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Diketopyrrolopyrrole (DPP) is one of the most common building blocks for small molecules and conjugated polymers designed for organic electronic applications. Transient absorption spectroscopy (TAS) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy were used to examine bulk heterojunction blend films of a small diketopyrrolopyrrole-based molecule, TDPP, with the common fullerene derivatives PC60BM and PC70BM. Following pulsed laser excitation, the spectral signatures of a fullerene anion and a TDPP triplet state are observed on the picosecond timescale by TAS. The presence of these species imply the formation of a TDPP:PCBM charge transfer state that subsequently undergoes ultra-fast spin-mixing and geminate recombination to produce a TDPP triplet state. The overall photophysical mechanism is confirmed by TR-EPR spectroscopy, which unambiguously shows that the TDPP triplet is formed via spin-mixing in the TDPP:PCBM charge transfer state, rather than direct intersystem crossing from the excited singlet state.
Furthermore, ultra-low band gap polymers INDT were investigated further using transient absorption spectroscopy (TAS) and pump-push photocurrent measurements. Different fullerenes were trialled to assess the effect on charge photogeneration. The LUMO levels of the donor and acceptor are almost isoenergetic for PC60BM (implying virtually zero driving force for charge separation) and this is reflected in inefficient charge photogeneration. A ketolactam fullerene with a deeper LUMO produces a higher level of charge photogeneration. Interestingly, it was discovered that the INDT polymers may possibly generate an intramolecular CT state-like singlet exciton, which is only able to be efficiently separated in the presence of a fullerene with a deep enough LUMO.
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