ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells

Journal of Alloys and Compounds xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells Ganghua Zhang a, Mingjun Zhu a, Lianna Zhai a, Jianwu Cao a, Zhipeng Gao b, **, Tao Zeng a, c, * a

Shanghai Key Laboratory of Engineering Materials Application and Evaluation, Shanghai Research Institute of Materials, Shanghai, 200437, PR China National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, 621900, China c Advanced Science Research Laboratory, Saitama Institute of Technology, Okabe, Saitama, 369-0293, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 August 2019 Received in revised form 24 October 2019 Accepted 25 October 2019 Available online xxx

Two quaternary chalcogenides of ThCr2Si2-type K2FeCu3Q4 (Q for S and Se) were utilized as new type of Pt-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The X-ray photoelectron spectroscopy (XPS), Mott
Schottky analysis, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to clarify the differences in energy band alignments, electrocatalytic and photovoltaic performances. A photoelectric conversion efficiency (PCE) of 5.07% was achieved in DSSCs assembled with K2FeCu3Se4 CE, which was obviously superior to that of K2FeCu3S4 CE (3.42%) and comparable with that of Pt CE (6.25%) under one sun AM 1.5 G illumination (100 mW cm
2). Compared to K2FeCu3S4 CE, the higher PCE of K2FeCu3Se4 CE can be attributed to the better catalytic activity for I
3 reduction, stronger driving force for electron injection from CE to electrolyte, faster electron transfer and electrolyte diffusion channels. © 2019 Elsevier B.V. All rights reserved.

Keywords: Quaternary chalcogenides Pt-free Counter electrode ThCr2Si2-type structure Dye-sensitized solar cell

1. Introduction Over the past decades, DSSCs have attracted enormous attention due to their simple fabrication procedures, low-cost and excellent photovoltaic performance [1e4]. As well-known, the dye
sensitized TiO2 anode, the CE and the electrolyte (I
3 /I ) are three main components of DSSCs. Among of them, the CE is an indispensable one that takes charge of the catalytic reaction for triiodide ions and greatly affects the device performance [5,6]. Thus, the energy band alignment, conductivity, electrocatalytic activity and crystallinity are critical factors for an efficient CE in DSSCs. Currently, platinum (Pt) is still the most common and effective counter electrode (CE) in DSSCs due to its excellent electrocatalytic activity and high electrical conductivity [7,8]. But, its high cost and scarcity also limit the further development and commercial competitiveness of Pt-based DSSCs. Thus, it is highly desirable to

* Corresponding author. Shanghai Key Laboratory of Engineering Materials Application and Evaluation, Shanghai Research Institute of Materials, Shanghai, 200437, PR China. ** Corresponding author. E-mail addresses: [email protected] (Z. Gao), [email protected] (T. Zeng).

explore the non-Pt electrocatalysts constructed by low-cost, earth abundant and sustainable elements. Recently, many intriguing alternative materials to Pt have drawn widespread interest, such as carbon-based materials [9], conductive polymers [10], oxides [11,12] and chalcogenides [13,14]. Among of them, the chalcogenides have been demonstrated as a promising system because of their excellent catalytic activity for I
3 reduction, distinctive electronic properties and controllable crystalline morphologies. Typically, the binary transition metal chalcogenides have been usually utilized to replace Pt CE owing to their simple chemical compositions and high catalytic activities for I
3 reduction, such as WS2 [15], MoSe2 [16] and NiS [17]. Additionally, since the fascinate morphologies, robust redox activities and tuneable bandgap structures, the ternary transition metal chalcogenides have been also adopted as the promising CEs with excellent performances comparable to Pt CE, such as NiCo2S4 [18,19], MIn2S4 (M ¼ Co, Ni) [20] and CuInS2 [21]. In contrast to the rich structural models of binary and ternary chalcogenides CEs, the known quaternary transition metal chalcogenides CEs are almost focused on the chalcopyrite compounds, such as CIGS [22], Cu2ZnSnS4 [23] and Cu2FeSnS4 [24]. Other quaternary chalcogenides are rarely verified to be efficient CEs for

https://doi.org/10.1016/j.jallcom.2019.152797 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

