(CZTS) Counter Electrodes for Efficient and Low Cost Dye-Sensitized

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An In-situ and Green Method to Prepare Pt-free Cu2ZnSnS4 (CZTS) Counter Electrodes for Efficient and Low-cost Dye Sensitized Solar Cells Shanlong Chen, Aichun Xu, Jie Tao, Haijun Tao, Yizhou Shen, Lumin Zhu, Jiajia Jiang, Tao Wang, and Lei Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00585 • Publication Date (Web): 03 Oct 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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ACS Sustainable Chemistry & Engineering

An In-situ and Green Method to Prepare Pt-free Cu2ZnSnS4 (CZTS) Counter Electrodes for Efficient and Low-cost Dye Sensitized Solar Cells Shanlong Chen, Aichun Xu, Jie Tao,* Haijun Tao,* Yizhou Shen, Lumin Zhu, Jiajia Jiang, Tao Wang, and Lei Pan College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. Corresponding Author Information *Telephone: +86 5211 2911. E-mail: [email protected]. *Telephone: +86 5211 2911. E-mail: [email protected]. KEYWORDS: TiCl4 pre-treatment, In-situ solvothermal growth, Kesterite-structure Cu2ZnSnS4, Counter electrodes, Nanosheets, Nanoparticles, Dye-sensitized solar cells. ABSTRACT: A novel in-situ and green method with the combination of TiCl4 pre-treatment and solvothermal growth was proposed to prepare the Pt-free Cu2ZnSnS4 (CZTS) counter electrodes (CEs) on FTO substrate. It was found that TiCl4 pre-treatment towards the substrates was crucial to high quality and high efficiency CZTS film, which was consists of nanosheets and nanoparticles structure. The highly crystallized nanosheets were assembled from kesterite-

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structure CZTS nanocrystals, with an average diameter of 10-30 nm. The detailed morphology, crystal structure, and composition of the CZTS nanostructures were characterized by SEM, TEM, SAED, XRD, Raman, and EDS analysis respectively. Electrocatalytic abilities of the films toward I-/I3- were verified through cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization measurements. A maximum power conversion efficiency of 5.65 % was achieved for a cell with CZTS 3, under 100 mW/cm2, which was higher than that of a cell with a commercial Pt counter electrode (4.96%). This high efficiency was mainly attributed to the good bonding strength between CZTS films and FTO substrate (RS=4.41 Ω cm2), and higher electron transfer process at the electrolyte/CE interface (high JSC), along with superior electrochemical catalytic ability (RCt=2.40 Ω cm2). The in-situ prepared CZTS CEs are proved to be suitable for high efficiency Pt-free dye-sensitized solar cells (DSSCs), leading to significantly decrease of the cell cost and simplification of preparation process. INTRODUCTION Dye-sensitized solar cells (DSSCs) have become a “top runner” among third-generation solar cells, in view of the advantages of low cost, environmental friendliness, easy fabrication, and relatively high efficiency (η=13%).1-3 The conventional DSSCs have a sandwich-type structure, consisting of a platinized fluorine-doped tin oxide (FTO) glass as the counter electrode (CE), a liquid electrolyte that traditionally contains I-/I3- redox couples as a conductor to electrically connect the two electrodes, and a porous-structured oxide film with adsorbed dye molecules as the photosensitized anode.2-9 As an essential and crucial component in DSSCs, the counter electrode (CE) behaves as a catalyst for the redox couples regeneration in the electrolyte, as well as an electron collector from the external circuit. The CE materials should possess two advantages of high catalytic activity and electrical conductivity.2,10-12 Generally, platinum (Pt)


