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Abstract title Fullerene Domains with Low Donor Concentration Enable Hole Transport by Tunneling in Organic Solar Cells
Author Melianas, Armantas, Linköping University, Linköping, Sweden (Presenting Author)
Co-author(s) Pranculis, Vytenis
Spoltore, Donato
Benduhn, Johannes
Inganäs, Olle
Gulbinas, Vidmantas
Vandewal, Koen
Kemerink, Martijn
Abstract text

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and mixed donor-acceptor phase are generally regarded as detrimental. Here, we show that a continuous donor network is not strictly necessary – hole motion between isolated donor sites can occur efficiently by direct 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 alpha-sexithiophene (a‑6T) dispersed in a buckminsterfullerene (C60) matrix (Fig. 1). Using vacuum deposition, we carefully vary the molar fraction of a‑6T in C60 from homogeneously diluted (<10% molar), to a point where a‑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 a‑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.