Free-standing polypyrrole films as substrate-free and Pt-free counter electrodes for quasi-solid dye-sensitized solar cells P. Veerender, Vibha Saxena, P. Jha, S.P. Koiry, Abhay Gusain, S. Samanta, A.K. Chauhan, D.K. Aswal ⇑ , S.K. Gupta Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India article info Article history: Received 9 July 2012 Received in revised form 20 August 2012 Accepted 20 August 2012 Available online 13 September 2012 Keywords: DSSC Free-standing polypyrrole TCO-free counter electrodes Pt-free counter electrodes abstract Substrate-free and Pt-free counter electrodes (CE) were utilized for dye sensitized solar cells for the first time using free-standing polypyrrole (PPy) films. The PPy films were syn- thesized by liquid–liquid biphasic interfacial polymerization. These films have unique mor- phologies as they have a dense base (that provides mechanical strength) and porous top (resulting in large surface area for catalytic activity). The electrochemical impedance spec- troscopy measurements revealed that free-standing PPy exhibit reasonable catalytic activ- ity for I  3 =I  redox couple. The dye-sensitized solar cells were fabricated using N3 dye, gel polymer electrolyte and free-standing PPy films as CE. The power conversion efficiency measured under AM 1.5, 100 mW/cm 2 illumination was 3.5%. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSSC) have emerged as promising technology for harvesting solar energy since their inception in 1991 by O’Regan and Gratzel [1]. In re- cent years, DSSCs have been extensively studied owing to their potential for enabling simpler fabrication process at lower cost. Typically, DSSCs comprise a photoanode (sensi- tized dye adsorbed on mesoporous TiO 2 ), a redox electro- lyte (usually I  3 =I  redox couple), and a platinized counter electrode (CE). A Pt film coated on transparent conducting oxide (TCO) glass substrates is employed as CE because of its excellent catalytic ability to reduce the triiodide into io- dide. However, the commercial viability of Pt as CE has been limited because of (i) its high cost and limited flexi- bility, (ii) corrosion of the Pt in presence of small traces of water in the electrolyte, (iii) high temperature process- ing, and (iv) dissolution of Pt films and formation of PtI 4 [2]. To address these issues, a great deal of research is ori- ented towards the development of Pt-free counter elec- trodes. In this regard, conducting polymers hold the promise for enabling the replacement of Pt CEs in DSSC be- cause of their unique properties, including low cost, ease of synthesis, good catalytic activity, and remarkable stability [3]. Many conducting polymers such as, poly(3,4-ethylene- dioxythiophene) (PEDOT) [4], poly(3,3-diethyl-3,4-dihy- dro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et 2 ) [5], polyaniline [6], and polypyrrole [7] were studied as alter- native CEs for DSSCs. In most cases, the films of conducting polymers were prepared by spin casting on TCO substrates and annealed at high temperatures in order to improve film adhesion on TCO substrate. However, the conducting polymer films prepared in this manner have limited thick- ness. Additionally, the poor film adhesion on the substrate requires high temperature processing. In order to avoid the use of high temperature processing, Xia et al. [8] deposited conducting polymer films on TCO substrates by vapour phase polymerization in two steps: first an oxidant mate- rial was spin casted on TCO substrate and then the sub- strate was kept in monomer saturated vapour to polymerize the monomer on the substrate. However, the conducting polymer films formed this way have low surface area owing to the limited thickness of the film. 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.039 ⇑ Corresponding author. E-mail address: dkaswal@yahoo.com (D.K. Aswal). Organic Electronics 13 (2012) 3032–3039 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel