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

Mechanically stable porous thick film electrodes of nano-crystal line 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 I-sc 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 450degreesC. 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. (C) 2004 Elsevier B.V. All rights reserved.