, E. Saucedo, A. Pérez-Rodríguez, Secondary phases

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dependence on composition ratioin sprayed Cu2ZnSnS4 thin films and its impact on the

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high power conversion efficiency, Sol. Energy Mater. Sol. Cells. 117 (2013) 246-250. [27] Z.L. Hou, Z.J. Zhou, S.J. Yuan, W.H. Zhou, S.X. Wu, D.F. Xue, Magnetron

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Sputtering Route to EfficiencyEnhanced Cu2ZnSnS4 Thin Films as the Counter Electrode

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of Dye-Sensitized Solar Cells, Sci. Adv. Mater. 5 (2013) 1764-1769.

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ACCEPTED MANUSCRIPT Figure Captions: Table 1. Respective solvent volumes (mL) used in CZTS-A to CZTS-E.

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Table 2. Compositional (atomic percent) analysis and grain size calculated by

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corresponding XRD diffraction pattern of CZTS-A to CZTS-E.

Fig. 1. EDS elemental mapping images of CZTS-A (a), CZTS-C(b) and CZTS-E(c) nanoparticle thin films prepared by spray-coating method on Mo substrates.

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different solvent ratio before annealing.

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Fig. 2. XRD (a) and Raman (b) spectra of CZTS nanoparticles synthesized with

Fig. 3. XRD (a) and Raman (b) spectra of CZTS nanoparticles synthesized with

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different solvent ratio after annealing at 500 ˚C for 30 mins.

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Fig. 4. Time dependent EDS, XRD, and Raman of CZTS-A (a1,a2,a3) and CZTS-C

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(b1,b2,b3).

Fig. 5. Low-resolution TEM, HRTEM and SAED pattern of CZTS-A (a,b,c) and CZTS-C

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(d,e,f)

Fig. 6. High-resolution XPS of CZTS-A and CZTS-E. Fig. 7. UV-Vis absorption spectra of as-synthesized CZTS nanoparticles (a); the extrapolation of the spectra identifies the bandgaps of CZTS nanoparticles (b). Fig. 8. SEM charactrizations of CZTS-A (a1,a2), CZTS-C (b1,b2) and CZTS-E (c1,c2) nanoparticle thin films on Mo/glass substrates before (a1-c1) and after annealing process (a2-c2). The insets show the corresponded cross-sectional view SEM images. Fig. 9. Current-time response curves of as-synthesized (a) and annealed (b) CZTS (CZTS-A to CZTS-E) nanoparticle thin films 22

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CZTS-B

OAm vol/ml

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12.5

En

0

5.5

CZTS-D

CZTS-E

9

5.5

0

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12.5

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size

before

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Table 1

Metal and sulfur sources

Composition determined

Grain

/mmol Cu:Zn:Sn:S

by EDS

annealing process (nm)

process (nm)

1.77：1.13：1：3.80

11.56

33.02

1.98：1.02：1：3.66

13.41

38.84

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Cu:Zn:Sn:S

Grain size after annealing

1.81：0.86：1：6.89

CZTS-B

1.92：0.91：1：7.21

CZTS-C

1.97：0.99：1：7.24

1.91：0.95：1：3.55

13.22

37.83

CZTS-D

1.91：0.92：1：7.22

1.90：1.12：1：3.80

15.66

39.48

CZTS-E

1.89：0.90：1：7.17

1.83：1.06：1：3.79

16.76

42.09

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CZTS-A

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Table 2

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Figure 3

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Figure 4

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(100)

d=0.331 nm (100)

Figure 5

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d=0.312 nm (112)

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(102) (101)

Figure 6

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ACCEPTED MANUSCRIPT Graphical Abstract Cu2ZnSnS4 (CZTS) nanoparticles with bandgap of about 1.5 eV were synthesized

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by a simple solvothermal method with oleylamine (OAm) and ethanediamine (En) as the

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reaction solvents. Raman spectra and X-ray photoelectron spectroscopy examined the phase purities of CZTS nanoparticles. Kesterite structured CZTS were gradually changed into the wurtzite structured CZTS by varying the volume ratios of OAm and En, which

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was confirmed by X-ray diffraction measurements. Time-dependent experiments were

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performed to study the mechanism of the phase selection of CZTS, which indicated that En played an important role in the formation of wurtzite structured CZTS. Annealing

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process improved the crystallinities of CZTS nanoparticle thin films, but wurtzite

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structured CZTS was changed to more stable kesterite phase. Photo-electrochemical

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Highlights

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• Structural controlled Cu2ZnSnS4 nanoparticles were synthesized by solvothermal method.

• The reaction rate of Zn2+ with sulphur precursors determined the structure of

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Cu2ZnSnS4.

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• Cu2ZnSnS4 nanoparticles synthesized with ethanediamine have better optoelectronic

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properties.

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