In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and the mixed donor-acceptor phase are generally regarded as detrimental. Here, we show experimental evidence that a continuous donor network is not strictly necessary – hole motion between isolated donor sites can occur efficiently by long-range tunneling.
Using Time-Resolved Electric-Field-Induced Second Harmonic generation (TREFISH) combined with photocurrent measurements we have measured the motion of photo-generated charges from first hopping events (with sub-picosecond time resolution) to full extraction in complete solar cell devices based on α-sexithiophene (α-6T) dispersed in a buckminsterfullerene (C60) matrix. Using vacuum deposition, we carefully vary the molar fraction of α-6T in C60 from homogeneously diluted (<10% molar), to a point where α-6T begins to form isolated aggregates (>10-25% molar) or is strongly aggregated (50% molar). We thus vary the distance between the donor sites in a controlled manner. We quantitatively show that even highly diluted α-6T sites (5.7-10% molar) in a C60 matrix enable hole transport, which occurs between isolated donor sites by hole tunneling through several C60 molecules (tunneling distance ≈ 4 nm). Furthermore, at such low donor amounts (<10% molar) electron transport in the buckminsterfullerene phase remains unperturbed, thus facilitating ambipolar transport.
These results question the relevance of ‘pristine phases’ and whether a continuous interpenetrating donor-acceptor network is the ideal morphology for charge transport. The limits of this charge transport mechanism are yet to be explored.