Highly Efficient Blue Electroluminescence Using Delayed-Fluorescence Emitters with Large Overlap Density between Luminescent and Ground States

Abstract

The use of thermally activated delayed-fluorescence (TADF) allows the realization of highly efficient organic light-emitting diodes (OLEDs) and is a promising alternative to the use of conventional fluorescence and phosphorescence. Recent research interest has focused on blue TADF emitters. In this study, we use quantum mechanics to reveal the relationship between the molecular structures and the photophysical properties of TADF emitters and derive a direction for the molecular design of highly efficient blue TADF emitters. Theoretical analyses show that the luminescence efficiency of TADF emitters largely depends on the overlap density (ρ10) between the electronic wave functions of the ground state and the lowest excited singlet state. By increasing ρ10, we develop an efficient sky-blue TADF emitter material, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)­phenyl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BCzT). When doped into a host layer, BCzT produces a high photoluminescence quantum yield of 95.6%. From the transient photoluminescence decays of the doped film, the efficiency of excited triplet state conversion into light is estimated to be 76.2%. An OLED using BCzT as a sky-blue emitter produces a maximum external quantum efficiency (EQE) of 21.7%, which is much higher than the EQE range of conventional fluorescent OLEDs (5–7.5%). The high EQE is a result of the high triplet-to-light conversion efficiency of BCzT. Our material design based on ρ10 distribution provides a rational approach for developing TADF emitters for high-efficiency blue OLEDs.

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

“…However, a structurally similar emitter PhCzTAZ (PhCzTAZ = 3‐(2′‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐[1,1′‐biphenyl]‐2‐yl)‐9‐phenyl‐9H‐carbazole) does not show TADF because of the absence of charge‐transfer emission, probably due to limited HOMO and LUMO communication restricted by steric hindrance around the biphenyl bridge. To improve on the low PLQY of the CzT emitter, BCzT (λ max : 483 nm; PLQY: 96%; τ d : 33 µs in 6 wt% DPEPO; Δ E ST : 0.29–0.33 eV) was then developed, in which the overlap density between the excited state and the ground state (ρ 10 ) was increased . The presence of an additional phenyl‐ring bridge increases the overlap integral between the HOMO and the LUMO as its presence mediates increased conjugation.…”

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

“…TADF emitters typically cover the green-yellowr egion, including 4CzIPN, [32,43] DPTZ-DBTO2, [33] [Cu I (POP)(tmbpy)][BF 4 ], [44] and TPA-QXN(CN)2, [45] and there have been reports of efficient blue TADF systems, but their stability is limited. [46][47][48] There are not many reports of orange-to-red TADF emittersa nd they are mainly based on anthraquinone [49] spironaphthalenone (DCPSO), [50] phenazine (POZ-DBPHZ [51] and DPXZ-BPPZ), [52] or naphthalenediimide (TPA-cNDI) [53] acceptors (Figure 4). The problem with red TADF emitters is fluorescenceq uenching and the difficulty of distinguishing between the TTAa nd TADF mechanisms, owing to the small energy differences.…”

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

“…However, the lifetime of blue phosphorescent OLEDs (PhOLEDs) is too short for use in practical applications. As an alternative to phosphorescent emitters, a thermally activated delayed fluorescent (TADF) emitter has been investigated . Although the lifetime of blue TADF devices is short, TADF OLEDs are candidate high‐efficiency blue OLEDs.…”