Element substitution of kesterite Cu2ZnSnS4 for efficient counter electrode of dye-sensitized solar cells

Development of cost-effective counter electrode (CE) materials is a key issue for practical applications of photoelectrochemical solar energy conversion. Kesterite Cu2ZnSnS4 (CZTS) has been recognized as a potential CE material, but its electrocatalytic activity is still insufficient for the recovery of I−/I3− electrolyte in dye-sensitized solar cells (DSSCs). Herein, we attempt to enhance the electrocatalytic activity of kesterite CZTS through element substitution of Zn2+ by Co2+ and Ni2+ cations, considering their high catalytic activity, as well as their similar atomic radius and electron configuration with Zn2+. The Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) CEs exhibit smaller charge-transfer resistance and reasonable power conversion efficiency (PCE) (CCTS, 8.3%; CNTS, 8.2%), comparable to that of Pt (8.3%). In contrast, the CZTS-based DSSCs only generate a PCE of 7.9%. Density functional theory calculation indicate that the enhanced catalytic performance is associated to the adsorption and desorption energy of iodine atom on the Co2+ and Ni2+. In addition, the stability of CCTS and CNTS CEs toward electrolyte is also significantly improved as evidenced by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy characterizations. These results thus suggest the effectiveness of the element substitution strategy for developing high-performance CE from the developed materials, particularly for multicomponent compounds.

Herein, we investigate the effect of element substitution on improving the electrocatalytic activity of kesterite CZTS CEs. We prepare kesterite Cu 2 XSnS 4 (X = Zn, Co, Ni) CEs by simple spin-coating method. Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) tests indicate that the Cu 2 CoSnS 4 (CCTS) and Cu 2 NiSnS 4 (CNTS) CEs possess decreased change-transfer resistance and improved stability toward iodide electrolyte. CCTS-and CNTS-based DSSCs exhibit enhanced efficiency (8.3% and 8.2%) compared with that of CZTS (7.9%), which is comparable with traditional Pt (8.3%). In addition, the highly-effective catalytic activity is related to the adsorption and desorption energy of iodine (I) atom calculated by the density functional theory [44][45][46][47] .

