Hydrothermal preparation of porous nano-crystalline TiO2 electrodes for flexible solar cells

Mechanically stable porous thick film electrodes of nano-crystalline TiO2 have been prepared at a low temperature from mixed pastes of nano-crystalline TiO2 particles and Ti-monomers such as TiCl4, TiOSO4, and Ti-tetraisopropoxide, which are converted into crystalline rutile or anatase TiO2 by hydrothermal treatment at the solid/gas interface and chemically connect the TiO2 particles. Rigid and flexible dye-sensitized solar cells (DSSCs) employing these materials achieved conversion efficiencies as high as 4.2 and 2.5% under AM 1.5 simulated sunlight (1 sun), respectively. These electrodes exhibited a linear increase of Isc against the light intensity beyond 2 sun level, suggesting an ideal porous structure of the film. However, imperfectness of the low-temperature materials has been indicated from the significant improvement of the cell efficiencies by post-heat treatment of the films at 450 °C. Incomplete necking of the particles as well as the presence of residual organics are thought as the reasons which limit the efficiencies of the low-temperature materials.