Mechanism of Primary Charge Photogeneration in Polyfluorene Copolymer/Fullerene Blends and Influence of the Donor/Acceptor Lowest Unoccupied Molecular Orbital Level Offset

Abstract

Primary charge photogeneration dynamics in the films of polyfluorene copolymer PFDTBT (poly([2,7-(9,9-bis(3,7-dimethyloctyl)fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)])) blended with fullerene derivatives, with variation of fullerene/polymer lowest unoccupied molecular orbital level offsets (−ΔEL) over 110–320 meV, are investigated by using near-infrared transient absorption spectroscopy under low excitation photon fluence. Time-resolved spectra combined with spectroelectrochemical characterization enable us to verify the signature spectra of the S1 exciton of the polymer, the interfacial charge transfer (ICT) state, and the charge-separated (CS) state, with which the species-associated kinetics are derived via decomposition of the time-resolved data matrices. For a neat film the interchain CT state is generated irrespective of the excitation photon energy; however, the CS state forms merely upon the above-gap excitation with a quantum yield of 15%, which subsequently recombines into the S1 exciton within 1 ps. For blend films a dual-path scheme of free charge formation is revealed: The S1 exciton transforms in a parallel manner into the ICT and CS states with branching ratios ΦICT:ΦCS ≈ 3:1 and time constants of 180–800 fs depending on −ΔEL. When −ΔEL is relatively large, i.e., 0.29 or 0.32 eV for PFDTBT blended with PC61BOE (4-(pentyl-[6,6]-C61)benzene octyl ether) or PC61BM (phenyl-[6,6]-C61-butyric acid methyl ester), respectively, the ICT state further dissociates into the CS state with a time constant of ∼300 ps and an efficiency exceeding 50%. This channel, however, closes when −ΔEL < 0.20 eV, and the ICT state fully recombines back to the ground state within a few hundred picoseconds. The large ICT-to-CS branching ratio and high dissociation efficiency of the ICT state consolidate the crucial role of this state in photocurrent generation. On the other hand, the process of exciton-to-CS transition is found to obey Marcus’s nonadiabatic electron transfer mechanism with a coupling strength V = 55 ± 7 cm–1 and a reorganization energy λ = 0.33 ± 0.02 eV, whereas the dissociation of the ICT state can be accounted for by Braun–Onsager’s model of e––h+ dissociation. The dynamic properties of the ICT state and roles of −ΔEL in yielding charge species (ICT and CS) revealed in the present work may shed light on the development of new photovoltaic materials.