Ultrafast exciton dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films

Ultrafast dynamics of excitons in organic semiconductors is essential for a deep understanding of the working mechanism of plastic opto-electronic devices. In this work, excited state dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films has been studied with femtosecond transient absorption and time-resolved photoluminescence spectroscopy. Upon the excitation with a femtosecond pulse at 400 nm, a broad positive absorption band at 1.5-2.4 eV is observed that contains two components: one decays with a time constant of a few ps and the other with 67 ± 7 ps. Because the decay curve of the latter coincides with that of photoluminescence, the slow decay component is ascribed to the lowest singlet exciton. The former fast decay component is ascribed to mixed states between charge transfer (CT) and Frenkel excitons, because it is accompanied by a feature due to the Stark effect caused by transient charged species: a combination of bleach and positive absorption at hνprobe > 2.4 eV which looks like derivative modulations of the ground state absorption spectrum. A pronounced polarization dependence is observed for the derivative-like features; this is due to anisotropic distributions of the dipole moments formed by the CT excitons. The derivative-like feature changes its shape after the decay of the mixed Frenkel-CT exciton and grows with a pump-probe delay time of up to 1 ns due to a thermal effect. The decay rate of the mixed Frenkel-CT exciton strongly depends on its density because of exciton-exciton annihilation at high density.