Transient analysis on stored charges in organic light-emitting diodes and their application in alternating current driven electroluminescence

Abstract

AC-driven organic light-emitting diodes (OLEDs) can overcome some reliability-related drawbacks to traditional DC-OLEDs. They imply the use of insulating layers in the device. In this work OLEDs containing an insulating layer of poly (methyl methacrylate) (PMMA) in storing charges with the thickness of 2, 6, 8, 10 nm have been prepared and investigated. The emission mechanisms of the device are analyzed considering transient and AC electroluminescence (EL) measurements. We show that charges are stored in the PMMA layer as surface charges and bulk charges. The former contribute to the occurrence of EL spike after the driving pulse with a decay tail for about 80 μs to 10nm PMMA device, and the latter can be released to emit light under reversed voltage more than 2 V because they are immobile unless under the stronger reversed field. Stored charges commonly are harmful for the performance of OLEDs devices due to quenching, nonradiative transition and the thermal energy accumulation even degradation. Whereas when operating under alternating current (AC) stress, we not only obtain the EL peak from injection charges under forward voltage, but also obtain another peak below the built-in voltage 4.3 V, whose peak point lies in −0.2 V. It turns to originate from stored charges so they become beneficial instead of weakening EL for reusing to produce light. All the results pave the way to realize AC driven OLEDs devices using these stored charges, to uncover the AC EL mechanisms and to improve their EL performance.