Selenization of Cu2ZnSnS4 Enhanced the Performance of Dye

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Selenization of Cu2ZnSnS4 enhanced the performance of dyesensitized solar cells: improved Zn-site catalytic activity for I3Xiuwen Wang, Ying Xie, Buhe Bateer, Kai Pan, Yanqing Jiao, Ni Xiong, Song Wang, and Honggang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09642 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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ACS Applied Materials & Interfaces

Selenization

of

Cu2ZnSnS4

enhanced

the

performance

of

dye-sensitized solar cells: improved Zn-site catalytic activity for I3Xiuwen Wang, Ying Xie, Buhe Bateer, Kai Pan,* Yanqing Jiao, Ni Xiong, Song Wang, and Honggang Fu* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, Harbin 150080, People’s Republic of China Tel.: +86 451 86604330; Fax: +86 451 8667 3647; E-mail: [email protected], [email protected] KEYWORDS: Cu2ZnSn(S, Se)4, catalytic performance, dye-sensitized solar cells, the adsorption energy, the work function ABSTRACT Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S, Se)4 (CZTSSe) as promising photovoltaic materials have drawn much attention because they are environmentally benign and earth abundant elements. In this work, the monodispersed, low-cost Cu2ZnSnS4 nanocrystals with small size have been controllably synthesized via a wet chemical routine. And CZTSSe could be easily prepared after selenization of CZTS. When they are employed as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs), the power conversion efficiency (PCE) has been improved from 3.54 % to 7.13 % as CZTS converted to CZTSSe, which is also compared to that of Pt (7.62 %). The exact reason for the enhanced catalytic activity of I3- is discussed with the work function and density functional theory (DFT) when CZTSSe converted from CZTS. The results of Kelvin probe suggest that the work function of CZTSSe (5.61 eV) is closer to that of Pt (5.65 eV), and higher than that of CZTS, which matched the redox shuttle potential better. According to the theory calculation, all the atomic and bond populations changed significantly when Se replaced partly of S on CZTS system, especially in Zn site. During the catalytic process as CEs, the adsorption energy obviously increased than other sites when I3- adsorbed on the Zn site in CZTSSe. So, Zn plays an important role for the reduction of I3- after CZTS converted to CZTSSe. Based on above analysis, the reason of enhanced performance of DSSCs when CZTS converted to

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CZTSSe is mainly due to the enhancement of Zn-site activity. This work is beneficial to understand the catalytic reaction mechanism of CZTS(Se) as CEs of DSSCs. 1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have received much attention due to their low cost, easy fabrication, and respectable power conversion efficiency (PCE).1-4 The counter electrode (CE) is a critical component of DSSCs that plays an important key role in catalyzing the reduction of I3- to I- to regenerate the sensitizer. Generally, CE with both superior catalytic activity and good conductivity is indispensable for getting better PCE.5 Platinum (Pt) has been widely used as CE material for DSSCs owing to above-mentioned characteristics.6, 7 However, the use of Pt is limited by its relatively scarce and resulting expensive price. Recently, extensive effort has been made to develop low-cost and high-efficiency platinum-free CE materials. Among them, transition metal sulfides have been explored as CE owing to both their excellent electrocatalytic activity.8-10 Among these transition metal sulfides, Cu2ZnSnS4 (CZTS), Cu2ZnSn(S, Se)4 (CZTSSe) and Cu2ZnSnSe4 are a class of earth-abundant and environmental friendly materials,11 which has been regarded as one of the most promising photovoltaic materials in solar-energy conversion and energy storage.12-14 One key attention is derived from positive synergetic effect of different components in CZTS(Se), which is more conducive to improve the catalytic activity in solar cell. Besides, it could significantly cut down the cost of cell. In the recent years, there have lots of reports about CZTS(Se) as CEs for solar cell.15-27 According to the reported papers, CZTS(Se) could be prepared with different structure and morphology (nanocrystals16, 20, two-dimensional plate arrays17, hierarchical nanostructures21, nanofibers22, and nanospheres23) using different methods (hot-injection16, solvothermal process23,

26, 27

, spray deposition25,

electrospun22, and precursor process26), which effected the catalytic performance for reduction of I3-. Wozny17 et. al directly synthesized CZTS nanoplates on a FTO substrate as an efficient CE for DSSCs, the PCE of it is comparable to that of a Pt. Fan16 et. al prepared the CZTS with various [Cu]/[Sn]+[Zn] molar ratio, which show superior catalytic activity for I3-. Chen18 et. al focuses on the improvement of crystallization of CZTSSe film for better catalytic activity for I3- in DSSCs. Xin15 et. al reported CZTS nanocrystals (NCs) followed by selenization to obtained CZTSSe, and em-

