Ultra-Small Magnetic Field Effects of Organic Light-Emitting Diodes Based on Tris(8-hydroxyquinoline)aluminium

Abstract

Magnetic field effects (namely, magnetoconductance (MC) and magnetoelectroluminescence (MEL)) in organic devices have been extensively studied in the last two decades and several theories and models have been developed to explain these interesting phenomena. In experiments the MC and MEL results are dependent on device drive conditions, materials, structures, etc. and different results can be attributed to different mechanisms under different conditions. Specifically, for Ultra-Small Magnetic Field Effects (USMFE) in organic devices, one of the most popular models is the Polaron Pair Model (PP model) which has been investigated for a decade. It is based on the effect of an external magnetic field on the singlet-triplet polaron pair interconversion and changes in ultimate singlet and triplet yields. However, in most of the Polaron Pair Model related literature, the quantitative connection between the model simulations and the experimentally obtained data (MC and MEL) is not directly made despite the polaron pair model successfully generating the USMFE MC (and MEL) typical functional forms ("W" shape). In this work, prototype fitting of the Polaron Pair model to experimentally obtained MC (and MEL) has been carried out yielding fitting parameters, in particular, the relevant local hyperfine field Bh f experienced by one or more polarons in tris-(8- hydroxyquinoline)aluminium (Alq3). Hyperfine field values are physically significant, and can be compared to experiments and calculations from the literature. The singleproton and two-proton Polaron Pair models are applied to the high resolution (mT), high sensitivity ( 10ð€€€6) and high reproducibility experimentally obtained MC (and MEL) data by fitting the model to obtain different physical parameters. In particular, hyperfine field(s) obtained are: Bh f =(0.34 0.04)mT using the single-proton PP model and Bh f 1=(0.63 0.01)mT, Bh f 2=(0.24 0.01)mT using the two-proton PP model with high reproducibility across devices and independent of drive current. These values are in accord with local hyperfine fields associated with the HOMO and LUMO probability densities in the Alq3 molecule where the electron polaron is experiencing a larger local hyperfine field while the hole polaron experiences a smaller hyperfine field. Additionally, in the single-proton PP model, a weight factor dTS is used todescribe the relative contribution of triplets and singlets to MC and the fitting yields a dTS smaller than 1, meaning singlets contribute more to the MC than triplets. However, in the developed two-proton PP model, the weight factor dTS is replaced with more explicit decay rates for dissociation and recombination of singlet and triplet PP with slightly higher dissociation rate for singlet ( 44.59MHz) than triplet ( 43.97MHz), and the higher dissociation rate means higher contribution to the MC, which agrees with the yielded weight factor dTS smaller than 1 from the single proton PP model. In particular, all the yielded parameters are obtained through a global fitting between the two-proton PP model and the experimentally obtained MC and MEL. Additionally, under constant current mode, the measured MC and MEL display different functional shapes instead of the same "W" shapes under constant voltage mode reported from literature. This indicates two different path ways for polaron pairs to decay with different rates. The magnitude of MEL is 100 times larger than MC, indicating that recombination process between polaron pair dominates in the whole PP dynamics, and this is also in accord with the much larger yielded singlet PP recombination rate (~87.97MHz) than the dissociation rate (~43.97MHz for triplet PP and 44.59MHz for singlet PP). The work presented not only helps to better understand the microscopic mechanisms operating within organic devices under weak external magnetic fields, but can also function as a probe to measure the local hyperfine environments for electron and hole polarons in organic semiconductors through the macroscopic electrical and optical measurement of a working device.