2

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

DSSCs application. Considering the alternative component elements and adjustable stoichiometric ratio, the extraordinary performances can be reasonably expected in quaternary chalcogenides by turning the crystal structure and electronic configuration [25,26]. As one type of important quaternary transition metal chalcogenides, the ThCr2Si2-type chalcogenides AM2
xCuxQ2 (A for K, Rb, Cs and M for Co, Fe, Mn) have exhibited unique properties, such as ferromagnetism, metallic or semimetallic behaviour, and photoelectric response [27e31]. Moreover, in view of the layered structure and alternative ionic intercalation, the ThCr2Si2-type chalcogenides possessed specific electronic transmission channels, which could be definitely different from two- and threedimensional compounds already demonstrated as efficient CEs in DSSCs (such as MoSe2 [16] and CIGS [22]). However, none of them has been utilized in the area of DSSCs so far, in spite of the earth abundant elements and low manufacturing cost. With the aim to expand the variety of the low-cost and sustainable CEs materials, the ThCr2Si2-type chalcogenides were thus employed as the CEs in DSSCs. Herein, the quaternary chalcogenides K2FeCu3Q4 CEs were fabricated by spraying the nanoparticles directly on indium tin oxide (ITO) glass, followed by vacuum sintering. A PCE of 5.07% was achieved in the DSSCs based on K2FeCu3Se4 CE, which was much higher than that of K2FeCu3S4 CE (3.42%) and close to that of Pt CE (6.25%) under simulated AM 1.5G illumination (100 mW/cm2). All these bring us a new avenue to explore new efficient CEs materials for novel energy-harvesting application.

C 1s reference of 284.5 eV. The optical properties of the samples were analyzed using a UV-3100 Shimadzu ultraviolet visibleinfrared spectrophotometer equipped with an integrating sphere, in reflectance mode. The absorption coefficient satisfies the equation (ahn)2 ¼ A(hn
Eg) for a direct bandgap material, where a, n, A, and Eg are the absorption coefficient, light frequency, proportionality coefficient, and bandgap energy, respectively.

2. Experimental details

2.5. Photoelectric measurements

2.1. Preparation of K2FeCu3Q4 and Pt counter electrodes

Solar cell performances were recorded on a Digital Multimeter, under AM 1.5G simulated sunlight (100 mW/cm2) at an ambient atmosphere. The intensity of the CEL-S500 solar simulator was calibrated by a standard Si photovoltaic cell. The active cell area for the light irradiation was about 0.25 cm2, which was controlled by a black mask. Each J-V curve was measured basing on at least different 10 pieces of CEs.

The K2FeCu3Q4 (Q for S and Se) nanoparticles were synthesized according to the methods reported previously [28]. The fabrication of K2FeCuQ4 films can be described as follows: 200 mg K2FeCu3Q4 powders and 5 g zirconium dioxide pearls were firstly dispersed in 5 mL isopropanol and milled for 8 h. The well-mixed suspension was then sprayed on an ITO glass. The as prepared films were sintered under N2 atmosphere at 450  C for 30 min in a tube furnace. Pt electrodes were prepared by sputtering Pt onto ITO followed by annealing at 450  C for 30 min. 2.2. Photoanode preparation and DSSCs assembly 20 nm-sized TiO2 particles were screen printed onto the ITO substrates and sintered at 450  C for 30 min to form TiO2 films. And then, the TiO2 films were immersed into a 0.3 mM solution of ruthenium dye N719 in anhydrous ethanol overnight to obtain the photoanodes. The DSSC was assembled by injecting electrolyte into the space between the electrodes with an active area of about 0.25 cm2 in the light of previous method. The symmetrical cell was assembled with two identical CEs filling the electrolyte (active area ~0.25 cm2), and then sealed with double-faced insulated adhesive tapes for the EIS experiments and Tafel-polarization tests. 2.3. Characterizations The purity and crystallinity of the films were confirmed with a lab X-ray diffractometer (a Rigaku D/Max-2000 diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) at 40 kV, 100 mA and a graphite monochromator at the secondary beam). The morphology and composition of the pristine sample were checked by a scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) with a FEI Quanta 200F microscope operating at 20 kV. XPS was carried out on an Axis Ultra spectrometer. The binding energies were collected for K 2p, Fe 2p, Cu 2p, S 2p and Se 3d regions with the

2.4. Electrochemical performance measurements Flat band potentials of K2FeCuQ4 films were determined on an electrochemical workstation (CHI750E, CH Instruments) with a typical three-electrode system under the frequency of 1 kHz, in which Pt and Ag/AgCl electrodes were used as the counter and reference electrodes. 1.0 M Na2SO3 aqueous solution was used as the supporting electrolyte to maintain the films stability. The cyclic voltammetry (CV) spectra were recorded with a typical threeelectrode system at a scan rate of 50 mV/s. The electrolyte contained 0.1 M LiClO4, 10 mM LiI and 1 mM I2. The EIS and Tafel polarization measurements of dummy cells were tested on a computer-controlled potentiostat (CHI750E, CH Instruments) in the dark. The EIS was tested by applying an AC voltage with 10 mV amplitude in the frequency range from 0.01 Hz to 105 Hz at ambient pressure and temperature. Tafel polarization measurements for the CEs were tested at a scan rate of 10 mV/s with the voltage ranging from
0.5 V to 0.5 V.

