Photocarrier relaxation through the manifold of localized statesin a polymer-fullerene bulk heterojunction material

We investigate the transport and relaxation of photogenerated carriers in a polymer-fullerene bulk-heterojunction (BHJ) material by measuring the transient photoconductivity as a function of the sample temperature. Data in the nanosecond time regime indicate that following the initial trapping (within ∼ 100 ps ), the photogenerated carriers relax into the manifold of disorder-induced localized states near the band edge, and the transport becomes thermally activated. By measuring the temporal dependence of the transport activation energy, we are able to monitor the relaxation of carriers into deeper traps, with a corresponding reduction in carrier mobility. A comparison of BHJs made from different molecular weight poly(3-hexylthiophene) (P3HT) (21 and 95 kDa), indicates that higher molecular weight polymer in the BHJ material yields a consistently higher activation energy. The reduction in the carrier mobility in BHJ materials made with higher molecular weight P3HT is confirmed by steady-state photoconductivity measurements. In contrast to disordered inorganic semiconductors (e.g., amorphous silicon), the activation energies associated with the density of states in the disorder-induced band tails are a small fraction ( < 10 % ) of the band gap energy of P3HT.