Improving the performance of dye-sensitized solar cells with TiO2/graphene/TiO2 sandwich structure
© Chen et al.; licensee Springer. 2014
Received: 24 June 2014
Accepted: 24 July 2014
Published: 3 August 2014
This study investigates the extent to which the TiO2/graphene/TiO2 sandwich structure improves the performance of dye-sensitized solar cells (DSSCs) over that of DSSCs with the traditional structure. Studies have demonstrated that the TiO2/graphene/TiO2 sandwich structure effectively enhances the open circuit voltage (Voc), short-circuit current density (Jsc), and photoelectrical conversion efficiency (η) of DSSCs. The enhanced performance of DSSCs with the sandwich structure can be attributed to an increase in electron transport efficiency and in the absorption of light in the visible range. The DSSC with the sandwich structure in this study exhibited a Voc of 0.6 V, a high Jsc of 11.22 mA cm-2, a fill factor (FF) of 0.58, and a calculated η of 3.93%, which is 60% higher than that of a DSSC with the traditional structure.
Dye-sensitized solar cells (DSSCs) are attracting attention globally because of their low cost, high energy conversion efficiency and potential applications[1–4]. Graphene has been extensively utilized in organic photovoltaic (PV) cells owing to its excellent optical and electrical characteristics, which are exploited in transparent conductive films or electrodes[5–8]. Some researchers have reported on composite graphene-TiO2 photoelectrodes in DSSCs[9–12]. Fang et al.[9, 10] discussed the effect of the amount of graphene on the structures and properties of DSSCs. DSSCs with the optimal composite TiO2 film can achieve a photoelectrical conversion efficiency of 7.02%. Graphene is also commonly used in graphene-based counter electrodes in DSSCs[13–15]. The conventional counter electrode is platinum (Pt) because of its outstanding conductivity, catalytic activity, and stability when in contact with an iodine-based electrolyte. The expensive Pt can be replaced with graphene films in DSSCs without significantly sacrificing photoelectrical efficiency. This replacement can simply reduce the cost of the fabrication process. Zhang et al. grew DSSCs with graphene-based counter electrodes, which exhibited a photoelectrical conversion efficiency of as high as 6.81%. Double-layer photoelectrodes have been used to increase the photoelectrical conversion efficiency of DSSCs. Many investigations have focused on modifying the nanostructures of TiO2 photoelectrodes to nanospheres, nanospindles, nanorods, nanowires, and others[16–20]. Many special nanostructures of photoelectrodes can increase the scattering of light and improve the performance of DSSCs[16, 17].
This work develops a new TiO2/graphene/TiO2 sandwich structure for photoelectrodes. A thin layer of graphene was inserted into the traditional TiO2 photoelectrode layer, making it a double layer. DSSCs with the traditional structure were also fabricated and the characteristics of the prepared DSSCs were compared. The DSSC with the TiO2/graphene/TiO2 sandwich structure exhibited excellent performance and higher photoelectrical conversion efficiency. This improvement is associated with the increase in electron transport efficiency and the absorption of light in the visible range.
Preparation of TiO2 photoelectrodes
The TiO2 slurry was prepared by mixing 6 g of nanocrystalline powder (P25 titanium oxide; Evonik Degussa Japan Co., Ltd., Tokyo, Japan), 0.1 mL Triton X-100, and 0.2 mL acetylacetone. The slurry was then stirred for 24 h before being spin-coated on ITO glass substrate at a rotation rate of 2,000 or 4,000 rpm. Following the deposition of graphene, the above procedure was carried out in the fabrication of DSSCs with the TiO2/graphene/TiO2 sandwich structure. The as-prepared TiO2 photoelectrodes were dried and annealed at 450°C for 30 min.
Preparation of graphene
The graphene film was prepared using a radio-frequency magnetron sputtering system with a carbon target (99.99%, Optotech Materials Co., Ltd, Taichung, Taiwan). The graphene film was deposited on the surface of the first photoelectrode layer. The working pressure of the chamber was maintained at 3 mTorr. The constant RF power was 90 W; the flow rate of argon was 90 sccm, and the deposition time was 2 min.
The electrolyte was composed of 0.05 M iodide, 0.5 M lithium iodide, and 0.5 M 4-tert-butylpyridine (TBP) in propylene carbonate. A 100-nm-thick layer of platinum was sputtered onto the ITO substrate as an electrochemical catalyst to form the counter electrode. Cells were fabricated by placing sealing films between the two electrodes, leaving two via holes through which the electrolyte could be injected. The sealing process was performed on a hot plate at 100°C for 3 min. Then, the electrolyte was injected into the space between the two electrodes through via holes. Finally, the via holes were sealed using epoxy with a low-vapor transmission rate. DSSCs with different structures were prepared to examine the effect of structure on the properties of the DSSC. Sample 1 was fabricated with a traditional structure and a single TiO2 photoelectrode layer, which was spin-coated at a rotation rate of 4,000 rpm. Sample 2 also had the traditional structure with a single TiO2 photoelectrode layer, which was spin-coated at a rotation rate of 2,000 rpm. Sample 3 had the sandwich structure of TiO2/graphene/TiO2 on ITO glass, and the deposition of the TiO2 photoeletrodes was performed at rotation rate of 4,000 rpm.
The crystalline microstructure of the products was elucidated using a PANalytical X'Pert Pro DY2840 X-ray diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu-Kα radiation (λ = 0.1541 nm) in the scanning range 2θ = 30° and 70°. The surface morphology and vertical structure were analyzed using a LEO 1530 field-emission scanning electron microscope (One Zeiss Drive Thornwood, New York, USA). The optical absorption properties were measured in the range of 300 to 900 nm using a Hitachi U-2001 ultraviolet-visible spectrophotometer (Chiyoda, Tokyo, Japan). The photocurrent voltage (I-V) characteristics were measured using a Keithley 2420 programmable source meter under 100 mW cm-2 irradiation (Keithley Instruments Inc., Cleveland, OH, USA). Simulated sunlight was provided by a 500-W xenon lamp (Hong Ming Technology Co, Ltd, Taiwan) that had been fitted with an AM-1.5 filter. The active area of each DSSC, which was exposed to the light, was 0.3 × 0.3 cm2.
Results and discussion
Photovoltaic parameters of DSSCs fabricated with different structures
This work proposed a simple and convenient method to enhance the performance of DSSCs using a low-cost and easy fabrication process. DSSCs with three structures were fabricated, and the characteristics of these DSSCs, including the Jsc, Voc, and photoelectrical conversion η of these DSSCs, were investigated. Clearly, the induced graphene film and sandwich structure markedly improve the performance of the DSSCs. This improvement in performance is associated with an increase in the absorption of light, a wide range of absorption wavelengths, shorter charge transportation distances, and the suppression of charge recombination when the graphene is applied.
The financial support of this paper was provided by the National Science Council of the Republic of China under Contract No. NSC 102-2622-E-027-021-CC3.
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