Promoting Effect of Layered Titanium Phosphate on the Electrochemical and Photovoltaic Performance of Dye-Sensitized Solar Cells
© The Author(s) 2010
Received: 25 February 2010
Accepted: 8 May 2010
Published: 20 May 2010
We reported a composite electrolyte prepared by incorporating layered α-titanium phosphate (α-TiP) into an iodide-based electrolyte using 1-ethyl-3-methylimidazolium tetrafluoroborate(EmimBF4) ionic liquid as solvent. The obtained composite electrolyte exhibited excellent electrochemical and photovoltaic properties compared to pure ionic liquid electrolyte. Both the diffusion coefficient of triiodide (I3 −) in the electrolyte and the charge-transfer reaction at the electrode/electrolyte interface were improved markedly. The mechanism for the enhanced electrochemical properties of the composite electrolyte was discussed. The highest conversion efficiency of dye-sensitized solar cell (DSSC) was obtained for the composite electrolyte containing 1wt% α-TiP, with an improvement of 58% in the conversion efficiency than the blank one, which offered a broad prospect for the fabrication of stable DSSCs with a high conversion efficiency.
KeywordsDiffusion Charge transfer Layered α-titanium phosphate Ionic liquid
Dye-sensitized solar cell (DSSC) has attracted considerable attention as a high-efficiency and low-cost alternative to conventional inorganic photovoltaic devices . Generally, DSSC comprises a dye-sensitized nanocrystalline porous TiO2 film immobilized on a transparent conducting oxide (TCO)–coated glass substrate, an electrolyte containing an I−/I3 − redox couple, and a platinized TCO-coated glass substrate as the counter electrode. When the sensitizer dye absorbs solar energy, electrons are injected rapidly from the excited state of the dye into the conduction band of TiO2. Injected electrons diffuse in TiO2 and reach the outer circuit through the back contact. Oxidized dye molecules are reduced by I− in the iodide-based liquid electrolyte via the reaction 3I− → I3 − + 2e, where I− changes to I3 − by losing two electrons. At the counter electrode, the reverse reaction takes place, where I3 − is reduced to I− by gaining two electrons from the counter electrode. In the iodide-based liquid electrolyte, I− diffuses from the counter electrode to the dye, and I3 − diffuses from the dye to the counter electrode. Among the above reactions, both the charge-transfer process at the Pt/electrolyte interface and the diffusion process of I3 − in the electrolyte depend on the properties of the electrolyte. Therefore, the electrolyte plays an important role in the photovoltaic performance of DSSC by affecting the kinetics of electronic or ionic processes [2–4].
Although an impressive 11% light-to-electricity conversion efficiency has been obtained for photovoltaic devices with organic solvent-based electrolytes , these volatile organic solvents are of questionable durability due to their evaporation and leaking, especially for DSSC with a flexible matrix. Recently, room temperature ionic liquids have attracted growing attention due to their negligible vapor pressure and high ionic conductivity [6–8]. They had the advantage of keeping their stability for a long time because ionic liquid electrolytes do not evaporate in normal temperature. However, the energy conversion efficiency could not reach that of DSSCs using volatile liquid electrolytes. This was mainly because that the viscosity of the ionic liquids was higher than that of volatile liquid solvents, which resulted in the low diffusion constant of I3 − in the electrolyte and the large charge-transfer resistance at the counter electrode/electrolyte interface [9, 10].
Recently, many efforts have been made to improve the photovoltaic properties and the stability of DSSC filled with ionic liquid-based electrolyte by adding silica nanoparticles, carbon nanotubes, carbon nanoparticles and titanium dioxide nanoparticles into various ionic liquid electrolytes [11, 12]. Wang et al. reported that layered α-zirconium phosphate enhanced the exchange current density and the diffusion coefficient of triiodide in the electrolyte . These striking and significant observations have triggered our interest to explore new layered materials to improve the electrochemical performances of the electrolyte and the photovoltaic characteristics of DSSCs. Crystalline α-titanium phosphate (α-TiP) has a two-dimensional layered structure similar to that of layered α-zirconium phosphate . However, for layered α-TiP, P atoms in the lower sandwich lie along a perpendicular line drawn from the Ti atom of the upper sandwich. This arrangement renders it with larger inter-laminar cavities and a greater ion exchange capacity than α-zirconium phosphate . The enlarged spacing would facilitate the diffusion of I3 − in the ionic liquid electrolyte, thus enhancing the photovoltaic performance of DSSC. In addition, α-TiP has the common Ti ions with TiO2 photoanode, which can avoid the possible effect of foreign zirconium ions on the photovoltaic characteristics of DSSC. Here, we firstly reported a composite electrolyte prepared by adding α-titanium phosphate (α-TiP) into 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) ionic liquid-based electrolyte showing a lower melting point and a higher conductivity . The composite electrolyte exhibited very good electrochemical and photovoltaic properties.