2


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ployed it as CE for DSSC, the PCE significantly increased. The above studies have made achievements in the improved performance via employing CZTS or CZTSSe as CE for DSSCs. Although CZTS(Se) showed excellent catalytic ability for I3-, the exact reason for good electro-catalytic activity of CZTS(Se) is not clear, and the role of each atom of the material is also not be clarified. It must be greatly meaningful to explore catalytic mechanism of each active site in an effective way. The first-principles calculations based on the density functional theory (DFT) is an extremely effective method to investigate the catalytic mechanism at the I3- molecules/catalyst interface by the adsorption energy, density of states (DOSs), and electron density difference (EDD). Wang and co-workers have investigated the charge transfer and adsorption energy between FeS2 nanocluster and I3- molecule to explain catalytic activity of FeS2 for I3- in DSSCs.9 Our group28 has previously investigated the adsorption energy of I3- complex on the (111) surface of α-NiS. Furthermore, the active sites and catalytic ability of FeS with different facets were also evaluated.29 The results indicated that the (001) facet of FeS possesses a higher reactive site density and a relatively higher single catalytic site capacity for the reduction of I3-. The results above suggested that theoretical calculations are very helpful for elucidating the catalytic mechanism, especially when the conventional experimental ways are not available. Herein, CZTS and CZTSSe were synthesized and used as CE material for DSSCs. The PCE of DSSC from 3.54 % soared to 7.13 % when CZTS converted to CZTSSe, which is compared to that of Pt-based DSSCs (7.62 %). DFT calculation was used to investigate the role of each element of the CZTS(Se) for excellent catalytic activity for I3- in DSSCs, which is our greatest concern. When I3- adsorbed on Sn, Cu, and Zn site of CZTSSe, the adsorption energy on Sn and Zn sites increased compared to that of CZTS. And, the adsorption energy on Zn site in CZTSSe increased most than other sites. The hybrid interactions between p orbital of I and 3d states of Zn revealed by the DOSs and EDD, which indicated that I1-I2 bond were effectively activated and dissociated. Therefore, selenization resulted in the enhanced catalytic activity of Zn-site for I3- when CZTS converted to CZTSSe. This is because the electro-negativity of Se is weaker than that of S, the structure (atom and bond populations) of CZTS significantly changed into that of CZTSSe after selenization. Meanwhile, the work function (φ) value of CZTSSe is 5.61 eV, which is closed to that of Pt (5.65 eV), 3



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and significantly higher than CZTS (5.34 eV). So, the CZTSSe has the similar nature to Pt (for example, the ability of electron transfer, catalytic activity, etc.). The suitable φ is in favor of the faster photoelectron transfer across the interface of CZTSSe and redox couple (triiodide/iodide), which could result in better catalytic activity. This study may provide guidance for us to understand catalytic mechanism of CZTS(Se) for the reduction of I3-, as far as we known, it is the first time to report this conclusion. 2. EXPERIMENTAL SECTION 2.1 Materials and Methods 1-octadecene (ODE), copper (II) chloride dehydrate (CuCl2·2H2O), zinc oxide (ZnO), tin (IV) chloride pentahydrate (SnCl4·5H2O), dodecanethiol (DDT), selenium powder, oleylamine (OAm), cyclohexane, ethanol absolute and tetrahydrofuran (THF) were obtained from Aladdin corporation. The material used for DSSCs is same to our previous reports.28, 29 2.2.1 Synthesis of CZTS NCs The monodispersed CZTS NCs were prepared using the reported method with a little modification.30 The detailed preparation process is as follows: 630 mg of SnCl4·5H2O, 391 mg of ZnO and 920 mg of CuCl2·2H2O were dissolved in a certain amount of THF. And then, ODE (12 mL) and OAm (5 mL) were added to above solution. Solution was heated under flow of argon to 170 °C and maintained at this temperature for 1 hour to remove low boiling point impurities and water. The mixture was cooled to 100 °C and 60 mmol DDT was injected. The temperature of reaction was raised to 250 °C and kept at this temperature for 45 minutes. Finally, the obtained samples were thoroughly purified by multiple precipitation and redispersion using ethanol and cyclohexane,30 and then dried in a vacuum oven. As a control, the CZTS NCs with different Zn atom contents were prepared with the similar preparation process, just adjusting the contents of reactant ZnO (361, 376, 406, and 421 mg). And these CZTS samples were labeled as CZTS-1, CZTS-2, CZTS-3, and CZTS-4, respectively. 2.2.2 Synthesis of CZTSSe The CZTS converted to CZTSSe through selenization process.31 In briefly, the furnace warmed up to 560 °C under flow of nitrogen and kept for 20 minutes. At last, the products were naturally cooled to room temperature. 4


