Thermally Activated Delayed Fluorescence Amorphous Molecular Materials for High-Performance Organic Light-Emitting Diodes

Abstract

Small-molecule thermally activated delayed fluorescence (TADF) materials have been extensively developed to actualize efficient organic LEDs (OLEDs). However, organic small molecules generally compromise thin film quality and stability due to the tendency of crystallization, aggregation, and phase separation, which hence degrade the efficiency and longterm stability of the OLEDs. Here, for the first time, we exploit the unique molecular configuration of the bimesitylene scaffold to design two highly efficient TADF amorphous molecular materials with excellent thermal and morphological stabilities. The twisted and rigid bimesitylene scaffold thwarts regular molecular packing and crystallization, thereby guaranteeing homogeneous and stable amorphous thin films. Meanwhile, the highly twisted geometry of the bimesitylene scaffold efficiently breaks the molecular conjugation and thus conserves the high energies of the lowest locally excited triplet states ( 3 LE) above the lowest charge transfer states ( 1 CT and 3 CT), leading to small singlet-triplet energy splitting and fast reverse intersystem crossing. These TADF emitters exhibit high photoluminescence quantum yields of 0.90 and 0.69 and short TADF lifetimes of 4.94 and 1.44 μs in doped films, based on which the greenish-blue and greenish-yellow OLEDs achieve external quantum efficiencies of 23.2 and 16.2%, respectively, with small efficiency roll-off rates and perfect color stability.

Systematic Substituent Control in Blue Thermally Activated Delayed Fluorescence (TADF) Emitters: Unraveling the Role of Direct Intersystem Crossing between the Same Charge‐Transfer States

“…Among the emitters with highly twisted structure of various TADF colors, those bearing stronger donor fragments with electron-releasing substituents, and/or acceptor fragments with electron-withdrawing groups as well as other structural modifications providing stabilization of the CT states, exhibit generally faster rISC, shorter TADF lifetimes and/or higher external quantum efficiency (EQE) values. [14][15][16][17][18][19] Such investigations question the key role of 3 LE-state in achieving efficient rISC as they evidence that TADF can occur via direct two-state spin-flip mechanism either. In this context, to understand the reasons of rISC enhancement under finely controlled energy separation of 1 CT and 3 LE states, we have recently developed a rotationally and vibronically assisted SOC model which explains TADF in a DMAC-TRZ emitter and its derivatives as a result of direct 3 CT- 1 CT twostate model.…”

Boosting emission efficiency and suppressing device-efficiency roll-off for TADF emitters by modulating molecular conformation and intra–intermolecular interactions

“…The combined effect of these two factors results in different energy level alignments of the excited states and thus distinct Δ E ST values for the two emitters. To clarify the main photophysical processes of these emitters, we calculated the relevant rate constants using a previously reported method 56 (see ESI† for details). As summarized in Table 1, CF 3 -Pym-DMAC and Ph-Pym-DMAC exhibit radiative rate constants ( k S r ) of the same order of magnitude, but their nonradiative rate constants ( k S nr ) differ significantly, with values of 8.9 × 10 5 s −1 and 2.6 × 10 7 s −1 , respectively.…”

Distinguishing the respective determining factors for spectral broadening and concentration quenching in multiple resonance type TADF emitter systems

“…The only expectation is that the 1 wt% DtBuCzB-based device only realizes a poor EQE of 17.7%, which might be due to the less ideal evaporation process that may cause phase segregation, thus disturbing the energy transfer process and deteriorating the device performance. 55 Fig. 6(b) depicts the device EQE curves of these compounds at optimized (1 or 5 wt%) and 20 wt% doping concentration.…”