Substitution Conformation Balances the Oscillator Strength and Singlet–Triplet Energy Gap for Highly Efficient D–A–D Thermally Activated Delayed Fluorescence Emitters

Abstract

Four donor-acceptor-donor region isomers (2,3-TXO-PhCz, 2,6-TXO-PhCz, 2,7-TXO-PhCz, and 3,6-TXO-PhCz) are designed. The substitution positions of the two PhCz units significantly impact the photophysical properties of the isomers, especially for the singlet-triplet energy splitting (ΔE ST ) and oscillator strength (f ). 2,3-TXO-PhCz exhibits weak emission due to the large steric hindrance of the two PhCz units. While 2,6-TXO-PhCz, 2,7-TXO-PhCz, and 3,6-TXO-PhCz all exhibit strong emission. The four emitters possess small ΔE ST of 0.01-0.24 eV; the corresponding f values are 0.064, 0.107, 0.026, and 0.134. Consequently, the photoluminescence quantum yields (PLQYs) of the doped films in CBP host are: 62.1% for 2,3-TXO-PhCz, 83.8% for 2,6-TXO-PhCz, 89.0% for 2,7-TXO-PhCz, and 85.4% for 3,6-TXO-PhCz. Although notable divergences of f and ΔE ST exist between 2,6-TXO-PhCz and 2,7-TXO-PhCz, similar PLQY of doped film in the CBP host, and exciton utilization and external quantum efficiency (EQE) of the corresponding devices can be achieved: 2,6-TXO-PhCz and 2,7-TXO-PhCz endow the organic lightemitting devices with high EQE of 23.2% and 24.4%. This proximity can be attributed to the synergistic effect of f and ΔE ST . This finding highlights the beneficial role of the different linking positions on the acceptor unit in facilitating the adjustment of f and ΔE ST in order to improve the device efficiency.

Highly Efficient, Red Delayed Fluorescent Emitters with Exothermic Reverse Intersystem Crossing via Hot Excited Triplet States

“…The curves of EQE versus current density were fitted according to either triplet–triplet annihilation (TTA) or the triplet-polaron quenching mechanism . The obtained J 0 values of three devices are about 10.4 mA cm –2 for Ben-MI-2TPA, 15.1 mA cm –2 for MI-2TPA, and 20.2 mA cm –2 for Ben-MI-2PhCz (Figure S25), which are comparable to those of the green TADF emitters with low-efficiency roll-off. − The low-efficiency roll-off of the devices based on our emitters can be attributed to the relatively short lifetimes of the delayed components that is accelerated via exergonic T 2 → S 1 RISC.…”

Small Energy Loss and Broad Energy Levels Offsets Lead to Efficient Ternary Polymer Solar Cells from a Blend of Two Fullerene‐Free Small Molecules as Electron Acceptors

“…[19,60,61] Next, the photogenerated charge carrier of 3TT-FIC molecules are dissociated into free electron-hole pairs at the IEICO-4F/3TT-FIC interface, leading to a more efficient exciton dissociation and high photocurrent. According to Förster resonance energy transfer theory, the energy transfer efficiency (E) formula is given below, where τ Ternary is the fluorescence lifetime of the PBDB-T:IEICO-4F:3TT-FIC ternary films and τ Pristine is the fluorescence lifetime of pristine materials films [19,61,62] η η = − E 1 Ternary Pristine (7) After incorporating the blended acceptor into PBDB-T, it exhibits further energy transfers (the E value is 74.5%) within the PBDB-T:IEICO-4F:3TT-FIC ternary films, the E value is only 71.2% for the PBDB-T:IEICO-4F binary films. There are the second light absorption of 3TT-FIC by the PL of PBDB-T and IEICO-4F, and enhance the efficiency of visible light absorption.…”

Recent progress of sulphur-containing high-efficiency organic light-emitting diodes (OLEDs)

“…17). 69 The substitution positions of the two PhCz units significantly impacted the photophysical properties of the isomers, especially for the Δ E ST and oscillator strength ( f ) (Fig. 21).…”