Triplet Exciton Diffusion and Phosphorescence Quenching in Iridium(III)-Centered Dendrimers

Abstract

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...

Kinetic Monte Carlo study of triplet-triplet annihilation in organic phosphorescent emitters

“…Namdas et al 12 and Ribiere et al 13 adopted this point of view, and deduced from transient PL studies for a series of dendrimers with a tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) core that the exciton hopping rate decreases exponentially with the hopping distance (R), consistent with an exchange (Dexter-type) interaction with a rate proportional to expðÀ2R=kÞ. For the two sets of materials studied, including in Ref.…”

Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductors

“…3 fit well with a single value of ␥ = ͑6 Ϯ 1͒ ϫ 10 −9 cm 3 / s, which indicates that the annihilation is diffusion-controlled. In this case 15,17 ␥ =4 DR a , where R a is the annihilation radius. The diffusion coefficient D in the neat CBP film is expected to be the same as in the blend because the diffusion occurs by incoherent hopping and is therefore insensitive to scattering effects.…”

“…We assume that the IrG1 dendrimers are homogeneously dispersed in the CBP host at a molecular scale, which was inferred in a previous study of triplet-triplet annihilation. 15 In this case the average center-to-center distance between IrG1 dendrimers is N d −1/3 = 2.5 nm. We have modeled the direct energy transfer in the blend assuming a cubic unit cell with a point dipole of the acceptor molecule located in the center of the cube.…”

“…When the excitation density increases, the PL decays become faster due to exciton-exciton annihilation. The PL intensity is proportional to the density of singlet excitons N͑t͒, which in the case of a short excitation pulse and a time-independent annihilation constant ␥ can be described by 15,17 N͑t͒ = ͓N 0 exp͑−t / ͔͒ / ͕1+␥ · · N 0 ͓1 − exp͑−t / ͔͖͒, where N 0 is the initial excitation density and = 0.5 ns is the decay time in the absence of annihilation. The results in Fig.…”

“…11,12 Increasing the distance between light emitting guest chromophores helps to reduce concentration quenching [13][14][15] and losses associated with exciton dissociation, exciton-exciton annihilation, and exciton-charge interactions, which occur at high electric fields and high current densities. 11,[15][16][17][18][19][20] A particularly widely used host for organic LEDs is the organic semiconductor 4 , 4Ј-bis͑N-carbazolyl͒biphenyl ͑CBP͒. [2][3][4][5][6][7][8][9] CBP is also of current interest as a host for laser gain media, with the potential for low threshold or even electrical pumping.…”