Sol-gel titanium dioxide blocking layers for dye-sensitized solar cells: electrochemical characterization

Compact, thin TiO2 films are grown on F-doped SnO2 (FTO) by dip-coating from precursor solutions containing poly(hexafluorobutyl methacrylate) or hexafluorobutyl methacrylate as the structure-directing agents. The films are quasi-amorphous, but crystallize to TiO2 (anatase) upon heat treatment at 500 °C in air. Cyclic voltammetry experiments performed using Fe(CN)6(3-/4-) or spiro-OMeTAD as model redox probes selectively indicate the pinholes, if any, in the layer. The pinhole-free films on FTO represent an excellent rectifying interface at which no anodic faradaic reactions occur in the depletion state. The flat-band potentials of the as-grown films are upshifted by 0.2-0.4 V against the values predicted for a perfect anatase single-crystal surface, but they still follow the Nernstian pH dependence. The optimized buffer layer is characterized by a combination of quasi-amorphous morphology (which is responsible for the blocking function) and calcination-induced crystallinity (which leads to fast electron injection and electron transport in the conduction band). The latter manifests itself by a reversible charging of the chemical capacitance of TiO2 in its accumulation state. The capacitive-charging capability and pinhole formation significantly depend on the post-deposition heat treatment.