Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell
© Liu et al.; licensee Springer. 2014
Received: 19 May 2014
Accepted: 9 July 2014
Published: 20 July 2014
Large-sized titanium dioxide (TiO2) nanotube arrays with an outer diameter of approximately 500 nm have been successfully synthesized by potentiostatic anodization at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. It is found that the synthesized large-diameter TiO2 nanotube array shows a superior light scattering ability, which can be used as a light scattering layer to significantly enhance the efficiency of TiO2 nanoparticle-based dye-sensitized solar cells from 5.18% to 6.15%. The remarkable light scattering ability makes the large-diameter TiO2 nanotube array a promising candidate for light management in dye-sensitized solar cells (DSSCs).
Dye-sensitized solar cells (DSSCs) have received considerable interest since 1991  with the growing concern on sustainable and renewable energies. The highest power conversion efficiency (PCE) of DSSCs based on TiO2 nanoparticle mesoporous films has been reported , and to further improve the PCE, plenty of research has been carried out, such as the development of new dyes with broadband absorption [3, 4], the increase of the sensitized surface area of the TiO2 film [5, 6], and the use of a scattering layer for enhanced light harvesting [7–13]. Among them, the introduction of a scattering layer with different structures has been widely studied and proven to be effective in light harvesting enhancement. TiO2 nanorods with a length of 180 to 250 nm have been used as scattering centers in DSSCs by Yoon et al. . Liu et al. had dispersed TiO2 nanospheres into nanocrystallites for increased light harvesting in DSSCs .
However, scattering centers of large-scale micrometer particles embedded in the absorbing layer of DSSCs would reduce the dye loading amounts. Hence, a bi-layer structure with the scattering layer beneath the absorbing layer to increase the optical path length is more favorable. Hierarchical TiO2 hollow spheres with an outer diameter of 300 to 700 nm  and size-tunable mesoporous spherical TiO2 have been tried as the scattering layer in bi-layer-structured DSSCs.
While the scattering of nanofibers and nanotubes was found to satisfy the Mie theory, which was originally proposed to describe the scattering of particles of a size similar to the wavelength [13–15], there are only few relevant reports on applying TiO2 nanotubes with a subwavelength-sized diameter as the scattering layer. Herein, we succeeded in a straightforward approach to the fabrication of large-diameter (comparable to wavelength) TiO2 nanotubes and characterized the light scattering effect by transmittance spectra measurement and finite-element full wave simulation. The anodization was processed at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. The transmittance spectra showed that the large-diameter nanotubes have a superior light scattering property. When the large-diameter TiO2 nanotube membrane was successfully peeled off and used as the scattering layer in DSSCs, the PCE was found to increase from 5.18% to 6.15% under 1 Sun (or 5.23% to 6.36% under 0.5 Sun) and increased by 19% (or 22%) due to the strong light scattering of large-diameter TiO2 nanotubes.
The large-diameter TiO2 nanotubes were fabricated through potentiostatic anodization in a conventional two-electrode electrochemical cell. Titanium sheets (0.125 mm in thickness, Strem Chemicals, Newburyport, MA, USA) were used as the working electrode while Pt foil was the counter electrode, with the distance between electrodes being 2 cm. The anodization process was divided into three steps: (1) The Ti foil was electrochemically pretreated for 0.5 h at 60 V in an ethylene glycol electrolyte containing 0.5 wt% NH4F and 3 vol% H2O (anodization electrolyte #1). After anodization, the anodized layer was peeled off by intense ultrasonication to expose the substrate. (2) The surface-exposed Ti was processed in another ethylene glycol electrolyte with 0.5 wt% NH4F and 10 vol% H2O, added with 1.5 M lactic acid (LA) (anodization electrolyte #2). Electrolyte #2 was aged by anodization reaction at 60 V for about 10 h before usage. To fabricate large-diameter nanotubes, the anodization voltage was fixed at 120 V for 10 min and then gradually increased to 180 V for 10 min at a rate of 0.1 V/s. (3) The as-grown large-diameter nanotubes were annealed at 450°C for 2 h and then detached from the Ti substrate by a third anodization at 60 V in electrolyte #1 to obtain the freestanding membranes . For comparison, freestanding TiO2 nanotube membranes of the same thickness but with smaller diameters were also fabricated by anodization at 60 V for 10 min in electrolyte #1.
