Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films

Abstract

Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate-mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor.

Effect of Interlayer Coupling on Ultrafast Charge Transfer from Semiconducting Molecules to Mono- and Bilayer Graphene

“…The approximately 0.7-eV separation between the S 1 and P peak can be accounted for by the exciton binding energy. A similar peak assignment has been made for α-sexithiophene, in which the exciton peak is located at approximately 0.9 eV below the LUMO peak [40]. The position of this state is also consistent with previous inverse photoemission measurements in which the bottom of the LUMO band is found to be about 2.4 eV above the HOMO peak [41].…”

“…4(a). The initial intensity drop is fitted with an exponential decay convoluted with the finite width of the laser pulses to metals is reported to have a transfer time in the range of 10-50 fs [40,60], which is comparable to our CT rate for Au. For interfaces dominated by π-orbital coupling, e.g., CT at organic-organic interfaces [49,50,54,55,61], the CT times are around 100 fs, which can be compared to our CT rate for graphene.…”

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.…”

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

“…Applications cover the ultrafast dynamics in image potential states, [1][2][3][4][5] electron relaxation in metals, 6-9 semiconductors 10-13 and more recently topological insulators, [14][15][16][17][18][19] charge transfer processes at solid state interfaces [20][21][22][23][24] and in adsorbate on surfaces [25][26][27][28][29][30][31] , and the formation and dynamics of excitons. 10,11,[32][33][34][35] The theoretical description of excitons constitutes the main focus of the present work.…”