The accumulation of diffusive charges in organic light-emitting diodes

• We propose the presence of diffusive charges in the emission layer of OLED devices.
• Diffusive charges react to AC voltage with delay, leading to an extra capacitance.
• The amount of diffusive charges is quantized by impedance and transient EL.
• Interaction between diffusive charges and excitons leads to triplet-polaron quenching.
• Correlation between the amount of diffusive charges and device lifetime is verified experimentally.

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.

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