Ultrafast singlet and triplet dynamics in microcrystalline pentacene films

The exciton dynamics of microcrystalline pentacene films is investigated by femtosecond pump-probe experiments. Measurements are performed with ultrashort laser pulses applied at normal incidence and at an angle of incidence of $65°$ to disentangle singlet and triplet contributions by exploiting the different orientations of the molecular transition dipoles. The results indicate that the initial 70 fs fast relaxation step transforms the optically excited excitons in a reasonable mobile species with strongly reduced radiative transition strength. Fission into triplet excitons takes place on the picosecond time scale as a secondary, thermally activated process and with a small total yield of approximately 2%. Evidence is provided that the dominant species are singlet excitons with excimer character. To the subsequent dynamics contribute diffusion driven exciton-exciton annihilation and trapping of singlet and triplet excitons. Values for diffusion constants and trap densities are extracted by modeling the measurements with rate equations which include singlet and triplet dynamics.

Figure 1

(a) Steady-state absorption spectrum of a microcrystalline pentacene film, (b) transient absorption spectra at different delay times after photoexcitation at 670 nm measured with laser beams normal to the substrate, and (c) transient spectra with beams at an angle of incidence of $65°$.

Figure 2

Time traces measured with tilted angle of incidence $\left(65°\right)$ of the pump and probe pulses at different probe wavelengths. The solid lines represent simulations with a model that describes the exciton dynamics on the ps time scale subsequent to the initial ultrafast relaxation process (for details see text).

Figure 3

Ab initio calculation (TDDFT) of the S0, S1, and S2 energies of the pentacene dimer depending on the distance between two coplanar pentacene molecules.

Figure 4

Time traces measured at 685 nm with different excitation energies modeled with Eqs. (6, 7, 8, 9, 10) (solid lines). The initial exciton concentrations are obtained by the fitting procedure and scale correctly with the pump energies. For details see text.

Figure 5

Time traces at normal incidence of the laser pulses probed at 685 nm and at different wavelengths in the spectral region of the ESA. The dashed line in the most upper graph results from a fit with a model of pure exciton-exciton annihilation. The dotted lines in the two upper graphs denote a fit with a model additionally accounting for the immobilization of excitons at traps. The solid lines show a consistent fit of all measured time traces achieved with Eqs. (6, 7, 8, 9, 10). For details see text.

Figure 6

Energy diagram and relaxation pathways.

Figure 7

Time traces measured with tilted angle of incidence of the laser pulses after photoexcitation at 670 nm at different probe wavelengths. The traces can be fitted with Eqs. (6, 7, 8, 9, 10) (black lines) with the same parameter values as for the measurements at normal incidence.

Figure 8

Amplitude spectra resulting from the fit with Eqs. (6, 7, 8, 9, 10) of the time traces measured at tilted angle of incidence. The break in the ordinate accounts for the very different amplitudes of the singlet and triplet species.