Unraveling the Important Role of High‐Lying Triplet–Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep‐Blue AIE‐Based OLEDs

Abstract

Aggregation‐induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light‐emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE‐based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high‐lying triplet state (Tn) and the lowest excited singlet state (S1) in AIE materials. Moreover, the relative positions of Tn (n < 4) and S1 are shown to have a significant impact on the spin‐conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep‐blue fluorescent OLED with CIE coordinates of (0.15, 0.08), a maximum external quantum efficiency of 10.2%, low efficiency roll‐off, and a high brightness of 16817 cd m−2 is developed. This is one of the most efficient deep‐blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high‐lying triplet excitons.

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

“…[11] Herein, the MEL line shapes of the TPB-PAPC nondoped devices first increase in the low-field region and then decrease in the highfield region under the external magnetic field, and do not change with current density (Figure 2a), which is consistent with our previously reported AIE materials with T n →S 1 transition. [12] Here, the MEL responses can be well fitted by the twopart experienced function: [13] Table S2, Supporting Information), and the second part is fitted by Δg model, indicating the existence of T n →S 1 transition in TPB-PAPC. In addition, the separation of electronic structures on HOMO and LUMO further illustrates that Δg model is suitable for representing the T n →S 1 transition in this system.…”

“…[ 11 ] Herein, the MEL line shapes of the TPB‐PAPC nondoped devices first increase in the low‐field region and then decrease in the high‐field region under the external magnetic field, and do not change with current density ( Figure a), which is consistent with our previously reported AIE materials with T n →S 1 transition. [ 12 ] Here, the MEL responses can be well fitted by the two‐part experienced function: [ 13 ] , where A and C are the two constants, and B 0 is the value correlated with the size of hyperfine coupling interaction (HFI) in organic materials. The fitting results show that the first part MEL is fitted well by HFI model at low magnetic field ( B 0 < 5 mT; Table S2, Supporting Information), and the second part is fitted by Δ g model, indicating the existence of T n →S 1 transition in TPB‐PAPC.…”

Robust Luminescent Molecules with High‐Level Reverse Intersystem Crossing for Efficient Near Ultraviolet Organic Light‐Emitting Diodes

“…The energy splitting between the lowest singlet (S 1 ) and triplet (T 1 ) states (Δ E ST ), calculated from the fluorescence and phosphorescence spectra at 77 K (Figure S9 and S10), are as large as 0.46–0.57 eV for the neat films and 0.49–0.60 eV for the doped films. These results disclose that the common TADF process does not exist in these molecules, and their delayed components should be induced by the hRISC from high‐lying triplet excited states to the S 1 state [3c,e] …”

High‐Performance Ultraviolet Organic Light‐Emitting Diode Enabled by High‐Lying Reverse Intersystem Crossing

“…On the other hand, the magnetic field effect (MFE) can be used as an effective method to discern the EL mechanism behind OLEDs with efficient triplet exciton utilization [21] . It is noted that the magneto‐EL (MEL) traces at varied applied bias present a sharp rise at the low magnetic field (<50 mT) and continuous saturation at the higher magnetic field (Figure 4 f), which also demonstrates that the triplet excitons harvesting originates from the RISC rather than TTA process [15b, 22] . Since the Δ E S1T1 is too large to facilitate the thermally activated RISC process from T 1 to S 1 , the triplet states at higher energy level should play a key role in the triplet‐to‐singlet conversion.…”