Transparent Graphene-Platinum Films for Advancing the Performance of Dye-Sensitized Solar Cells
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Transparent Graphene-Platinum Films for Advancing the Performance of Dye-Sensitized Solar Cells P. T. Shih1, R. X. Dong1, K. C. Ho1,2,* and J. J. Lin1,* 1 Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan. 2 Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan. ABSTRACT Transparent films of platinum nanoparticles on graphene nanohybrids were synthesized in a two-step process. Reduction of homogeneously dispersed Pt precursor and graphene in water and solution coating/annealing afforded thin films with high catalytic performance as counter electrodes in dye-sensitized solar cells (DSSC). The requisite dispersant consisting of poly(oxyethylene)-(POE) segments and cyclic imide functionalities allowed the in-situ reduction of dihydrogen hexachloroplatinate by ethanol and the formation of nanohybrids of graphenesupported Pt nanoparticles at 4.0 nm diameter. Characterizations of polymeric dispersants by Fourier-transform infrared spectroscopy, thermogravimetric analysis, and nanohybrids by transmission electron microscope were performed. After screening various compositions of Pt/graphene, the nanohybrid film at the specific ratio of 5/1 by weight was fabricated into a counter electrode (CE) for DSSC by the solution casting method. The evaluation of cell performance demonstrated the most improved power conversion efficiency of 8.00%. This is significant achievement in comparison with 7.14% for the DSSC with the conventional platinum sputtered CE. Furthermore, the solution casting method allows the preparation of transparent CE films that are suitable for using as rear-illuminated DSSC. The approach was proven to be feasible by measuring the cell efficiency under rear light illumination. The power efficiency up to 7.01%, comparable to 8.00% by a normally front illumination, has been accomplished. In contrast, the rear illumination at merely 2.36% efficiency was obtained for the DSSC with sputtered platinum CE. Analyses of cyclic voltammetry, electrochemical impedance spectra were well correlated to the high efficiency of the performance caused by this nanohybrid film. INTRODUCTION Recent advances in synthesizing new nanomaterials allow the controls of nanoparticle size and structural morphology. For example, the manipulation and utilization of carbonbased nanofillers including carbon black, carbon nanotube [1], carbon nanofiber [2], and graphene [3] have been well documented. Among these materials, graphene is unique for the 2D geometric shape and chemical compositions of hexagonally arrayed carbon atoms in single-atom-thick sheet. Owing to its high surface area [4], electrical conductivity [5] and transparency, graphene is potentially useful for fabrication into solar cell devices. However, the sheet-like graphene has the inherent force of aggregating through interlayer piling and difficulty for being dispersed in organic mediums [6,7] in the process of fabricating homogeneous films. In addition, unlike carbon nanotube, graphene is poor in cataly
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