Highly efficient through-space charge transfer TADF molecule employed in TADF- and TADF-sensitized organic light-emitting diodes

Abstract

The inception and harnessing of excitons are paramount for the electroluminescence performance of organic light-emitting devices (OLEDs). Through-space charge transfer (TSCT) via intramolecular interaction has proved to be one of the most potent techniques employed to achieve 100% internal quantum efficiency. However, molecular strategies utilized to comprehensively enhance the electroluminescent performance of TSCT emitters regarding improving the photoluminescence quantum yield (PLQY) and elevating the light out-coupling efficiency remain arduous. To surmount this challenge, we deliberately designed and synthesized a proof-of-concept TSCT emitter called CzO-TRZ by incorporating an extra carbazole donor into spiro-heterocyclic architecture. The introduction of rigid spiral fragments can immensely boost the horizontal orientation dipole ratio and establish an extra through-bond charge transfer (TBCT) radiative decay channel. As a result, a very high PLQY of 98.7%, fast k_RISC of 2.2×10⁵ s⁻¹ and high k_r^s of 2.2×10⁷ s⁻¹, and an ultrahigh horizontal dipolar ratio of 90% were concurrently achieved for CzO-TRZ blended films. Furthermore, corresponding thermally activated delayed fluorescence (TADF)- and TADF-sensitized fluorescence (TSF)-OLEDs based on CzO-TRZ demonstrated external quantum efficiencies (EQEs) of 33.4% and 30.3%, respectively, highlighting its versatile applications as both an emitter and sensitized host.