Polypyrrole-coated multi-walled carbon nanotubes for the simple preparation of counter electrodes in dye-sensitized solar cells Pavol Gemeiner a, *, Jaroslav Kuli9 cek b , Milan Mikula a , Michal Hatala a ,  Lubomír Švorc c , Lenka Hlavatá c , Matej Mi9 cušík b , Mária Omastová b a Department of Graphic Arts and Applied Photochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovakia b Polymer Institute, Slovak Academy of Sciences, 845 41 Bratislava, Slovakia c Institute of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovakia A R T I C L E I N F O Article history: Received 15 July 2015 Received in revised form 14 October 2015 Accepted 21 October 2015 Available online 12 November 2015 Keywords: Dye-sensitized solar cell Counter electrode Polypyrrole Multi-walled carbon nanotubes low-cost deposition A B S T R A C T Polypyrrole-coated multi-walled carbon nanotubes (PPy/MWCNT) were synthesized using a three-step chemical oxidative polymerization and were simply deposited by an ex situ method from n-butyl alcohol dispersions onto conducting FTO glasses to be used as a low-cost replacement for Pt counter electrodes (CE) in dye-sensitized solar cells (DSSCs). PPy/MWCNT CEs, compared to PPy CEs, have significantly higher interlayer cohesion and adhesion to the FTO substrate, which increases with the MWCNT ratio. The network structure of PPy/MWCNT CEs, with a larger surface area compared to PPy CEs (characterized by SEM), has a positive effect on the charge transfer resistance (R ct ), and the PPy/MWCNT CE sample with a ratio of 70:30 reaches a lower R ct than the standard Pt CE, as measured by electrochemical impedance spectroscopy. PPy/MWCNT CEs have reasonable catalytic activities and good stability in the presence of an iodide/triiodide electrolyte, as measured by cyclic voltammetry. The conversion efficiency of the DSSC with PPy/MWCNT (70:30) CEs reaches a 10 higher value (h = 4.9%) than the DSSC with pristine PPy CEs (h = 0.5%) and a 30% lower value than the standard DSSC with Pt CEs (h = 7.1%). ã 2015 Elsevier B.V. All rights reserved. 1. Introduction Since 1991 [1], as one of the most promising hybrid solar cell technologies, dye-sensitized solar cells (DSSC) have drawn the attention of many researchers [2]. This is mainly due to their various advantages, such as a simple and low-cost preparation process, the possibility of using a large variety of inexpensive materials, illumination from both sides, their light weight and low toxicity, the possibility of using flexible substrates and their good photovoltaic performance in diverse light conditions [3,4]. Moreover, from a mass production point of view and compared to conventional photovoltaics, DSSCs can be prepared in ambient and low-temperature conditions on large areas by production techniques such as doctor-blade, drop casting, screen printing and roll-to-roll [5]. In general, the standard structure of DSSCs is based on three crucial components: a TiO 2 mesoporous semiconductive electrode sensitized with a monolayer of dye, a liquid electrolyte with an iodide/triiodide redox mediator, and a platinum counter electrode (CE) [6]. The fundamental DSSC principle is based on charge separation in the TiO 2 -dye interface and electron transpor- tation through a nano-oxide semiconductive electrode and external circuit to the counter electrode, where I 3  anions of the electrolyte are reduced; the process is completed by electron transportation via a redox mediator to the dye in an oxidized state [7]. DSSCs with Pt as the standard catalytic layer of the counter electrode deposited onto fluorine-doped tin oxide (FTO) glass have the highest photovoltaic performance with a conversion efficiency over 13% [8]. This is mainly due to the high catalytic activity of platinum and low charge transport resistance (R ct ) of I 3  reduction [9]. Moreover, Pt is highly transparent in visible and near infrared spectra (>80%), which is beneficial for bifacial cell illumination [10]. However, the high-cost preparation of CEs with sufficient catalytic properties requiring high temperatures and corrosion of Pt in the presence of a liquid electrolyte is a serious drawback for its large-scale application [2]. * Corresponding author. E-mail address: pavol.gemeiner@gmail.com (P. Gemeiner). http://dx.doi.org/10.1016/j.synthmet.2015.10.020 0379-6779/ ã 2015 Elsevier B.V. All rights reserved. Synthetic Metals 210 (2015) 323–331 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/sy nmet