Measurement of the intersystem crossing rate in aluminum tris(8-hydroxyquinoline) and its modulation by an applied magnetic field

Zhang, Sijie; Song, Jiaxin; Kreouzis, Theo; Gillin, W. P.

doi:10.1063/1.3204015

Cited by 17 publications

(11 citation statements)

References 20 publications

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“…The phosphorescence emission is not observed, indicating that the SOC strength is very weak. Although we did not get the k ISC value of Alq 3 from PL measurements, k ISC of Alq 3 was measured by the time-resolved photoluminescence technique to be of the order of 1.7 μ s −1 [26]. Since k ISC is proportional to the SOC strength [27], these results clearly suggest that the SOC strength in Ir(ppy) 3 is four orders of magnitude larger than in Alq 3 .…”

Section: Photoluminescence Spectramentioning

confidence: 88%

The role of heavy metal ions on spin transport in organic semiconductors

et al. 2015

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It is generally believed that spin-orbit coupling (SOC) strength and the associated spin relaxation can be enhanced by introducing heavy metal ions in organic semiconductors. Here, we systematically study the spin transport in two organic semiconductors, tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) and tris-(8-hydroxyquinoline) aluminum (Alq 3 ), which have similar chemical structures except that Ir(ppy) 3 contains a heavy metal ion Ir. As expected, the photoluminescence spectroscopy measurements show that the SOC strength in Ir(ppy) 3 is several orders of magnitude larger than in Alq 3 . Surprisingly, the spin diffusion length in Ir(ppy) 3 , deduced from magnetoresistance measurements in Ir (ppy) 3 -based organic spin valves, is longer than in Alq 3 . Considering the lower carrier mobility in Ir (ppy) 3 , the spin relaxation time in Ir(ppy) 3 is much longer than in Alq 3 , implying that the SOC strength in Ir(ppy) 3 is weaker than in Alq 3 . The seemingly contradictory results of photoluminescence spectroscopy and magneto-transport can be explained by the SOC strength depending on the electronic states of a material. The weak SOC strength in Ir(ppy) 3 observed in magneto-transport measurements is due to the strong ligand field induced orbital moment quenching for Ir 3+ and the polarons transporting in the ligands. However, the excitons involved in photoluminescence spectroscopy overlap with the Ir ion and transforms Ir 3+ to Ir 4+ , which has non-zero spin and orbital moments and hence results in high SOC strength.

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Section: Photoluminescence Spectramentioning

confidence: 88%

The role of heavy metal ions on spin transport in organic semiconductors

et al. 2015

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“…Although we refer here to ISC the exchange between singlets and triplets can occur at both the exciton and pair-state level. 40 The effect of the magnetic field on the ISC rate has two effects, first it is responsible for the changes in device efficiency that we have previously reported in a number of devices, [14][15][16] second by changing the concentration of triplets it can affect the magnitude of the polaron trapping and hence affect the mobility. 39 Given that the change in mobility, and hence the current through the device, caused by either of these processes should be dependent on the triplet concentration, we have plotted in Fig.…”

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confidence: 99%

“…The magnetic field dependence of the polaron trapping processes is identical to the change in efficiency, caused by a change in the intersystem crossing rate between singlets and triplets which could occur at either the excitonic or pair-state level. 40 The magnetic field dependence of the interaction process is also Lorentzian, although measurements to higher magnetic fields will be required to fix the precise value for the saturation field. The observation that the triplet-polaron interaction is Lorentzian is consistent with Ern and Merrifield's work on triplet quenching in anthracene.…”

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confidence: 99%

“…As the triplet population can affect mobility through both trapping and the triplet-polaron interaction, the current in the sample would be expected to show a dependence on magnetic field ͑due to the field-dependent ISC at either the pairstate or excitonic level 3,14,40 ͒, and this would be expected to scale with the absolute triplet concentration. Not only would the magnitude of the process be expected to scale with the triplet population but the magnetic field dependence of the trapping component should mirror any change in the triplet population.…”

