Enhanced Photovoltaic Properties of a Cobalt Bipyridyl Redox Electrolyte in Dye-Sensitized Solar Cells Employing Vertically Aligned TiO2 Nanotube Electrodes

Photovoltaic performances of TiO2 nanoparticle (NP) electrodes and TiO2 nanotube (NT) electrodes in dye-sensitized solar cells (DSSCs) employing a cobalt bipyridyl redox electrolyte were compared. The TiO2 NP electrodes had pore sizes ranging from 15 to 20 nm while the NT electrodes had a lager pore size of 80 nm. Highly ordered and vertically oriented TiO2 NT electrodes were prepared by a two-step anodization method. In application to DSSCs employing the cobalt redox electrolyte, the 11-μm-thick NP electrode exhibited an efficiency of 1.60% with a Jsc of 3.96 mA/cm2. Meanwhile, despite nearly half of the amount of adsorbed dye molecules, the 11-μm-thick NT electrode exhibited a slightly enhanced efficiency of 1.84% with a Jsc of 5.86 mA/cm2. In addition, the 35-μm-thick NT electrode showed an efficiency of 2.38% with a Jsc of 9.80 mA/cm2. Compared to the 11-μm-thick NP electrode, the 35-μm-thick NT electrode exhibited a 1.5 times higher efficiency with a 2.5 times higher Jsc in spite of having a similar amount of adsorbed dye molecules. Photocurrent transient measurements revealed that the mass transport limitation of the cobalt redox electrolyte within the conventional NP electrodes was greatly alleviated within the NT electrodes. In addition, the electrochemical impedance spectra indicated that the interfacial contact between the cobalt redox electrolyte and TiO2 electrode was prominently enhanced in the NT electrodes. Furthermore, the electron lifetime and electron diffusion length were all greatly longer within the NT electrodes. These superior photovoltaic properties may be attributed to the large pore size and vertically oriented structures of the NT electrodes.