Solar absorption and microstructure of C-doped and H-co-doped TiO2 thin films

This work examined the effect of different levels of C doping on the microstructure and optical properties of TiO2 thin films consisting of mostly rutile and amorphous phases. The increase in the substitutional C doping from 0.32 to 8.57 at% was found to reduce the long-range order through the refinement of the crystalline phase, and increases in the concentration of the extended defects and fraction of the disordered phase, where C gets segregated due to its low solubility in the crystalline phase. As expected, the main absorption threshold red-shifted with the increase in C doping (from 3.22 to 2.48 eV) and additional thresholds existed in the visible and NIR ranges. However, at the same time only a small reduction in the separation between the extended states (by up to 0.27 eV) was achieved. The significant increase in the visible absorption originated mostly from the C-induced spread of localized states within the band gap. Higher levels of C doping introduce localized states deeper into the band gap, which may increase the carrier recombination as well. Co-doping with hydrogen nearly fills the band gap with localized states and further increases the visible and NIR absorption.