“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 (τ_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 τ_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 (k_F), intersystem crossing (k_ISC), internal conversion (k_IC) and reverse intersystem crossing (k_RISC) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, k_F, k_ISC, k_IC and k_RISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small k_F, low photoluminescence quantum efficiency (Φ) and large k_ISC, molecules with a small singlet–triplet splitting energy (ΔE_ST) and lowest charge transfer triplet excited state (³CT) eventuate in shortening the τ_TADF. Herein, k_RISC, which is inversely proportional to ΔE_ST, turns out to be the rate-limited factor in tuning the τ_TADF (“rate limited effect” of the RISC process). As revealed by flexible potential surface scanning, PyCN–ACR exhibited a moderate k_F, reduced k_IC and enlarged k_RISC, resulting in a short τ_TADF and a moderate Φ 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.