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deposited on a transparent conducting oxide (TCO) such as indium tin oxide (ITO) or fluorinedoped tin oxide (FTO) has been always employed as the CE for DSSCs, presenting the high efficiency and long-term stability. However, on the viewpoint of cost cutting and commercial applications, Pt is not the appropriate CE catalyst due to the high cost and low abundance (0.0037 ppm). Several kinds of low-cost Pt-free materials have been proposed to be used as CE catalysts, including carbon materials (graphite,13 carbon nanotubes,14 graphene, etc.), conducting polymers (polyaniline,15 poly-(3,4-ethylene dioxythiophene) (PEDOT),16 etc.), and the inorganic compounds (transition metal in the form of oxides,17 nitrides,14 sulfides,18-20 and carbides21). Among the inorganic sulfide compounds, kesterite-structure Cu2ZnSnS4 (CZTS) is a quaternary chalcogenide p-type inorganic semiconductor, with the advantages of earth abundance (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm), low-toxicity of the materials, a direct band gap of 1.5 eV, and high absorption coefficient (>104 cm-1), and it has been proved to be a high catalytic material. Recently, many approaches have been developed to fabricate CZTS thin films, such as vacuum based deposition (evaporation and sputtering), chemical vapor deposition, electrodeposition, and solution-based processing. Meanwhile, thin films of CZTS/CZTSe have been applied as effective photocathode materials to replace Pt in low-cost DSSCs. Generally, CZTS nanoparticles synthesized by a solution based method, then spin coated or drop casted on FTO substrate, were always used as the CE in DSSCs.20,21-25 CZTS powders with various hierarchical nanostructures prepared by solvothermal method were spray-casted onto FTO as a thin film. The above CZTS thin film, under following 500 oC sulfurization heat treatment, exhibited both high electrocatalytic activity comparable to Pt, and efficient reduction of I3- in the electrolyte, showing an efficiency of 6.98% (Pt=6.91%).22 At the same time, similar Cu2ZnSnSe4 (CZTSe) nanoparticles with diameters of 200-300 nm synthesized by solvothermal were drop-casted on



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FTO substrate as CE, yielding an efficiency of 3.85%.23 A spin-coated method with post annealed in an N2 gas was used to prepare ultrathin CZTS thin film CE, with the η of 5.63%.24 In addition, porous CZTS thin film was in-situ prepared on Mo substrate via a solvothermal approach, with 500 oC sulfurization following, yielding an efficiency of 1.23% (Pt= 1.15%).25 However, most of the above reported strategy has a number of limitations, such as multiple steps (relying on coating method and post-annealing), and rigorous synthesis conditions (excessive organic ligands or sulfurization).21-26 Therefore, it is still a great challenge to develop a simple, environment friendly and feasible method to synthesize stable CZTS thin film on FTO substrate. In this paper, it is firstly reported that the CZTS thin film, composed of nanosheets and nanoparticles, is directly grown on FTO glass substrate as Pt-free CE, via an in-situ and one-step low-cost solvothermal method, with the assistance of TiCl4 pre-treatment. An efficiency of 5.65% with CZTS CE film proves the effectiveness of the TiCl4 pre-treatment, possessing the advantages of easy deposition, increasing chemical stability and mechanical stability. Furthermore, it is potential to apply this novel and environment friendly method to in-situ synthesize thin films of other chalcopyrite semiconductors. EXPERIMENTAL DETAILS Materials. Stannic (IV) chloride pentahydrate, thiourea, lithium perchlorate (LiClO4), anhydrous ethanol, acetonitrile, and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. Cupric (II) acetate anhydrous, zinc (II) acetate anhydrous, lithium iodide (LiI), and iodine (I2) were purchased from Aladdin Industrial Corporation. Titanium (IV) tetrachloride was purchased from Shanghai Meixing Chemical Reagent Corporation. FTO glass (sheet resistance 7~8 Ω sq-1),


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N719 dye (0.3 mM (Bu4N)2[Ru(dcbpyH)2(NCS)2]), and electrolyte (DHS-Et23) were purchased from OPV Tech Co., Ltd (China), Dyesol (Australia), and HeptaChroma (China) respectively. All the used reagents were analytical grade, without further purification. Preparation of Cu2ZnSnS4 (CZTS) Precursor Solution. Cupric (II) acetate anhydrous, zinc (II) acetate anhydrous, Stannic (IV) chloride pentahydrate and thiourea were added in anhydrous ethanol with sonification and stirring to achieve transparent as-prepared precursor solution. The concentration of the Cu, Zn, Sn precursors was fixed at 0.2 mmol, 0.1 mmol, 0.1 mmol, and the S precursor (0.8 mmol) was kept higher than stoichiometric ratio for the complete sulfurization of the compound. In order to obtain different thickness of the CZTS thin film, the as-prepared precursor was diluted into different concentration of CZTS alcohol solution, through adding different volume of anhydrous ethanol (as shown in table 1). Table 1. The CZTS Samples under In-situ Solvothermal for Different Precursor Concentrations Counter Electrode CZTS 1 CZTS 2 CZTS 3 CZTS 4 CZTS 5

CZTS precursor:Anhydrous ethanol (ml) 35:0 30:5 25:10 20:15 15:20

Reaction temperature/time (oC/h)

