Binder-Free Pastes Fabricated from Sol–Gel-Derived TiO₂ by Ammonia Addition for Low-Temperature Photoelectrodes

To obtain high-efficiency dye-sensitized solar cells (DSSCs) on plastic substrates, we used a viscous, binderfree TiO₂ paste composed of TiO₂ nanoparticles and ammonia water that is processable at low temperatures. Nanocrystalline, spherical anatase TiO₂ colloids were synthesized with narrow size distributions (∼20 nm) using a hydrothermal method and were transformed to a binder-free TiO₂ paste when ammonia water was added to the solution. This process results from the flocculation of the TiO₂ nanoparticles by CH₃COO^– and NH⁺ ₄ ions. The high loading (20 wt%) of TiO₂ nanoparticles in the solution resulted in a viscous TiO₂ paste despite the addition of a small amount of ammonia (0.015 wt%), which enabled the formation of uniform, thick, mesoporous TiO₂ films without aggregates. Using this approach, we achieved an energy conversion efficiency of 3.38% in the DSSC when using an 8-μm-thick mesoporous TiO₂ film that was dried at 150 °C without any additional treatment.