Titanium dioxide, a well-characterized photocatalyst, can be transformed into a transparent conducting oxide (TCO) by incorporating cationic dopants with an oxidation state greater than that of titanium (Ti⁴⁺). This produces a blue shift in optical absorption and a dramatic reduction in resistivity. Phosphorus can be incorporated as a cationic (P⁵⁺) or an anionic (P^3-) dopant, and it is much more abundant than other common dopants, including Nb or Ta.

TiO₂ materials exhibit a higher refractive index, greater chemical stability, and a lower production cost compared with indium-based TCO materials.

Ivan Parki, University College, London, UK, and colleagues used atmospheric pressure chemical vapor deposition (APCVD) to produce the first example of TiO₂ films doped with both P⁵⁺ and P^3– species. The relative proportion depends on the synthesis conditions. Undoped TiO₂ films showed superior photocatalytic activity to doped films containing primarily P^3– because the phosphorus anion reduced photogenerated charge carrier lifetimes. Doped films with only P⁵⁺ species increased charge carrier concentrations by several orders of magnitude over undoped films.

• Multifunctional P-Doped TiO₂ Films: A New Approach to Self-Cleaning, Transparent Conducting Oxide Materials,
  Carlos Sotelo-Vazquez, Nuruzzaman Noor, Andreas Kafizas, Raul Quesada-Cabrera, David O. Scanlon, Alaric Taylor, James R. Durrant, Ivan P. Parkin,
  Chem. Mater. 2015.
  DOI: 10.1021/cm504734a