200/24

Fabrication of CZTS Counter Electrodes (CEs). CZTS/FTO CEs were prepared by an in-situ solvothermal method, with TiCl4 pre-treatment, as shown in Scheme 1. The FTO glass (2×1.5 cm2) as conductive substrate was first pre-treated with 0.05 M TiCl4 aqueous solution at 70 oC for 30 min, subsequently rinsed with water for several times and dried under air stream, while the treated area was 1.5×1.5 cm2. Then, the



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above-substrate was placed in the bottom of the autoclave, with the conductive area facing up. The CZTS precursor was added to autoclave up to 70% of the total volume (50 mL) subsequently. The autoclave was sealed and maintained at 200 oC for 24 h under autogenous pressure and then allowed to cool to room temperature naturally. After CZTS growth processes, the as-prepared film was washed with ethanol and dried at 70 oC without annealing, then the CEs with nanostructure were thus obtained. Pt CE was purchased from Dyesol (Australia) for comparison.

Scheme 1. Schematic representation of in-situ solvothermal synthesis of CZTS CEs. DSSCs Device Fabrication. For a fair comparison, all the DSSCs were fabricated with TiO2 photoanode materials relying on a standardized fabrication process. To clean the FTO substrate, it was sequentially washed with a detergent solution, distilled water, acetone, and ethanol in an ultrasonic bath for 15 min. The dry substrate was subsequently subjected to TiCl4 aqueous solution (0.05 M) treatment for the duration of 30 min at 70 oC. A commercial TiO2 paste (P25, Degussa, Germany) was coated on a clean FTO substrate using doctor blading method. A portion of 0.25 cm2 was selected as the active photo-anode area. The TiO2-coated FTO glass was gradually heated to 500 oC (rate=2


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o

C/min) in air and subsequently sintered at that temperature for 30 min to create TiO2 film. The

total thickness of the TiO2 film was about 18 µm (as presented in Figure S1). After the heat treatment, the substrate was subjected to TiCl4 aqueous solution treatment again to acquire more dye absorbing. Thereafter, the 80 oC baked electrode was immersed in solution, containing 0.3 mM N719 dye in anhydrous ethanol for 24 h at room temperature under dark condition. The TiO2 electrode was coupled with one of the various CEs (CZTS and commercial Pt) to fabricate the DSSCs. The electrolyte was injected into the gap between the two electrodes by capillarity, and the hole was sealed with hot-melt glue after the electrolyte injection. Characterization and Measurements. The crystalline phase and purity of CZTS samples were examined by a X-ray diffractometer (RIGAKU, Smartlab TM9 KW, Cu Kα radiation at λ=1.541 Å,) operated at 40 kV/200 mA, and a confocal Raman microscope (Horiba Jobin Yvon HR800) using the 488 nm laser line of an air cooled Ar-ion laser. The morphologies of synthesized nanostructures were observed by using field-emission-scanning electron microscopy (FE-SEM, HITACHI, SU-4800). Energy-dispersive spectroscopic (EDS) mapping and spot scanning from FE-SEM were also used for the elemental analysis of the CZTS samples. The detailed nanostructures and crystal structure were further investigated by a transmission electron microscopy (TEM, FEI Tecnai G2), coupled with selected area electron diffraction (SAED). The thin film was scraped from their substrates and dispersed in ethanol to form the suspensions for TEM observation. The assembled DSSCs were illuminated by a solar simulator (94042A, AM1.5 G, Newport, America), and the incident light intensity (100 mW/cm2) was calibrated with a standard Si cell (91150V, Newport, America), so as to measure the J-V curves of the solar cells devices. The electrochemical measurements including cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and