Results and Discussion
Structure and morphology characterizations. We prepared porous CXTS films by spin-coating precursor solutions based on water and ethanol mixed solvent and annealing them in N 2 atmosphere at 540 °C for 15 minutes 48 . To avoid the signals interference of FTO (SnO 2 : F) to CXTS films, we recorded X-ray diffraction (XRD) patterns and Raman spectra through CXTS films on quartz prepared by the same method. The diffraction peaks at 28.53°, 47.33°, and 56.18° were indexed to (112), (220), and (312) planes respectively, which were in good agreement with those of previously reported kesterite CZTS 48-50 ( Fig. 1(a)). This measurement indicated that the element substitution did not change the crystal structure of CZTS. Furthermore, three peaks at 288, 336 and 372 cm −1 were observed in the Raman spectra ( Fig. 1(b)) of CZTS, which were indexed to CZTS materials. The CCTS and CNTS spectra showed peaks at 288, 325 and 350 cm −1 , which were observed in the CCTS and CNTS materials of previous literatures [51][52][53][54] . In addition, we used Energy-dispersive X-ray spectroscopy (EDX) to analyze the composition of CXTS films ( Fig. S1(a-c) in the Supplementary Information). The elemental composition ratio was 1.8:1:1.3:4.7, 1.5:1:1.3:4.4 and 1.5:1:1.1:4, respectively. These results indicated that the CXTS CEs was successfully synthesized. In addition, CXTS CEs showed above 75% transmittance in the range of visible wavelengths as shown in the UV-Vis spectra of Fig. 1(c). Figure 1(d-f) exhibited the top-view scanning electron microscope (SEM) images of the CXTS films. It was obvious that the CXTS films showed a porous structure, which was beneficial to the high catalytic activity because of the high specific surface area 55 . The Atomic force microscope (AFM) measurements also showed similar morphology (see Fig. S1(d-f) in the Supplementary Information). We performed step profiler test to accurately measure the thickness of CXTS films. The thickness of CZTS, CCTS and CNTS CEs were calculated to be 189 ± 13 nm, 125 ± 3 nm and 148 ± 27 nm, respectively, from nine measure points. The thickness of CXTS films was carefully optimized by spin-coating 1 layer of precursor solution (see electrolyte//CE). The exchange current density (J 0 ) of CEs could be acquired from the intercept of a tangent to Tafel polarization curves (Tafel), the variation of which could be inverse with the charge-transfer resistance (R ct ) values fitted from EIS through eq. 1: where R is the gas constant, T is the temperature, F is Faraday's constant and n is the electron number involved in the electrochemical reduction of triiodides at the electrode 56 . As shown in Fig. 2(a), the anodic and cathodic branches of CCTS-and CNTS-Tafel curves exhibited larger slopes than those of CZTS, revealing a higher J 0 and more efficient catalytic activity of CCTS and CNTS CEs for reducing triiodides. In addition, we also prepared Cu 2 MnSnS 4 and Cu 2 FeSnS 4 CEs by similar method. However, their performance was poor because of the bad activity and unstable chemical property of these two films.
The EIS test was used to further evaluate the catalytic activity of CXTS CEs. The left arcs of EIS spectra in Fig. 2(b) reflects the R ct and series resistance (R s ) whose exact values are obtained by fitting the equivalent circuit in the inset of Fig. 2(b). As shown in Table 1, the R ct values were reduced after substituting Zn 2+ by Co 2+ and Ni 2+ ions (CCTS, 5.3 Ω; CNTS, 5.5 Ω; CZTS, 6.5 Ω), which was consistent with the variation of J 0 values in Fig. 2(a). In addition, the R s values of CCTS and CNTS CEs were close to that of CZTS, indicated their similar electron transport ability. Thus, the variation of R ct led to the enhancement of the catalytic activity of CCTS and CNTS CEs, compared with CZTS. This catalytic activity trend was also observed on dense CXTS films prepared by spin-coating the dimethyl sulphoxide-based precursor solution (see Fig. S3 in the Supplementary Information). All the electrochemical data suggested that the substitution of Zn 2+ by Co 2+ and Ni 2+ effectively improved the electrocatalytic activity of kesterite CZTS CEs for reducing triiodides.
Photovoltaic performance of DSSCs. The current density-voltage (J-V) curves of DSSCs containing Pt or CXTS CEs and N719-sensitized TiO 2 photoanode in iodide electrolyte were shown in Fig. 2(c). Table 1 summarizes the resultant photovoltaic parameters. The CCTS-and CNTS-based DSSCs revealed comparable power  Table S1 in the Supplementary Information). Therefore, the electrochemical data of dummy cells and photovoltaic performance of DSSCs confirm that substitution of Zn 2+ by Co 2+ and Ni 2+ is effective for improving the electrocatalytic ability of kesterite CZTS CEs.
Density functional theory calculation. Considering that the catalytic activity of CEs strongly correlates with the adsorption and desorption processes of redox species, we perform density functional theory calculation to explore the origin of catalysis-activity enhancement caused by element substitution. First, we checked the change of adsorption energy toward I atom (E I ad ) during the substitution of Zn 2+ by Co 2+ and Ni 2+ . We found that the I atom was preferentially adsorbed on Zn 2+ of CZTS, as the calculated E I ad value of Sn 4+ (0.295 eV) was significantly lower than that of Zn 2+ (0.975 eV) (see Fig. S5 in the Supplementary Information). And the E I ad value ( Fig. 3 and Fig. S5 in the Supplementary Information) remarkably increased to 1.428 eV (CCTS) and 1.953 eV (CNTS) after the element substitution. This change indicated the stronger adsorption ability toward I atom of CCTS and CNTS CEs, resulted in their more efficient catalytic activity for reducing triiodides. Moreover, the calculated bond length between I atom and metal ions for the transition state ( − d I M TS ) decreased from 0.247 nm of CZTS to 0.245 nm of CCTS and 0.240 nm of CNTS, which could result in more difficult desorption of the adsorbed I atom (I * ). These theoretical calculation data showed that the enhanced performance of CCTS and CNTS CEs compared CZTS was associated to the improved adsorption and desorption energy.
Furthermore, we compared the amounts of I atom adsorbed on the CXTS surface by XPS 57-60 (see Fig. S6 in the Supplementary Information and Fig. 4). We immersed CXTS CEs in the iodide electrolyte for 30 minutes and rinsed them with ethanol. The peak area ratio of I 3d to Cu 2p spectra was marked as the normalized peak area of I 3d spectra. No signals of I 3d were found in XPS results before immersing. But, after immersing, the normalized peak area of I 3d spectra of CCTS (0.1894) and CNTS (0.1621) were significantly larger than that of CZTS (0.0443) (Fig. 4(b)), indicating more I atom adsorbed on CCTS and CNTS CEs surface. This change was consistent with the enhanced E ad I values and decreased bond length. The electrochemical, photovoltaic and theoretical results all indicated that the substitution of Zn 2+ by Co 2+ and Ni 2+ was effective to improve the catalytic activity of kesterite CZTS.

Durability test.
The stability is one of the major factors to evaluate the property of CEs [61][62][63] . Herein, we used Tafel, EIS and XPS tests to examine the stability of CXTS CEs. First, the current density in Tafel curves at −0.40 V (see Fig. S7 in the Supplementary Information) of the CZTS CEs decreased by 6% compared with the original ones after 1800 s test (Fig. 5). Whereas, the current density of Pt, CCTS and CNTS CEs only decreased by less than 3%, indicating the better stability of CCTS and CNTS CEs. The fitted R s and R ct values obtained from EIS  spectra of CXTS CEs before and after immersing in the iodide electrolyte for 30 minutes (see Fig. S8 and Table S2 in the Supplementary Information) also showed the good stability of CCTS and CNTS. In addition, after immersing, the peak area of Co and Ni XPS spectra decreased by 2.52% and 24.98% of the original ones, respectively ( Fig. 4(c)). This result was significantly smaller than that of Zn spectra in CZTS (39.03%). Different stability measurements all suggested that the CCTS and CNTS CEs possessed better stability toward the iodide electrolyte compared with CZTS CE.

Conclusions
In conclusion, we proved that the substitution of Zn 2+ by Co 2+ and Ni 2+ was a convenient but effective approach to enhance the electrocatalytic performance of kesterite CZTS CEs in DSSCs. After substitution, CCTS and CNTS CEs exhibited decreased charge transfer resistance (CCTS, 5.3 Ω; CNTS, 5.5 Ω; CZTS, 6.5 Ω) and improved electrocatalytic activity (PCE: CCTS, 8.3%; CNTS, 8.2%) compared with CZTS (7.9%) toward iodide electrolyte, which was comparable with the traditional Pt-based cells (8.3%). The enhanced activity was associated to the change of adsorption and desorption energy (the bond length between I atom and metal ions for the transition state ( − d I M TS ) of I atom by theoretical calculation. Furthermore, the stability of kesterite CXTS CEs was also significantly improved. The results indicated that this element substitution method without changing the materials  structure was effective to improve potential catalysts performance, especially for the multicomponent compounds.