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2.3 Cell assembly and Characterizations The preparation process of CZTS, CZTSSe, Pt electrodes, assembly of DSSCs devices, and related characterizations were similar to our previous reports.28, 29, 32 2.4 First-principles calculations of populations (atomic, bond) and I3- adsorption of on CZTS(Se) with different sites The detailed theory calculation conditions were similar to our previous reports .28, 29 In order to explain electro-catalytic activity of the CZTS(Se) CEs in DSSCs, the adsorption energy of I3- molecule at different sites in the CZTS(Se) system has been investigated. The absorption energy (Eb) is defined as eqn (1) −

Eb = EI 3

Where

−

EI 3

− CZTS ( Se )

− CZTS ( Se )

− ( E CZTS ( Se ) + E I

− 3

)

(1)

means the total energy of I3- adsorpted on the CZTS(Se)

surface, ECZTS(Se) is the total energy of CZTS(Se) surface, and EI 3

−

is the total

energy of the isolated I3- complex. 3. RESULTS AND DISCUSSION 3.1 Structural characterization of CZTS and CZTSSe CZTS NCs were synthesized under argon atmosphere using standard Schlenk techniques with the 391 mg of reactant ZnO. During the growth process of CZTS NCs, DDT was injected the purified precursor solution with the syringe. And then the solution raised the reaction temperature to 250 °C, monomer concentration was gradually increased and reached the minimum nucleation concentration. As long as the nucleation occurred, the concentration of monomer was sharply decreased to below the nucleation threshold, and the monomer was continuously consumed for particle growth.33 Long-chain organic ligands will be coated in particle surface rapidly, which is helpful to get monodisperse CZTS NCs without further particle growth. CZTS NCs were characterized by a quasi-spherical geometry, as shown in the representative transmission electron microscopy (TEM) micrograph in Figure 1a. They were highly monodispersed, with an average particle size of 12 nm. The high-resolution transmission electron microscope (HRTEM) image of CZTS NCs in Figure 1b shows clearly visible lattice fringes, suggesting well crystalline of material. The lattice spacing of 0.32 nm correspond to the (002) planes of CZTS. Typical power X-ray diffraction (XRD) displayed in Figure 1c, which identifies these NCs as CZTS. The diffraction 5



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peaks at 26.85°, 28.26°, 30.46°, 39.14°, 49.41°, 51.40°, 56.19°, 57.49° and 63.33°can be observed for CZTS NCs, which is assigned to the (100), (002), (101), (102), (110), (103), (112), (201) and (202) planes of the wurtzite Cu2ZnSnS4 phase without impurities.30, 34 The EDS spectrum confirms the coexistence of Cu, Sn, Zn, and S elements in CZTS nanocrystals (Figure 1d). Meanwhile, the detailed atom ratio of sample can be obtained from the results of EDS by measuring randomly five spots in CZTS (Table S1). And the corresponding average atomic percentage of Cu, Zn, Sn, and S is 24.77 %, 12.13 %, 13.29 %, and 49.81 %, respectively. So, the component of CZTS is Cu1.99Zn0.97Sn1.07S4. And the atomic ratio of Cu: Sn: Zn: S is closed to 2:1:1:4. CZTS-1, CZTS-2, CZTS-3 and CZTS-4 samples with different Zn contents were prepared by adjusting the amount of reactant ZnO (361, 376, 406, and 421 mg). Compared with the obtained XRD pattern of CZTS (Figure 1c), the diffraction peak intensity ratio of respective CZTS-1, CZTS-2, CZTS-3, CZTS-4 changed remarkably (Figure S1). If the amount of reactant ZnO is lower than 391 mg, besides wurtzite Cu2ZnSnS4 nanocrystals, the tin copper sulfide (Cu2SnS3 or Cu3SnS4) was also produced as by-product (CZTS-1, CZTS-2). And if the amount of reactant ZnO is higher than 391 mg, ZnS and the tin copper sulfide (Cu2SnS3 or Cu3SnS4) were produced as by product (CZTS-3, CZTS-4). Furthermore, EDS was used to determine the component of the samples (Figure S2 and Table S2). And then obtained samples were Cu3.47Zn0.42Sn0.76S4, Cu3.48Zn0.56Sn0.82S4, Cu3.46Zn0.77Sn0.81S4 and Cu2.60Zn0.91Sn0.81S4 for CZTS-1, CZTS-2, CZTS-3 and CZTS-4, respectively. The components of above samples have changed remarkably compared to Cu1.99Zn0.97Sn1.07S4. This also indicated the impurities (ZnS and tin copper sulfide) existed in the above samples. So, the amount of reactant ZnO plays an important role for the formation of CZTS without impurities. Only with the specific amount of reactant ZnO (391 mg), the CZTS nanocrystals were prepared without impurities. On the basis of the above analysis, the monodispersed CZTS NCs were obtained. The CZTS sintering in selenium vapor to yield CZTSSe NCs were characterized by TEM mapping (Figure 2 a-e). The results of mapping show that Cu, Sn, and Zn elements are homogeneously distributed in specific region. While the S and Se elements are inhomogeneously distributed because Se atoms randomly replaced S atoms during the selenization process. The EDS analysis of CZTSSe showed in Figure 2f, revealing the coexistence of Cu, Zn, Sn, S and Se in the CZTSSe system. On the basis of the above analysis, CZTSSe NCs were prepared with selenization of CZTS NCs. 6


