Efficiency Breakthrough of Fluorescence OLEDs by the Strategic Management of “Hot Excitons” at Highly Lying Excitation Triplet Energy Levels

Abstract

Aggregation‐induced emission (AIE) and hybridized local and charge‐transfer (HLCT) materials are two kinds of promising electroluminescence systems for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs) by harnessing “hot excitons” at the high‐lying triplet exciton states (Tn, n ≥ 2). Nonetheless, the efficiency of the resulting OLEDs did not meet expectations due to the possible loss of Tn→Tn−1. Herein, experimental results and theoretical calculations demonstrate the “hot exciton” process between the high‐lying triplet state T3 and the lowest excited singlet state S1 in an AIE material 4⁗‐(diphenylamino)‐2″,5″‐diphenyl‐[1,1″:4′,1″:4″,1′″:4′″,1⁗‐quinquephenyl]‐4‐carbonitrile (TPB‐PAPC) and it is found that the Förster resonance energy transfer (FRET) between two molecules can facilitate the “hot exciton” process and inhibit the T3→T2 loss by doping a blue fluorescent emitter in TPB‐PAPC. Finally, the doped TPB‐PAPC blue OLEDs achieve a maximum external quantum efficiency (EQEmax) of 9.0% with a small efficiency roll‐off. Furthermore, doping the blue fluorescent emitter in a HLCT material 2‐(4‐(10‐(3‐(9H‐carbazol‐9‐yl)phenyl)anthracen‐9‐yl)phenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d] imidazole (PAC) is used as the emission layer, and the resulting blue OLEDs exhibit an EQEmax of 17.4%, realizing the efficiency breakthrough of blue fluorescence OLEDs. This work establishes a physical insight in the design of high‐performance “hot exciton” molecules and the fabrication of high‐performance blue fluorescence OLEDs.

Deep-red electro-fluorescence based on an excimer emission with hot-exciton channels

“…As shown in Fig. 4a and b, the energy diagram of the two materials demonstrates the typical “hot-exciton” model, which should meet three typical demands: 24–36…”

“…As shown in Fig. 4a and b, the energy diagram of the two materials demonstrates the typical ''hot-exciton'' model, which should meet three typical demands: [24][25][26][27][28][29][30][31][32][33][34][35][36] (1) A large energy gap between T 2 and T 1 for forbidden internal conversion from T 2 to T 1 .…”

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

“…12 In recent years, a great deal of endeavors have been devoted to the OLEDs based on pure organic small molecules which can utilize triplet excitons. The main mechanisms for harvesting triplet energy include TTA, [13][14][15] hot excitons, [16][17][18] and thermally activated delayed fluorescence (TADF). 19,20 TTA is a characteristic mechanism to up-convert the triplet excited states with low energy into singlet excited states (Fig.…”

Improved transient electroluminescence technique based on time-correlated single-photon counting technology to evaluate organic mobility

“…The time-correlated single-photon counting (TCSPC) technique has been proved as effective for measuring transient EL [22][23][24][25]. Instead of detecting the analog signal intensity, the photon counting technique allows for the quantification of single-photon pulses caused by the excitation of dispersed individual photons [26].…”