Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives

Abstract

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A (14.8%), 34.5 cd A (16.9%), 12.8 cd A (10.1%), and 13.2 cd A (15.1%), with Commission Internationale de L'Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.

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.…”

“…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. By using an integrating sphere, the absolute Φ PL of 18 wt% DPXZ‐BPPZ‐doped CBP film was measured to be an extremely high value of 97.1 ± 1.1% under oxygen‐free condition, which is the highest one among the reported red TADF emitters 15, 17, 25, 26, 27, 33. Therefore, according to Equation (1), the k F value is almost 33 times larger than that of k IC for DPXZ‐BPPZ, resulting in an negligible IC process contribution to exciton loss.…”

“…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.…”

“…Similarly, in 2017, Wang and co‐workers reported a series of long‐wavelength TADF emitters, in which, the D‐Ph‐A structure red TADF emitters also achieved better device performance than the D–A structure ones. The red device using DPA‐Ph‐DCPP (Figure S1d, Supporting Information) as the emitter shows a maximum EQE of 15.1% 26. However, with the enhanced conjugation between D and A segments, all these molecules suffer large Δ E ST s higher than 0.17 eV, which weaken the RISC process and lead to the energy loss from triplet excitons 28, 29.…”

“…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).…”