High-efficiency dye-sensitized solar cells based on robust and both-end-open TiO2 nanotube membranes
© Lin et al; licensee Springer. 2011
Received: 15 April 2011
Accepted: 27 July 2011
Published: 27 July 2011
In the present work, dye-sensitized solar cells (DSSCs) were fabricated by incorporating transparent electrodes of ordered free-standing TiO2 nanotube (TNT) arrays with both ends open transferred onto fluorine-doped tin oxide (FTO) conductive glass. The high-quality TiO2 membranes used here were obtained by a self-detaching technique, with the superiorities of facile but reliable procedures. Afterwards, these TNT membranes can be easily transferred to FTO glass substrates by TiO2 nanoparticle paste without any crack. Compared with those DSSCs consisting of the bottom-closed membranes or attached to Ti substrate, the carefully assembled and front-side illuminated DSSCs showed an enhanced solar energy conversion efficiency as high as 5.32% of 24-μm-thick TiO2 nanotube membranes without further treatments. These results reveal that by facilitating high-quality membrane synthesis, this kind of DSSCs assembly with optimized tube configuration can have a fascinating future.
The application of semiconductor TiO2 in dye-sensitized solar cells (DSSCs) was extensively investigated for its low cost and high energy conversion efficiency. The high-efficiency DSSCs were first reported in 1991 by O'Regan and Grätzel with a power conversion efficiency of 7.12% . The photoanodes are consisted of disordered TiO2 anatase nanoparticle films, with sufficient dye anchored for high light harvesting, on transparent conducting oxide glass. The latest certified efficiency of DSSCs, which are based on the nanoparticulate TiO2 photoanode, is 11.2% . However, the losses in nanoparticulate DSSCs were large because of the carrier recombination at grain boundaries and long carrier diffusion paths through the TiO2 nanoparticle network . Compared with nanoparticles, highly ordered vertically oriented TiO2 nanotube (TNT) arrays cannot only offer a large internal surface area but also reduce recombination probabilities and provide a directed electron traveling path . DSSCs with pure TiO2 nanotubes [5–9] and treated TiO2 nanotubes [10, 11] (treated with TiCl4 for instance) both show good performance on photon-to-current conversion efficiency. The best efficiency records are 5.2% and 7%, respectively .
DSSC with the photoanode of TNTs grown on Ti foil requires backside illumination, which may cause light reflection and absorption by the counter electrode and the electrolyte [12, 13]. To resolve this drawback, Grimes and co-workers demonstrated a transparent TiO2 nanotube-based photoanode by an anodization of Ti thin film sputtered on fluorine-doped tin oxide (FTO) conductive glass . However, procedures for photoanode fabrication were complex and costly, which required special treatment of the metal layer in contact with the electrolyte surface  and strict process control . In addition, the increase in film thickness will lead to the poor adhesion to the substrate [17–19]. Free-standing nanotubes detached from Ti substrate and fixed onto the FTO glass is another approach to prepare photoanode of front-side illuminated DSSCs. Chen and Xu developed a two-step anodization process to fabricate large-area free-standing TiO2 nanotube arrays and transferred them onto FTO glass . Lei et al. reported the formation of large-scale free-standing TNT arrays via sonication of TNT arrays on Ti foil and transferred them onto the FTO glass . In these cases, the bottom ends of the nanotubes are closed. Recently, Lin et al. introduced a transparent photoanode made of ordered opened-end TNT arrays transferred onto FTO glass and observed an increase in efficiency than closed-end TNTs . However, it needed an additional chemical etching step to open the closed bottom end, and due to the complex fabrication procedures, this cell configuration has not been paid extended attention.
Recently, we reported a facile fabrication rote to synthesis free-standing TiO2 nanotube membranes, with both the closed and open bottom-side morphologies, through the so-called self-detaching anodization . The films with high-quality surfaces and both-side-open tubes could be obtained by appropriate thermal treatment during the process. This self-detaching process is easy and efficient without additional chemical dissolution or etching. So we expect that by facilitating the synthesis process and improving film quality, high-quality DSSCs based on both-end-open tubes can have broad prospects. Herein, we report our results on application of these transparent, free-standing TNT membranes for use on the photoanodes in DSSCs. Photoanodes consisting of high-quality TiO2 membranes well attached to the FTO can be successfully fabricated. Under air mass (AM) 1.5 G solar light, the DSSC based on the 24-μm-thick tubes without TiCl4 treatment and under not-optimized conditions shows the best solar cell efficiency of 5.32%, which is consistent with the current recorded efficiencies. By further optimizing the tube length, annealing temperature and electrolyte composition, the efficiency can be expected to be further improved.