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XPS was measured to verify the oxidation states of the chemical elements in CZTS before and after treatment with Se vapor by characterizing the full-range, Zn 2p, S 2p and Se 3d (Figure 3). The peaks of Zn 2p for CZTS appeared at binding energy of 1021.6 and 1045.1 eV, which can be assigned to Zn (Ⅱ).17 But for CZTSSe, the Zn 2p spectra unambiguously shift to the lower binding energy of 1021.35 and 1044.4 eV, which suggests that the oxidation state of Zn becomes lower. The main peak for CZTSSe located at 54.2 eV is representative of the Se 3d binding energy for lattice Se2-.35, 36 It confirmed that Se inset in the lattice of CZTS. Interestingly, sulfur spectrum of the CZTS NCs can be assigned to binding energy of 162.6 eV and 161.4 eV, corresponding to the S2-.37-39 But the S peaks of CZTSSe became broad and the value of binding energy (161.2 eV) becomes smaller. This is due to the insertion of Se, which led to the change of binding energy between S and Zn, Sn, and Cu. 3.2 Photovoltaic performance In order to estimate the catalytic performance of CZTS, CZTSSe versus Pt, DSSCs devices were fabricated. The current-voltage (J-V) performances of three DSSCs are shown in Figure 4, and corresponding characteristic parameters including Jsc, Voc, FF and PCE are listed in Table 1. Actually, the Voc is determined by the quasi-Fermi level of dye-sensitized TiO2 photoanode and redox potential of electrolyte. In this work, the DSSCs fabricated with same photoanode and redox electrolyte. So, Voc is almost similar for above three samples, and it has nothing to do with the counter electrode material CZTS(Se) or Pt. But, the cell based on the EPt presents the best PCE of 7.62 % with Jsc of 17.11 mA cm-2 and FF of 0.64, which is obviously higher than those of the cell fabricated with a ECZTS, with an PCE, Jsc and FF of 3.54%, 10.87 mA cm-2 and 0.47, respectively. The poor PCE of the cell with ECZTS was mainly due to CZTS lacked reaction active sites. Therefore, the value of PCE, Jsc and FF were limited. However, the PCE of DSSC with ECZTSSe reached to 7.13%, which compared to that of Pt-based DSSCs. This may be due to the insertion of Se into CZTS for improved catalytic activity for I3-. In order to establish the statistics and errors for DSSCs based on CZTSSe, the histograms of photovoltaic parameters (PCE, Jsc, Voc, and FF) for a series of DSSCs based on CZTSSe electrodes, composed of 16 separate devices, were shown in Figure S3. The PCE of device resembled a normal distribution, and had a standard deviation of 0.05 %.40 So, CZTSSe is an efficient earth-abundant alternative to Pt as CE material for DSSCs. 7



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Cyclic voltammetry (CV) experiments were applied to study the reaction kinetics and catalytic activity of CZTS, CZTSSe, and Pt electrodes. Figure 5a shows the CV curves using three-electrode system. The major reduction of I3- at the CE/electrolyte interface can be represented as: −