3. Results and discussion With the advantages of the mild reaction conditions, simple apparatus and convenient operation process, hydrothermal synthesis has been widely adopted to synthesize the nano-materials [32e35]. In this work, the K2FeCu3Q4 nanoparticles were also prepared by a facile hydrothermal route. By shortening the reaction time, the as-prepared K2FeCu3Q4 nanoparticles also show a biscuitlike appearance as reported previously [28], but possess smaller particle sizes around 30e150 nm for K2FeCu3S4 and 50e200 nm for K2FeCu3Se4 (see Fig. 1). The structure and purity of the K2FeCu3Q4 powders and films were checked by powder XRD. From Fig. 2a and b, all Bragg peaks of K2FeCu3Q4 films could be well indexed to the tetragonal structure (ThCr2Si2-type, space group I4/mmm) as reported previously [28], which also confirmed thermal stability of the samples after the annealing treatment. SEM photographs have been taken to check the surface morphology of K2FeCu3Q4 films. As shown in Fig. 2c and d, it can be clearly found that both films were constructed from the incompact stacking of nanocrystallites, because the films were prepared by spraying the as-synthesized K2FeCu3Q4 nanoparticles directly on ITO glass. For both samples, the EDX analysis showed the presence of all four elements in the selected regions with a constant ratio (Fig. S1), indicating the uniform composition of these films. The XPS measurements were performed to check the oxidation states of all elements in K2FeCu3Q4. As given in Fig. 3, the Cu 2p core split into 2p3/2 (~932.11 eV for K2FeCu3S4 and 931.13 eV for K2FeCu3Se4) and 2p1/2 (~952.05 eV for K2FeCu3S4 and 951.98 eV for

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

Fig. 1. SEM images of crystalline samples: (a) for K2FeCu3S4 and (b) for K2FeCu3Se4.

K2FeCu3Se4) peaks, indicating the oxidation state of Cuþ in K2FeCu3Q4 by reference to the previous literature [36]. The binding energies of Fe 2p1/2 and 2p3/2 peaks (794.31 eV and 779.28 eV for K2FeCu3S4; 794.16 eV and 779.12 eV for K2FeCu3Se4) correspond to those of the previously reported Fe3þ oxidation state [37]. The oxidation states of Kþ, S2
and Se2
were also determined from XPS spectra by reference to their characteristic binding energies (see Fig. S2). Moreover, the valence band positions of K2FeCu3Q4 samples could be estimated by linear extrapolating the initial peaks of the full XPS spectra to the baselines (see Fig. S3 and Fig. 4a and b). The valence band maximum (VBM) of K2FeCu3Se4 (~0.79 eV) is more positive than that of K2FeCu3S4 (~0.64 eV), suggesting the localized states above the valence band edge are sensitive to the coordinated anions. The conduction band minimums (CBM) of K2FeCu3Q4 samples were also determined by Mott
Schottky analysis. As shown in Fig. 4c and d, the Mott-Schottky plots of K2FeCu3Q4 electrodes possess positive slopes, indicating the n-type semiconductor nature of the samples. Estimated from the extrapolating of the linear portion of the plots, the flat band potentials of K2FeCu3S4 and K2FeCu3Se4 (vs. Ag/AgCl at pH ¼ 7) were identified at
0.08 V and
0.14 V, respectively. Generally, the bottom edge of the conduction band of an n-type semiconductor is about 0.2 V more negative than its flat band potential [38,39]. Thus, the CBM of K2FeCu3Q4 samples (vs. NHE) could be determined as
0.28 V for K2FeCu3S4 and ‒0.34 V for K2FeCu3Se4, respectively. To show a complete picture of the band alignment of K2FeCu3Q4, the optical bandgaps were determined to be 0.98 eV for K2FeCu3S4 and 1.22 eV

