NH3-treated WO3 as low-cost and efficient counter electrode for dye-sensitized solar cells
© Song et al.; licensee Springer. 2015
Received: 6 October 2014
Accepted: 23 December 2014
Published: 28 January 2015
A novel low-cost and efficient counter electrode (CE) was obtained by treating catalytic inert tungsten trioxide (WO3) nanomaterial in NH3 atmosphere at elevated temperatures. The formation of tungsten oxynitride from WO3 after NH3 treatment, as evidenced by X-ray photoelectron spectroscopy and X-ray diffraction, increases the catalytic activity of the CE. Correspondingly, the power conversion efficiency (PCE) of the DSC is significantly increased from 0.9% for pristine WO3 CE to 5.9% for NH3-treated WO3 CE. The photovoltaic performance of DSC using NH3-treated WO3 CE is comparable to that of DSC using standard Pt CE (with a PCE of 6.0%). In addition, it is also shown that NH3 treatment is more efficient than H2 or N2 treatment in enhancing the catalytic performance of WO3 CE. This work highlights the potential of NH3-treated WO3 for the application in DSCs and provides a facile method to get highly efficient and low-cost CEs from catalytic inert metal oxides.
KeywordsTungsten trioxide (WO3) NH3 treatment Counter electrode Catalytic Power conversion efficiency
Dye-sensitized solar cells (DSCs) have attracted great attention for their low cost, simple production, and acceptable energy conversion efficiency [1,2]. It typically consists of three parts: a dye-sensitized oxide layer, electrolyte, and a counter electrode (CE). As an important component of DSCs, the CE transfers the electrons from the external circuit to the internal electrolyte and thus reduces triiodide ions to iodide ions, which realizes the continuous operation of DSCs and greatly influences the photovoltaic performance of DSCs. For achieving the high performance of DSCs, the CEs should possess high conductivity and catalytic activity . High catalytic active platinized fluorine-doped tin oxide (FTO) is the most commonly used CE in DSCs. However, the high cost of scarce Pt limits the large-scale fabrication and application of DSCs, which promotes the exploration of Pt-free CEs [3-5].
Carbonaceous materials , conducting polymers , inorganic compounds (like sulfides , carbides , and nitrides ), and composite materials [11-13] have been reported as Pt-free materials in DSCs. The metal oxides were also studied as CEs for their facile synthesis and low cost, but the efficiencies were relatively low and not able to replace Pt . These oxides may be further improved by changing their electronic structure. Hydrogen (H2) or nitrogen (N2) treatments have been proved to be a facile and efficient method to change the electronic structure of oxides, with which the efficiencies were improved from 0.63% to 5.43% for WO3 by H2 treatment and from 1.84% to 6.09% for SnO2 by N2 treatment [14,15]. However, the DSCs using these CEs still yield low fill factors (FF) and low efficiencies as compared to conventional Pt CEs; further improvements need to be carried out.
In this work, we demonstrated that the electronic structure of the metal oxide (WO3) was able to be facilely changed by NH3 treatment and its catalytic activity was also improved. The DSC using NH3-treated WO3 exhibits superior photovoltaic performance with a power conversion efficiency (PCE) of 5.9%, which is similar to that using standard Pt CE (6.0%) and is much higher than that using pristine WO3 CE (0.9%). Moreover, we also demonstrated that NH3 treatment was more efficient than H2 or N2 treatment in improving the performance of DSCs using WO3-based CEs.
Preparation of WO3, NH3-treated WO3, and standard Pt CEs
The original WO3 nanopowders are commercial products with a particle diameter of about 30 nm. To prepare the WO3 slurry, 133 mg WO3 and 20 mg ethyl cellulose are dispersed in 1 ml alpha-terpineol and then stirred for 24 h to form a fluid mixture. The yellow-green slurry was deposited on pre-cleaned FTO/glass substrates by doctor blade method to form continuous films. The films were then dried at 110°C for 30 min to remove the organic solvents and the WO3 CEs were obtained. Atmosphere (including NH3, H2, and N2)-treated WO3 CEs were obtained by annealing the as-prepared WO3 CEs in different atmospheres at 480°C for 2 h. Standard Pt CE was also fabricated by sputtering thermodecomposition of H2PtCl6 on pre-cleaned FTO/glass at 450°C for 20 min.
Fabrication of DSCs
TiO2 films were prepared by doctor blading of TiO2 nanoparticle (P25) slurry on FTO/glass substrates. All of the TiO2 films were post-treated with TiCl4. After calcination, the TiO2 films were immersed in a 0.3 mmol/l ethanol solution of N719 dye for 24 h. The DSCs were fabricated by assembling dye-sensitized TiO2 photoanodes with as-fabricated CEs using 30-μm-thick Surlyn (DuPont, Wilmington, DE, USA). I−/I3 − electrolyte with acetonitrile as the solvent was used. The active area of solar cells was about 4 mm × 4 mm. Symmetric cells for electrochemical measurements were fabricated by assembling two identical CEs together using 30-μm-thick Surlyn.
