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

Abstract

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...

Tuning organic magnetoresistance in polymer-fullerene blends by controlling spin reaction pathways

“…Besides spin injection into organic semiconductors 3,4 , a lot of attention was drawn to non-spin-polarized organic semiconductor device. Despite the absence of magnetic elements, they show a large room-temperature magnetoresistance effect at relatively small magnetic fields of only a few millitesla, an effect sometimes referred to as organic magnetoresistance (OMAR) [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] . Cheap plastic sensor technology has been suggested as an example of its application potential 23 .…”

“…Cheap plastic sensor technology has been suggested as an example of its application potential 23 . However, because this MFE was discovered a decade ago, the desire to unravel the exciting new physics behind the intrinsically magnetic fielddependent charge-transport properties of organic semiconductors has been the major motivation for intensive experimental [5][6][7][8][9][10][11][12][13][14] and theoretical [15][16][17][18][19][20][21][22] research.…”

Modulating the line shape of magnetoconductance by varying the charge injection in polymer light-emitting diodes

“…The above observations agree well with the proposed model that the dissociation of electron-hole pairs under the applied magnetic field contributes to the positive MC response. 22 In order to examine the underlying mechanisms of MC responses in SY-PPV-based PLEDs, we performed the curve fitting by applying a combination of two empirical equations of a non-Lorentzian and Lorentzian function as shown in the equation below: 13 Fig. 3(a), the MC response can be fitted by the positive component (the non-Lorentzian function in Eq.…”

“…[6][7][8][9] This phenomenon has been widely used to study the magnetic field effect in many electronic devices such as photovoltaic cells, field effect transistors, and light-emitting diodes. [10][11][12][13] There are several studies to find MC effect in polymer light-emitting diodes (PLEDs) using different kind of polymer materials. The current in the devices can increase or decrease with the applied magnetic field, which is known as the positive or negative MC responses, respectively.…”

Unraveling the temperature and voltage dependence of magnetic field effects in organic semiconductors

“…The effect can be tuned by changing the operating conditions, such as voltage, temperature, and angle between the current and magnetic field. [3][4][5][6][7][8][9][10][11][12][13][14][15] Recently, cheap plastic sensor technology has been proposed as an example of its application potential. 16 Nevertheless, a fundamental understanding of the interactions of spins and charges in organic semiconductors currently remains the goal of extensive experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and theoretical [17][18][19] research.…”