“Rate-limited effect” of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes

Abstract

Issues concerning excited state lifetime (<i>τ</i>_TADF) tuning of thermally activated delayed fluorescence (TADF) materials are critical for organic light emitting diode (OLED) applications and other specific fields. For TADF-OLEDs, employing emitters with a short <i>τ</i>_TADF gives rise to suppressed singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA), leading to reduced efficiency roll-off at practical relevant brightness (100 and 1000 cd m^-2 for display and illumination applications, respectively). Through molecular design, exciton dynamic process rate constants including fluorescence (<i>k</i>_F), intersystem crossing (<i>k</i>_ISC), internal conversion (<i>k</i>_IC) and reverse intersystem crossing (<i>k</i>_RISC) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, <i>k</i>_F, <i>k</i>_ISC, <i>k</i>_IC and <i>k</i>_RISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small <i>k</i>_F, low photoluminescence quantum efficiency (<i>Φ</i>) and large <i>k</i>_ISC, molecules with a small singlet-triplet splitting energy (Δ<i>E</i>_ST) and lowest charge transfer triplet excited state (³CT) eventuate in shortening the <i>τ</i>_TADF. Herein, <i>k</i>_RISC, which is inversely proportional to Δ<i>E</i>_ST, turns out to be the rate-limited factor in tuning the <i>τ</i>_TADF ("rate limited effect" of the RISC process). As revealed by flexible potential surface scanning, PyCN-ACR exhibited a moderate <i>k</i>_F, reduced <i>k</i>_IC and enlarged <i>k</i>_RISC, resulting in a short <i>τ</i>_TADF and a moderate <i>Φ</i> with orange-red emission. OLEDs containing PyCN-ACR as the emitting guest achieved orange-red TADF-OLEDs with an emission peak at 590 nm and the best external quantum efficiencies (EQEs) of 12.4%/9.9%/5.1% at practical luminances of 100/1000/10 000 cd m^-2.