I 3 + 2e − ↔ 3I −

(2)

Generally, the cathode peak current density (Jcp) of a CV curve suggests that the catalytic ability of CEs for the reduction of I3- in DSSCs, that is to say, the higher value of Jcp reflects the superior electrocatalytic capacity of a CE.32, 41 The Jcp for CZTSSe (2.0 mA/cm2) is observed to be higher than that of CZTS (1.5 mA/cm2). This indicates that CZTSSe has a better electro-catalytic activity for I3- reduction than CZTS. Remarkably, the Jcp of CZTSSe electrode is compared to that of Pt cathode, suggesting that CZTSSe has superior electro-catalytic activity. To verify that the CZTSSe film is a reliable material for DSSC, long-term CV analysis was executed, which is shown in Figure 5b. The Jcp of CZTSSe show constant amplitudes after 100 cycles, which means its good long-term stability. It further confirmed that the earth-abundant CZTSSe is available material to Pt as CE. The correlation between the photovoltaic efficiency of DSSCs and CEs not only depends upon catalytic activity of catalysts layer, but also relies on their interfacial electron transfer. To understand the electrochemical characteristics of CZTS, CZTSSe and Pt CEs, EIS spectra were measured. EIS of symmetrical cells, which were fabricated with two identical electrodes (CZTS, CZTSSe or Pt CEs) adopting the sandwich structure, as shown in Figure 5c. Nyquist plots were fitted by an equivalent circuit (inserted in Figure 5c) where Rs represents the series resistance, Rct is the charge transfer resistance.42 Rs is defined by the high frequency intercept on the real axis. The semicircle in the high frequencies represents the Rct value. The values of Rs and Rct were obtained through fitting the EIS spectra in Figure 5c. The Rs of the CZTSSe CE was 19.57 Ω/cm2, smaller than that of the CZTS (22.02 Ω/cm2). It is well-known that Rct is the most important characteristic to demonstrate the electron transfer of different CEs on the reduction of triiodide.43 A lower Rct facilitates electron transfer from CE to electrolyte for catalytic reduction of I3- and consequently results in less interface recombination.44 The Rct of CZTSSe CE was closed to the Pt CE, which revealing that catalytic property of CZTSSe was compared to Pt. 8


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Tafel polarization curve is an efficient method to further confirm the electrochemical catalytic activities of different CEs.45 The exchange current density (J0) located in the Tafel zone, which is related to the catalytic capacity of the catalysts.46 It can be estimated from the extrapolated intercepts of the cathode and anodic branches of the corresponding Tafel curves. Generally, a larger J0 usually means higher ability for I3reduction. The value of J0 varies inversely with Rct, which are according to the equation (3):

J0 =

RT nFRct

(3)

Where R is the gas constant, T is the absolute temperature, n is the electron number involved in reaction, F is Faraday’s constant.47 The Tafel polarization curve was examined in a symmetrical dummy cell, which is similar to the one used in the EIS measurement.45 As shown in Figure 5d, it can be clearly seen that CZTSSe CE showed higher J0 than CZTS CE, indicating that the electrochemical catalytic performance of CZTSSe CE is stronger than CZTS CE. In other words, the electro-catalytic ability has been enhanced when CZTS converted to CZTSSe. Meanwhile, the catalytic activity of CZTSSe is compared to Pt under the same condition. The results of Tafel polarization were in good agreement with the data showed in CV cures and EIS analysis. 3.3 The possible catalytic principle for redox couple The constituent of copper zinc tin chalcogenide could affect the performance of DSSC in a large extent, especially when Se replaced partly of S existed in CZTS system. The catalytic activity of materials for the reduction of redox couple increased after treatment with Se vapor. As for this point, there are no related reports to explain this phenomenon. The SKP is a very sensitive instrument, which is usually used to discern subtle molecular interactions. The work function (φ) is depended on the measurement of a fundamental property of material.48 So, SKP measurement was taken to analysis the capability of electron transfer of CZTS before and after exposed to Se vapor at 560 °C. As shown in Figure 6, the φ value of CZTSSe is 5.61 eV, which is closed to that of Pt (5.65 eV), and significantly higher than CZTS (5.34 eV). That is to say, the CZTSSe has the similar nature to Pt (for example, the ability of electron transfer, catalytic activity, etc.). Meanwhile, barrier height at the FTO sub9



