Exciton annihilation in a polyfluorene: Low threshold for singlet-singlet annihilation and the absence of singlet-triplet annihilation

Ultrafast photoinduced absorption measurements have been used to directly investigate singlet-singlet annihilation in polyfluorene. The pump fluence threshold for annihilation to dominate the decay was measured to be $\sim 1\mu \mathrm{J}{\mathrm{cm}}^{-2}$ corresponding to an excitation density of $1.5\times {10}^{17}{\mathrm{cm}}^{-3}$. The annihilation rate was found to be faster than that expected from a simple dipole-dipole interaction. This is ascribed to the additional influence of diffusion which, because of the dispersive nature of the exciton migration, has strong time dependence as the singlet excitons thermalize in the density of states as well as the expected intrinsic time dependence from a diffusion controlled process. Also, a comparable background level of triplets was created in the film to study the effect of singlet-triplet annihilation, which surprisingly, given the low threshold for singlet-singlet annihilation, was found to be negligible.

Figure 1

(Color online) Time resolved photoinduced absorption spectra of a thin film of PF2/6 at various times after photoexcitation measured for an excitation density of $20\mu \mathrm{J}{\mathrm{cm}}^{-2}$.

Figure 2

(Color online) Decay of the PA singlet exciton feature $\left(1.60\mathrm{eV}\right)$ for excitation dose of $25\mu \mathrm{J}{\mathrm{cm}}^{-2}$ (엯) and $7\mu \mathrm{J}{\mathrm{cm}}^{-2}$ (∎) at $300\mathrm{K}$ (top panel) and $10\mathrm{K}$ (bottom panel).

Figure 3

(Color online) Excitation dose dependences for the intensity of the PA absorption at 1.6 and $2.4\mathrm{eV}$. Straight lines of gradient 0.5 and 1 are drawn as a guide to the eyes.

Figure 4

(Color online) Time dependence of the singlet-singlet annihilation parameter $\gamma$ at $10\mathrm{K}$. The curves are calculated from the transient absorption data (filled squares) and a theoretical plot of the expected annihilation parameter from Eq. (4), using the values of $R$ and $D$ given in the text (hollow circles).

Figure 5

Variation of the separation of singlet excitons at the point where SSA dominates the decay of the singlet excitons (solid) and singlet-triplet separation when a background of triplet is created with cw excitation (dots). Variation with depth into the sample (left) and triplet-triplet annihilation rate (right).

Figure 6

(Color online) Decay of the singlet exciton PA feature both with (엯) and without (∎) a background of triplet population, for cw excitation $\left(100\mathrm{mW}{\mathrm{cm}}^{-2}\right)$.