Dye-sensitized solar cells made from nanocrystalline TiO2 films coated with outer layers of different oxide materials

The material extensively used for construction of dye-sensitized solar cells is TiO2. Similar cells made from other familiar semiconductor oxide materials such as SnO2 and ZnO have yielded efficiencies far below the values corresponding to TiO2-based cells. The indication is that electrons injected to s-band materials (SnO2) are more susceptible to recombination compared to d-band materials (TiO2). However, an impressive improvement in efficiency has been noticed when SnO2 crystallites are coated with similar to0.5-1 nm ultra-thin shells of high band gap oxides which act as a barrier against recombination. Naturally a barrier would also lower the electron injection efficiency and oxide barriers of thickness similar to0.5-1 nm were noted to lower the efficiency of cells based on TiO2. Contrary to our observation, reports in literature gives instances of obtaining higher efficiencies when TiO2 crystallites in the film are coated with other oxide materials. In order to elucidate this problem an extensive series of experiments were conducted by coating TiO2 crystallites with outer shells of different materials of varying thickness. Although there is some evidence for marginal improvement, efficiencies distinctively above the optimized TiO2 could not be achieved by this technique. Experimental details and difficulties involved in making a clear conclusion are discussed. (C) 2004 Elsevier B.V. All rights reserved.