3

for K2FeCu3Se4 by constructing Tauc plots from UV-Vis spectra (Fig. S4). Combined with the optical bandgaps and valence-band XPS analysis, the CBM of K2FeCu3Q4 samples (vs. NHE) could be determined at
0.34 V for K2FeCu3S4 and ‒0.43 V for K2FeCu3Se4, respectively, corresponding to the results calculated from electrochemical flat band potential measurements. Accordingly, the energy band alignments could be constructed as shown in Fig. 5a, revealing that their valence band potentials were more negative than the oxidation potential of O2/H2O (~0.82 V at pH ¼ 7) [40]. Thus, both samples are inefficient for water splitting due to the photoinduced corrosion under illumination [41]. However, in view
of the redox potential of I
3 /I (~0.54 V at pH ¼ 7), the conduction band potentials of both samples are efficient to transfer the injected
electrons to the I
3 /I electrolyte, which means K2FeCu3Q4 could be utilized as the DSSC photocathodes. Moreover, the larger energylevel difference (DE) between the CBM of CE material and the redox potential of electrolyte always means stronger driving force of the electronic injection. Thus, in view of the DE (0.88 eV for K2FeCu3S4 and 0.97 eV for K2FeCu3Se4) between the CBM of
K2FeCu3Q4 CEs and the ɸ(I
3 /I ), the driving force of electron injection of K2FeCu3Se4 is much stronger than that of K2FeCu3S4, corresponding to the order of catalytic activity as demonstrated later. To verify the catalytic activities of the K2FeCu3Q4 films, CV measurements have been performed using a three-electrode system. Fig. 5b shows the CV curves of the K2FeCu3Q4 electrodes with an ITO-Pt electrode as a reference. Two pairs of redox peaks can be observed in the case of Pt as reported previously [16,20]. The cathodic peak at lower potential can be assigned to the reaction of e
I
(1), while the peak at higher potential can be 3 þ 2e ¼ 3I assigned to the reaction of 3I2 þ 2ee ¼ 2I
3 (2). For both samples, the cathodic peaks at lower potentials can be also found in the CV curves, suggesting the similar catalytic activities as to Pt. Thus, K2FeCu3Q4 samples are effective in catalyzing the reduction of triiodide to iodide as a new type of promising counter electrodes in the DSSCs. Commonly, the excellent catalytic activity and electron transfer efficiency for CEs can be estimated by the higher reduction peak current density (Jr) and lower peak-to-peak separation (Epp) [42,43]. As shown in Fig. 5b, the Jr is in an order of Pt > K2FeCu3Se4 > K2FeCu3S4 while the Epp values (see Table 1) are in an order of Pt > K2FeCu3S4 > K2FeCu3Se4, which reveals the higher catalytic activity of the K2FeCu3Se4 CE for I
3 reduction than that of K2FeCu3S4 CE. The photovoltaic performances of the DSSCs assembled with K2FeCu3Q4 CEs (see Fig. S5) were performed on a Digital Multimeter under simulated AM 1.5G illumination (100 mW/cm2). As depicted in Fig. 5c, the photo-generated electrons transits from the ground state to the excited state of N719 dye, and then transfers quickly to the external circuit through the CB of TiO2 [3,20]. Finally, the electrons will be collected by the K2FeCu3Q4 CE and injects into the electrolyte to achieve the I
3 reduction. Fig. 5d gives the photovoltaic performance curves of DSSCs with different CEs. The detailed photovoltaic parameters are compared in Table 1. The DSSCs based on K2FeCu3Se4 CE achieves an open-circuit voltage (Voc) of 0.715 V, a short-circuit current density (Jsc) of 9.843 mA/cm2, a fill factor (FF) of 0.568, and a PCE of 5.07% which is more outstanding than that of K2FeCu3S4 CE (3.42%) and comparable with that of Pt CE (6.25%). The larger JSC of K2FeCu3Se4 CE can be attributed to its higher catalytic activity as demonstrated by CV measurements. Compared with the metallic Pt, the conductivities of the semiconducting K2FeCu3Q4 films are definitely poor as confirmed by the EIS measurements (discussed later), which limit the diffusion and exchange of the photo-generated electrons. Besides, since the K2FeCu3Q4 films were fabricated by spraying technology followed by vacuum sintering, the small surface areas of the films could

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

4

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

Fig. 2. The XRD patterns (a, b) and SEM images (c, d) of the as-prepared K2FeCu3Q4 samples.