A detailed methodology of fabricating free-standing TNT membranes via self-detaching anodization has been mentioned elsewhere . Hence, we only summarized the key points of the fabrication processes here. To grow TNT arrays on Ti, a common two-electrode electrochemical cell was used with the working electrode Ti foil (0.2 mm thickness, Strem Chemicals, Newburyport, MA, USA) and the counter electrode Pt foil. Anodization was carried out in an ethylene glycol electrolyte containing 0.5 wt.% NH4F + 3 vol.% deionized water under a constant voltage of 60 V. The second-step anodization durations are 0.5 and 1 h for different film thicknesses, and the resulting oxide films were subjected to thermal treatments at 200°C and 400°C for both-end-opened and bottom-end-closed membranes, respectively. Afterwards, the as-formed films were completely detached by the third-step anodization in about 1 h under a temperature of 30°C.
A TiO2 nanoparticle (TNP) viscous paste was prepared as follows: mixed TiO2 nanoparticles (P25, Degussa, Borger, TX, USA) with 3 vol.% acetic acid solution at the weight rate of 3:10 and stirred for 1 h. The TNP paste was spin-coated onto FTO glass substrates, and the free-standing TiO2 nanotube membranes were transferred onto the paste layers immediately. After being dried in air, the films were sintered at 450°C for 3 h. The as-formed electrodes were then immersed in a 1:1 (v/v) acetonitrile and ethanol solution containing 3 × 10'4 M RuL2(NCS)2:2TBA (N719 dye, L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, TBA = tetrabutylammonium, Dyesol, Queanbeyan, New South Wales, Australia) for 24 h. The sensitized electrodes were further sandwiched with the sputtered-Pt FTO glass, separated by a 60-μm-thick hot-melt spacer. The intervening space was filled with a common kind of liquid electrolyte of DMPII/LiI/I2/TBP/GuSCN in acetonitrile (DHS-E23, Heptachroma, Dalian, China).
Four kinds of DSSCs were prepared for investigation: first, photoanode made of free-standing TNT membrane with both ends open transferred onto FTO glass (O-FTO); second, photoanode made of free-standing TNT membrane with the bottom ends closed transferred onto FTO glass (C-FTO); third, photoanode made of TNP layer (approximately 10 μm) pasted on FTO by doctor blade technique (NP-FTO); and fourth, photoanode made of TNT arrays on Ti substrate (NT-Ti), with a film thickness of about 24 μm. The third and fourth samples are for photoelectric performance comparison. All the TNTs in our experiments are pure without any treatment.
Field emission scanning electron microscope (FE-SEM, FEI Sirion 200, FEI Company, Hillsboro, OR, USA) was utilized for morphological and structural characterization. Photocurrent-voltage characteristics (J-V curves) were measured under AM 1.5 G solar simulator (Oriel Sol3A, Newport Corporation, Irvine, CA, USA) at a light intensity of 100 mW/cm2.
Results and discussion
The values of J sc, V oc, FF, and η for all the DSSC samples
J sc (mA/cm2)
V oc (V)
Front-illuminated DSSCs based on both-end-opened TNT membranes, which were prepared by a simple self-detaching method, were achieved with the photoanodes consisting of these membranes adhered on FTO glass by TNP paste. Compared with NT-Ti-based backside illuminated DSSCs with an efficiency of 3.04% and C-FTO-based DSSCs with an efficiency of 4.52%, the O-FTO-based DSSCs with a film thickness of 24 μm showed the best efficiency of 5.32%. The simple and reliable assembly of this high-efficiency solar cell configuration can open new prospect for future DSSC application.
This research was supported by the National Natural Science Foundation of China (contract no. 10874119), the National Basic Research Program "973" of China (contract no. 2007CB307000), and the Foundation for Development of Science and Technology of Shanghai (grant no. 10JC1407200).
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