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Tafel polarization measurements, were conducted by means of a CHI 660E potentiostat. The CV tests were performed at a scan rate of 100 mV s-1 in acetonitrile solution consisting of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 in a three-electrode system, in which the as-fabricated CEs acted as the working electrode, in addition to a Pt sheet (6 cm2) counter electrode and a Ag/Ag+ couple served as a reference electrode. The EIS tests of the symmetric thin layer CEs were carried out in the frequency ranging from 0.05 to 106 Hz with perturbation amplitude of 5 mV, under dark condition. The applied bias voltage, between the counter electrode and the working electrode, was set at the open-circuit voltage (VOC) of the DSSCs. The obtained impedance spectra were fitted with Zview software based on appropriate equivalent circuit. For Tafel polarization measurements (symmetrical CEs), the scan rate was set at 10 mV s-1. RESULTS AND DISCUSSION It is well known, the quality of counter electrode (CE) is of great concern for highly efficient DSSCs, including chemical stability, mechanical stability and catalytic activity toward the electrolyte. Amongst, mechanical stability is the fundamental requirement for the stable and high-efficiency CZTS electrodes. Figure 1(a)(b) shows the photos of CZTS films after 24 hour solvothermal growth on the FTO substrate with or without TiCl4 pre-treatment, respectively. It is clearly seen that, when the substrate is pre-treated by TiCl4, a homogeneous CZTS film is achieved in comparison with the untreated FTO glass, and the thicker films are acquired with the CZTS precursor concentration increasing (as demonstrated in Figure S3). Moreover, the CZTS film could barely grow on FTO glass, if the substrates are not treated by TiCl4, as shown in Figure 1(b). It has been widely reported by other researchers, the TiCl4 pre-treatment will increase the surface roughness of the substrate by forming a thin TiO2 seed layer and it is beneficial to generate an even film with strong contact with the substrate.27,28 Furthermore, the


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pre-treatment has increased the stability of the films in the iodic electrolyte. Besides, the asprepared films cannot be easily scratched off, indicating that the film surface is mechanically robust (good adherence to the substrate).29 FE-SEM images of CZTS films grown by in-situ solvothermal method are shown in Figure 1(c-k). It is observed that the CZTS films present the morphology of interconnected nanosheets. The hierarchical nanosheets structure could provide multiple channels to transfer the carriers from their surface to the conducting FTO substrate,30 and increase the optical path to capture photons efficiently. For the CZTS 5 precursor concentration, CZTS is spread continuously and uniformly on the FTO glass as interconnected nanosheets, with a thickness of 15-35 nm and a length of ~500 nm (seeing in Figure 1 (k)). It is interesting that the CZTS nanoparticles gradually cover the entire top surface of the films on increasing the precursor concentration, as shown in Figure 1 (c-i). The top layer of the CZTS film is composed of clusters of nanoparticles, with a diameter of 100-140 nm. The CZTS thin film becomes porous and uniform. This specific structure could extend the specific surface area for the catalytic site. The structural properties of the as-synthesized CZTS CEs are illustrated in Figure 2. The observed diffraction peaks show that the kesterite-structure CZTS films (CAS 26-0575) are obtained during solvothermal process. The three strong diffraction peaks around 28.5o, 32.9o, 47.3o and 56.2o are assigned to those crystal faces of (112), (200), (220) and (312), respectively. In XRD pattern of the CZTS 3-5, the diffraction peaks intensities present considerable increase, and the possible reason for this observation is ascribed from more crystalline nanoparticles covering on the top surface. Furthermore, it is noteworthy that the XRD profiles of tetragonal Cu2SnS3 (CAS 89-4714) and cubic ZnS (CAS 80-0020) are similar to that of kesterite CZTS phase.22,31,32 The further structural information of the sample on macroscopic scale is obtained



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from Raman spectrum. The peaks from Cu2SnS3 (352 cm-1), ZnS (346 cm-1), and Cu2-xS (475 cm-1) phases are not observed in Figure 2 (b). The biggest Raman peak at 338 cm-1 are from kesterite CZTS phase.22,31,32 These results exclude the presence of other binary or ternary sulfide compounds, with only CZTS existing, which are similar to those in previous reports.31,33 Hence, the pure kesterite-structure CZTS CEs are obtained through an in-situ solvothermal method.

Figure 1. The photos of CZTS films on FTO substrate with (a) and without (b) TiCl4 pretreatment, FE-SEM image of the CZTS films of CZTS 1 (c), CZTS 2 (f), CZTS 3 (i), CZTS 4 (j), CZTS 5 (k) at low magnification, and CZTS 1 (d,e), CZTS 2 (g,h) at high magnification.


