Spiro-based thermally activated delayed fluorescence emitters with reduced non-radiative decay for high quantum efficiency, low roll-off, light-emitting diodes

Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (PL), short delayed luminescence lifetime, high external quantum efficiency (EQE) and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX-, spiro-(fluorene-9,9’-xanthene), based emitters (SFX-PO-DPA, SFX-PO-DPA-Me and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg-Teller mechanism in the excited state. They possess short excited state delayed lifetimes (<10 s), high photoluminescence quantum yields (PL ~70%) and small singlet-triplet splitting energies (EST < 0.10 eV) in the doped films in an mCP host matrix. The OLEDs showed some of the highest EQEs using spiro-containing emitters where maximum external quantum efficiencies (EQEmax) of 11% and 16% were obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a spiro-based emitter coupled with a small efficiency roll off (19% at 100 cd m-2) was attained with SFX-PO-DPA-Me.