Fig. 3. XPS spectra of Fe 2p and Cu 2p for K2FeCu3S4 (a,b) and K2FeCu3Se4 (c,d).

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

5

Fig. 4. The valence-band XPS spectra (a,b) and Mott-Schottky plots (c,d) of the K2FeCu3Q4 samples.


Fig. 5. (a) Potentials for CB and VB in K2FeCu3Q4 relative to the redox potential of I
3 /I and the standard levels of hydrogen production and water oxidation at pH ¼ 7. (b) Cyclic voltammetry for K2FeCu3Q4 on ITO substrate and ITO-Pt electrodes in 10 mM LiI and 1 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte. (c) The schematic representation of K2FeCu3Q4 as counter electrodes (CEs) in DSSC. (d) Photovoltaic characteristics (JeV) of fabricated DSSCs based on K2FeCu3Q4 and Pt CEs under AM 1.5 G illumination (100 mW/cm2).

reduce their electrocatalytic activities and the random loose packing of the nanoparticles should result in high contact barriers, which could significantly influence the photovoltaic performance of the DSSCs. However, as new counter electrode materials for DSSCs, the higher PCE can be reasonably expected in K2FeCu3Q4 CEs

by further optimizing their conductivities and electrocatalytic activities via proper strategies as reported in other promising CEs [14e18]. To further study the charge transfer kinetics and evaluate the catalytic activities of different CEs, EIS has been performed with a

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

6

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

Table 1 Epp, photovoltaic parameters of DSSCs and simulated data from fitted EIS spectra based on different counter electrodes (CEs). CEs

Epp (V)

Voc (V)

Jsc (mA/cm2)

FF

h (%)

Rs (U)

Rct (U)

ITO-Pt K2FeCu3S4 K2FeCu3Se4

0.457 0.387 0.224

0.738 0.691 0.715

9.994 9.093 9.843

0.668 0.427 0.568

6.25 3.42 5.07

15.12 90.15 57.09

10.27 1558.13 221.64

symmetric cell configuration consisting of two identical CEs. Using the Randles-type equivalent circuit [44,45], EIS curves were fitted by the Zsimpwin software (see Fig. 6a and Fig. S6) and the fitting parameters were shown in Table 1. The intercept on the real axis presents the ohmic series resistance (RS) and the semicircles at high frequency and low frequency can be assigned to the charge transfer resistance (RCT) at the interface of CE/electrolyte. Generally, the charge transfer rate and catalytic activity of CEs can be evaluated by comparing the RCT, while FF is inversely proportional to Rs. Clearly, the attenuation trend of RCT and Rs (K2FeCu3S4 > K2FeCu3Se4 > Pt) further confirmed the faster charge transfer and higher catalytic activity in K2FeCu3Se4 CE than that in K2FeCu3S4 CE, corresponding to the CV results and their photovoltaic performances. To further evaluate the catalytic activities, the Tafel curves of the K2FeCu3Q4 CEs were measured as shown in Fig. 6b. Theoretically, the Tafel curve can be divided into three zones: polarization zone at low potential curve (<50 mV), Tafel zone at middle potential (with a sharp slope) and diffusion zone at high potential (horizontal part). From the Tafel plots of K2FeCu3Q4 CEs, the better catalytic activity of

K2FeCu3Se4 can be unambiguously affirmed by its higher limiting diffusion current density (Jlim) and exchange current density (J0). 4. Conclusions In summary, two semiconducting chalcogenides K2FeCu3S4 and K2FeCu3Se4 were exploited as new catalysts for the CEs in DSSCs system. The as-prepared K2FeCu3Q4 possessed suitable energy band alignments and showed a robust electrocatalytic activity for the triiodide reduction. An energy conversion efficiency of 5.07% was achieved in the DSSCs based on K2FeCu3Se4 CE, which was about 81% of that based on Pt CE (6.25%). These results demonstrated that ThCr2Si2-type chalcogenides could be promising lowcost candidate catalysts to replace the costly Pt CE in DSSCs system. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the NSF of China (Grant No. 51772184 ,11704353 and U1930124), the Program of Shanghai Technology Research Leader (Grant No. 17XD1420300), the LSD fund (Grant No. 6142A03010102), the LSD engineering project (Grant No. 2016Z-04), the Dean Fund of CAEP (Grant No. YZJJLX2016001), and the CSS project (Grant No. YK2015-0602006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152797. References

Fig. 6. (a) Nyquist plots and (b) Tafel curves of the symmetrical cells fabricated with two identical K2FeCu3S4, K2FeCu3Se4 and Pt CEs, respectively.