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Figure 2. The XRD patterns (a) and Raman spectra (b) of the CZTS nanostructures obtained at various CZTS precursor concentrations. The detailed structure and morphology of the nanosheets and nanoparticles were further investigated by TEM. It is interesting that an individual CZTS nanosheet consists of numerous interconnected nanocrystals with an average diameter of 10-30 nm, as shown in Figure 3(a). As seen in Figure 3 (g), the CZTS nanosheet is compact and the nanocrystals combine each other to form a whole unity. The interconnected nanocrystals provide more catalytic site for I-/I3electrolyte to improve the solar cells performance. The selected-area electron diffraction (SAED) of the nanosheet shows that the CZTS nanosheet is highly crystallized, with the diffraction spots indexed as (112), (220), and (312) planes respectively. The single crystalline structure facilitates faster electron transport than the polycrystalline structure. The excellent crystallization characterization of as-prepared CZTS could benefit for the higher chemical catalytic activity. As for the CZTS nanoparticles, it is in an irregular shape with diameter of 50-60 nm and consists of abundant of aggregation nanocrystals (diameter ~10 nm). More and smaller particles absorb each other to get bigger nanoparticles cluster. Furthermore, there is some space between the incompact crystals and it is in the characteristic of loose shape, as shown in Figure 3(h). Apparently, the SAED pattern of the nanoparticles, corresponding to the polycrystalline CZTS kesterite structure with the presence of the major diffraction rings (112), (105), and (312), is shown in Figure 3(d). A blurry diffraction ring also appears in SAED pattern of nanosheets, corresponding to (112) plane, as seen in Figure 3(b). It may be inferred that the nanosheet is grown from the nanoparticles, with more nanocrystals connecting together tightly and growing in nearly same direction. The representative HR-TEM images taken from the edge of nanosheet and nanoparticles are shown in Figure 3(e)(f). The parallel lattice fringes across almost all of the



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nanosheets are clearly observed, corresponding to the (112) lattice fringes. These results are well in accordance with the perfectly oriented aggregation (oriented attachment) between the nanocrystals, with assistance of the FTO substrate. Notably, all the elements are homogeneously distributed over the whole area of the nanosheets and nanoparticles, as shown in Figure S2, S4 and Table S1. 34,35

Figure 3. The TEM image and SAED pattern of nanosheets (a,b) and nanoparticles (c,d), HRTEM image and schematic representation of the CZTS nanosheets (e,g) and nanoparticles (f,h). The current density-voltage (J-V) curves of DSSCs assembled with the same batch of photoanodes and CZTS, Pt CEs were recorded. The detailed photovoltaic parameters were summarized in Table 2, and every CE sample was measured with three cells. The solar cell based on the CZTS 5 shows a short circuit current density (JSC) of 10.74 mA cm-2, an open-circuit voltage (VOC) of 0.78 V, a low fill factor (FF) of 0.37, and an unexpected η of 3.15. This is probably because the film is too thin (~350 nm thickness) to provide enough catalytic sites for I3reduction, as demonstrated in Figure 1 and S3. With the extension in the precursor concentrations, the efficiency becomes higher and reaches the highest value of 5.65% with the CZTS 3 (~580 nm thickness), higher than the efficiency (4.96%) of DSSCs based on commercial


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Pt. This is because the more CZTS nanoparticles are well-dispersed on the top surface of the substrate. The increased number of active catalytic sites of CZTS nanoparticles and faster electron-transfer pathway of nanosheets enhance the I3- reduction and photogenerated electrontransfer at the counter electrode/redox electrolyte interface.36,37 Furthermore, the DSSCs based on the nanosheets and nanoparticles composite have the higher FF (up to 0.63). It is suggested that the CZTS 3 has the lower diffusion impedance for the redox species, which could be verified in Figure 8 later. With a continued extension in precursor concentrations, the efficiency and FF becomes lower, which is attributed to the weak conductivity arising from the thicker CZTS film.38

Figure 4. J-V curves of the DSSCs with the films of CZTS 1, CZTS 2, CZTS 3, CZTS 4, CZTS 5, and commercial Pt measured under a light intensity of 100 mW/cm2. Table 2. Photovoltaic parameters for different CEs. Counter Electrode Pt CZTS 1 CZTS 2 CZTS 3 CZTS 4 CZTS 5