The structure and morphology properties of the samples were measured by X-ray diffraction (XRD; XRD-6000, Shimadzu Corp., Kyoto, Japan) and scanning electron microscopy (SEM; S-4800, Ltd., Tokyo, Japan). The element distribution was tested by X-ray photoelectron spectroscopy (XPS) and electron diffraction spectroscopy (EDS). The photovoltaic performance of DSCs was characterized using a source meter (2400, Keithley Instruments, Inc., Beijing, China) under AM 1.5G irradiation (100 mW/cm2) generated by a solar simulator (XES-301S + EL-100, San-ei Electric Co., Ltd., Osaka, Japan). Electrochemical impedance spectroscopy (EIS) was carried out using the electrochemical workstation (CHI660D), performed on symmetric cells.
Results and discussion
The W 4f XPS spectra of WO3 and NH3-treated WO3 samples are shown in Figure 2b. The peaks at 35.77 eV (W 4f7/2) and 37.97 eV (W 4f5/2) from WO3 can be ascribed to the binding energy of high oxidation state of W. In comparison, one additional peak at 33.32 eV (W4f7/2), which is associated with lower oxidation states of W, can be observed from the NH3-treated WO3 sample, indicating the formation of W-N bonds in NH3-treated WO3 as might be expected in tungsten oxynitrides . In addition, the peaks located at 35.37 and 37.47 eV from NH3-treated WO3 are lower compared with those from the pristine WO3 (35.77 and 37.97 eV), which probably result from the existence of less electronegative atoms into the oxide lattice considering the fact that N has smaller electronegativity (3.04) than O (3.44). From the above results, it can be concluded that WOxNy, other than tungsten nitrides, were formed, as in good accordance with the previous XRD analysis.
Photovoltaic parameters of DSCs using WO 3 , NH 3 -treated WO 3 , and standard Pt CE
J sc (mA/cm 2 )
V oc (V)
The excellent performance of NH3-treated WO3 CE can be ascribed to the change of electronic structure from tungsten oxide to tungsten oxynitride by NH3 treatment. NH3-treated WO3 CE possesses similar W-N bonds to tungsten nitride which is a catalytic active site for the reduction of triiodide [10,20]; hence, it is also able to provide Pt-like electrocatalytic properties. Meanwhile, as the reduction ability of NH3 also provides a reduction atmosphere for WO3, which will create oxygen vacancies as similar to the case of H2 treatment , the catalytic activity can also be improved in the presence of oxygen vacancies. Therefore, NH3-treated WO3 CE exhibits the best performance among the WO3 CEs treated in different atmospheres.
Moreover, NH3 treatment may also vary the energy level of WO3 by introducing oxygen vacancies. As the conduction band level of WO3 (approximately 0.7 V versus normal hydrogen electrode (NHE)) is larger than the potential of I−/I3 − (approximately 0.3 V versus NHE), the overpotential for triiodide reduction in WO3 CE will be inevitable, leading to a low V OC in DSCs using WO3 CE (as shown in Figure 5). However, the V OC values of DSCs using Pt CE and NH3-treated WO3 CE are nearly identical, which indicates that the overpotential for triiodide reduction in NH3-treated WO3 CE is negligible and the Fermi level of WO3 is varied by NH3 treatment. It is proposed that hydrogen incorporation in WO3 favors the occupation of gap states near the Fermi level and the maintenance of a high work function, which facilitate the charge transport and enhance charge extraction in organic solar cells . NH3 treatment may also play a similar role in affecting the electronic structure of WO3 and can be explored as a hole-extracting layer for organic solar cells.
In conclusion, it is demonstrated that NH3 treatment can significantly improve the catalytic performance of WO3 in the use of CE material for DSCs. By annealing commercial WO3 in a NH3 atmosphere, the oxygen atoms in WO3 can be partially substituted by nitrogen to form tungsten oxynitrides, which obviously enhance the catalytic activity of the CEs. Correspondingly, the DSC using NH3-treated WO3 CE exhibits excellent performance, which is comparable to the DSC using standard Pt CE. The findings in this work also provide new insights into the exploration of low-cost and highly efficient CE materials with metal oxynitrides for DSCs.
This work was supported partially by the National Natural Science Foundation of China (Grant nos. 51372082, 51172069, 50972032, 61204064, 51202067, and 91333122), Ph.D. Programs Foundation of Ministry of Education of China (Grant nos. 20110036110006, 20120036120006, and 20130036110012), Science and Technology Program Foundation of Suzhou City (SYG201215), and the Fundamental Research Funds for the Central Universities.
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