Ionic liquid-based electrolyte solidified with SiO 2 nanoparticles for dye-sensitized solar cells M. Berginc a , M. Hočevar a , U. Opara Krašovec a, ⁎ , A. Hinsch b , R. Sastrawan b , M. Topič a a University of Ljubljana, Faculty of Electrical Engineering, 1000 Ljubljana, Slovenia b Fraunhofer Institute for Solar Energy Systems ISE, D-79110 Freiburg, Germany Available online 14 June 2007 Abstract The performance of dye-sensitized solar cells (DSSC) based on the propyl-methyl-imidazolium iodide (PMII) ionic liquid (IL) with and without the addition of SiO 2 nanoparticles is studied. Results confirm that the presence of SiO 2 nanoparticles in PMII electrolyte improves the charge transport of iodide/tri-iodide redox couple in the electrolyte and consequently increases the efficiency of DSSC up to 20%, relatively. Short circuit current density (J SC ) of the DSSC under illumination may be limited by the charge transport of the redox couple in the IL-based electrolytes and a theoretical maximum of J SC can be evaluated from the cyclic voltammetry measurements of simple symmetric cells (TCO-Pt$electrolyte$Pt- TCO). The results show a strong temperature dependence of the DSSC performance if the PMII/I 2 -based electrolytes are used. © 2007 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cell; Ionic liquid; Electrolyte; Nanoparticle; Charge transport 1. Introduction Dye-sensitized solar cells (DSSC) have been extensively studied in the last decade as a promising renewable energy source, because of their potentially inexpensive manufacturing technology compared to silicon solar cells [1]. The active electrode of the DSSC consists of a transparent conductive oxide (TCO) glass coated with nanoporous TiO 2 covered by a monolayer of the Ruthenium complex-based dye. The counter electrode is a TCO covered with a thin platinum layer. The gap between the electrodes is filled with electrolyte containing iodide/iodine (I - /I 3 - ) redox couple. Nowadays, research is focusing on exchanging volatile acetonitrile electrolytes with non volatile ionic liquids (IL) [2] in order to enhance the lifetime of the DSSC. Typical examples of ionic liquids are 1,3 alkyl imidazolium salts (iodide, dicyanamide, tetrafluor- oborate, …). In addition, many attempts are made to solidify the electrolyte either with an organic [3,4] and/or organic– inorganic [5] network or by incorporation of inorganic nanoparticles [6]. Due to the lower viscosity of IL (100–1000 mPa s) compared to conventional electrolytes (e.g. acetonitrile), a slower mass transport is expected. Therefore, the charge transport in the IL electrolyte from the active electrode to the counter electrode in DSSC becomes diffusion limited under one sun (100 mW/cm 2 ) operation. On the other hand, a unique charge transport mech- anism, not based only on physical diffusion, is reported for IL electrolytes [7]. In some particular conditions of iodine redox composition, a significant enhancement of charge transport rate is described to be due to conjugation of physical diffusion and exchange reaction (electron hopping along a continues I - /I 3 - chain) in ionic liquid [7]. Recently, a relative improvement of the DSSC conversion efficiency by up to 20% was reported, if nanoparticles were added to the IL electrolyte although the electrolyte was solidified [6]. This improvement could be explained by ordering of the electrolyte structure being beneficial for the electron hopping (exchange reaction). The structure of such an electrolyte– nanoparticle composite is schematically presented in Fig. 1 [6]. Imidazolium cations are adsorbed on the surface of the nanoparticles, which are surrounded with continues chains of I - and I 3 - anions. In this paper, the charge transport in the electrolyte, consisting of propyl-methyl-imidazolium iodide Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 4645 – 4650 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: ursa.opara@fe.uni-lj.si (U. Opara Krašovec). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.079