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strate/CZTS(Se) interface was affected by the difference of φ. Because CZTSSe possesses a higher φ, a suitable built-in field is formed for electron transfer. The faster photoelectron transfer across the interface of CZTSSe and redox couple (triiodide/iodide) may occurred, which reduced the probability of electron and hole recombination. From the view of electron transport, CZTSSe is more favorable as CE material for DSSCs than CZTS. It is well agreed with the electrochemical tests. In generally, catalytic activity of material is strongly dependent on chemical composition and structural characteristics. The CZTSSe showed better catalytic ability for I3- than CZTS in the same conditions, but the exact reason for excellent catalytic activity of CZTS(Se) for use in DSSCs is not clear, even the role of each element of the material is also not clarified. It hinders us to understand the catalytic property of CZTS(Se), which is known as one of the promising photovoltaic material. Now, theory calculaion was used to study the change of chemical composition, structure and adsorption properties of I3- in CZTS before and after treatment with Se vapor. In order to analysis the essential reason of improved catalytic activity when CZTS converted to CZTSSe. The atomic and bond populations have been firstly calculated. Atomic populations of CZTS and CZTSSe are shown in Table 2. In the CZTS system, Zn and Sn atoms were positively charged because of losing electrons (0.66, 0.95); while the Cu and S atoms were negatively charged for getting electrons (-0.05, -0.38). Once converting to CZTSSe by partial replacement of S, the positive charges of Zn and Sn atoms decreased (0.49, 0.79), while the negative charge of Cu atom increased (-0.12). This is because the electro-negativity of Se (0.09) is weaker than that of S (-0.38). Table 3 shows the bond population of CZTS and CZTSSe, the Sn-S bond is changed little after selenization (0.41, 0.46), but the covalent bonds between Se and Cu (0.2) is obviously weaker than S-Cu bond (0.47), Zn-Se bond becomes antibonding (-2.12), accompanied by the improvement of Zn-site catalytic capability. phenomenon is crucial for the enhancement of the catalytic activity for

Such a

I3-.

To clarify the role of catalytic site, the adsorption energies and bond lengths were also calculated for I3- complex adsorbed on CZTS and CZTSSe. The results were shown in Table 4 and Figure 7. When I3- adsorbed on the Sn, Cu and Zn sites in CZTS system, the adsorption energy was -1.202 eV, -2.060 eV and -2.343 eV, respectively. The results indicated that I3- possesses a strong interaction with all these sites,

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which leads to the elongated bond length of I1-I2. So, Sn, Cu and Zn sites display positive contributions for I3- activation. More interestingly, when I3- adsorbed on the Zn site in CZTS system, the adsorption energy is much negative than other sites, and the corresponding bond length of I1-I2 (3.298 Å) is also the longest than those at Sn site (3.036), Cu site (3.152) and the corresponding I3- free molecule (2.903 Å). This means that Zn site in CZTS is the dominant active site for I3- catalysis. After S atoms were replaced partly by Se atom, the Zn-Se anti-bond was formed in CZTSSe (Table 3), which increased the catalytic ability. Because of the substitution, the adsorption energy at Zn site increased significantly (-3.727 eV), and the bond length of I1-I2 is further elongated (3.373 Å). Meanwhile, the adsorption energy and bond length at Sn-site did not change much in CZTSSe (-1.419 eV, 3.004 Å), but the adsorption energy and bond length at Cu site significantly reduced (-1.418 eV, 3.090 Å) after selenization. As a result, the activity of Zn site is enhanced for catalytic reduction of I3- after selenization. It can be concluded that the enhanced interactions between Zn and I3complex will make the activation of I3- molecule more easily due to the selenization, leading to the dissociation of the molecule and thus responsible for the catalytic reaction. Furthermore, as the interaction between Zn and I3- mainly depends on their molecular orbital interactions, to further reveal the origin of such interactions, the corresponding total and partial density of states (DOSs) was calculated (Figure 8a).28 The Fermi energy was taken as zero point (E-Ef = 0 eV).49 The total DOSs (TDOS) of I3apparent shifted to lower energy position when it adsorbed on the Zn-site in CZTSSe. It means that charge transfer occurred between I3- molecules and Zn site. It significantly found that the p state of I3- was splitted obviously after adsorption from -6.0 to 0.0 eV, especially for the contacting I1 atom. At the same time, the DOSs of the Zn atom that connect I1 were also changed due to the adsorption of I3-, and it can be confirmed that its peak intensity at -7.5 eV decreases to a large extent. According to the PDOSs, it can be identified that the peaks at [-8.0 eV, 7.0 eV] are mainly originated from Zn 3d states. So, the bonding between p orbital of I and 3d of Zn is confirmed to be helpful for the activation and dissociation of I3-. The electron density difference (EDD) diagrams for I3-/Zn-site in CZTSSe system are shown in Figure 8b. The negative or positive region represents the electron density was enriched or depleted, respectively.50 The results indicated that the depletion 11



