Electrospun Carbon Nanofibers as Low-Cost Counter Electrode for Dye-Sensitized Solar Cells Prakash Joshi, † Lifeng Zhang, ‡ Qiliang Chen, † David Galipeau, † Hao Fong,* ,‡ and Qiquan Qiao* ,† Center for Advanced Photovoltaics, Department of Electrical Engineering, South Dakota State University, 020 EECS, Brookings, South Dakota 57007, and Department of Chemistry, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701 ABSTRACT Electrospun carbon nanofibers (ECNs) have been explored as an electrocatalyst and low-cost alternative to platinum (Pt) for triiodide reduction in dye-sensitized solar cells (DSCs). The results of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements indicated that the ECN counter electrodes exhibited low charge-transfer resistance (R ct ), large capacitance (C), and fast reaction rates for triiodide reduction. Although the efficiency (η) of ECN-based cells was slightly lower than that of Pt- based cells, their short circuit current density (J sc ) and open circuit voltage (V oc ) were comparable. The ECN-based cells achieved an energy conversion efficiency (η) of 5.5 % under the AM 1.5 illumination at 100 mW cm -2 . The reason for lower cell performance using the ECN electrode was because of its lower fill factor (FF) than that of Pt-based cells, probably caused by high total series resistance (R Stot ) at ∼15.5 Ω cm 2 , which was larger than that of ∼4.8 Ω cm 2 in the Pt-based devices. Simulated results showed that the fill factor (FF) and η could be substantially improved by decreasing R Stot , which might be achieved by using thinner and highly porous ECNs to reduce the thickness of the ECNs counter electrode. KEYWORDS: electrospinning • carbon nanofibers • dye-sensitized solar cells • counter electrode INTRODUCTION D ye-sensitized solar cells (DSCs) have attracted ex- tensive attention as a low-cost alternative to Si solar cells (1-7). A typical DSC consists of a photoanode and a counter electrode separated by an electrolyte contain- ing an iodide/triiodide (I - /I 3 - ) redox couple. The photoanode is usually a transparent conducting oxide (TCO) coated with a film of TiO 2 nanoparticles, while the counter electrode is a TCO coated with a thin layer of platinum (8). When dye molecules adsorbed onto TiO 2 surface are exposed to sun- light, photoelectrons are generated and injected into the photoanode; subsequently, the electrons move to the counter electrode through an external circuit. The oxidized dye molecules regain electrons from I - ions, resulting in the oxidation of I - ions into I 3 - ions; and the I 3 - ions are reduced into I - ions by obtaining electrons from the counter elec- trode. Pt counter electrodes have been widely used in DSCs, because Pt is an efficient electrocatalyst for reduction of I 3 - ions (9-11). Nonetheless, Pt is an expensive metal and the corrosive I - /I 3 - redox couple can reduce its catalytic activity, which raises concerns about the long-term stability of DSCs (12). Previous studies have revealed that carbonaceous materi- als including graphite, carbon black, carbon nanotubes, and poly(3,4-ethylenedioxythiophene) (PEDOT) doped with p- toluenesulfonate (PEDOT-TsO) or polystyrenesulfonate (PEDOT-PSS), can exhibit comparable electrocatalytic per- formance to Pt for the reduction of I 3 - ions (3, 12-25). It is noteworthy that carbonaceous materials are abundant and low-cost and also possess high resistivity against corrosion (22). Therefore, the replacement of Pt with low-cost carbon- aceous materials could facilitate the commercialization of DSCs (16-18, 26). Herein, we report for the first time that electrospun carbon nanofibers (ECNs) can be used as a low-cost alterna- tive to Pt counter electrodes in DSCs. The ECN-based DSCs had similar performance as that of Pt-based devices in terms of short circuit current density (J sc ) and open circuit voltage (V oc ). Electrochemical measurements indicated that the ECN counter electrode exhibited low charge-transfer resistance (R ct ), large capacitance (C), and fast reaction rates for I 3 - reduction, indicating that the ECN electrode is an efficient electrocatalyst for the application in DSCs. EXPERIMENTAL SECTION Preparation of Electrospun Carbon Nanofibers (ECN). The ECNs were developed through electrospinning of polyacryloni- trile (PAN) nanofibers, followed by thermal treatments of stabilization and carbonization (27). Prior to electrospinning, the PAN (S.A.F 3K, Courtaulds, U.K.) was first dissolved in N,N- dimethylformamide (DMF) to prepare a 14 wt. % solution. Subsequently, the solution was filled in a 30 mL BD Luer-Lok tip plastic syringe having a stainless-steel needle with 18 gauge * To whom correspondence should be addressed. Tel.: 605-688-6965 (Q.Q); 605- 394-1229 (H.F.). Fax: 605-688-4401 (Q.Q); 605-394-1232 (H.F.) E-mail: Qiquan.Qiquan@sdstate.edu (Q.Q); Hao.Fong@sdsmt.edu (H.F.). Received for review August 16, 2010 and accepted October 26, 2010 † South Dakota State University. ‡ South Dakota School of Mines and Technology. DOI: 10.1021/am100742s 2010 American Chemical Society ARTICLE 3572 VOL. 2 • NO. 12 • 3572–3577 • 2010 www.acsami.org Published on Web 11/12/2010