Highly Efficient Organic Light‐Emitting Diode Based on a Hidden Thermally Activated Delayed Fluorescence Channel in a Heptazine Derivative

Abstract

An orange-red organic light-emitting diode containing a heptazine derivative exhibits high performance with a maximum external quantum efficiency of 17.5 ± 1.3% and a peak luminance of 17000 ± 1600 cd m⁻² without any light out-coupling enhancement. The high electroluminescence performance can be ascribed to the presence of an efficient up-conversion channel from the lowest triplet state to the lowest singlet state.

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

“…Thus excited state formation from charge recombination directly generates the CT charge transfer states, not the unrelaxed local singlet and triplet states. This is well documented in previous work on interfacial exciplex devices17 and also gives rise to the very low turn on voltages observed here and in other TADF devices 18. This then avoids losses associated with the excitonic states localized on the D and A fragments, contributing to a much higher effective PLQY, as confirmed by direct absorption to the 1 CT state.…”

Red Organic Light‐Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

“…Meanwhile, in nitrogen atmosphere, a double‐component emission decay profile was obtained in the time range of 400 µs with a delayed lifetime (τ d ) of 10.3 µs. This result is significantly shorter than the most of reported red TADF emitters,17, 26, 27 suggesting a more efficient RISC process can be expected. In addition, as shown in Figure 3d, with the temperature increasing from 100 to 300 K, the delayed lifetime of DPXZ‐BPPZ is gradually declined, which is consistent with its TADF characteristic.…”

“…And the OLED based on AQ‐TPA exhibits a maximum EQE of 12.5% 17. They also reported a red TADF emitter HAP‐3TPA (Figure S1c, Supporting Information) using the same method 27. And HAP‐3TPA exhibits a high maximum EQE of 17.5% in the device, which is the highest result among the red TADF‐based OLEDs till now.…”

“…In the past several years, great progresses have been made in blue and green TADF‐based OLEDs, approaching or even exceeding the external quantum efficiencies (EQEs) of 20% 21, 22, 23, 24. However, with the decreasing of Δ E of TADF emitters into the long‐wavelength area (i.e., red emitters), significantly increased k IC s would become comparable with or even higher than k F s 25, 26, 27. Thus, as shown in Figure 1 a, when developing red TADF emitters based on the design strategy of high‐performance blue and green TADF emitters, the IC process would play an evident role in energy transfer diagrams, leading to terrible energy loss.…”

“…As shown in Figure 2 , DPXZ‐BPPZ is constructed with a conventional D–A structure by using dibenzo[ a , c ]dipyrido[3,2‐ h :2′,3′‐ j ]phenazine (BPPZ) as the A segment and phenoxazine (PXZ) as the D segment. Compared with the A segments used in reported red TADF emitters,17, 25, 26, 27 BPPZ possesses a significantly larger, completely planar and fully conjugated structure without any branched group, thus, it can evidently suppress harmful structural relaxation. On the other side, electron‐donating PXZ does not only exhibit suitable HOMO energy level to realize red emission cooperating with BPPZ, but is also more rigid than other D segments, like diphenylamine (DPA) and 9,9‐dimethyl‐9,10‐dihydroacridine (DMAC).…”