The resulting nanotube membrane was used as a scattering layer by adhering to the fluorine-doped tin oxide (FTO) substrate with TiO2 NP paste via a doctor blade method, followed by sintering at 450°C for 2 h. The sintered photoanodes were immersed in a dye-containing solvent (N719 dye, Dyesol, Queanbeyan, New South Wales, Australia) for 3 days. A 25-μm-thick hot-melt spacer was used to separate the sensitized photoanode and the counter electrode which was prepared by thermal decomposition of H2PtCl6 isopropanol solution on FTO glass at 380°C for 30 min. The interspace was filled with a liquid electrolyte of DMPII/LiI/I2/TBP/GuSCN in 3-methoxypropionitrile.
The structure and morphology of the TiO2 nanotubes were analyzed using field-emission scanning electron microscopy (FESEM; JEOL JSM-6335 F, JEOL Ltd., Tokyo, Japan). The current density-voltage (J-V) characteristics were measured by a sourcemeter (Model 2420, Keithley Instruments, Inc., Cleveland, OH, USA) under AM 1.5G illumination (100 mW cm−2) which was provided by a 300-W solar simulator (Model 91160, Newport Corporation-Oriel Instruments, Irvine, CA, USA). Transmittance spectra were acquired using a UV–Vis spectrophotometer (Model UV-2550, Shimadzu Corporation, Tokyo, Japan). The amount of dye was measured by desorbing the attached dye molecules in 0.1 M NaOH aqueous solution, with the concentration determined by a UV–Vis spectrophotometer. The normalized incident photon-to-current conversion efficiency (IPCE) values were measured with an IPCE system equipped with a xenon lamp (Oriel 66902, 300 W), a monochromator (Newport 66902), and a dual-channel power meter (Newport 2931_C) equipped with a Si detector (Oriel 76175_71580).
Results and discussion
Photovoltaic properties of the DSSCs with and without the scattering layers
Relative dye loading
η(%) 1 Sun
η(%) 0.5 Sun
TP (3 L)
TP (3 L) + LTNA
14 + 1.8
TP (3 L) + STNA
14 + 1.8
The incident photon-to-current conversion efficiency (IPCE) spectra are depicted in Figure 3b to provide detailed information on light harvesting. It is observed that the main light harvesting enhancement caused by the scattering layer occurs not only in the dye absorption range but also in the long wavelength side [24, 25], which is exactly the wavelength range for the small dye absorption. Consequently, the TP (3L) + LTNA cell is able to more efficiently recapture the unabsorbed light which resulted from the efficient light scattering capability of the LTNA layer. A further insight into the electrochemical behavior was provided by the EIS measurement in the dark at different applied bias voltages. The electron recombination time (τn) was calculated from the Bode phase plots by τn = 1/(2πfpeak), where fpeak is the characteristic peak frequency in the mid-frequency (1 to 100 Hz) region [5, 26]. As shown in Additional file 1: Figure S4, the use of the light scattering layer does not significantly influence the τn and hence does not affect the electron transport.
Large-diameter TiO2 nanotube arrays were successfully synthesized. The outstanding scattering power of the LTNA layer was demonstrated by the transmittance spectra and the optical simulation. The LTNA layer is superior to the STNA one in terms of light scattering. The use of the LTNA as the scattering layer in DSSCs enhances the PCE (from 5.18% to 6.15%) and the short-circuit current density much more than the STNA does. It is believed that the large-diameter nanotubes can be applied to other types of solar cells and higher conversion efficiency can be achieved by further optimization.
The work was supported by grants received from the Research Grants Council of the Hong Kong Special Administrative Region (Project Nos. PolyU5159/13E and PolyU5163/12E) and the Hong Kong Polytechnic University (Project No. G-YL06). The work was also supported by the National Natural Science Foundation of China (Grant No. 61125503) and the Foundation for Development of Science and Technology of Shanghai (Grant No. 11XD1402600).
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