ZnO-treated TiO 2 inverse opal electrodes for dye-sensitized solar cells Hye-Na Kim, Jun Hyuk Moon * Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea article info Article history: Received 23 August 2012 Received in revised form 1 November 2012 Accepted 17 December 2012 Available online 5 January 2013 Keywords: ZnO Dye-sensitized solar cells TiO 2 Inverse opals Porous electrodes abstract We report that the photovoltaic properties of inverse opal TiO 2 (io-TiO 2 ) electrodes in dye-sensitized solar cells can be enhanced by ZnO treatment of the inverse opal structures. ZnO was coated on the surface of io-TiO 2 via the solegel reaction of ZnO precursors. Energy dispersive X-ray spectroscopy (EDX) measurements showed that the amount of ZnO on the io-TiO 2 surface was measured to be 0.12e0.50 wt% of zinc, depending on the number of coatings. Compared to bare inverse opal electrodes, the energy conversion efficiency of cells increased for the 0.35 wt% ZnO-coated electrodes, and then decreased for the 0.50 wt% ZnO-coated electrodes. The maximum efficiency of 5.3% was achieved, corresponding to a 23% increase in efficiency compared with bare io-TiO 2 electrodes. The enhanced efficiency was mainly attributed to the improvement of the open-circuit voltage (V OC ). EIS and dark current measurements confirmed that this enhancement in V OC was due to the movement of the conduction band edge in a negative direction after ZnO treatment, rather than the formation of a barrier layer for electron recombination. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSSCs) have been developed based on nanocrystalline TiO 2 electrodes, which are sensitized by ruthenium dye and soaked in an iodine/iodide electrolyte solution. They are attractive as next-generation photovoltaic devices, due to their low cost, the simplicity of their fabrication, and, most of all, their potential to allow the construction of transparent devices, which would enable their application in building materials [1]. In DSSCs, TiO 2 electrodes are one of the key components, in that the TiO 2 electrode is involved in the lifetime of the photo-generated electrons, encompassing injection, transport and recombination. Specifically, in DSSCs, the electrons are generated by the dye coated on the TiO 2 electrodes, and are injected into the electrodes. The TiO 2 electrodes also allow the transport of electrons to a collecting electrode (i.e., the FTO substrate), and provide an interface where the electrons are recombined with a redox couple in the electrolyte solution. Randomly packed TiO 2 nanoparticles have been widely applied; results have suggested that the efficiency of DSSCs can be further enhanced by providing a more efficient electron transport path through the TiO 2 electrodes. In this regards, recent researches have focused on applying microstructured electrodes (i.e., more ordered structures compared to the randomly packed TiO 2 nanoparticle structure). For example, the use of nanotubes or nanowires produced a much improved electron lifetime [2]; in the case of nanotubes, the lifetimes measured were approximately 10 times longer than those measured for conventional nanocrystalline TiO 2 electrodes, due to the more direct electron pathways [2,3]. An inverted structure of monodisperse colloidal crystals (or inverse opal structure) was applied as an alternative microstructured electrode. These electrodes showed improved electron transport compared with conventional electrodes, due to their fully three- dimensionally connected structure [4,5]. Most significantly, the pore size could be easily controlled over a range of several hun- dreds of nanometers, via the use of different sizes of colloidal particles in the colloidal crystal template [6]. This facile control over the macroscale pores has been advantageous for the application of solid-state electrolytes such as polymer electrolytes containing mesoscale polymer molecules [5]. In this work, we extended our previous work on the bilayer inverse opal electrodes to examine the improvement of efficiency of DSSCs by introducing ZnO treatment [14]. Metal oxide shell coatings such as Nb 2 O 5 , Al 2 O 3 and ZnO have been applied to nanocrystalline TiO 2 electrodes [7e9]. Since the individual particle size of these electrodes is small enough not to generate the space charge layer, the formation of such an energy barrier layer have been applied to decrease electron recombination losses; this results in the increase of the photocurrent density (J SC ), or/and the open- circuit voltage (V OC ) [8]. ZnO has been widely applied, since it has * Corresponding author. E-mail address: junhyuk@sogang.ac.kr (J.H. Moon). Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.12.014 Current Applied Physics 13 (2013) 841e845