Revealing ultrafast relaxation dynamics in six-thiophene thin film and single crystal

The excited state dynamics of six-thiophene (6T) thin film and single crystal was studied by time-resolved fluorescence and femtosecond transient absorption techniques under different excitation conditions. The dominant process in 6T system is the generation of ion pairs, dissociated to polarons staying for up to 130-3700 ns in film and 1 ms in crystal. Singlet fission (SF) was directly observed from the upper vibrational levels of the first excited singlet state S-1 in 6T thin film/single crystal within 30 fs, which competes with intramolecular vibrational relaxation. Triplet state lifetime is dramatically shortened from 6T single crystal (1 ms) to thin film (4 ns) due to triplet-triplet annihilation, which is influenced by structural defects in amorphous regions of 6T film. Compared with 6T crystal, the partially ordered semicrystalline morphology of 6T film suggests the impact of well-structured crystallinity and molecular packing on the photocarriers transport dynamics and lifetime of triplet state. Moreover, excitation to upper excited singlet state (4.96 eV) leads to a higher yield of polarons (factor of 4), while SF in film/crystal is fully suppressed. Existence of long-lived photogenerated polarons may count pi-conjugated oligomers as promising materials for developing organic-molecule-based optoelectroinc devices.

Automated summary

The excited state dynamics of six-thiophene (6T) thin film and single crystal differ under different excitation conditions, with the generation of ion pairs being the dominant process and triplet state lifetime being influenced by structural defects. Additionally, the competition between singlet fission and intramolecular vibrational relaxation in thin film and single crystal has implications for developing organic-molecule-based optoelectronic devices.