Triplet Exciton Diffusion Length Research Articles

Diffusion of triplet excitons in an operational organic light-emitting diode

Measurements of the diffusion length L for triplet excitons in small molecular-weight organic semiconductors are commonly carried out using a technique in which a phosphorescent-doped probe layer is set in the vicinity of a supposed exciton generation zone. However, analyses commonly used to retrieve $L$ ignore microcavity effects that may induce a strong modulation of the emitted light as the position of the exciton probe is shifted. The present paper investigates in detail how this technique may be improved to obtain more accurate results for L. The example of 4,4'-bis(carbazol-9-yl)1,1'-biphenyl (CBP) is taken, for which a triplet diffusion length of L=16 +/- 4 nm (at 3 mA/cm2) is inferred from experiments. The influence of triplet-triplet annihilation, responsible for an apparent decrease of L at high current densities, is theoretically investigated, as well as the 'invasiveness' of the thin probe layer on the exciton distribution. The interplay of microcavity effects and direct recombinations is demonstrated experimentally with the archetypal trilayer structure [N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)]-4,4'-diaminobiphenyl (NPB)/CBP/ 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (named bathocuproine, BCP). It is shown that in this device holes do cross the NPB/CBP junction, without the assistance of electrons and despite the high energetic barrier imposed by the shift between the HOMO levels. The use of the variable-thickness doped layer technique in this case is then discussed. Finally, some guidelines are given for improving the measure of the diffusion length of triplet excitons in operational OLEDs, applicable to virtually any small molecular-weight material.

Triplet exciton diffusion in fac-tris(2-phenylpyridine) iridium(III)-cored electroluminescent dendrimers

We have studied triplet-triplet annihilation in neat films of electrophosphorescent fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3]-cored dendrimers containing phenylene- and carbazole-based dendrons with 2-ethylhexyloxy surface groups using time-resolved photoluminescence. From measured annihilation rates, the limiting current densities above which annihilation would dominate in dendrimer light-emitting devices are found to be >1A∕cm2. The triplet exciton diffusion length varies in the range of 2–10 nm depending on the dendron size. The distance dependence of the nearest-neighbor hopping rate shows that energy transfer is dominated by the exchange mechanism.

Triplet exciton–charge carrier interaction in crystalline tetracene

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Diffusion Length and Lifetime of Triplet Excitons and Crystal Absorption Coefficient in Tetracene Determined from Photocurrent Measurements

AbstractBy comparing the decrease of the i+ and i− currents as function of the exciting wavelength and from the maximum of the i− current the crystal absorption coefficient of tetracene can be determined. With the penetration depth of the radiation thus known the diffusion length and lifetime of triplet excitons are determined from the dependence of the currents on the wavelengths. The diffusion length is found to be (0.4 ± 0.2) μm, the lifetime about 100 μs.

Diffusion Length of Triplet Excitons in Anthracene Crystals

Recently two different experimental principles have been used to determine the diffusion length of triplet excitons and the results have differed by an order of magnitude. Experiments have now been performed using (i) singlet excitation and (ii) triplet lifetime dependence on thickness, and the results are shown to be in accord with results from other techniques. The triplet diffusion length l is shown to be 20±10 μ. Also it is found that l is slightly temperature dependent, with an apparent activation energy of approximately 10−2 eV.

Über die Reichweite von Triplett-Excitonen in reinem geschmolzenem Naphthalin

In connection with the preceeding paper measurements are reported concerning the photocurrent arising in molten naphthalene with intense illumination. It has been shown that irradiation in the spectral regions of the forbidden singlet-triplet absorption band and of the longwave edge of the singlet absorption band can produce triplet excitons in the bulk of the liquid. If the incident light is parallel to the electrodes, it is able to build up a nearly homogenous distribution of excitons in a layer parallel to the electrodes. By variation of the distance of the layer from the anode one can get information about the migration and the diffusion length of the triplet-excitons, because they can only contribute to the conductivity if they have reached the anode and have dissociated there. The experiments demonstrate that in forty-times zone-refined molten naphthalene (oxygen pressure less than 10-2 Torr) the diffusion length of triplet excitons is about 1 cm. The migration obeys a diffusion law with a diffusion constant D= 1.5 cm2 sec-1 (error: factor 2). From the rise and decay times of the photocurrent it follows a mean life time τ = (0.4 ± 0.2) sec of the triplets in the limiting case of neglegible dissociation. The probability of primary dissociation is about 0.3 (error: factor 2). Because of the high recombination probability only a small part of the produced cations can move to the cathode, so that the resulting dissociation probability is much lower. The results cannot be explained by assuming diffusion of an excited molecule but demand the assumption of a high hopping probability of the triplet state in a pure liquid aromatic compound.