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confidence: 99%

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Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation

et al. 2010

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The perturbation of the current transport in aluminum tris͑8-hydroxyquinoline͒ ͑Alq 3 ͒-based organic light emitting diodes has been investigated as a function of magnetic field. The change in current, or organic magnetoresistance, with applied field has been fitted using two Lorentzian functions corresponding to polaron trapping by triplets and the interaction between polarons and triplets as suggested in the triplet polaron interaction model. The model has been applied to a number of devices with Alq 3 thicknesses from 50 to 90 nm and with two different cathodes. In all cases the data could be fitted using just these two processes, the prefactors for which were found to scale linearly with the triplet population over 6 orders of magnitude. This work demonstrates that the magnitude and shape of the organic magnetoresistance can be predicted and illustrates the importance of magnetic field measurements as a tool for understanding the processes affecting current transport in organic devices. 4 observed that the application of a magnetic field could also perturb the photoconductivity in organic devices and they then showed that for organic light emitting diodes ͑OLEDs͒, the application of a weak magnetic field could substantially alter the current in the device as well as its light output 5 and hence its efficiency. Since then the study of these phenomena has increased dramatically 6-36 but there is still no consensus as to the mechanism responsible and there has been little work aimed at successfully predicting the trends observed in the magnetic field effects as the operating conditions of the devices are changed. This is an essential prerequisite for developing a successful model for organic magnetoresistance ͑OMR͒.Although there is a proposed model for OMR in unipolar structures, 34 the majority of the current models are primarily based on the effect of magnetic fields on excitons or the pair states prior to exciton formation. 4,[12][13][14] This is because the majority of experiments suggest that OMR can only be seen in devices above turn-on, i.e., with an applied voltage above the built in potential of the device. The exception to this is for devices that contain a poly͑3,4-ethylenedioxythiophene͒ poly͑styrenesulfonate͒ hole transport layer. 9,21The model for OMR that we have proposed in our previous work is based on the effect of excitons ͑primarily the long-lived triplets͒ on charge transport.14 This triplet polaron interaction ͑TPI͒ model suggests that triplets can act to reduce the mobility through two mechanisms. The first is through simple trapping and spin blocking. Agranovich et al. 37 have demonstrated theoretically that excitons should act as shallow traps for polarons, either through Frenkel-type trapping where the exciton and polaron are on adjacent molecules or through the formation of charged excitons. If a polaron has the same spin state as the corresponding charge carrier on the triplet then the site is spin blocked and only the Frenkel exciton can be formed. This will still affect the mobil...

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“…Transitions from a triplet to singlet state can occur if the triplet is raised in to a higher excited state, which can occur due to excited state absorption. 44 These changes in triplet population could then affect the current through a device via a number of processes that affect the mobility, for example, the role of weak electrostatic trapping of polarons at triplets or spin blocking; the role of excitons as shallow traps was theoretically proposed by Agranovich et al, 45 and the effect of excitons on mobility has recently been experimentally verified using dark injection experiments. 46,47 Our initial work focused on the role of site blocking and polaron interactions with excitons, which we suggested as a cause for the increase in current through a device with magnetic field.…”

Section: Introductionmentioning

confidence: 99%

Modeling of positive and negative organic magnetoresistance in organic light-emitting diodes

et al. 2012

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The organic magnetoresistance of aluminium tris(8-hydroxyquinoline) based organic light-emitting diodes has been modeled using the triplet polaron interaction coupled with exciton dissociation. We have demonstrated that each of the processes is proportional to the exciton concentration over a wide range of operating conditions for a number of devices with a wide range of layer thicknesses. This work demonstrates that using a magnetic field to perturb the operation of a working organic device is particularly valuable in that it provides a new technique for studying a range of processes affecting current transport such as polaron trapping, triplet-polaron interactions, and exciton dissociation in fully working devices.

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Measurement of the intersystem crossing rate in aluminum tris(8-hydroxyquinoline) and its modulation by an applied magnetic field

Cited by 17 publications

References 20 publications

The role of heavy metal ions on spin transport in organic semiconductors

The role of heavy metal ions on spin transport in organic semiconductors

Determining the influence of excited states on current transport in organic light emitting diodes using magnetic field perturbation

Modeling of positive and negative organic magnetoresistance in organic light-emitting diodes

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