Rigidity and Polymerization Amplified Red Thermally Activated Delayed Fluorescence Polymers for Constructing Red and Single‐Emissive‐Layer White OLEDs

Abstract

Polymerization could be a feasible method to overcome the rigid structure induced self-quenching effect in conventional thermally activated delayed fluorescence (TADF) emitters. Despite steady progress in TADF polymer research, developing an efficient red TADF polymer still remains a great challenge because of the large non-radiative internal conversion rate governed by the energy gap law. Herein, a novel strategy for constructing a red TADF conjugated polymer is presented by means of embedding quinoxaline-6,7-dicarbonitrile (QC) as an acceptor into a polycarbazole (PCz) backbone and attaching donor 9,10-dihydroacridine (A) as a pendant. The obtained polymers PCzAQCx with the appropriate molar content of the AQC unit (x ≥ 0.5) exhibit efficient TADF features with a dominant emissive peak at 627−661 nm and a photoluminescence quantum yield of up to 76% in neat film. The non-doped electroluminescent devices with the poly mers produce red emissions with a maximum external quantum efficiency (EQE) of up to 12.5% and the emission peak at 620 nm, which represents state-of-the-art performance for solution-processed devices based on red TADF polymers. Furthermore, combined with a blue TADF emitter, the bright white devices with tunable spectra cover the whole visible-near infrared range from 400 to 900 nm and a record-high EQE of up to 22.4% is achievable.

Backbone‐Acceptor/Pendant‐Donor Strategy for Efficient Thermally Activated Delayed Fluorescence Conjugated Polymers with External Quantum Efficiency Close to 25% and Emission Peak at 608 nm

“…The LUMO+1 has an apparently raised energy level but is well delocalized on the QF and two adjacent S groups, whereas the HOMO-1 and HOMO are degenerate orbitals with the identical distributions of electron cloud. The transition energy gap of 2.03 eV is very close to that (2.04 eV) of another model compound in our previous work, [35] indicating the long wavelength emission of orange/red in the designed polymers. It is worthy noted that the angle of 77° between acridine and phenylene rings is smaller than those (80°-90°) of the reported literatures (Figure S1, Supporting Information).…”

“…The OLEDs using the polymers with the 10% and 20% molar content of the AQF unit achieve the record-high maximum EQE of around 24% with the emission peak at 608 nm (Figure 6), [6,14,22,23,35,[37][38][39][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] after the defect in hole injection/transport is balanced by simply optimizing doped device structure. Therefore, the BAPD strategy should be a promising approach to develop efficient TADF conjugated polymers with long wavelength emission from yellow to red.…”

“…It is worthy noted that the angle of 77° between acridine and phenylene rings is smaller than those (80°-90°) of the reported literatures (Figure S1, Supporting Information). [35,36] The decreased twisted degree corresponds to a larger overlap of HOMO and LUMO on the phenylene bridges, probably producing high PLQY. [35] Hence, the calculated results suggest that the polymers PSAQFx constructed from S-AQF-S would have a delocalized backbone with the strong electron-withdrawing ability and thus readily realize orange/red emissions with TADF mechanism.…”

“…The redshifted phenomenon from the extended conjugation is similar to the polymers constructed by BDPA strategy, [39] whereas it contrasts with the polymers involving acceptor moieties embedded into donor backbone. [35] Therefore, a preliminary consequence is that the entirely conjugated acceptor backbone is helpful to the redshift of PL spectra because the acceptor strength of the polymer emitters is further enhanced after polymerization. The effect can be firmly verified by the PL spectra of AQF and S-AQF-S, in which a blue emission at 441 nm contributed from the acridine moieties of AQF is clearly observed but disappears in the PL spectrum of S-AQF-S (Figure S3d, Supporting Information).…”

Modulating the Electron‐Donating Ability of Acridine Donor Units for Orange–Red Thermally Activated Delayed Fluorescence Emitters

“…The k nr,s values were calculated to be 8.18×10 6 s −1 for BFDMAc‐PhNAI, 5.34×10 7 s −1 for BTDPAc‐PhNAI, and 5.40×10 7 s −1 for BTDMAc‐PhNAI. The higher k nr,s values of BTDPAc‐PhNAI and BTDMAc‐PhNAI compared with that of BFDMAc‐PhNAI can be ascribed to the electron‐donating benzothiophene unit fused to the donor units, which leads to the approach of the S 0 and S 1 surfaces of BTDPAc‐PhNAI and BTDMAc‐PhNAI, according to the energy‐gap law [39, 40] . Therefore, more emissive singlet excitons may be wasted through the nonradiative deactivation pathway for BTDMAc‐PhNAI and BTDPAc‐PhNAI, which makes the Φ PL values of BTDMAc‐PhNAI and BTDPAc‐PhNAI lower than that of BFDMAc‐PhNAI.…”