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of electron density around the I1–I2 bond caused by the formed covalent bonds both I1 and the Zn atoms. Furthermore, the EDD around I1 sphere displayed the p characteristics, while Zn atoms in contact with it showed unambiguous characteristics of d orbital. It further confirmed that a high degree hybridization between p orbit of iodine and d orbit of Zinc. Taking the above results into consideration, theoretical calculation perspicuous explained the reason of the improved catalytic activity when CZTS converted to CZTSSe, that is, selenization enhanced Zn-site electrocatalytic activity for reduction of I3-. It is a primary reason that the DSSCs based on CZTSSe electrode show higher PCE than that based on CZTS. 4. CONCLUSIONS When CZTSSe are employed as counter electrodes (CEs) materials for dye-sensitized solar cells (DSSCs), the PCE has been improved to 7.13 %, which is higher than that of CZTS (3.54 %), and it also compared to that of Pt (7.62 %). A theoretical assessment shows that selenization enhanced Zn-site catalytic activity for the reduction of I3-. Once I3- molecular adsorbed on the sites in CZTS(Se) system, the adsorption energy of Zn obviously increased than other sites, which means that higher catalytic activity of Zn site. The DOSs and EDD were calculated to reveal the hybrid interactions between p orbital of I and 3d states of Zn, which lead to I3- was activated and dissociated easily. The discussion concerning the role of Zn site for reduction of I3- after CZTS converted into CZTSSe is meaningful to understand the catalytic reaction mechanism of CZTS(Se) as CEs of DSSCs. ACKNOWLEDGMENTS This project is supported by National Natural Science Foundation of China (21473051, 21371053, and 21631004), Natural Science Foundation of Heilongjiang Province (E2016056), the International Science & Technology Cooperation Program of China (2014DFR41110), the Excellent Youth of Common Universities of Heilongjiang Province (1252G045). REFERENCES (1) O'Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. 12


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(34) Lu, X.; Zhuang, Z.; Peng, Q.; Li, Y. Wurtzite Cu2ZnSnS4 Nanocrystals: a Novel Quaternary Semiconductor. Chem. Commun. 2011, 47, 3141-3143. (35) Wang, J.-J.; Wang, Y.-Q.; Cao, F.-F.; Guo, Y.-G.; Wan, L.-J. Synthesis of Monodispersed Wurtzite Structure CuInSe2 Nanocrystals and Their Application in High-Performance Organic-Inorganic Hybrid Photodetectors. J. Am. Chem. Soc. 2010, 132, 12218-12221. (36) Liu, Y.; Yao, D.; Shen, L.; Zhang, H.; Zhang, X.; Yang, B. Alkylthiol-Enabled Se Powder Dissolution in Oleylamine at Room Temperature for the Phosphine-Free Synthesis of Copper-Based Quaternary Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 7207-7210. (37) Yu, P.; Wang, L.; Wang, J.; Zhao, D.; Tian, C.; Zhao, L.; Yu, H. Graphene-Like Nanocomposites Anchored By Ni3S2 Slices for Li-Ion Storage. RSC Adv. 2016, 6, 48083-48088. (38) Xu, J.; Yang, X.; Yang, Q.-D.; Wong, T.-L.; Lee, C.-S. Cu2ZnSnS4 Hierarchical Microspheres as an Effective Counter Electrode Material for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 19718-19723. (39) Patil, A. M.; Lokhande, A. C.; Shinde, P. A.; Kim, J. H.; Lokhande, C. D. Vertically Aligned NiS Nano-Flakes Derived from Hydrothermally Prepared Ni(OH)2 for High Performance Supercapacitor. J. Energy Chem. 2017, DOI: org/10.1016/j. jechem. 2017. 05. 005. (40) Luber, E. J.; Buriak, J. M. Reporting Performance in Organic Photovoltaic Devices. ACS Nano 2013, 7, 4708-4714. (41) Dwivedi, P.; Das, S.; Dhanekar, S. Wafer-Scale Synthesized MoS2/Porous Silicon Nanostructures for Efficient and Selective Ethanol Sensing at Room Temperature. ACS Appl. Mater. Interfaces 2017, 9, 21017-21024. (42) Miao, X.; Pan, K.; Pan, Q.; Zhou, W.; Wang, L.; Liao, Y.; Tian, G.; Wang, G. Highly Crystalline Graphene/Carbon Black Composite Counter Electrodes with Controllable Content: Synthesis, Characterization and Application in Dye-Sensitized Solar Cells. Electrochimica. Acta 2013, 96, 155-163. (43) Zhang, Y.-Z.; Li, H.-H.; Zhou, Z.-J.; Kou, D.-X.; Zhou, W.-H.; Wu, S.-X. The Effect of the Selenization Process on Grain Size and Performance of CuIn(Sx,Se1-x)2 Counter Electrodes. RSC Adv. 2015, 5, 11599-11603. (44) Yuan, S.-J.; Zhou, Z.-J.; Hou, Z.-L.; Zhou, W.-H.; Yao, R.-Y.; Zhao, Y.; Wu, S.-X. Enhanced Performance of Dye-Sensitized Solar Cells Using Solution-Based In 16


