Mesoporous nitrogen-doped TiO2 sphere applied for quasi-solid-state dye-sensitized solar cell
© Xiang et al; licensee Springer. 2011
Received: 27 September 2011
Accepted: 24 November 2011
Published: 24 November 2011
A mesoscopic nitrogen-doped TiO2 sphere has been developed for a quasi-solid-state dye-sensitized solar cell [DSSC]. Compared with the undoped TiO2 sphere, the quasi-solid-state DSSC based on the nitrogen-doped TiO2 sphere shows more excellent photovoltaic performance. The photoelectrochemistry of electrodes based on nitrogen-doped and undoped TiO2 spheres was characterized with Mott-Schottky analysis, intensity modulated photocurrent spectroscopy, and electrochemical impedance spectroscopy, which indicated that both the quasi-Fermi level and the charge transport of the photoelectrode were improved after being doped with nitrogen. As a result, a photoelectric conversion efficiency of 6.01% was obtained for the quasi-solid-state DSSC.
Since a mesoscopic TiO2-based dye-sensitized solar cell [DSSC] was reported by O' Regan and Grätzel in 1991 , DSSC has emerged as a promising candidate for the practical photovoltaic application due to its low manufacturing cost and high energy conversion efficiency. Even though DSSC is now commercially available, market expansion is still limited because of the use of a liquid electrolyte, which always signifies instability due to the leakage of the solvent and the erosion of the electrode. It seems a good way to replace the liquid electrolyte by a solid-state or quasi-solid-state medium [2, 3]. Unfortunately, compared with liquid electrolyte, solid-state or quasi-solid-state DSSC still presents lower energy conversion efficiency. Therefore, how to improve the performance of DSSC composing of solid-state or quasi-solid-state medium is still a big issue.
As one of the key components, mesoscopic TiO2 film plays an important role in determining the performance of DSSC, which assumes both the task of dye anchorage and charge carrier transport. Over the past decade, substantial efforts have been made to improve the performance of DSSC through the reformation of the TiO2 film. The first strategy is to increase the light absorption efficiency of the photoanode by increasing the surface area of the TiO2 film, which provides sufficient room for dye loading. Another strategy is to improve the electron injection efficiency by adjusting the conduction band edge to match the LUMO of the dye . The last strategy is to increase the charge collection efficiency through the improvement of the electron transport or lifetime .
For an effective DSSC, the key parameters of the mesoscopic TiO2 film such as porosity, pore size distribution, light scattering, electron percolation, and conduction band edge should be coordinated to the characterization of the dye and electrolyte medium, which could be controlled by precursor chemistry, hydrothermal growth temperature, binder addition, doping materials, and sintering conditions . Recently, Chen et al.  reported a mesoporous anatase TiO2 bead with high surface area and controllable pore size for DSSC, which indicated that the DSSC employing mesoporous TiO2 sphere has demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 titania electrodes due to the favorable interconnected, densely packed nanocrystalline TiO2 particles inside the spheres, resulting in the improvement of the parameters. In 2005, Ma et al.  reported nitrogen-doped TiO2 particles for DSSC that enhanced the incident photon-to-current conversion efficiency within the spectrum of 380 to 520 nm and 550 to 750 nm; as a result, the short-circuit photocurrent density was pronouncedly improved. After that, Tian et al.  found that the position of the flatband potential edge was shifted to negative when doping nitrogen into the TiO2 film, which is attributed to the formation of an O-Ti-N bond and results in the improvement of the open-circuit voltage in DSSC.
Therefore, it could be expected to promote the performance of DSSC from the improvement of both the short-circuit photocurrent density and the open-circuit voltage with the nitrogen-doped TiO2 sphere. Herein, we attempted to synthesize a crystalline nitrogen-doped TiO2 sphere under the hydrothermal condition and apply it in quasi-solid-state DSSC. The results indicated that with the nitrogen-doped TiO2 sphere, the parameters of quasi-solid-state DSSC were improved in both the short-circuit photocurrent density and the open-circuit voltage. As a result, a power conversion efficiency up to 6.01% of quasi-solid-state DSSC was obtained under air mass [AM] 1.5 sunlight at 100 mW/cm2.
Mesoporous TiO2 spheres were synthesized with the hydrothermal method as follows: 8 g dodecylamine and 8 g titanium isopropoxide [TIP] were mixed with 360 mL ethanol. The urea solution was added into the dodecylamine-TIP mixture solution under vigorous stirring at ambient temperature. The molar ratio of the urea to TIP was adjusted to 0, 4, 8, and 16. Twelve hours later, the white TiO2 suspension was transferred into a Teflon-lined autoclave and then heated at 210°C. After 12 h, the nitrogen-doped TiO2 spheres were obtained and marked as TN0, TN1, TN2, and TN3, respectively.
