Discrete spherical polypyrrole (PPy) nanoparticles with a uniform diameter of $85 nm and electrical conductivity of $10 S cm À1 were fabricated by chemical oxidative polymerization within micelles composed of myristyl trimethyl ammonium bromide (MTAB) and decyl alcohol as the nanoreactors. A methanol-based colloidal dispersion containing 5 wt% PPy was dropcast directly onto fluorine-doped tin oxide (FTO) glass to use as counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). The surface resistivity of the PPy layer on the FTO glass decreased from 624 to 387 U sq À1 after post-doping with concentrated HCl vapor for 1 min. The DSSCs made of PPy/FTO and HCl-doped PPy/FTO CEs exhibited power conversion efficiencies of $5.28 and $6.83%, respectively under standard AM 1.5 sunlight illumination. The post-doped and highly oxidized PPy allowed the electrons to move into the PPy layer readily and facilitated the electrocatalytic reaction of the I 3 À /I À redox couple, giving enhanced cell performance. Moreover, the cell efficiency was enhanced to $7.73% with further fine tuning of the electrolyte composition, which is comparable to the value ($8.2%) using conventional Pt CEs.

Poly (3,4-ethylenedioxythiophene) nanofibers (PEDOT-NFs) with 10-50 nm diameters and high electrical conductivity (up to 83 S cm À1 ) have been fabricated chemically within micelles composed of sodium dodecyl sulfate (SDS) as nanoreactors. Dimethyl sulfoxide (DMSO, 5 wt%) was added to a methanol-based colloidal dispersion containing PEDOT-NFs to improve dispersibility. The dye-sensitized solar cells (DSSCs) using PEDOT-NFs as a counter electrode (CE) with low surface resistance and a highly porous surface exhibited enhanced power conversion efficiency (9.2%) compared to bulk PEDOT (6.8%). To date, PEDOT-NFs have shown the highest performance among CEs using PEDOT. It also surpassed the efficiency of platinum (Pt) CEs (8.6%). Electrochemical characterization, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), showed that PEDOT-NFs efficiently reduced triiodide due to the enhanced electrical and morphological properties, which contributed to the high current density (17.5 mA cm À2 ) and fill factor (72.6%).

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b S Supporting Information ' INTRODUCTION Solar energy has been considered a green and renewable alternative energy source to fossil fuels. 1 Photovoltaic systems are among the best means of harvesting energy directly from sunlight. In particular, dye-sensitized solar cells (DSSCs), based on sensitizer dye adsorbed nanocrystalline TiO 2 anode, electrolyte, and, traditionally, platinized counter electrodes, are interesting photovoltaic devices because of their powerful sunlightharvesting efficiency, low cost, and ease of fabrication. 2,3 Until recently, much effort had been focused on the optimization 4 of sensitizer dyes, 5À7 photoanode materials, 8,9 and electrolytes 10À12 for more efficient DSSCs; however, counter electrodes 13,14 have received less attention. Generally, platinum (Pt) layered transparent conductive oxide (TCO) substrates produced by sputtering, electrochemical deposition, or thermal deposition methods are currently used as counter electrodes because of their high electrocatalytic activity toward I À /I 3 À redox couples commonly used for DSSCs. 13,14 "Champion cells" with power conversion efficiencies higher than 11% also use Pt counter electrodes. 15 To further improve the power conversion efficiency of DSSCs, new types of efficient counter electrodes with lower charge-transfer resistance (R ct ) and higher reduction rates of redox couples should be developed. Efficient counter electrodes can reduce inner energy losses in DSSCs, improving current density and fill factors.Recently, electrically conducting polymers 16À22 and various carbonaceous materials 23À27 have been widely studied for potential use as DSSC counter electrodes because of their low material cost and high electrocatalytic activity for reducing I 3 À