Double layered nanoarchitecture based on anodic TiO2 nanotubes for dye-sensitized solar cells

Based on the TiO2 nanotube (TONT) with three different thicknesses (10, 18, 28 μm) prepared by electrochemical anodization, a double layered electrode consisting of TONT and TiO2 nanoparticles (TONPs) with a size of approximately 20 nm was developed. The anodic TONT electrode has been actively investigated due to its beneficial properties such as a favorable electron transport rate and light scattering ability. However, the small specific surface area caused by the large pores and vacant space between 1-D NTs impedes the fabrication of high efficient DSSCs. Hence, the TONP layer with a thickness of 10 μm is additionally deposited on the top surface of the TONT film to offer a sufficient surface area for large dye loading. Compared to a single TONT electrode, the double layered TONT/NP electrodes provide a high short-circuit current density (Jsc) and fill factor (FF) while maintaining a constant open-circuit voltage (Voc), finally disclosing a higher conversion efficiency (η), attributed to the high loading of dye molecules, light scattering ability, low resistance in the photoanode/electrolyte interface, and longer electron lifetimes. In particular, the optimal η was attained with the TONT/NP (18 μm) electrode having a Voc of 0.80 V, a Jsc of 7.80 mA/cm2, a FF of 62.7%, and a η of 3.93%. Incidentally, when using a longer TONT film (28 μm), η decreases steadily due to the fast charge recombination process. A more detailed discussion is included below.