[1] Y. Ren, D. Sun, Y. Cao, H.N. Tsao, Y. Yuan, S.M. Zakeeruddin, P. Wang, M. Gr€ atzel, A stable blue photosensitizer for color palette of dye-sensitized solar cells reaching 12.6% efficiency, J. Am. Chem. Soc. 140 (2018) 2405e2408. [2] M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua, €tzel, A. Hagfeldt, Dye-sensitized solar S.M. Zakeeruddin, J.-E. Moser, M. Gra cells for efficient power generation under ambient lighting, Nat. Photonics 11 (2017) 372e378. [3] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595e6663. [4] S. So, I. Hwang, J. Yoo, S. Mohajernia, M. Ma ckovi c, E. Spiecker, G. Cha, A. Mazare, P. Schmuki, Inducing a nanotwinned grain structure within the TiO2 nanotubes provides enhanced electron transport and DSSC efficiencies >10%, Adv. Energy Mater. 8 (2018) 1800981. [5] J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, G. Luo, Y. Lin, Y. Xie, Y. Wei, Counter electrodes in dye-sensitized solar cells, Chem. Soc. Rev. 46 (2017) 5975e6023. [6] Y. Di, Z. Xiao, Z. Zhao, G. Ru, B. Chen, J. Feng, Bimetallic NiCoP nanoparticles incorporating with carbon nanotubes as efficient and durable electrode materials for dye sensitized solar cells, J. Alloy. Comp. 788 (2019) 198e205. [7] T. Liu, K. Yu, L. Gao, H. Chen, N. Wang, L. Hao, T. Li, H. He, Z. Guo, A graphene quantum dot decorated SrRuO3 mesoporous film as an efficient counter electrode for high-performance dye-sensitized solar cells, J. Mater. Chem. A 5 (2017) 17848e17855. [8] J. Briscoe, S. Dunn, The future of using earth-abundant elements in counter electrodes for dye-sensitized solar cells, Adv. Mater. 28 (2016) 3802e3813. [9] H. Wang, K. Sun, F. Tao, D.J. Stacchiola, Y.H. Hu, 3D honeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dyesensitized solar cells, Angew. Chem. Int. Ed. 52 (2013) 9210e9214. [10] M.S. Su’ait, M.Y.A. Rahman, A. Ahmad, Review on polymer electrolyte in dyesensitized solar cells (DSSCs), Sol. Energy 115 (2015) 452e470. [11] D. Ursu, M. Vajda, M. Miclau, Investigation of the p-type dye-sensitized solar cell based on full Cu2O electrodes, J. Alloy. Comp. 802 (2019) 86e92. [12] O. Langmar, E. Fazio, P. Schol, G. de la Torre, R.D. Costa, T. Torres, D.M. Guldi, €chen-ladungstransfers und des fill-factors in CuOSteuerung des Grenzfla €tzel-Tandemzellen, Angew. Chem. Int. Ed. 131 (2019) basierten Gra 4097e4102.

