Certain dyes, especially those containing the triarylamine moiety, can perform hole transport via intermolecular hole hopping [see, for example, Bonhôte et al., JACS 121:1324 (1999)]. We shall report on our observations of hole transport in the triarylamine-containing organic dye D5 and a related family of dyes under development.

Calculations have shown that the positive charge in oxidized D5 resides primarily on the triarylamine moiety [Hagberg et al., Chem. Commun. 2006:2245 (2006)], and hole transport is presumed to occur between these groups. Cyclic voltammetry of D5 adsorbed to a nanostructured TiO₂ film shows a reversible oxidation peak near +0.45 V vs. Fc⁺/Fc, which, since this level lies within the bandgap of the TiO₂, must be due to hole transport through the dye (see figure; inset shows peak current vs. square root of scan rate, indicating diffusion-like oxidation and hopping).

Hole transport in a dye is not necessarily beneficial: it can provide an additional route for recombination as holes are conducted to the collector. However, this effect appears minor in D5, which gives high efficiencies in a DSC. In any case, a TiO₂ blocking layer between the substrate and semiconductor film should minimize any recombination problem.

On the other hand, a hole-transporting dye opens the possibility of constructing a solid-state DSC with no additional hole transport material (HTM). Alternatively, when used in cells containing an HTM such as spiro-OMeTAD, a hole-transporting dye can alleviate problems due to incomplete HTM pore filling by shuttling holes from unfilled pores to the HTM.