Mesoscopic nitrogen-doped or undoped TiO2 sphere pastes for screen print were prepared as reported . Hydroxypropyl cellulose was dissolved in ethanol to form 10 wt.% ethanolic mixture. Two grams of mesoscopic nitrogen-doped TiO2 sphere, 5 g 10 wt.% ethanolic mixture, 8.1 g terpineol, and 10 mL ethanol were mixed by ball milling for 12 h. Then, the slurry was sonicated for 30 min and concentrated to a paste with 20 wt.% TiO2.
The mesoscopic nitrogen-doped or undoped TiO2 sphere-based photoelectrode was coated on a fluorine-doped tin oxide [FTO] glass (TEC15; Pilkington TEC Glass™, IL, USA) by using a doctor blade and then sintered at 450°C for 30 min. The thickness of nitrogen-doped or undoped TiO2 sphere film was controlled by the thickness of scotch tape. The photoelectrode with a 2-μm TiO2 sphere layer was used for Mott-Schottky [MS] analysis. The electrode with a 12-μm TiO2 sphere film was dyed with N719 ethanolic solution. A sandwich structure quasi-solid-state DSSC containing the mesoscopic TiO2 sphere-based electrode, platinized counter electrode, and quasi-solid-state electrolyte was performed for photocurrent-voltage testing, which was sandwiched with a 100-μm film of Surlyn (DuPont Surlyn, Wilmington, DE, USA) and then heated at 100°C for sealing.
The surface appearance of their films on FTO glass was performed using a Sirion 200 field emission scanning electron microscopy [FE-SEM] (FEI Company, Eindhoven, the Netherlands). Veeco Dektak 150 (Veeco Instruments Inc., NY, USA) was used to detect the thickness of the TiO2 film. MS analysis was performed in a three-electrode cell under a dark condition, where the TN0- and TN2-based electrodes without dye (ca. 2 μm) were used as the work electrode, a saturated calomel electrode [SCE] served as the reference electrode, a platinum wire was used as the counter electrode, and the active area was 0.36 cm2. The ethylene carbonate and propylene carbonate (1:1 volume ratio) solution containing 0.01 M iodine [I2] and 0.1 M potassium iodide was used as an electrolyte for MS analysis. Intensity modulated photocurrent spectroscopy [IMPS] was characterized within TN0- and TN2-based DSSC and performed with a green light emitting diode (λ max = 535 nm) driven by Zahner Zenium. The photocurrent density-voltage curves of the quasi-solid-state DSSC were perfomed with a Keithley 2400 SourceMeter (Keithley Instruments, Inc., Ohio, USA) under illumination with an Oriel solar simulator (Newport Corporation, CA, USA) composed of a 1, 000-W xenon arc lamp and AM 1.5 G filters. Light intensity was calibrated with a normative silicon cell. The electrochemical impedance spectroscopy [EIS] of the quasi-solid-state DSSC was characterized by a potentiostat (M2273; EG&G Inc., MD, USA) with a frequency range from 100 mHz to 1 MHz.
Results and discussion
where C sc is the space charge capacity, N D is the electron density, ε is the dielectric constant, ε 0 is the permittivity of vacuum, and A is the active surface area. E fb is the intercept extrapolating a straight line from the plot of the square of the space charge capacity against the applied voltage, which found that the E fb of the electrode using TN0 and TN2 are about -0.55 V and -0.68 V (vs. SCE), respectively. Compared with the undoped TiO2 sphere-based electrode, the E fb of the nitrogen-doped TiO2 sphere-based electrode is shifted to about 0.13 V negatively, which suggests that a higher quasi-Fermi level could be obtained by doping nitrogen, resulting in the enhancement of V oc of the mesoscopic nitrogen-doped TiO2 sphere-based DSSC.
A series of mesoscopic nitrogen-doped and undoped TiO2 spheres for quasi-solid-state DSSC have been prepared by the hydrothermal method. The effects of nitrogen within TiO2 spheres on the photovoltaic performance have been investigated, which show that the introduction of nitrogen increases J sc, V oc, and efficiency. The photoelectrochemistry of nitrogen-doped and undoped TiO2 sphere-based electrodes was performed with MS, IMPS, and EIS. The results indicated that the quasi-Fermi level and the charge transport of the photoelectrode were improved, and the interfacial recombination was retarded after being doped with nitrogen within a mesoscopic sphere-based electrode. As a result, a DSSC with a power conversion efficiency of 6.01% under AM 1.5 sunlight at 100 mW/cm2 has been obtained with N719 dye in combination with a quasi-solid state electrolyte. Higher parameter DSSC could be expected with this mesoscopic nitrogen-doped TiO2 sphere.
The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 61106056), the Nature Science Foundation of Hubei Province (2008CDA042), and the Fundamental Research Funds for the Central Universities (Hust, 2011TS020). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology [HUST] for field emission scanning electron microscopy [FE-SEM] testing.
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