Fitting the magnetoresponses of the OLED using polaron pair model to obtain spin-pair dynamics and local hyperfine fields

Organic light-emitting diode (OLED) displays a sign reversal magnetic field effect (MFE) when the applied magnetic field range is reduced to the sub-milliTesla range and the Polaron Pair Model has been successful in explaining the ultra-small MFE. Here, we obtained high resolution (~ 1 µT) magnetoconductance (MC) and magnetoelectroluminescence (MEL) of a tris-(8-hydroxyquinoline)aluminium-based (Alq(3)) OLED within the magnetic field range of ± 500 µT with the earth magnetic field components cancelled. A clear “W” shaped MC with a dip position of ± 250 µT and a monotonic MEL were observed. We demonstrate a fitting technique using the polaron pair model to the experimentally obtained MC and MEL. The fitting process extracts physically significant parameters within a working OLED: the local hyperfine fields for electron and hole in Alq(3): B(hf1) = (0.63 ± 0.01) mT (electron), B(hf2) = (0.24 ± 0.01) mT (hole); the separation rates for singlet and triplet polaron pairs: k(S,s) = (44.59 ± 0.01) MHz, k(T,s) = (43.97 ± 0.01) MHz, and the recombination rate for singlet polaron pair k(S,r) = (88 ± 6) MHz. The yielded parameters are highly reproducible across different OLEDs and are in broad agreement with density functional theory (DFT) calculations and reported experimental observations. This demonstrates the feasibility of this fitting technique to approach any working OLED for obtaining significant microscopic parameters.