JSC (mA/cm2) 10.70±0.30 12.11±0.40 10.20±0.25 12.45±0.34 9.19±0.38 10.74±0.43

VOC (V) 0.81±0.05 0.75±0.03 0.76±0.02 0.72±0.03 0.80±0.02 0.78±0.03


FF 0.58±0.02 0.48±0.05 0.51±0.03 0.63±0.02 0.47±0.03 0.37±0.03

η (%) 4.96±0.02 4.46±0.04 3.97±0.02 5.65±0.02 3.45±0.03 3.15±0.03


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Schematic diagram of possible electron transfer and recombination of the TiO2/CZTS cells is also presented in Figure 5. The green arrows indicate the possible electron transfer and the dashed yellow arrows show the possible electron recombination. As for the normal DSSCs device, VOC (0.7-0.8 V) is usually determined by the Fermi energy around the conduction band (ECB) of TiO2 (ECB: -0.5 V vs. normal hydrogen electrode (NHE)), and the oxidation reduction potential of I-/I3- (0.33 V vs. NHE).39,40 But, VOC of the TiO2/CZTS DSSCs is determined by the quasi-Fermi levels above the valence band (VB) of CZTS and below the CB of TiO2.41 In many cases of p-type bulk semiconductors, the flat band potential (EFB) from the current-voltage curve of the photocurrent response can be comparable to the VB.42 The reported EFB of CZTS thin film is roughly 0.3 V vs. NHE, thus, VB of CZTS is roughly 0.3 V vs. NHE.25,42 As the quasi-Fermi level is lower than the oxidation reduction potential of I-/I3-, the VOC of the TiO2/CZTS DSSCs is slightly lower than that of the TiO2/Pt DSSCs, resulting in a decrease of VOC from 0.81 V to ~0.72 V.

Figure 5. Scheme diagram for the TiO2/CZTS dye-sensitized solar cells. The long-term stability test of DSSCs based on CZTS 3 CE was implemented for 8 days, as shown in Figure 6. The device was stored in ambient air, under dark condition. After 8 days, the parameters of the FF, and efficiency retained 95, and 93% of their initial values, respectively. It


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is thus indicated that, the CZTS CEs synthesized by in-situ solvothermal method, could be further applied to industrial application.

Figure 6. Long-term stability test for DSSCs based on CZTS 3 CE.

Figure 7. Cyclic voltammograms for CZTS 3 and Pt counter electrodes at a scanning rate of 100 mV s-1 in the voltage range of -2.0 to 2.0 V vs. Ag/AgCl at room temperature. To investigate the electrocatalytic activity for CZTS counter electrode, cyclic voltammetry (CV) was carried out in a three-electrode system. As shown in Figure 7, the cyclic voltammogram curve of the CZTS 3 CE displays obvious cathodic peaks and similar shape of the curve for CZTS to that of Pt determines an analogous electrochemical behavior. It is deduced that the insitu CZTS thin film is an excellent choice to replace the expensive Pt in DSSCs. EIS experiments, constructed with two identical CEs (CE/electrolyte/CE), were used to further study the electron transport behavior at the interface of the CE/iodic electrolyte. The typical



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Nyquist plots and the equivalent circuit are shown in Figure 8 (a). The equivalent circuit consists of a series resistance (RS), indicated by the high frequency nonzero intercept of the real axis, and the charge transfer resistance (RCt) parallel with the corresponding constant phase element (CPE) at the CE/electrolyte interface.43,44 The RS is mainly composed of the bulk resistance of CE materials, resistance of FTO glass substrates, and contact resistance, etc. All the Nyquist plots were fitted using Z-view software, summarized in Table 3. The RS values of the different CZTS CEs are 5.20, 6.78, 4.41, 5.74, and 5.83 Ω cm2, respectively, which are much smaller than the RS value of the commercial Pt electrode (10.67 Ω cm2). The CZTS 3 CE has the smallest RS value, reflecting a good bonding strength between CZTS films and FTO substrate. Thus, the lightgenerated electrons could effectively transfer from catalytic material to electrolyte.43,45 The RCt values of CZTS CEs are 2.91, 2.98, 2.40, 8.88, and 109.02 Ω cm2, respectively. The lower value of RCt usually corresponds to an increase in the electrocatalytic activity of the CE, resulting in acceleration of the higher electron transfer process at the electrolyte/CE interface.43,45,46 This result demonstrates that the CZTS 3 electrode has a superior electrocatalytic activity than that of Pt CE (3.83 Ω cm2) for I-/I3- redox reaction, further proving that this CZTS nanostructures are good candidate for Pt-free CE fabrication in DSSCs, with a smaller charge-transfer resistance in electrolyte. The electrocatalytic activity of the CEs was further investigated by Tafel polarization measurement in iodic electrolyte. The slope of the cathodic and anodic branches of the CZTS 3 CE shows larger values than that of Pt. The elevated slope of the CZTS 3 CE shows a higher exchange current density (J0) and limiting current density (Jlim), which is in good agreement with the above EIS values, in terms of Eq. (1).47 The J0 value is directly related to the RCt value and is calculated by the following Eq. (1). J0=RT/nFRCt

