Acridine-Based TADF Molecules in Organic Light-Emitting Transistors (OLETs)

Organic light-emitting transistors are photonic devices combining the function of an electrical switch with the capability of generating light under appropriate bias conditions. Achieving high-performance light-emitting transistors requires high mobility organic semiconductors, optimized device structures and highly efficient emissive layers. Thermally-activated delayed-fluorescence (TADF) has enabled the possibility to achieve heavy-metal-free phosphorescent emitter-based devices with high brightness and state-of-the-art color coordinates, with efficiency exceeding 30%. Since the first report on delayed fluorescence in 2012, TADF molecules have been successfully exploited mainly in OLEDs emissive layers, where host(s)-guest configurations are commonly adopted, mainly to suppress concentration quenching arising from the long-lived triplet excitons.<br/>Here, we present our recent results on the use of an acridine-based TADF molecule (DMAC-DPS) in a host-guest configuration and its optoelectronic behavior in the limit of <i>field-effect</i> devices. First, we studied the optical response of these blends by tuning the doping concentration to optimize light generation, for which we observed a quenching of the photoluminescence signal for increasing dye concentration. These blends are then implemented in multilayer organic light-emitting transistors; by engineering the structure (<i>i.e.</i>, organic stack), we achieved a large improvement in the light output (~ 4 times). We analyzed our results in terms of balanced charge transport in the emissive layer as well as the device, which, in the limit of horizontal charge transport, leads to an improved exciton formation and decay process.<br/>While the efficiency of our devices is yet to achieve state-of-the-art diode counterpart, this work demonstrates that using TADF molecule while engineering the emissive layer is a promising approach to enhance the light emission in field-effect devices. This opens the way for a broader exploitation of organic light-emitting transistors as alternative photonic devices in fields ranging from display technology to flexible and wearable electronics.