Abstract
The Polaron Pair (PP) model has been successfully applied to magnetoconductance (MC) in organic semiconductor devices under ultra-small magnetic fields (USMFE). We report µT resolution MC measurements carried out with high sensitivity (better than 10−6) on the common organic semiconductor tris-(8-hydroxyquinoline)aluminium in the range ±500 µT displaying clear minima at ~±240 µT. Unlike traditional approaches, where device MC is simply evaluated using the PP model using nominal parameters for microscopic quantities such as the local hyperfine magnetic field, we have carried out actual fitting of the PP MC model to the experimentally obtained data. The fitting procedure yields physically realistic values for the polaron pair decay rate, local hyperfine magnetic field and triplet contribution to dissociation namely: k = 28.6 ± 9.7 MHz, {B}_{hf} = 0.34 ± 0.04 mT and {delta }_{TS} = 0.99 ± 0.01 respectively. The local hyperfine field obtained by fitting is in excellent agreement with independently calculated values for this system and is reproducible across different devices and independent of drive conditions. This demonstrates the applicability of the fitting approach to any organic USMFE MC data for obtaining microscopic parameter values.
Highlights
Organic magnetoconductance (MC) is the change in device conductance when exposed to an external magnetic field
The local Bhf of 0.34 mT, is much smaller than typical hyperfine fields quoted in the literature[27,28,29], but is comparable to local hyperfine fields calculated by Marumoto et al using Density Functional Theory (DFT) for an Alq[3] anion[31] where different local hyperfine field magnitudes between 0.01 mT and 1.43 mT are reported
In trying to assess the relevant local hyperfine field experienced by a polaron one has to take into account the spatial distribution and location of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Summary
Organic magnetoconductance (MC) is the change in device conductance when exposed to an external magnetic field. These traditional approaches have been successful in reproducing the functional forms of experimentally obtained MC data for a number of systems[15,20,21,22,24], but they are based on calculating the MC resulting from PP models using historically reported or “typical” hyperfine field values for organic systems This differs fundamentally from our approach where PP model fitting is carried out on MC data with no assumptions regarding microscopic parameter values such as the (average local) hyperfine field experienced by the polaron. This raises the exciting prospect that PP model fitting to experimental MC data can be used more generally as a method of obtaining microscopic parameters (e.g. average local Bhf) in a variety of organic systems
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