Effect of Nitrogen Doping on the Performance of Dye-Sensitized Solar Cells Composed of Mesoporous TiO2 Photoelectrodes

Nitrogen-doped mesoporous TiO2 (NMP TiO2) nanoparticles are synthesized using a soft triblock copolymer template by TiCl4 hydrolysis with ammonia water and applied to the photoelectrodes of dye-sensitized solar cells (DSSCs). The large surface area of a TiO2 mesoporous structure is favorable for dye uptake, and nitrogen doping of TiO2 is expected to increase the charge transport in the photoelectrode as well as the scattering of visible light. Structural characterizations for NMP TiO2 nanoparticles by XRD, XPS, BET, and BJH analyses revealed successful synthesis. However, the photovoltaic performances of the DSSCs prepared from NMP TiO2 were not improved, as had been expected: the photo-conversion efficiency (eta) of DSSCs from undoped mesoporous TiO2 (MP TiO2) was 4.69%, an improvement over the 4.15% with the application of P25 TiO2, but the efficiency of DSSCs from NMP TiO2 decreased to 3.2 similar to 3.6%. The measured amounts of adsorbed dye showed that nitrogen doping did not significantly affect dye adsorption. Therefore, it can be concluded that nitrogen doping increases isotropic charge transport in a TiO2 nanoparticle to promote charge recombination into an electrolyte, despite its advantages. The full benefits of nitrogen doping may be obtained through measures such as the deposition of a thin barrier layer of oxide onto the TiO2 surface to prevent charge recombination during charge transport in the TiO2 network.