Revealing Ultrafast Relaxation Dynamics Of Organic Semiconductor Thin Films And Single Crystals

The organic oligomer six-thiophene(6T)in solid phase is the first discovered organic semiconducting compound which exhibits extraordinary electronic and optical properties;indolo[3,2-b]carbazole(ICZ)as another ?-conjugated material,has also been used in various electronic applications and photovoltaic devices due to its excellent electron-donating properties.In this paper,the excited state dynamics of 6T thin film/single crystal and ICZ thin film/single crystal were studied by ultrafast time-resolved spectroscopy,e.g.time correlated single photon counting,fluorescence up-conversion and femtosecond transient absorption technics.Main conclusions are as followed by explaining the singlet fission,charge transfer and polaron formation processes in detail:1.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 was directly observed from the upper vibrational levels of the first excited singlet state S1 in 6T thin film/single crystal within 30 fs.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 e V)leads to a higher yield of polarons,while SF in film/crystal is fully suppressed.Until now singlet fission in 6T thin film and single crystal was observed for the first time by femtosecond pump-probe spectroscopy.Existence of long-lived photogenerated polarons may count ?-conjugated oligomers as promising materials for developing organic-molecule-based optoelectroinc devices.2.The wavelength dependent fluorescence lifetime of ICZ system indicates that the excited singlet state has a fine structure and includes many well-arranged vibrational levels,with lifetimes ranging from 1.5 to 4.6 ns of film and 2.5 to 6.6 ns of crystal.In ICZ thin film,singlet fission was directly observed both from the upper excited singlet state SN within 480-850 fs and the upper vibrational levels of the first excited singlet state S1 within 6-11 ps,which fully suppressed intramolecular charge transfer process in S1 state.In ICZ single crystal,the exciton relaxation through singlet fission and ion formation competing in the ultrafast timescale(<30 fs),which is accompanied by a broadening of S1-SN absorption band in transient absorption spectra.Compared with ICZ thin film,the triplet state staying for up to 1 ms in single crystal,which once again deepens the understanding of the influence of well-structured crystallinity and molecular packing on the triplet lifetime.This is the first time to perform ultafast time-resolved spectroscopy for ICZ system,which provides unique informtion and valuable reference in constructing advanced organic optoelectronic and photovoltaic applications.