Interface Exciplex Formation in TADF Organic Light-Emitting Transistors

Abstract

Thermally-activated delayed fluorescence (TADF) materials have received large attention for their ability to harvest both singlet and triplet excitons via reverse intersystem crossing (rISC), enabling near 100% internal quantum efficiency without relying on scarce heavy-metal complexes. While TADF emitters are largely used in organic light-emitting diodes (OLED), their implementation in organic light-emitting transistor (OLET), a device platform that uniquely combines transistor switching and light emission, remains relatively underexplored. In this work, we fabricated and investigated the integration of 2CzPN, a blue-emitting TADF molecule, doped into the high-triplet-energy host DPEPO in different architectures, to explore their potential for efficient and colour-tunable light emission. We explored multilayer heterostructures which include (or not) an electron transport layer, and we found: an approximate 10 wt% doping of 2CzPN in the DPEPO host yielded optimal performance in all cases. However, two-layer devices (no electron transport layer) exhibited intrinsic emission, typically of 2CzPN, while the addition of the electron transport layer produced both a redshift of the emission (green emission) and the onset of an additional spectral contribution due to the formation of interfacial exciplexes at the emissive layer/e-transport interface. Our results demonstrate the dual advantage of TADF emitters in field-effect devices: efficient triplet harvesting and tunable emission via interface engineering, thus suggesting the importance of optimizing both emitter design and device architecture for high-performance, color-tunable organic transistors.