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797

G. Zhang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx [13] Z. Jin, M. Zhang, M. Wang, C. Feng, Z. Wang, Metal selenides as efficient counter electrodes for dye-sensitized solar cells, Acc. Chem. Res. 50 (2017) 895e904. [14] Y. Xie, C. Zhang, G. Yang, J. Yang, X. Zhou, J. Ma, Highly crystalline stannitephase Cu2XSnS4 (X ¼ Mn, Fe, Co, Ni, Zn and Cd) nanoflower counter electrodes for ZnO-based dye-sensitised solar cells, J. Alloy. Comp. 696 (2017) 938e946. [15] C. Tan, Z. Lai, H. Zhang, Ultrathin two-dimensional multinary layered metal chalcogenide nanomaterials, Adv. Mater. 29 (2017), 1701392. [16] H. Chen, Y. Xie, H. Cui, W. Zhao, X. Zhu, Y. Wang, X. Lü, F. Huang, In situ growth of a MoSe2/Mo counter electrode for high efficiency dye-sensitized solar cells, Chem. Commun. 50 (2014) 4475e4477. [17] X. Wang, Y. Xie, Y. Jiao, K. Pan, B. Bateer, J. Wu, H. Fu, Carbon nanotubes in situ embedded with NiS nanocrystals outperform Pt in dye-sensitized solar cells: interface improved activity, J. Mater. Chem. A 7 (2019) 10405e10411. [18] S.Y. Khoo, J. Miao, H.B. Yang, Z. He, K.C. Leong, B. Liu, T.T.Y. Tan, One-step hydrothermal tailoring of NiCo2S4 nanostructures on conducting oxide substrates as an efficient counter electrode in dye-sensitized solar cells, Adv. Mater. 2 (2015), 1500384. [19] Y. Jiang, X. Qian, C. Zhu, H. Liu, L. Hou, Nickel cobalt sulfide double-shelled hollow nanospheres as superior bifunctional electrocatalysts for photovoltaics and alkaline hydrogen evolution, ACS Appl. Mater. Interfaces 10 (2018) 9379e9389. [20] W. Hou, Y. Xiao, G. Han, An interconnected ternary MIn2S4 (M¼Fe, Co, Ni) thiospinel nanosheet array: a type of efficient platinum-free counter electrode for dye-sensitized solar cells, Angew. Chem. Int. Ed. 129 (2017) 9274e9278. [21] P. llaiyaraja, B. Rakesh, T.K. Das, P.S.V. Mocherla, C. Sudakar, CuInS2 quantum dot sensitized solar cells with high VOC z 0.9 V achieved using microspherenanoparticulate TiO2 composite photoanode, Sol. Energy Mater. Sol. Cells 178 (2018) 208e222. [22] C. RaviDhas, A.J. Christy, R. Venkatesh, K.S. Anuratha, K. Ravichandran, A.M.E. Raj, B. Subramanian, S.K. Panda, Nebulizer spray-deposited CuInGaS2 thin films, a viable candidate for counter electrode in dye-sensitized solar cells, Sol. Energy 157 (2017) 58e70. [23] S.S. Nemala, K. Mokurala, P. Bhargava, S. Mallick, Titania nanobelts as a scattering layer with Cu2ZnSnS4 as a counter electrode for DSSC with improved efficiency, Mater. Today 5 (2018) 23351e23357. [24] R.R. Prabhakar, N.H. Loc, M.H. Kumar, P.P. Boix, S. Juan, R.A. John, S.K. Batabyal, L.H. Wong, Facile water-based spray pyrolysis of earth-abundant Cu2FeSnS4 thin films as an efficient counter electrode in dye-sensitized solar cells, ACS Appl. Mater. Interfaces 6 (2014) 17661e17667. [25] D. Hobbis, K. Wei, H. Wang, G.S. Nolas, Synthesis and transport properties of Cu-excess Cu(Zn,Cd)2InTe4 quaternary chalcogenides, J. Alloy. Comp. 743 (2018) 543e546. [26] J. Zhao, S.M. Islam, O.Y. Kontsevoi, G. Tan, C.C. Stoumpos, H. Chen, R.K. Li, M.G. Kanatzidis, The two-dimensional AxCdxBi4exQ6 (A ¼ K, Rb, Cs; Q ¼ S, Se): direct bandgap semiconductors and ion-exchange materials, J. Am. Chem. Soc. 139 (2017) 6978e6987. [27] J. Yang, B. Chen, H. Wang, Q. Mao, M. Imai, K. Yoshimura, M. Fang, Magnetic properties in layered ACo2Se2 (A ¼ K, Rb, Cs) with the ThCr2Si2-type structure, Phys. Rev. B 88 (2013), 064406. [28] G. Zhang, H. Chen, Y. Xie, F. Huang, Facile synthesis, magnetic, electrical and photoelectric properties of layered quaternary chalcogenides K2FeCu3Q4 (Q¼ S and Se), CrystEngComm 16 (2014) 1810e1816. [29] G. Zhang, J. Cao, C. Zhao, B. Han, C. Ma, Z. Gao, T. Zeng, Facile synthesis and

[30]

[31]

[32] [33]

[34]

[35]

[36]

