Disorder-Induced Dispersive Magneto-Electroluminescence of Blue Emitters in Organic Light Emitting Diodes

Disorder-induced inhomogeneity in blue-fluorescent-based organic light-emitting diodes (OLEDs) based on mixtures of host and guest molecules is studied using magneto-electroluminescence, MEL(B), response based on the so called "Delta g mechanism", where Delta g is the difference in the Lande g-factor of electrons and holes. The disorder in the organic active layer is manifested by a unique non-Lorentzian MEL(B) response that is analyzed using a distribution of spin lifetimes for the injected electron-hole pairs that is determined by a dispersive parameter, alpha (<1). The carriers' inhomogeneous response also influences the free carrier absorption spectrum, which shows characteristic properties described by a dispersive parameter beta (<1). From the measured MEL(B) response at various injection conditions it is found that alpha is robust at increasing current density showing that the inhomogeneity is governed by intrinsic disorder in the device active layer. Also the obtained increase in alpha at low temperature indicates that the organic layer becomes more ordered, where longer-lived electron-hole spin pairs are formed.