Electrochemical impedance spectra (EIS) were measured using an IM6/IM6e (Zahner, Germany) electrochemical analyzer in a two-electrode configuration. A 10 mV AC perturbation was applied and the frequency range was 0.01 Hz–100 kHz. The limiting current density was determined by steady-state current–voltage curve of the Pt thin-layer cell. The scan rate was 10 mV/s. Morphology of α-TiP powders was determined by field emission scanning electron microscope (FESEM, JEOL JSM-6301F). X-ray diffraction (XRD, RIGAKU D/MAX-2400) was used to characterize the crystal structure and interlayer distance of α-TiP crystalline. Photocurrent-voltage curves were recorded using a source meter (Keithley-2400, Keithley Co. Ltd., USA) under an illumination of 100mW/cm2 (globe AM1.5, 1sun) from a Xenon lamp (Oriel) at room temperature.
Results and Discussion
Figure 1a showed the FESEM images of layered α-TiP nanoparticles. It can be seen that the particles were predominantly hexagonal platelet shaped with an average diameter of about 20 nm and an average thickness of about 3 nm. Figure 1b showed wide-angle XRD pattern of layered α-TiP nanoparticles. From the corresponding characteristic 2θ values of the diffraction peaks in Fig. 1b, it was confirmed that the as-prepared sample was identified as α-TiP phase (JCPDS 80-1067).
The Influence of α-TiP on the Charge-Transfer Resistance of Rct
Fitted parameters of the EmimBF4-based electrolytes with various contents of layered α-TiP
Content of α-TiP (wt%)
The Influence of Layered α-TiP on the Diffusion Coefficient of I3 −
Limiting current density (jlim) and diffusion coefficient (DI3 −) for EmimBF4-based electrolyte with various contents of layered α-TiP
Content of α-TiP (wt%)
DI3 − (10−7 cm2/s)
The Mechanism for the Improved Electrochemical Properties
However, when the α-TiP content was above 1wt%, both the diffusion of I3 − and charge-transfer reaction were suppressed with the increase in the α-TiP content, which might be explained by the following two aspects. On one hand, excess α-TiP probably exhausted all the Emim+ ions in the electrolyte and no more Emim+-pillared α-TiP could be formed. On the other hand, excess α-TiP powders would increase the viscosity of the electrolyte to some extent. Both the aspects described previously are harmful and would counteract the diffusion of I3 − and the charge transfer. Therefore, the optimal content of α-TiP was 1wt% in this study, where the diffusion of I3 − and the charge transfer were the most efficient.
The Influence of Layered α-TiP on the Photovoltaic Performance of DSSC
Photovoltaic parameters of DSSCs based on the composite electrolytes with various contents of layered α-TiP
Content of TiP (wt%)
In summary, we reported a composite electrolyte prepared by incorporating layered α-titanium phosphate into EmimBF4-based electrolyte, which exhibited excellent electrochemical and photovoltaic properties. The charge-transfer reaction at the Pt/electrolyte interface and the diffusion coefficient of I3 − in the electrolyte were both enhanced markedly, both of which was ascribed to the intercalation behavior of Emim+ cations into layered α-TiP. The confinement effect by the nanochannels between the sandwiches of layered α-TiP was proposed to be responsible for the enhanced diffusion coefficient of I3 − in the electrolyte. The increased active area of α-TiP/electrolyte interface and the higher local concentration of I3 − around the Pt/electrolyte interface might contribute to the improved charge-transfer reaction. The optimum of 1 wt% α-TiP was obtained for the EmimBF4-based electrolyte, where both the diffusion of I3 −, the charge-transfer reaction and photoelectric conversion efficiency were the most efficient. These quasi-solid-state electrolytes offer a broad prospect for the application of ionic liquids in DSSC. They will enable the fabrication of flexible, compact, laminated solid-state devices free of leakage and available in various geometries.
The authors acknowledge financial support from National Nature Science Foundation of China (50802051), National High Technology Research and Development Program of China (863 Program, 2006AA03Z218) and China Postdoctoral Science Foundation (20060400055).
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