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

7

magnetic and electrical properties of layered chalcogenides K2CoCu3Q4 (Q for S and Se), Dalton Trans. 47 (2018) 14968e14974. M. Oledzka, K.V. Ramanujachary, M. Greenblatt, Synthesis and characterization of new quaternary selenides with ThCr2Si2-type structure: ACuMnSe2 (A ¼ K, Rb, Cs), Mater. Res. Bull. 33 (1998) 855e866. €ther, W. Bensch, Bridging O. Tiedje, E.E. Krasovskii, W. Schattke, P. Stoll, C. Na from ThCr2Si2-type materials to hexagonal dichalcogenides: an ab initio and experimental study of KCu2Se2, Phys. Rev. B 67 (2003), 134105. M. Yin, Y. Yao, H. Fan, S. Liu, WO3-SnO2 nanosheet composites: hydrothermal synthesis and gas sensing mechanism, J. Alloy. Comp. 736 (2018) 322e331. G. Zhang, J. Cao, G. Huang, J. Li, D. Li, W. Yao, T. Zeng, Facile fabrication of wellpolarized Bi2WO6 nanosheets with enhanced visible-light photocatalytic activity, Catal. Sci. Technol. 8 (2018) 6420e6428. G. Zhang, H. Chen, Z. Gu, P. Zhang, T. Zeng, F. Huang, Facile synthesis, magnetic and electric characterization of mixed valence La0.75K0.25AMnTiO6 (A ¼ Sr and Ba) perovskites, Inorg. Chem. 56 (2017) 10404e10411. J. Qi, X. Lai, J. Wang, H. Tang, H. Ren, Y. Yang, Q. Jin, L. Zhang, R. Yu, G. Ma, Z. Su, H. Zhao, D. Wang, Multi-shelled hollow micro-/nanostructures, Chem. Soc. Rev. 44 (2015) 6749e6773. Y. Zhang, Z. Qiao, X. Chen, Microwave-assisted elemental direct reaction route to nanocrystalline copper chalcogenides CuSe and Cu2Te, J. Mater. Chem. 12 (2002) 2747e2748. C. Chen, S. Hu, Y. Fu, Effects of surface hydroxyl group density on the photocatalytic activity of Fe3þ-doped TiO2, J. Alloy. Comp. 632 (2015) 326e334. Q. Xu, J. Yu, J. Zhang, J. Zhang, G. Liu, Cubic anatase TiO2 nanocrystals with enhanced photocatalytic CO2 reduction activity, Chem. Commun. 51 (2015) 7950e7953. T. Oshima, T. Ichibha, K.S. Qin, K. Muraoka, J.J.M. Vequizo, K. Hibino, R. Kuriki, S. Yamashita, K. Hongo, T. Uchiyama, K. Fujii, D. Lu, R. Maezono, A. Yamakata, H. Kato, K. Kimoto, M. Yashima, Y. Uchimoto, M. Kakihana, O. Ishitani, H. Kageyama, K. Maeda, Undoped layered perovskite oxynitride Li2LaTa2O6N for photocatalytic CO2 reduction with visible light, Angew. Chem. Int. Ed. 57 (2018) 8154e8158. C. Liu, D. Kong, P. Hsu, H. Yuan, H. Lee, Y. Liu, H. Wang, S. Wang, K. Yan, D. Lin, P.A. Maraccini, K.M. Parker, A.B. Boehm, Y. Cui, Rapid water disinfection using vertically aligned MoS2 nanofilms and visible light, Nat. Nanotechnol. 11 (2016) 1098e1104. S. Chen, L. Wang, Thermodynamic oxidation and reduction potentials of photocatalytic semiconductors in aqueous solution, Chem. Mater. 24 (2012) 3659e3666. W.J. Hou, Y.M. Xiao, G.Y. Han, D.Y. Fu, R.F. Wu, Serrated, flexible and ultrathin polyaniline nanoribbons: an efficient counter electrode for the dye-sensitized solar cell, J. Power Sources 322 (2016) 155e162. Y. Yang, J. Wei, G. Zhang, W. Sun, W. Zhou, One step hydrothermal synthesis of vertical Ni-Mo-S nanosheet array as the counter electrode for FDSC, J. Alloy. Comp. 764 (2018) 890e894. X.J. Zheng, J. Deng, N. Wang, D.H. Deng, W.H. Zhang, X.H. Bao, C. Li, Podlike Ndoped carbon nanotubes encapsulating FeNi alloy nanoparticles: highperformance counter electrode materials for dye-sensitized solar cells, Angew. Chem. Int. Ed. 53 (2014) 7023e7027. C. Bao, F.X. Li, J.L. Wang, P.P. Sun, N. Huang, Y.H. Sun, L. Fang, L. Wang, X.H. Sun, One-pot solvothermal in situ growth of 1D single-crystalline NiSe on Ni foil as efficient and stable transparent conductive oxide free counter electrodes for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 8 (2016) 32788e32796.

Please cite this article as: G. Zhang et al., ThCr2Si2-type quaternary chalcogenides as efficient Pt-free counter electrodes for dye-sensitized solar cells, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152797