A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay. DOI: 10.1103/PhysRevLett.100.017402 PACS numbers: 78.66.Qn, 73.50.Gr, 78.40.Me, 78.55.Kz Excitation energy transfer is an important process in organic semiconductors and has to be taken into account when designing new optoelectronic materials and devices. In photovoltaic devices, the neutral excited state is generated by light absorption and must diffuse to a heterojunction with another material to be separated into charge carriers and so provide photocurrent. In organic light emitting diodes (OLEDs), exciton diffusion can lead to a decrease of the electroluminescence efficiency due to quenching of the emitting state by intermolecular interactions and defects [1,2], exciton interactions with charge carriers [2,3], and exciton-exciton annihilation [4,5]. These quenching effects are more pronounced in phosphorescent OLEDs than in fluorescent devices because of the longer excited state lifetime. Nevertheless, phosphorescent OLEDs show much higher internal quantum efficiencies due to their ability to convert both singlet and triplet excitons into light [6 -8]. A photoluminescence (PL) study of iridium(III) complexes dispersed into a wide energy gap host suggested that intermolecular quenching of phosphorescence in films is controlled by the dipole-dipole interactions between emitters [9]. However, the impact of exciton migration on phosphorescence quenching has not been considered.In this Letter, we compare the dynamics of triplet exciton diffusion and quenching in several fac-tris(2-phenylpyridyl)iridium(III) [Ir ppy 3 ]-cored phosphorescent dendrimers. Dendrimers provide a convenient way of changing the spacing of the core chromophores in the solid state and hence of studying the effect of spacing on the physics of exciton diffusion and light emission. The triplet exciton diffusion rates are extracted from the measurements of triplet-triplet annihilation and have an exponential dependence on chromophore spacing. This shows that diffusion is controlled by nearest-neighbor electron exchange interactions [10]. Nonradiative decay in 180 nm thick films is governed by much weaker dipole-dipole interactions and does not depend on the triplet diffusion rate. In much thinner films (<20 nm), phosphorescence quenching is found to depend on exciton diffusion and can be modeled using the diffusion equation together with quenching at the film surface.Five green Ir ppy 3 -cored dendrimers were used in this study and their chemical st...
We study photoluminescence and triplet-triplet exciton annihilation in a neat film of a fac-tris͑2-phenylpyridyl͒iridium͑III͒ ͓Ir͑ppy͒ 3 ͔-cored dendrimer and in its blend with a 4 , 4Ј-bis͑N-carbazolyl͒biphenyl host for the temperature range of 77-300 K. The nearest neighbor hopping rate of triplet excitons is found to increase by a factor of 2 with temperature between 150 and 300 K and is temperature independent at lower temperature. The intermolecular quenching rate follows the Arrhenius law with an activation energy of 7 meV, which can be explained by stronger dipole-dipole interactions with the donor molecule in the higher triplet substate. The results indicate that energy disorder has no significant effect on triplet transport and quenching in these materials.
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