Understanding Fundamental Processes in Poly(9,9-Dioctylfluorene) Light-Emitting Diodes via Ultrafast Electric-Field-Assisted Pump-Probe Spectroscopy

Femtosecond electric-field-assisted pump-probe measurements are a new approach to the study of fundamental processes within organic optoelectronic devices. Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene), using both temporal and spectral information to investigate polaron generation, due to field-induced singlet dissociation, and their subsequent recombination. The fundamental event in electroluminescence is time resolved: we find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. Our results indicate that the efficiency of singlet formation, $\beta \approx 0.7$, is much higher than expected from simple state degeneracy arguments ($\beta \approx 0.25$).

Figure 1

(a) The chirp-free transient absorption spectrum ($\Delta T/T$) of a layer of PFO at 1 ps pump-probe delay. The chemical structure of PFO is shown in the inset. The nature of the spectral features (labeled ${\mathrm{P}\mathrm{A}}_1$, ${\mathrm{P}\mathrm{A}}_2$, SE) is discussed in the text. (b) The field-induced change (${\Delta }^{2}T/T$) in the differential transmission spectrum for a PFO film at two different pump-probe delays at 2 ps (filled triangles + dashed line) and at 200 ps (open squares + solid line).

Figure 2

${\Delta }^{2}T/T$ dynamics at 680 nm (solid line) and at 820 nm (open squares $+$ solid line). The $\Delta T/T$ dynamics are shown at the same wavelengths in the inset (same symbols apply).

Figure 3

The temporal evolution of (a) the polaron population (probed at 580 nm) in a LED structure (${\Delta }^{2}T/T$, solid squares) and the trace decay at 820 nm (${\Delta }^{2}T/T$, open squares); (b) the directly photogenerated singlet population (probed at 680 nm) without ($\Delta T/T$, filled squares) and with (${\Delta }^{2}T/T$, open squares) an applied field. Fits to the data are shown by solid lines and are discussed more extensively in the text.

Figure 4

The temporal evolution of the polaron population in a LED structure with the presence of an applied electric field at three different excitation densities: $1.67\mathrm{m}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$ (filled squares), $0.44\mathrm{m}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$ (open squares), and $0.18\mathrm{m}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$ (solid line).