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Metal Selenides as Efficient Counter Electrodes for Dye-Sensitized Solar Cells Zhitong Jin, Meirong Zhang, Min Wang, Chuanqi Feng, and Zhong-Sheng Wang* Department of Chemistry, Lab of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China S Supporting Information *

CONSPECTUS: Solar energy is the most abundant renewable energy available to the earth and can meet the energy needs of humankind, but efficient conversion of solar energy to electricity is an urgent issue of scientific research. As the third-generation photovoltaic technology, dyesensitized solar cells (DSSCs) have gained great attention since the landmark efficiency of ∼7% reported by O’Regan and Grätzel. The most attractive features of DSSCs include low cost, simple manufacturing processes, medium-purity materials, and theoretically high power conversion efficiencies. As one of the key materials in DSSCs, the counter electrode (CE) plays a crucial role in completing the electric circuit by catalyzing the reduction of the oxidized state to the reduced state for a redox couple (e.g., I3−/I−) in the electrolyte at the CE− electrolyte interface. To lower the cost caused by the typically used Pt CE, which restricts the large-scale application because of its low reserves and high price, great effort has been made to develop new CE materials alternative to Pt. A lot of Pt-free electrocatalysts, such as carbon materials, inorganic compounds, conductive polymers, and their composites with good electrocatalytic activity, have been applied as CEs in DSSCs in the past years. Metal selenides have been widely used as electrocatalysts for the oxygen reduction reaction and light-harvesting materials for solar cells. Our group first expanded their applications to the DSSC field by using in situ-grown Co0.85Se nanosheet and Ni0.85Se nanoparticle films as CEs. This finding has inspired extensive studies on developing new metal selenides in order to seek more efficient CE materials for low-cost DSSCs, and a lot of meaningful results have been achieved in the past years. In this Account, we summarize recent advances in binary and mutinary metal selenides applied as CEs in DSSCs. The synthetic methods for metal selenides with various morphologies and stoichiometric ratios and deposition methods for CE films are described. We emphasize that the in situ growth method exhibits advantages over other methods for fabricating stable and efficient CEs. We focus on the effect of morphology on the electocatalytic and photovoltaic performance. Application of transparent metal selenide CEs in bifacial DSSCs and the superiority of in situ-grown metal selenide nanosheet fiber CEs used for fiber DSSCs are presented. In addition, we show that metal selenides with a hollow sphere structure can function not only as an efficient electrocatalyst but also as a light-scattering layer. Finally, we present our views on the current challenges and future development of metal selenide CE materials.

1. INTRODUCTION Since the pioneering work on dye-sensitized solar cells (DSSCs),1 substantial research on DSSCs has been conducted over the last 25 years, aiming at their industrialization. A DSSC consists of a dye-adsorbed TiO2 film as the photoanode and a catalyst-coated conductive substrate (e.g., fluorine-doped tin oxide, FTO) as the counter electrode (CE) with an electrolyte containing a redox couple (e.g., I3−/I−) filled between the two electrodes.1 The photovoltaic performance is evaluated in terms of the power conversion efficiency (PCE), which is calculated from the short-circuit photocurrent density (Jsc), the opencircuit photovoltage (Voc), and the fill factor (FF). As a crucial material of DSSCs, the main function of the CE is to catalyze the reduction of I3− to I−. Electrochemical impedance spectroscopy (EIS) of a dummy cell composed of © 2017 American Chemical Society

two identical CE electrodes sandwiching the electrolyte is always employed to characterize the electrocatalytic activity.2 The series resistance (Rs), charge transfer resistance (Rct), constant phase element (CPE) at the CE−electrolyte interface, and Nernst diffusion impedance (ZN) of the redox electrolyte can be derived from the fitted EIS spectrum. In addition, cyclic voltammetry (CV) and Tafel curves are used as supplementary means to assess the catalytic activity. A material with higher catalytic activity has the characteristics of smaller Rct, larger CV peak current density, smaller potential separation between anodic and cathodic CV peaks (Epp), and higher exchange current density (J0). Although Pt is the typically used CE for Received: December 14, 2016 Published: March 10, 2017 895

DOI: 10.1021/acs.accounts.6b00625 Acc. Chem. Res. 2017, 50, 895−904

Article

Accounts of Chemical Research

Figure 1. (a, b) SEM images of in situ-grown (a) Co0.85Se nanosheet and (b) Ni0.85Se nanoparticle films. (c) EIS spectra with the equivalent circuit (inset) and (d) Tafel curves for the dummy cells. (e) CV curves of I3−/I− species for various electrodes. (f) Current−voltage curves of DSSCs. Adapted from ref 7. Copyright 2012 American Chemical Society.

