High Electron Mobility and Its Role in Charge Carrier Generation in Merocyanine/Fullerene Blends

Charge carrier generation and drift dynamics have been investigated in two types of dye:fullerene heterojunctions: vacuum-deposited merocyanine:C-60 and solution-processed merocyanine:PC61BM blends by combining electric-field-induced fluorescence quenching and ultrafast time-resolved carrier drift measurements. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. Interfacial CT states on large C-60 domains have low energies, while CT states on PC61BM domains have larger energies, which are weakly dependent on the domain sizes. We distinguish two interfacial CT state dissociation pathways: (i) ultrafast, weakly dependent on the electric field and (ii) slow field-assisted dissociation during entire CT state lifetime. We attribute process i to low-energy, weakly bound CT states on large fullerene domains and process ii to strongly bound CT states on small domains or single fullerene molecules. The electron mobility in films with 50% of C-60 is several times higher than in the films with PC61BM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons; thus, fast motion and extraction of electrons are crucial for efficient solar cells.