Time-Resolved Charge Carrier Generation from Higher Lying Excited States in Conjugated Polymers

Sub-ps three-pulse transient differential transmission spectroscopy using two excitation pulses is used to directly investigate the generation of charge carriers in ladder-type poly(para)phenyl in bulk film. The role of higher excited singlet states of both even and odd symmetry is examined and the dynamics of the major processes involved is described quantitatively. The charge generation efficiency is found to depend strongly on the delay between the two excitation pulses. This is explained by the interplay between internal conversion, excitation energy migration, and on-site vibronic relaxation.

Figure 1

Transient differential transmission spectrum of $m$-LPPP at pump-probe delays 300 fs for excitation at 3.2 eV (solid line), respectively, 4.8 eV (dash-dotted line), and 10 ps (dashed line for 3.2 eV, dotted line for 4.8 eV). Upper inset shows larger part of spectrum for 3.2 eV; lower inset shows chemical structure of $m$-LPPP.

Figure 2

Transient transmission dynamics at 2.5 eV with (triangles) and without (squares) reexcitation 300 fs after pump pulse. Diamond curve shows the difference between the two cases; lines show fit (parameters, see text). Inset shows the energy level structure of $m$-LPPP.

Figure 3

Double-differential transmission spectrum of $m$-LPPP for reexcitation 300 fs after pump, probed after 10 ps.

Figure 4

Double-differential transmission dynamics at 1.9 eV for reexcitation 300 fs after pump. Dashed line shows fit as discussed in the text. Inset shows the behavior on a longer time scale.

Figure 5

Double-differential signal at 10 ps after push in function of pump-push delay (lower curve). Upper curve shows transient transmission dynamics at 1.6 eV.