Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 1–10 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Electrochemical impedance and X-ray photoelectron spectroscopic analysis of dye-sensitized liquid electrolyte based SnO 2 /ZnO solar cell G.R.R.A. Kumara a,∗ , Kenji Murakami b , Masaru Shimomura b , K. Velauthamurty a , E.V.A. Premalal b , R.M.G. Rajapakse a,b , H.M.N. Bandara a a Department of Chemistry, Post-Graduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka b Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan article info Article history: Received 6 November 2009 Received in revised form 8 June 2010 Accepted 15 July 2010 Available online 22 July 2010 Keywords: Dye-sensitized solar cell SnO2/ZnO Electrochemical impedance spectroscopy Electron traps abstract A dye-sensitized solar cell based on interconnected SnO 2 nanoparticle matrix covered with a thin outer shell of ZnO, N719 dye, I - /I 3 - in acetonitrile liquid electrolyte system and lightly platinized FTO counter electrode shows significantly enhanced performance when compared to similar cells made with either pristine SnO 2 or pristine ZnO interconnected nanoparticles. Attempts have been made to investigate the reasons for such an improvement using the information obtained from X-ray photoelectron spectroscopy (XPS) and the electrochemical impedance spectroscopy (EIS). The XPS results reveal that the intercon- nected nanoparticluar SnO 2 matrix surfaces are fully covered by a ∼1 nm thick outer shell of a ZnO layer. EIS results disfavour the idea of direct injection of electrons from the excited dye molecules across the thin outer shell of ZnO into the conduction band of SnO 2 but supports the fact that electrons are first injected to the CB of ZnO and subsequently to the CB of SnO 2 particles both involving trapping and detrapping at each stage. The electron transport along the interconnected SnO 2 nanoparticles also involves anomalous diffusion characterized by a straight line of inclination greater than 45 ◦ in the complex impedance plot. This anomalous diffusion is attributed to the trap mediated electron transport. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The dye-sensitized nanocrystalline solar cell (DSSC) invented by O’Reagan and Gratzel [1] is a device of tremendous potential for practical applications which has already achieved solar con- version efficiencies as high as 11.5% [2]. The DSSC is composed of an interconnected nanocrystalline TiO 2 particle matrix deposited on a transparent conducting tin oxide glass surface (TCO, com- monly used TCO is fluorine-doped tin oxide, FTO) with a typical thickness of less than 15 m which functions as the active elec- trode of the solar cell. The commonly used TiO 2 particles known as p-25 are comprised of highly porous spheres of 15–20 nm diam- eters such that a film of typical thickness of 10 m to have a roughness factor greater than 1000 which in turn to result in a porosity of 50–70% for sufficient electrolyte film penetration [3]. The interconnected particle surfaces are fully covered with light- absorbing dye molecules [usually Ru(II) dyes with bipyridyl and thiocyanate ligands] which are anchored to TiO 2 surface by means of dative coordination through two carboxylate groups. Lightly platinized TCO plate acts as the counter electrode and the cell is completed by sandwiching a non-aqueous electrolyte (acetonitrile) ∗ Corresponding author. Tel.: +94 81 239 4420; fax: +94 81 238 8018. E-mail address: grakumara2000@yahoo.com (G.R.R.A. Kumara). containing a redox couple (I - /I 3 - in acetonitrile) between the two electrodes. While the Gratzel cell is performing so well giving such high energy conversion efficiency, researchers have found that it is not possible if TiO 2 in the DSSC is replaced with other similar oxide semiconductors such as SnO 2 or ZnO [4–7]. The maximum effi- ciencies recorded are of the order of 1% for the latter systems. However, Tennakone and co-workers have demonstrated that if interconnected SnO 2 nanoparticles are covered with less than 1 nm thin layer of ZnO or interconnected nanoparticles of ZnO are cov- ered with less than 1 nm thin layer of SnO 2 , the performance of such DSSCs are dramatically improved compared to those with pristine semiconductor particles [8–10]. Similar results have been obtained when an insulator material is used as the thin outer layer [4,7,11,12]. They have concluded that the main character- istics intrinsic to a semiconductor determining its suitability for DSSCs are the effective electron mass (EEM) and the conduction band position. TiO 2 has a high EEM (10m e and 50m e according to some reports) and hence is capable of suppressing the recom- bination of electrons in the TiO 2 matrix with the redox species in the electrolyte (e.g., I 3 - ) in contact with it [13]. The EEM val- ues of SnO 2 and ZnO are 0.17m e and 0.19m e respectively and are more than an order of magnitude less than that of TiO 2 [13]. Since the maximum obtainable charge carrier (electrons in this case) mobility is inversely proportional to the effective mass (of 1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2010.07.013