curves, and CV curves shown in Figure 1c−e. The PCE obtained with the Co0.85Se nanosheet CE (9.40%) is thus higher than those with the Ni0.85Se nanoparticle CE (8.32%) and the Pt CE (8.64%), as shown in Figure 1f. As Co0.85Se and Ni0.85Se show comparable PCEs at the same morphology, the performance difference of the Co0.85Se nanosheets and Ni0.85Se nanoparticles should be attributed to the morphology rather than to the metal.7 The morphology also influences the electrochemical stability. Since nanosheets can adhere to FTO more strongly with a larger contact area than nanoparticles, the Co0.85Se nanosheet CE has better electrochemical stability than the Ni0.85Se nanoparticle CE.7 Moreover, the Nernst diffusion impedance is reduced (Figure 1c) in going from Ni0.85Se nanoparticles to Co0.85Se nanosheets because of the increased contact area between the catalyst and the electrolyte.7 One attractive feature of the in situ-grown Co0.85Se nanosheet and Ni0.85Se nanoparticle CEs is the high optical transparency (>85%) in the visible spectral region because excellent catalytic performance can be achieved at pretty small loading ( MoSe2.48 The SILAR method is conducive to the study of the metal cation effect. Moreover, spin-coating,19,32,34 spray,23,36,40 and electrodeposition49 are also employed to fabricate thin CE films. Taken together, each of the deposition methods has advantages and disadvantages. We emphasize that the in situ growth method is a cost-effective strategy to prepare stable and efficient CE films. For comparison, the efficiencies of DSSCs with various metal selenide CEs are summarized in Table S1.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.6b00625. PCEs of various metal selenide CEs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhong-Sheng Wang: 0000-0002-3816-0726 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies

5. CONCLUSIONS AND OUTLOOK In this Account, we have reviewed recent advances in metal selenides toward their electrocatalytic effects on the electron transfer from the CE to the electrolyte in DSSCs and the photovoltaic performance. Diverse metal selenides with various metal cations and stoichiometric ratios have been synthesized, mainly through hydrothermal and solvothermal reactions, by simply varying the metal cation and the metal/selenium ratio of the reactants. Adjusting the reaction conditions, such as solvent and surfactant, can control the morphologies of metal selenides. Metal selenide CEs can be prepared via several methods, among which the in situ growth strategy is superior to other methods because it offers more homogeneous dispersion on the substrate, stronger adhesion to the substrate, and more effective catalytic sites. In addition to the excellent catalytic activity, metal selenide nanosheets are also attractive for high-performance fiber DSSCs in combination with the in situ growth method. The transparent feature of efficient metal selenide CEs makes them applicable to bifacial DSSCs, which have superiority of harvesting light from both the photoanode and CE sides. Metal selenides demonstrate excellent catalytic activity with low cost and good stability, and they will become strong competitors as CE materials for large-scale applications. Despite the huge achievements of metal selenides as the CEs of DSSCs, there still remain some challenges with some aspects. The catalytic mechanism for the metal selenides remains unclear to date. Elucidation of the mechanism may help to guide the design and synthesis of more efficient metal selenides. The effect of the stoichiometric ratio on the catalytic performance is an interesting issue. To study this effect, the morphology should be kept the same, which relies on the controllable synthesis methodology. Metal selenides have been mainly studied with the iodine-based electrolyte, but it is also important to understand their catalytic activity on iodine-free electrolytes in order to get higher Voc. Although the electrochemical stability of metal selenide CEs has been welladdressed, one should employ a series of characterization techniques, which are detailed in a recent review,50 to assess the comprehensive stability of metal selenide CEs for practical applications.

Zhitong Jin is currently pursuing his Ph.D. degree on DSSCs under the supervision of Professor Zhong-Sheng Wang at Fudan University, China. Meirong Zhang is currently pursuing her Ph.D. degree on DSSCs under the supervision of Professor Zhong-Sheng Wang at Fudan University, China. Min Wang is currently pursuing her Ph.D. degree on quantum-dot solar cells under the supervision of Professor Zhong-Sheng Wang at Fudan University, China. Chuanqi Feng is currently pursuing his Ph.D. degree on DSSCs under the supervision of Professor Zhong-Sheng Wang at Fudan University, China. Zhong-Sheng Wang received his Ph.D. degree from Peking University. He is currently a Full Professor of Chemistry at Fudan University. His research interests include dye-sensitized and perovskite solar cells.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (21673049) and the STCSM (168014342).

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REFERENCES

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