Exciton dissociation mechanisms in the polymeric semiconductors poly„9,9-dioctylfluorene… and poly„9,9-dioctylfluorene-co-benzothiadiazole… Mark A. Stevens, Carlos Silva, David M. Russell, and Richard H. Friend Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom Received 19 September 2000; revised manuscript received 10 January 2001; published 6 April 2001 We present femtosecond transient absorption measurements on the semiconductor conjugated polymers poly9,9-dioctylfluoreneF8 and poly9,9-dioctylfluorene-co-benzothiadiazoleF8BT. Detailed photophysi- cal modeling reveals that, in F8, sequential excitation, first to the lowest singlet excited state, and then to a higher-energy state resonant with the pump photon energy, is predominantly responsible for the rapid ( 150 fs) dissociation of photoinduced excitons. Resonant sequential excitation accesses high-energy states that can promptly evolve to charged or triplet states. In F8BT, however, we find that sequential excitation plays a lesser role in fast polaron-pair generation, and that exciton bimolecular annihilation can explain the charge population. We suggest that the electrophilic benzothiadiazole groups in F8BT facilitate charge formation by dissociation of the excited state formed by exciton-exciton annihilation. Modeling also reveals that exciton bimolecular annihilation can occur via two separate and competing processes. We find that in F8, the dominant mechanism involves exciton diffusion and collision. In F8BT, however, additional annihilation of spatially separated excitons occurs when they interact through the Fo ¨rster transfer mechanism, where the critical dis- tance for annihilation in F8BT is 4 nm. DOI: 10.1103/PhysRevB.63.165213 PACS numbers: 78.47.+p, 78.66.Qn, 78.20.Bh I. INTRODUCTION Polyfluorenes have emerged recently as a class of conju- gated polymer which show promise for applications in opto- electronics. The prospects of high luminescent efficiency, good charge-transport characteristics, 1 thermal stability, and tunability of physical parameters through chemical modifica- tion and copolymerization 2 have made these polymers inter- esting from a commercial aspect. Many members of the polyfluorene family are high-efficiency blue-emitting materi- als, which are desirable both because they are required for full-color displays, and because they can act as energy do- nors in blends with other conjugated polymers. Efficient light-emitting diodes, 3 and photovoltaic diodes 4 based on polyfluorene, have been demonstrated. The simplest materials in this large class of polymers are those based on a fluorene monomer unit with alkyl side chains to confer solubility. One of the most interesting prop- erties of these materials is their ability to form a liquid crys- talline phase. 5–7 This has allowed for the preparation of aligned films, which in turn have been used to produce po- larized photoluminescence 5,7 and electroluminescence, 8 and to greatly increase the carrier mobility in the alignment direction. 9 The morphology of the polymers can be modified drastically when they are cooled from the liquid crystalline phase. It was reported 5 that slow cooling allows crystalliza- tion to occur, while rapid cooling, or ‘‘quenching,’’ results in the morphology of the solid film retaining that of the liquid crystal phase. In addition to their excellent properties for device appli- cations, polyfluorenes and related materials also provide a convenient system in which to study the effects of film morphology. 10 Modifications to the side chains can have strong effects on the packing structure at the molecular level, without significantly altering the electronic structure of the polymer. A recent comprehensive study by Kraabel et al. showed that in phenylene-based conjugated polymers, two types of photoexcitations are produced; an intrachain exciton, and weakly coupled, interchain polaron pairs, created at the ex- pense of the singlet exciton. 11 Numerous additional ultrafast studies have unambiguously identified singlet excitons as be- ing responsible for the stimulated emission observed from solutions 12,13 and solid films of conjugated polymers. 14–16 More recently, singlet excitons have also been associated with the transient photoinduced absorption band which usu- ally appears around 1.5 eV. 16,17 However, the dynamics of the photoinduced absorption over a large range of wave- lengths and at various pumping intensities indicate that other species are also present. These other absorbing species have been variously attributed to triplet excitons, 18 interchain spatially indirect excitons, 13,17,19 and excimers. 20 It has been well documented that at a sufficiently high initial excitation density, the decay rate of singlet excitons is enhanced due to bimolecular annihilation dynamics. 17,21–25 A previous report from our research group showed that exciton bimolecular annihilation is an important source of photoin- duced charges in poly-p -phenylenevinylene PPV, 24 as probed by steady-state photocurrent. Exciton-exciton annihi- lation was modeled extensively, 22–25 but there is still dis- agreement on the mechanism responsible for such processes. The mechanism for exciton bimolecular annihilation was variably reported in two regimes. The first is diffusion limited, 21,24 where exciton encounter and collision is the rate- limiting process and the bimolecular rate constant is time independent. The second is characterized by a bimolecular rate ‘‘constant’’ that contains an explicit time dependence, 17,22,23,25 where dipole-dipole interactions be- tween pairs of excitons lead to a long-range resonance Fo ¨ r- ster energy transfer. The basis for the two mechanisms in- cludes kinetic modeling of femtosecond transient absorption PHYSICAL REVIEW B, VOLUME 63, 165213 0163-1829/2001/6316/16521318/$20.00 ©2001 The American Physical Society 63 165213-1