Journal of Power Sources 196 (2011) 10798–10805 Contents lists available at SciVerse ScienceDirect Journal of Power Sources jou rnal h omepa g e: www.elsevier.com/locate/jpowsour Carbon-nanoﬁber counter electrodes for quasi-solid state dye-sensitized solar cells Ganapathy Veerappan, Woosung Kwon, Shi-Woo Rhee ∗ System on Chip Chemical Process Research Center, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea a r t i c l e i n f o Article history: Received 14 June 2011 Received in revised form 29 August 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Dye-sensitized solar cells Carbon-nanoﬁbers Charge transfer resistance Flexible counter electrode Quasi-solid state electrolyte a b s t r a c t Carbon-nanoﬁbers (CNFs) with antler and herringbone structures are studied as a tri-iodide (I 3 - ) reduc- tion electrocatalyst in combination with the liquid electrolyte or an alternative stable quasi-solid state electrolyte. The catalytic properties of the counter electrode (CE) are characterized by cyclic voltamme- try (CV) and electrochemical impedance spectroscopy (EIS). The doctor bladed low temperature CNFs-CE has faster I 3 - reduction rate and low charge transfer resistance (R CT ) of ∼0.5  cm 2 than platinum (Pt) (∼2.3  cm 2 ) due to the nanoﬁber stacking morphology. Its herringbone and antler structures with graphitic layers lead to defect rich edge planes and larger diameter of CNFs facilitate the electron trans- fer kinetics. The cells with CNF counter electrodes are showing promising energy conversion efﬁciency greater than 7.0% for the glass based devices and 5.0% for the ﬂexible cells ﬁlled with the quasi-solid state electrolyte, which is similar to Pt performance. Application of CNFs-CE in ﬂexible and quasi-solid state electrolyte increases the possibility of roll to roll process, low cost and stable dye-sensitized solar cells (DSCs). © 2011 Elsevier B.V. All rights reserved. 1. Introduction Low cost, non-vacuum and high energy conversion efﬁ- ciency made dye-sensitized solar cells (DSCs) the most promising alternative to the conventional silicon-based solar cells [1–4]. Dye- sensitized solar cells main framework consists of two electrodes, namely photo electrode (PE) and a counter electrode (CE) and an electrolyte in between. Both electrodes were prepared on transpar- ent conducting oxide (TCO) glass substrate. Platinum (Pt) has been used as a catalytic material for tri-iodide (I 3 - ) reduction in DSCs due to its superior catalytic activity and good conductivity [5,6]. Though Pt has superior catalytic activity, it is a very expensive noble metal and scarce in the world. This necessitates the ﬁnding of an alternative catalytic material which is low-cost, low temperature processable, abundantly available in nature and at the same time, stable [7]. In recent years, some cost effective alternative materi- als such as carbon-nanotubes (CNTs), mesoporous carbon, graphite, carbon black, carbon composites, graphene and conducting poly- mers have been tried [8–18]. So far, the carbon ﬁlms prepared by the doctor blading method exhibited good electrochemical prop- erty and conversion efﬁciency but all those reports included high temperature sintering and volatile liquid electrolytes [12,13]. Prob- lems associated with the liquid electrolyte can be solved by using quasi-solid state electrolyte with good thermal stability [9,19,20]. ∗ Corresponding author. Tel.: +82 54 279 2265; fax: +82 54 279 8619. E-mail address: srhee@postech.ac.kr (S.-W. Rhee). Due to the high temperature process in CE, very few groups have reported the ﬂexible DSC. Low temperature DSCs have been made on ﬂexible polymer substrates such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) coated with conductive indium doped tin oxide (ITO) ﬁlms but most of the reports were mainly aimed for ﬂexible photo electrode [21]. Very few reports on the low temperature and ﬂexible CE are available and their device performances are not so good in comparison with the Pt cells [21–24]. Till now, not much attention has been paid to the ﬂexible CE and the quasi-solid state electrolyte in DSCs. In this research, attempts were made to replace conventional Pt with two different carbon-nanoﬁbers (CNFs) as CE catalysts. Also attempts were made to replace volatile liquid electrolyte with the quasi-solid state electrolyte as a redox mediator on ﬂexible ITO/PET substrates. Electrochemical analysis was used to scrutinize the cat- alytic property of the CNFs-CE which is known to have a large number of edge plane sites along the surface of the tube [25–29]. Under 1 sun illumination (AM 1.5), the cells assembled with CNF on ﬂexible ITO/PET or CNF on ﬂuorine doped tin oxide (FTO) glass will be evaluated both with the liquid and the quasi-solid state electrolyte. 2. Experimental 2.1. Preparation of electrodes and electrolyte Carbon-nanoﬁber counter electrodes (CNFs-CE) were prepared as follows. Two different types of commercially available TCOs; F: 0378-7753/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2011.09.004