Solar Cells: Improving solar cell efficiency with plasma-modified TiO2

• Current researchers: Dan Pulsipher
• Related Fisher group references:
  • O2 Plasma Treatment of Mesoporous and Compact TiO2 Photovoltaic Films: Revealing and Eliminating Effects of Si Incorporation, D.J.V. Pulsipher, E.R. Fisher, Surf. Coat. Technol., 203, 2236-2242 (2009).

Titanium dioxide (TiO2) is a versatile and very widely studied inorganic oxide. Among its many applications, TiO2 finds use in photovoltaic and photocatalytic devices. In photovoltaic devices, TiO2 serves as an efficient wide band gap semiconductor material used to promote the flow of electrons in the device by preventing the recombination of electron-hole pairs. In photocatalysis, electron-hole pairs generated by UV light can recombine at a TiO2 surface and cause the chemical reaction of species adsorbed on the TiO2. In this case, the recombination of electron-hole pairs increases photocatalytic efficiency. In the case of photovoltaics, however, electron-hole recombination limits the flow of electrons and inhibits efficiency. Structural defects called electron trap states can contribute to this effect by immobilizing electrons in the material. In the Fisher group, we’re interested in using plasma-modification of TiO2 materials to control the effects of trap states and further understand the role they play in photovoltaic devices. Early efforts to modify mesoporous TiO2 films with different plasma treatments have made use of photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS) as characterization techniques.

We’re also interested in dye-sensitized solar cells as a long-term application of this research. In these devices, photosensitive dyes are used to generate electron-hole pairs. Electrons are then transferred to a TiO2 surface and diffuse to an electrode to be used in a circuit. The TiO2 film may consist of a flat, extended surface, but the use of mesoporous TiO2 films allows for improved device efficiencies by providing a greater surface area for interaction of the TiO2 with the dye. However, electron trap states in the TiO2 reside largely on the material’s surface, so that their ability to inhibit device current can become especially pronounced. Plasmas are uniquely suited for the chemical modification of these mesoporous materials, and as we begin to understand how trap states can be controlled, we may be able to minimize their effect even on the inner surfaces of the pores.