The accumulation of diffusive charges in organic light-emitting diodes

Abstract We investigated the accumulation of diffusive charges within the emitting layer of organic light-emitting diodes (OLEDs). The presence of accumulated charges can be probed by both transient electroluminescence (transient EL) and impedance measurements (IM). As an example we studied OLED devices using a red phosphorescent emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate) [Ir(MDQ)2(acac)] and green emitter Ir(ppy)3 co-doped into a mixed host matrix. The results deduced from both transient EL and IM measurements are in good consistency, enabling us to compare the accumulation of diffusive charges in different devices quantitatively. Experimental results indicate that the lifetime of OLED devices is reduced by the diffusive charges. We ascribe this to the exciton–polaron annihilation where the diffusive charges play an important role. The quantification of the accumulation of diffusive charges in the emitting layer may help to improve the lifetime of OLEDs.