Thermally induced structural modifications of nano-sized anatase films and the effects on the dye-TiO2 surface interactions

Nano-sized TiO2 films have been grown by DC-Reactive Sputtering at ∼150 °C on [0 0 0 1] ZnO:Al substrates to be applied in Dye Sensitized Solar Cells. The effects of post-deposition thermal treatment at 500 °C on the optical and structural (density, grain size) properties of the TiO2/ZnO:Al bi-layers have been investigated in details. The effects of such modifications on the surface properties, have been evaluated by dye-sensitization with 5,10,15,20 Tetrakis(4-carboxyphenyl) porphyrin (TCPP). Different interactions of the probe-molecules with the as grown and annealed TiO2 surfaces have been observed. In particular, Quantum yield (QY) analysis of TCPP-sensitized films showed that the emitted/absorbed photons ratio in the as grown layers is a factor of ∼3 lower than in the annealed layers. Based on simplified test solar cells, the electron injection is thought to be the most appropriate mechanism to explain the QY results. Our approach gives particular emphasis to the role of the surfaces and the interfaces involved in the TCPP/TiO2/AZO multilayer structures with the intent of proposing the as sputtered TiO2 layer as blocking layer in complete architecture for low thermal budget solutions (<200 °C).