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Figure 1 TEM image (a), HRTEM micrograph (b) and XRD pattern of CZTS NCs (c). (d) Elemental composition of CZTS measured by EDS.

Figure 2 The Cu, Sn, Zn, S and Se elemental TEM mapping of CZTSSe (a-e). Composition of CZTSSe measured by EDS (f).

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Figure 3 Full-range (a), Zn 2p (b) and S 2p (c) XPS spectra of CZTS and CZTSSe, respectively. (d) XPS of Se 3d of CZTSSe.

Figure 4 Current density-voltage (J-V) characteristics of DSSCs with Pt, CZTS and CZTSSe CEs, measured under AM1.5 illumination.

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Table 1 photovoltaic parameters of the DSSCs with various CEs.a CEs

Voc (mV)

Jsc (mA cm-2)

FF

PCE (%)

Pt

704

17.11

0.64

7.62

CZTS

694

10.87

0.47

3.54

CZTSSe

701

16.08

0.63

7.13

a

Jsc: short-circuit photocurrent; Voc: open-circuit photovoltage; FF: fill factor; PCE: power conversion efficiency.

Figure 5 (a) Cyclic voltammograms (CVs) of CZTS, CZTSSe and Pt electrodes. The electrolyte solution was composed of LiClO4 (0.1 M), LiI (10 mM) and I2 (1 mM) in acetonitrile, the scan rate was 25 mV s-1. (b) 100 consecutive CV spectra for the CZTSSe electrodes at a scan rate of 50 mV s-1. (c) EIS Nyquist plots of dummy cells with a symmetric sandwich-like structure between two identical electrodes consisting of: CZTS, CZTSSe and Pt electrodes. (d) Tafel polarization curves for the symmetrical cells fabricated in the same way as the ones used in the EIS experiments. Inset in (c) is equivalent circuit and detail parameters of cells.

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Figure 6 Work function map of CZTSSe (i) and CZTS (ii).

Table 2 Atomic populations of CZTS and CZTSSe. Cu

Zn

Sn

S

Se

CZTS

-0.05

0.66

0.95

-0.38

-

CZTSSe

-0.12

0.49

0.79

-0.38

0.09

Table 3 Bond populations of CZTS and CZTSSe. Cu-S/Cu-Se

Zn-S/Zn-Se

Sn-S

CZTS

0.47/-

0.32/-

0.41

CZTSSe

0.48/0.2

0.41/-2.12

0.46

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Figure 7 3D view of I3- complex adsorption on CZTS substitution by Se in some atom sites

Table 4 The adsorption data of I3- complex on different atom sites and bond length of CZTS and CZTSSe. EAd. (eV)

Atom site

CZTS

CZTSSe

a

Bond length (Å) M-I1

I1-I2 a

Sn

-1.202

2.984

3.036

Cu

-2.060

2.479

3.152

Zn

-2.343

2.565

3.298

Sn

-1.419

3.003

3.004

Cu

-1.418

2.425

3.090

Zn

-3.727

2.515

3.373

The bond length of I3- free molecule is about 2.903 Å.

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Figure 8 (a) Density of states (DOSs) for I3- complex and Zn atoms in the I3-/CZTSSe. (b) Electron density difference (EDD) diagrams for I3-/CZTSSe.

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Table of Contents Selenization of Cu2ZnSnS4 enhanced the performance of dye-sensitized solar cells: improved Zn-site catalytic activity for I3-

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Selenization of Cu2ZnSnS4 Enhanced the Performance of Dye

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