Ultrafast Exciton Relaxation in Microcrystalline Pentacene Films

The exciton dynamics in microcrystalline pentacene films is investigated by transient absorption measurements with 30 fs time resolution. It is found that the emission from photoexcited Frenkel excitons decays within 70 fs due to the ultrafast formation of an excitonic species with a strongly reduced transition dipole to the ground state and an absorption dipole in the plane of the film. We propose that an excimer exciton is formed and stabilized by changes of the local crystal structure. The subsequent dynamics is dominated by diffusion controlled annihilation and trapping.

Figure 1

(a) Steady state absorption spectrum of the microcrystalline pentacene films, (b) transient absorption spectra 0.6, 8, and 600 ps after photoexcitation at 670 nm measured with laser beams vertical to the substrate and (c) transient spectra at 0.5 and 100 ps with an angle of incidence of 65°. $\Delta \mathrm{OD}=-\log (T/T_0)$ is the optical density change, $T$ the sample transmission with photoexcitation, and $T_0$ without photoexcitation.

Figure 2

Decomposition of the polarization averaged transient spectrum (short dashed line) at a delay of 8 ps into ground state bleach (solid line) and the absorption of the excitons (ESA, broken line).

Figure 3

(a) Transient spectra under normal incidence during the first 200 fs in the region of the first absorption band. (b) Time traces (circles) measured with 25 fs probe pulses centered at 635 and 676 nm. Monoexponential decays (solid lines) are convoluted with the cross correlation (dash-dotted lines, vertically shifted for better visibility) and fitted to the data. The inset shows the corresponding energy diagram.

Figure 4

Bleach recovery probed at 685 nm (circles). Modeling by exciton-exciton-annihilation only (broken line and left sketch) gives poor results, while good agreement is achieved if immobilization of excitons by traps is included (solid line and right sketch).