Fabrication of dye-sensitized solar cells by transplanting highly ordered TiO 2 nanotube arrays Hun Park a , Woong-Rae Kim b , Hyo-Tae Jeong b , Jae-Joon Lee c , Ho-Gi Kim a , Won-Youl Choi b,n a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea b Department of Metal and Materials Engineering, Kangnung-wonju National University, Kangnung 210-702, South Korea c Department of Applied Chemistry, Konkuk University, Chungju 380-701, South Korea article info Article history: Received 5 November 2009 Accepted 2 February 2010 Available online 2 March 2010 Keywords: TiO 2 nanotube Transplanting process Anodic oxidation Dye-sensitized solar cells abstract Highly ordered TiO 2 nanotube arrays fabricated by anodization are very attractive to dye-sensitized solar cells (DSCs) due to their superior charge percolation and slower charge recombination. However, the efficiency of TiO 2 -nanotube-based DSCs is 6.89%, which is still lower than that of TiO 2 -nanoparticle- based DSCs. We have suggested the transplanting the highly ordered TiO 2 nanotube arrays to FTO glass to improve the performance of TiO 2 -nanotube-based DSCs. DSCs based on transplanted TiO 2 nanotube arrays and TiO 2 nanoparticles were fabricated by same process and materials to exclude the unexpected factors. In TiO 2 thickness of ca. 15 mm, the efficiency of 2.91% in front-side illuminated DSCs based on TiO 2 nanotube arrays was higher than those in back-side illuminated DSCs based on TiO 2 nanotube arrays and in front-side illuminated DSCs based on TiO 2 nanoparticle. Front-side illuminated DSCs based on TiO 2 nanotube arrays having various thicknesses were successfully fabricated. The efficiency in DSCs having 20.0 mm thick TiO 2 nanotube arrays was improved to 5.36% by TiCl 4 treatment. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSCs) have a great potential to be an alternative device for conventional photovoltaics due to their low cost and easy fabrication [1,2]. The certified efficiency of DSCs, which are based on the nanoparticular TiO 2 photoelectrode, is 11.2% [3]. This efficiency is still low for commercialization of DSCs. It was reported that the TiO 2 nanoparticle structure, which shows the best efficiency in DSCs, has trap sites at the contact region between TiO 2 nanoparticles and these interrupt the transport of electrons in the TiO 2 film [4,5]. Several TiO 2 structures such as nanowire [6], nanofiber [7], hollow hemisphere [8], hollow sphere [9], nanotube [4,5,10–13], etc., have been applied to DSCs to overcome the limitation of TiO 2 nanoparticle structure. Recently, among these structures, TiO 2 nanotube arrays have received a great attention due to their one dimensional structure to provide a direct path for electrons [14,15]. In application to DSCs, vertically oriented TiO 2 nanotube arrays have higher charge collection efficiencies than a nanoparticle- based structure due to their faster transport and slower recombination of electrons [5]. TiO 2 -nanotube-based DSCs are classified into front-side illu- minated DSCs and back-side illuminated DSCs according to the direction of incident light [10,11]. Light is directly incident on the photoelectrode in front-side illuminated DSCs. On the other hand, in case of back-side illuminated DSCs, light is incident on the cathode side. To fabricate TiO 2 nanotube arrays for photoelec- trodes of front-side illuminated DSCs, Ti thin film was generally deposited to FTO glass by d.c. sputtering deposition. It is hard to obtain the TiO 2 nanotube arrays with conventional thickness of 15 mm in TiO 2 nanoparticle layer, because d.c. sputtering deposi- tion only provides us a thin film. Photoelectrodes of back-side illuminated DSCs are fabricated by anodizing Ti foil. Longer TiO 2 nanotube arrays can be easily grown because anodic oxidation is done directly on Ti foil. However, if the thickness of TiO 2 nanotube arrays is longer than ca. 15 mm, anodized TiO 2 nanotube arrays can be easily detached from Ti foil due to the local stress at the interface of TiO 2 nanotube arrays and Ti foil. Front-side illumi- nated DSCs usually have a better performance than back-side illuminated DSCs, which have the same thickness of TiO 2 nanotube arrays with front-side illuminated DSCs [11]. To increase the efficiency of TiO 2 -nanotube-based DSCs, the longer TiO 2 nanotube arrays in front-side illuminated DSCs have been requested. In this study, we suggest a transplanting process to fabricate front-side illuminated DSCs with longer TiO 2 nanotube arrays than ca. 15 mm. Fig. 1 shows the schematic diagram of transplanting process. When a longer TiO 2 nanotube arrays than ca. 15 mm are grown on Ti foil by anodic oxidation, they were easily detached from Ti foil. The detached TiO 2 nanotube arrays Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.017 n Corresponding author. Tel.: + 82 33 640 2483; fax: + 82 33 642 2245. E-mail address: cwy@kangnung.ac.kr (W.-Y. Choi). Solar Energy Materials & Solar Cells 95 (2011) 184–189