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

Abstract

(2007) 'Exciton annihilation in a polyuorene : low threshold for singlet-singlet annihilation and the absence of singlet-triplet annihilation. ', Physical review B., 76 (8) Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. 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 ϳ1 J cm −2 corresponding to an excitation density of 1.5ϫ 10 17 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.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

“…Fluorescence and electroluminescence intensities obviously satisfy these requirements. For both kinds of excitation (optical or electrical), these signals are truly time-independent and directly proportional to the singlet generation rates at normal excitation densities, only at high laser fluences do nonlinear effects start to emerge [98]. This can be seen in Figure 2, where the individual optically and electrically excited signals perfectly add up to the simultaneously excited one, that is, the optically excited fluorescence contribution in the presence of the electrical excitation is unchanged.…”

Photoluminescence quenching in films of conjugated polymers by electrochemical doping

“…They are either formed by the absorption of a photon or by the meeting of injected electrons and holes. Excitons can be quenched in the presence of sufficiently large electric fields [2], by other excitons [3], or by polarons [4]. Which of these is dominant depends on the device and the emitter used.…”

Photoluminescence sidebands of carbon nanotubes below the bright singlet excitonic levels

“…8 The strong confinement in the radial direction ͑ϳ1 nm͒ and the weak dielectric screening inside SWNTs give rise to very large exciton binding energies of ϳ0.5 eV, [9][10][11][12][13] which is much larger than those of InGaAs quantum wires 5,14 and larger than or comparable to those of -conjugated polymers. 15,16 So far, various groups have studied the ex-ph interactions in SWNTs both in terms of theory 17,18 and experiment [19][20][21][22] and have revealed the existence of a phonon sideband approximately 200 meV above the energy level of the singlet bright exciton ͑which has an s envelope and termed a "1u" state based on symmetry 12,13 but we simply call it "E ii " hereafter for the ith single-particle subband͒. [17][18][19][20][21][22] Techniques for preparing SWNT samples for optical studies have been rapidly improving recently.…”