Detachment and transfer of ordered TiO ₂ nanotube arrays for front-illuminated dye-sensitized solar cells

We report a method of detachment and transfer of one-dimensional TiO ₂ nanotube (NT) arrays to fabricate dye-sensitized solar cells (DSSCs) with illumination from the front side. The ordered NT arrays (tube length 20 μm) were detached from the NT-Ti substrate, as annealed, by means of a second anodization at 20 V for 4 h and then transferred, inverted, onto a transparent substrate of conducting oxide (TCO) with an interface of TiO ₂ nanoparticles of thickness ∼2 μm. Dry etching of the NT surface in a highly dense plasma reactor under BCl ₃/Cl ₂ for 90 s opened previously closed ends of the tubes. The inverted (bottom-up) NT-TCO substrate was fabricated into a DSSC device that shows a cell performance (η = 6.24%) significantly improved over those of a front-illuminated counterpart with an upright (face-up) structure (η = 4.84%) and of a conventional back-illuminated device (η = 4.61%). Electrochemical impedance spectra (EIS) of the devices under one-sun irradiation were measured to rationalize the cell performances that are consistent with the corresponding interfacial impedances between the NT arrays and the electrode for collection of electrons.