Narrowband, angle-stable, and highly efficient polariton organic light emitting diodes employing thermally activated delayed fluorescence

Abstract

Narrowband emission is crucial for next generation optoelectronic devices to satisfy demands for high color brilliance. Microcavities can narrow the emission spectra of organic light-emitting diodes (OLEDs) through the creation of resonant standing waves, independent of emitter material, enabling flexibility in molecular and device design. However, this introduces a strong angle-dependence of the perceived emission color. Utilizing strong light-matter coupling of cavity photons with the virtually angle-independent exciton, leads to exciton-polariton emission which offers a further reduction in linewidth and suppresses angular dispersion if tuned correctly. Creating polaritons in highly efficient materials such as thermally activated delayed fluorescence (TADF) molecules is however difficult as their low oscillator strengths intrinsically disfavor light-matter interaction. Here, we present the successful combination of a highly efficient but intrinsically broadband TADF emitter, CzDBA, with a strongly absorbing assistant strong coupling material, C545T, in a modified microcavity OLED structure. Through optimizing the assistant strong coupling layer architecture and integrating it with a highly conductive p-doped hole transport layer, we demonstrate polariton OLEDs with exceptionally narrowband, angle stable emission at external quantum efficiencies above 20% for both bottom- and top-emitting designs, more than doubling the performance compared to previously reported polariton OLEDs. Importantly, the fabrication of our polariton OLED stack does not require significant structural changes relative to existing microcavity OLEDs. Combined with their high efficiency, this paves the way for future utilization of polaritonic emission in commercial display applications.