Carbon-doped freestanding TiO₂ nanotube arrays in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated with both closed- and open-ended freestanding TiO₂ nanotube arrays that were doped with carbon via chemical vapor deposition to improve their electron transport properties. The energy conversion efficiencies of DSSCs increased from 5.07% to 6.21% after doping with a small amount of carbon, i.e., an enhancement of 22.4%. This suggests that the π-π conjugation introduced by carbon doping improved the efficiency of electron transport. However, energy conversion efficiencies of DSSCs with a large amount of carbon doping on the TiO₂ nanotube arrays decreased from 5.07% to 2.87%, with a lower short-circuit current, open circuit voltage, and fill factor because of the reduced level of dye adsorption on the TiO₂ nanotube arrays.