Band Gap Engineering and Carrier Transport in TiO2 for Solar Energy Harvesting

TiO2 has been used in photocatalysis and photovoltaics because of its comprehensive combination of energy band structure, carrier transport, and inertness. However, wide band gap and relative slow carrier transport limits its full potential in these applications. 3.2 eV band gap of anatase indicates its low efficiency of utilizing full spectrum of solar light, and, band gap engineering was employed to address this issue. Specifically, nitrogen doping, iron doping, and N/Fe codoping were investigated for their photocatalytic effect. Doping was carried out in hydrothermal reactor by adding aliovalent ions to TiO2 precursor. Both N and Fe doping show the narrowing of band gap compared with the pristine TiO2. N-doping enhances its visible light photocatalytic performance, while Fe-doping and codoping result in poor photocatalysis. Further low temperature fluorescence spectra indicate the high recombination in Fe-doped and co-doped samples. The next issue is the relatively slow carrier transport in TiO2 nanoparticle-based dye sensitized solar cell (DSSC). Single crystalline rutile nanorod was studied in order to overcome this drawback. Synthesis was achieved with the assistance of microwave heating, and reaction rate was boosted due to this unique heating method. The carrier diffusion coefficient and life time of TiCl4 treated nanorod were systematically measured, and surface diffusion model is proposed to interpret observed phenomena. Combining band gap engineering (doping) and carrier transport (nanorod) research, doped nanorod was investigated. It is found that Nb doped nanorod shows higher conductivity, better back contact between FTO and nanorod, easy injection of electron from dye sensitizers to nanorods, and, consequently, less recombination. The efficiency of solar cell from Nb doped nanorod increases by almost 80% comparing with pure nanorod. What’s more, the conductive-AFM with nanoscale resolution provides unprecedented image of current path in nanorod, which could verify carrier transport model and shed light on engineering of nanostructure for superior performance