Dark photovoltaic effects of Ti-rooted TiO2 nanopillars as the anode in CdSe-sensitised solar cell

Abstract

TiO2 nanopillars (NPs) rooted in Ti were cultivated hydrothermally with the strongest organic base tetramethylammonium hydroxide (TMAOH) in aqueous solution in Ti substrate. TiCl4 was used to coat TiO2 NPs with a nanoscale compact thin TiO2 layer against charge recombination with oxidised species in electrolyte. A consecutive layer of CdSe nanoparticles was induced by successive ionic layer adsorption and reaction (SILAR) mode for sensitisation by light harvesting. J–V curves surprisingly indicate that untreated TiO2 NPs photoanode rooted in Ti produces considerable currents in the dark (thermal photovoltaic power generation) even higher compared to that under intense UV illumination, signifying TiCl4–CdSe treatment does not profit. Electrochemical impedance spectroscopy fitting parameters further reveal that the parameters associated with injection and recombination issues have been improved, as a comprehensive electrochemical result of TiCl4–CdSe treatment. Thus, the current-limiting problem probably arises from the remarkable continuous absorption bands in a wide spectrum range from visible to far-infrared, which is confirmed by UV–vis absorption spectra. However, the reason for the substantial photocurrent-limiting is that the intense irradiating UV light causes the innumerable electrons distributed in the inter-band-gap states to transit to much higher levels above the CB edge of TiO2 with excessive energy, then they collide with TiO2 crystall lattices to release thermal energy eventually to raise the transport resistance.