Modelling and fitting the Polaron Pair Magnetoconductance model to obtain a realistic local hyperfine field in Tris-(8-hydroxyquinoline) aluminium based diodes

The Polaron Pair (PP) model has been successfully applied to magnetoconductance (MC) in organic semiconductor devices under ultra-small magnetic fields (USMFE). We report mu T 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 mu T displaying clear minima at -+/- 240 mu T. 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.