Flexible and platinum-free dye-sensitized solar cells with conducting-polymer-coated graphene counter electrodes.

Dye-sensitized solar cells (DSSCs) have emerged as a high-efficiency, low-cost alternative to solid-state silicon solar cells. Typically, DSSCs are composed of a mesoporous titania nanocrystal electrode on a transparent conductive oxide (TCO) substrate with ruthenium-based sensitizers on the titania nanocrystals, platinum on the TCO substrate as a counter electrode, and iodine/iodide electrolyte between the two TCO substrates. The high costs of these materials is an obstacle and prevents the commercialization of DSSCs. In particular platinum, which has a high conductivity, high catalytic activity, and is stable, is one of the most expensive components in DSSCs. To reduce costs, the development of alternative materials for use in DSSCs is needed. Another important issue for commercialization is finding a method to produce flexible DSSCs, because this would enable the fabrication of light-weight, thin, and low-cost DSSCs through roll-to-roll mass production. Several methods for fabricating titania nanoparticle films (for use as photoanode) or platinum (for use as counter electrode) on a TCO-coated plastic film have been devised. Poly(ethylene terephthalate) coated with indium-doped tin oxide (ITO–PET) is one of the best flexible TCO substrates. However, ITO–plastic substrates are well-known to easily lose their conductivity during bending tests, resulting in low stabilities compared to electrodes that use glass substrates. Hence, the development of next-generation flexible DSSCs is focused on replacing platinum and inorganic material-based TCOs simultaneously. The first to be considered as alternatives to platinum were carbon-based materials, such as activated carbon, carbon nanotubes, graphite, carbon black, and graphene, which have been used as catalysts for DSSCs. In a recent study, a carbon-based counter electrode achieved a power conversion efficiency of ca. 9%, which is comparable to that of platinumbased DSSCs. Another approach to platinum-free counter electrodes used several conducting polymers and obtained maximum conversion efficiencies of ca. 7.8% from poly(3,4alkylenedioxythiophene), 7.1% from microporous polyaniline, 7.07% from a nanographite/polyaniline composite, and 7.93% from poly(3,4-alkylenedioxythiophene) nanoporous layers prepared by electro-oxidative polymerization. However, there have been few reports on platinumand TCOfree counter electrodes achieving cell efficiencies comparable to Pt/TCO counter electrodes. Typical platinum-free counter electrode materials have been prepared on TCO substrates and showed efficiencies of over 9%. Because the TCO is also expensive, the next development for cost-effective counter electrodes should be the simultaneous omission of platinum and the TCO. Our group has reported DSSC counter electrodes with conducting polymers, replacing both platinum and the TCO substrate. Because the conducting polymer should, in addition to inducing an electrochemical reaction, also transport charges, highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) films are essential. When only PEDOT films without TCO were used as counter electrode in the fabrication of DSSCs, a power conversion efficiency of 5.08% was obtained. In the J–V curves, the Jsc and Voc values were comparable values to those of Pt/FTO based DSSCs, but the main obstacle were the low FF values, due to the low surface conductivity of PEDOT. Given that the polymer does not have a conductivity comparable to that of commercialized TCOs, there is still scope to improve the cell efficiency without resorting to the use of a TCO as substrate. Owing to its high conductivity, graphene has recently attracted attention as an alternative to TCOs for various electronic applications, such as displays, solar cells, and sensors. Several developments in its large-area synthesis and the transfer of high-quality graphene films onto a target substrate have created new pathways for the application of graphene to flexible devices. Herein, we report the cell performances of DSSCs with graphene, overcoated with a PEDOT film, on flexible target substrates. These were used as counter electrode without using additional TCO. The high conductivity of the graphene layer provides a further decrease of the surface resistance. Furthermore, the graphene/PEDOT counter electrodes fabricated on plastic substrates show very good mechanical flexibility. Figure 1 is an illustration detailing the fabrication steps of the graphene/PEDOT counter electrode on a PET substrate. First large-area, high-quality graphene films were grown on a rectangular piece of copper foil (thickness 25 mm) by using procedures described elsewhere. In brief, an annealing process with hydrogen gas cleans a surface of the copper catalyst material and expands grain size of the copper to synthesize high-quality graphene. After hydrocarbon atoms are ad[a] K. S. Lee, Prof. J. Y. Lee, Prof. J. H. Park School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology Sungkyunkwan University Suwon 440-746 (Korea) Fax: (+82)31-290-7272 E-mail : [email protected] [b] Y. Lee, Prof. J.-H. Ahn School of Advanced Materials Science and Engineering and SKKU Advanced Institute of Nanotechnology Sungkyunkwan University Suwon 440-746 (Korea) Fax: (+82)31-290-7400 E-mail : [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100430.