Balanced Hole and Electron Transport in Ir(ppy)3:TCTA Blends

Abstract

Balanced charge injection and transport in organic light-emitting diodes (OLEDs) is essential for highly efficient devices with small efficiency roll-off at high luminance. However, there are few reports on the measurement of charge mobility within the emissive blend layer of an OLED. In this paper, we show that photoexcitation in conjunction with Metal–Insulator–Semiconductor Charge Extraction with Linearly Increasing Voltage (photo-MIS-CELIV) can be used to determine the hole and electron mobilities of the emissive blend layer in a single device architecture. We demonstrate the technique by studying the commonly used emissive blend of fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), as well as neat TCTA and Ir(ppy)3 films. It was found that Ir(ppy)3 and its blend films with TCTA have measurable electron mobilities and critically they are of similar magnitude as their hole mobilities, irrespective of the Ir(ppy)3 doping ratio. Such balanced charge mobility suggests that the transport of both holes and electrons occurs mostly on the Ir(ppy)3 guest molecules in the blend. Additionally, we demonstrate that photo-MIS-CELIV can be used to measure the quantum efficiency of exciton dissociation in organic semiconductor thin films.