Published: April 13, 2011 r2011 American Chemical Society 6311 dx.doi.org/10.1021/la200159w | Langmuir 2011, 27, 6311–6315 ARTICLE pubs.acs.org/Langmuir Bilayer Inverse Opal TiO 2 Electrodes for Dye-Sensitized Solar Cells via Post-Treatment Ju-Hwan Shin and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, South Korea ’ INTRODUCTION Since Gr € atzel and co-workers first reported the preparation of an efficient regenerative photoelectrochemical solar cell using a nanocrystalline TiO 2 (nc-TiO 2 ) electrode sensitized by a ruthe- nium dye soaked in an iodine/iodide electrolyte solution, dye- sensitized solar cells (DSSCs) have been pursued as next-genera- tion photovoltaic devices with simple and low-cost fabrication requirements. 1 Recently, the engineering of TiO 2 electrodes has been intensively studied because TiO 2 layers have adsorption sites for dye molecules, transfer photogenerated electrons, and provide a diffusion path for ions in an electrolyte solution. 2,3 Among the various new electrode microstructure architectural designs, 3D ordered porous electrodes made from colloidal crystal templates or inverse opals have been attractive due to the following advantages. 4À7 First, inverse opals offer flexibility in the optical design of electrically active TiO 2 electrodes. Lee et al., and more recently Guldin et al., fabricated bilayers of nc-TiO 2 and inverse opal TiO 2 (io-TiO 2 ) and found improvements in the light-harvesting efficiencies of the photonic band gaps due to the presence of ordered macropores. 4,7 Second, instead of using io- TiO 2 as an optical element, fully connected pores with extended diameters in the io-TiO 2 layers allow transport of ions and infiltration of the electrolyte solution. More importantly, the io-TiO 2 electrode itself can be advantageous in applications that include polymeric electrolytes with high viscosities and relatively large molecular weight. Third, 3D connected TiO 2 networks can provide an organized electron path, which may facilitate charge transport and thus enhance the collection efficiencies of back- contact electrodes. 5 Although these approaches have shown promise, inclusion of io-TiO 2 in bilayer electrodes increased the photocurrent by 10% but decreased the open circuit voltage and fill factor due to the higher film thickness. These results together decreased the net efficiency. 4 The efficiencies of applications that used io-TiO 2 directly as an electrode were in the range 2À3%, 6 which is lower than the efficiencies of applications that used conventional electro- des (11%) 8 or other engineered electrodes such as TiO 2 ordered nanotube or rod arrays (3À6%). 9,10 Thus, further improvements are required if io-TiO 2 are to be applied as electrodes in DSSCs or as scattering or band ngap layers in nc-TiO 2 electrodes for DSSCs. Post-treatments have provided a simple and facile approach to improving the efficiencies of nc-TiO 2 electrodes. 11À14 Applica- tion of a post-treatment can enhance the efficiency of nc-TiO 2 between 20% and 50%. A typical post-treatment process involves soaking TiO 2 electrodes in a dilute TiCl 4 aqueous solution of concentration 0.02 MÀ0.5 M. The TiCl 4 solÀgel reaction results in deposition of TiO 2 nanoparticles or a TiO 2 coating layer on the surface of the nc-TiO 2 layer. Post-treatment improves the efficiency via several factors: the more light-scattering by overlayer of TiO 2 improves light-harvesting efficiency, 11 better interparticle connectivity enhances the charge collection efficiency, 15 the injec- tion efficiency improves, 13 and defect healing is introduced (i.e., removal of surface defects associated with oxygen vacancies). 14 However, post-treatments of nc-TiO 2 electrodes are widely considered of minor importance. The extensive growth of TiO 2 layers during post-treatment may clog the mesopores of nc-TiO 2 electrodes. Clogging limits permeation of liquid electrolytes, reduces the interfacial area between the TiO 2 surface and the electrolyte solution, and decreases the efficiency. Post-treatments have been applied to nanostructures other than nc-TiO 2 , including Received: January 13, 2011 Revised: March 22, 2011 ABSTRACT: We investigated the formation of bilayer inverse opal TiO 2 (io- TiO 2 ) structures via post-treatment with a TiO 2 precursor solution and characterized the photovoltaic performances of the resulting electrodes for use in dye-sensitized solar cells. The post-treatment of TiO 2 inverse opals in a precursor solution grew rutile TiO 2 nanoparticles on anatase crystalline phase io-TiO 2 surfaces, resulting in anatase/rutile bilayer structures. We achieved a maximum photovoltaic conversion efficiency of 4.6% using a 25 μm thick electrode formed with the post-treated io-TiO 2 under simulated AM 1.5 light. This efficiency represents a 183% improvement over the non-post-treated io-TiO 2 electrodes. The shell thickness was controlled by the post-treatment time. The effects of shell thickness on photovoltaic performance were investigated by measuring the morphologies and electrochemical impedance of the post-treated io-TiO 2 . We found that post-treatment up to a certain period of time increased the surface area and electron lifetime, but further treatment resulted in decreased area and saturated lifetimes. The optimal post-treatment time was identified, and the optimal io- TiO 2 electrodes were characterized. Downloaded via SOGANG UNIV on January 30, 2019 at 13:04:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.