Ultrafast Decay Dynamics of Excited and Charged States in α-Sexithienyl Film As Revealed by Femtosecond Transient Absorption and Picosecond Fluorescence Spectroscopy

Abstract

Primary processes of photoexcited states in an R-sexithienyl film are investigated by femtosecond transient absorption spectroscopy and picosecond time resolved fluorescence spectroscopy. Four species are observed in the transient absorption measurement. A broad absorption that shows a very rapid relaxation (within a few picoseconds) is ascribed to a singlet Frenkel exciton state. An oscillating structure is also apparent immediately after excitation and is ascribed to the Stark effect induced by a charged species (ion pair state). The latter state decays within 200 ps and is replaced by a different oscillating structure that is due to a thermal effect induced by a dissipation of excess energy. Another band owing to a triplet state appears via a very rapid conversion from a higher singlet exciton state. Fluorescence decay curves are well fitted with lifetimes of the charged state, which indicates that most of the emission is brought about by the charge recombination process, and furthermore, a charge-transfer (CT) emission band that has not been reported is observed.

Carrier and Polarization Dynamics in MonolayerMoS2

“…The exciton binding energy is estimated to be in the hundreds of meV range [11,12,14]. Systems with large exciton binding energies, such as organic films and carbon nanotubes, have intrinsic exciton radiative lifetimes on the order of a few ps [34][35][36]. In our measurements, the PL decay time τ does not change with the temperature within our time resolution; we do not observe any activation or any other typical signature of nonradiative processes.…”

Carrier injection dynamics in heterojunction solar cells with bipolar molecule

“…22 The robust carrier injection is perhaps ascribed to the efficient charge separation within the 6T layer under photo excitation at 400 nm. [25][26][27] Actually, we observed oscillatory structures in the neat 6T film around Fig. 2(a)], which are ascribed to the 6T þ -6T À pairs.…”

“…2(a)], which are ascribed to the 6T þ -6T À pairs. [25][26][27] The structures are discernible even at 1000 ps, suggesting that the individual 6T þ and 6T À are stable in the 6T layer. Then, some part of the 6T À species can reach the D/A interface and inject electrons to the SubPc layer.…”

Ultrafast spectroscopy of dark states in solid state sexithiophene

“…The model predicts that the initially photoexcited state quickly relaxes into the lowest exciton state, giving rise to spectral and temporal dynamics which are in stark contrast with those of the isolated molecule. Based on this observation a number of time resolved spectroscopical studies have been carried out in polycrystalline materials [8][9][10][11] and thin single crystals, 12 but there is no general agreement on the interpretation of the results.…”