Vertically aligned TiO2/ZnO nanotube arrays prepared by atomic layer deposition for photovoltaic applications

Vertically aligned TiO2/ZnO nanotube (NT) arrays were developed for application to photoanodes in mesoscopic solar cells. By a two-step anodic oxidation, vertically aligned TiO2 NT arrays with highly ordered surface structure were prepared, followed by deposition of a ZnO shell with a precisely controlled thickness using atomic layer deposition (ALD). When applied to a photoanode of dye-sensitized solar cells (DSSCs), the photovoltage is gradually enhanced as the ZnO shell thickness of the TiO2/ZnO NT electrodes is increased. Furtheremore, the electron lifetime in photoanodes is significantly enhanced due to the ZnO shell, which is examined by open-circuit voltage decay (OCVD) measurement. Photocurrent density-voltage (J-V) curves under the dark condition and OCVD spectra reveal that a negative shift in TiO2 conduction band potential and an energy barrier effect owing to the ZnO shell concurrently contribute to the enhancement of VOC and electron lifetime.