(1)


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where R is the gas constant, T is the absolute temperature in K, n is the number of electrons involved in the electrochemical reduction of triiodide in the electrode, F is the Faraday constant, and RCt is the charge transfer resistance. A larger J0 generally demonstrates a better catalytic ability to reduce I-/I3- electrolyte and a larger Jlim presents higher diffusion coefficient. More importantly, the high JSC value for the DSSCs with CZTS 3 CE can be also attributed to the decrease in the internal resistance elements (RS and RCt). However, CZTS 4 and 5, possessing the poorer electrical conductivity, have the lower exchange current density and it is agreement with worse catalytic ability as well as unsatisfactory photovoltaic performance (as shown in Figure 4 and 8 (a)). Therefore, the optimized CZTS thin film, comparable to Pt, exhibits high photovoltaic performance, outstanding electrochemical properties and has a great potential for the fabrication of highly efficient DSSCs.

Figure 8. Nyquist plots of EIS (a) and Tafel polarization curves (b,c,d) for symmetric cells assembled with CZTS coated FTO and Pt CEs.



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Table 3. Electrochemical parameters of different CEs. Counter Electrode Pt CZTS 1 CZTS 2 CZTS 3 CZTS 4 CZTS 5

RS (Ω cm2) 10.67 5.20 6.78 4.41 5.74 5.83

RCt (Ω cm2) 3.83 2.91 2.98 2.40 8.88 109.02

CONCLUSIONS A novel feasible method with the combination of TiCl4 pre-treatment and an in-situ solvothermal growth has been developed to prepare the CZTS electrodes on FTO substrate for the first time. It was found that the TiCl4 pre-treatment was indispensable to the formation of homogenous and continuous CZTS films with good mechanical stability and high-efficiency. The introduction of the kesterite-phase CZTS nanosheets and nanoparticles structure morphology to the counter electrode provided multiple channels to transfer the carriers from their surface to substrate and extended the specific surface area for the catalytic site. An optimized concentration of the precursor solution was acquired and the corresponding CZTS counter electrode exhibited efficiency of 5.65%, which was high than the efficiency (4.96%) of DSSCs based on commercial Pt. The fabrication ideas reported here could be further adapted to prepare other efficient counter electrodes for not only DSSCs but also QDSSCs. ASSOCIATED CONTENT Supporting Information Cross-sectional SEM images of the TiO2 photoanode. SEM image of CZTS 3 films and corresponding EDS elemental mappings for Cu, Zn, Sn, and S. The cross-sectional SEM images of the CZTS films for CZTS 1 (a), CZTS 2 (b), CZTS 3 (c), CZTS 4 (d), and CZTS 5 (e). SEM image (a) of CZTS 3 film, and elemental spectrum (b-f) of several dots on the thin film.


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Elemental ratios of several dots on the CZTS 3 thin film. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Telephone: +86 5211 2911. E-mail: [email protected]. *Telephone: +86 5211 2911. E-mail: [email protected]. Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grant no. 51202112), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Foundation of Graduate Innovation Center in NUAA (Grant no. KFJJ201442), the Fundamental Research Funds for the Central Universities (Grant no. NJ20150027), and the Open Fund of Jiangsu Key Laboratory of Materials and Technology for Energy Conversion (Grant no. MTEC-2015M04). ` REFERENCES (1) Mathew S.; Yella A.; Gao P.; Humphry-Baker R.; Curchod B. F. E.; Ashari-Astani N.; Tavernelli I.; Rothlisberger U.; Nazeeruddin Md. K.; Grätzel M. Dye-sensitized solar cells with



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For Table of Contents Use Only

An In-situ and Green Method to Prepare Pt-free Cu2ZnSnS4 (CZTS) Counter Electrodes for Efficient and Low-cost Dye Sensitized Solar Cells Shanlong Chen, Aichun Xu, Jie Tao,* Haijun Tao,* Yizhou Shen, Lumin Zhu, Jiajia Jiang, Tao Wang, and Lei Pan A novel in-situ and green method with the combination of TiCl4 pre-treatment and solvothermal growth was proposed to prepare the Pt-free Cu2ZnSnS4 counter electrode on FTO substrate.


(CZTS) Counter Electrodes for Efficient and Low Cost Dye-Sensitized

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