Understanding Interfacial Charge Transfer between Metallic PEDOT Counter Electrodes and a Cobalt Redox Shuttle in Dye-Sensitized Solar Cells Byung-wook Park, † Meysam Pazoki, † Kerttu Aitola, † Seunghee Jeong, ‡ Erik M. J. Johansson, † Anders Hagfeldt, †,§ and Gerrit Boschloo* ,† † Department of Chemistry, Ångströ m Laboratory, and ‡ Division of Solid-State Electronics, Ångströ m Laboratory, Uppsala University, Box 532, SE 751-20 Uppsala, Sweden § School of Chemical Engineering, Sungkyankwan University, Suwon 440-746, Korea * S Supporting Information ABSTRACT: Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with iron(III) tris-p-toluenesulfonate (PEDOT:Tos) having metallic conductivity was coated onto fluorine-doped tin oxide (FTO) glass and plain glass substrates and used as a counter electrode (CE) in a dye-sensitized solar cell (DSC) with a [Co(bpy) 3 ] 3+/2+ complex redox shuttle. DSCs with PEDOT:Tos/glass CE yielded power conversion efficiencies (PCE) of 6.3%, similar to that of DSCs with platinized FTO glass CE (6.1%). The PEDOT:Tos-based counter electrodes had 5 to 10 times lower charge-transfer resistance than the Pt/FTO CE in DSCs, as analyzed by impedance spectroscopy. More detailed studies in symmetrical CE-CE cells showed that the PEDOT:Tos layers are nanoporous. Not all internal area can be used catalytically under solar cell conditions and effective charge-transfer resistance was similar to that of Pt/FTO. KEYWORDS: dye-sensitized solar cells, counter electrode, electrocatalytic activity, metallic PEDOT, tosylate, Co complex electrolyte 1. INTRODUCTION Dye-sensitized solar cells (DSC) are attracting a lot of attention because of their high indoor power conversion efficiency and low cost compared to inorganic solar cells. 1-3 The DSC consists of a mesoporous TiO 2 film on a transparent conducting oxide-coated (TCO) glass substrate working electrode (WE), sensitized with dye molecules, a redox electrolyte, and a counter electrode (CE) composed of a platinum-coated TCO plate. Among these components, the TCO glass used both at the WE and the CE is a rather expensive component. 4 The most frequently used TCO in DSCs is fluorine-doped tin oxide (FTO). A relatively expensive component in the DSC is also the Pt catalyst at the counter electrode. A number of investigations have been carried out to replace the Pt-coated FTO counter electrodes in DSCs by low-cost materials, such as different carbon species 5-11 and conducting polymers. 12-21 DSCs with carbon black-based counter electro- des have yielded record efficiencies of about 9% under 1 sun illumination. 11 With conducting polymers, micro-porous polyaniline CE for DSC resulted in a solar cells with a PCE of 7.1%. 13 DSCs with poly (3,4-propylenedioxythiophene) (PProDOT) coated onto FTO CE reached PCE of approximately 10%. 15 PEDOT-coated FTO CE in combination with a cobalt complex electrolyte have yielded a PCE of around 10%. 16 Carli et al. compared the catalytic properties for PEDOT on FTO with ClO 4 - , sodium dodecylsulfate (SDS), or polystyr- enesulfonate (PSS) as counterions to that of gold and platinum-coated FTO CE in cobalt complex-based electrolytes, where gold and PEDOT/ClO 4 - yielded the best results. 17 To further improve the counter electrode and the solar cell performance, researchers have investigated hybrid PEDOT counter electrodes, such as carbon nanotube/PEDOT, graphene/PEDOT, and metal/PEDOT. 18,19,27-29 PEDOT is a promising alternative counter electrode material due to its high conductivity, electrochemical stability, semi- transparency, and catalytic performance. Because of its metal- like high conductivity, PEDOT:Tos can even replace TCO on the counter electrode to lower the cost of DSC fabrication. 4,18 Solar cell efficiencies of cells with Pt/TCO free counter electrode in combination with I - /I 3 - redox mediator were comparatively good. In this study, we employ a PEDOT:Tos coated CE with a cobalt(III/II) tris(2,2′-bipyridine) ([Co(bpy) 3 ] 3+/2+ ) complex- based redox mediator and compared it with the I - /I 3 - redox mediator. The interfacial charge-transfer between the redox shuttles and the different CEs (Pt/FTO glass, PEDOT:Tos/ Received: November 16, 2013 Accepted: January 10, 2014 Published: January 10, 2014 Research Article www.acsami.org © 2014 American Chemical Society 2074 dx.doi.org/10.1021/am405108d | ACS Appl. Mater. Interfaces 2014, 6, 2074-2079