ZnO nanowire/TiO 2 nanoparticle photoanodes prepared by the ultrasonic irradiation assisted dip-coating method Xiaoyan Gan a , Xiaomin Li a, ⁎, Xiangdong Gao a , Fuwei Zhuge a,b , Weidong Yu a a State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai, 200050, People's Republic of China b Graduate School of the Chinese Academy of Sciences, People's Republic of China abstract article info Article history: Received 9 June 2009 Received in revised form 22 January 2010 Accepted 26 January 2010 Available online 2 February 2010 Keywords: Zinc oxide Titanium dioxide Hybrid Photoanodes Ultrasonic treatment Dye-sensitized solar cell Scanning electron microscopy Electrical properties and measurements Hybrid ZnO/TiO 2 photoanodes for dye-sensitized solar cells were prepared by combining ZnO nanowire (NW) arrays and TiO 2 nanoparticles (NPs) with the assistance of the ultrasonic irradiation assisted dip- coating method. Results show that the ultrasonic irradiation was an efficient way to promote the gap filling of TiO 2 NPs in the interstices of ZnO NWs. Hybrid ZnO NW/TiO 2 NP electrodes prepared with ultrasonic treatment exhibited better gap filling efficiency and higher visible absorptance. The overall conversion efficiency of the hybrid electrode was 0.79%, representing 35% improvement compared with that of the traditional one (0.58%). The enlarged surface area and improved attachments of TiO 2 NPs onto the walls of ZnO NWs induced by the application of ultrasonic irradiation may be the underlying reason. Electrochemical impedance spectroscopy measurements indicated that hybrid electrodes combined the advantages of improved electron transport along the ZnO NWs and increased surface area provided by infiltrated TiO 2 NPs, both of which are responsible for the improved cell efficiency. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSSCs) have emerged as one of the most promising candidates to replace conventional silica-based solar cells since 1991 [1–3]. These DSSCs are normally based on the photo- excitation of dye molecules adsorbed on the surface of semiconduct- ing TiO 2 nanoparticles (NPs). However, the performance of those nanoparticle-based DSSCs is greatly limited by their trap-limited diffusion process, in which photogenerated electrons repeatedly interact with a distribution of traps as they undertake a random walk through the film [4]. In order to improve the electron transport in TiO 2 NPs electrodes, composite electrodes which were formed by mixing TiO 2 NPs with ZnO nanorods (NRs) [5] or TiO 2 nanowires (NWs) [6] have been utilized. Although those NWs or NRs were not vertically attached on the conductive substrate, moderate success has been achieved by optimizing the ratio of NWs (or NRs) and NPs in the electrodes. In recent years, replacement of the typical NP films with arrays of oriented single-crystalline NWs, such as ZnO NW arrays, has attracted much attention due to their high electron mobility and the availability of low temperature synthesis [4,7,8]. However, efficiencies of the ZnO NWs cells are still low due to their inferior surface area for dye loading and the formation of ZnO/dye aggregates. Recently, ZnO NW array/NP composite electrodes have been fabricated and have shown much better properties than that of pure ZnO NW cells. For example, Baxter et al. [9] and Jiang et al. [10] have reported enhanced light harvesting by filling the voids between ZnO NWs with ZnO NPs. Ku et al. have achieved a significant improvement in the efficiency of ZnO NW DSSCs by in-situ chemical bath deposition dense ZnO NPs within the interstices of the vertical ZnO NW arrays [11–13]. However, gap filling of NPs in the aforementioned ZnO NWs remains deficient or requires a long reaction time. In this work, we have reported an easy and efficient method to promote the gap filling of TiO 2 NPs in the interstitial voids of the ZnO NWs by integrating the ultrasonic irradiation with the dip-coating method. Effects of the ultrasonic treatment on the microstructure, the sensitization, and the performance of hybrid ZnO NW/TiO 2 NP electrodes were investigated in detail. The electron transport properties in TiO 2 NPs, ZnO NWs, and hybrid ZnO NW/TiO 2 NP DSSCs were examined to explore the underlying mechanism. 2. Experimental details 2.1. Synthesis of hybrid ZnO NWs/TiO 2 NPs electrodes ZnO NW arrays were grown on ZnO-seeded fluorinated tin oxide (FTO, 20 Ω/□) substrates by chemical bath deposition. The ZnO seed layer was prepared by sol–gel method as described by Ohyama [14]. ZnO NW arrays were obtained by immersing the seeded substrates Thin Solid Films 518 (2010) 4809–4812 ⁎ Corresponding author. Tel.: +86 021 52412554; fax: +86 021 52413122. E-mail address: lixm@mail.sic.ac.cn (X. Li). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.01.043 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf