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A feasible strategy, backbone‐acceptor and pendant‐donor, for constructing thermally activated delayed fluorescence (TADF) conjugated polymer is presented. In the obtained polymers PSAQFx, the backbone consists of dibenzothiophene‐S,S‐dioxide (S) and 6,7‐difluoroquinoxaline (QF), while two 9,10‐dihydroacridine (A) groups as pendants are grafted on QF acceptor through phenylene bridges. Compared to the model compound AQF, the polymers exhibit redshifted emission owing to extended conjugation along the acceptor backbone. Photoluminescence quantum yields can be improved by managing the molar content (x) of the AQF unit and reach the maximum of 91% for PSAQF5 in neat film. Solution‐processed organic light‐emitting diodes (OLEDs) using these polymers as the emitters achieve a maximum external quantum efficiency (EQE) of 24.8% and a maximum power efficiency of 68.5 lm W−1 with the emission peak at 608 nm and luminance at 58 cd m−2. This is the first example of achieving EQE above 20% with emission peak over 600 nm in the polymer TADF OLEDs. [ABSTRACT FROM AUTHOR]

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 polymers 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. [ABSTRACT FROM AUTHOR]

• Effect of the polymer M n on the TADF property was explored for the first time. • More compacted stacking and better energy transfer is observed in low M n polymers. • Low M n polymer achieves a record-high maximum EQE of 21.2 % at 628 nm. Thermally activated delayed fluorescence (TADF) polymers have made remarkable progress in solution processed organic light-emitting diodes (OLEDs) due to their superior film-forming stability. However, the effect of some factors such as the molecular weight and end groups on the optoelectronic properties is never explored so far. Herein, a set of the TADF conjugated polymers with the different number-average molecular weights (M n s) of 5.4, 24.8, 41.8 and 146.1 kg/mol are synthesized and their M n effect on the photoluminescence (PL) and electroluminescence (EL) is systematically investigated. The microstructure models of the spin-coated polymer films based on the M n and chain length are proposed for realizing efficient non-doped OLEDs. Low M n produces more compacted polymer chain stacking and smoother film morphology, leading to restricted non-radiative loss and more effective energy transfer. In particular, the polymers with low M n s of 5.4 and 24.8 kg/mol achieve the higher PL quantum yields and more efficient reverse intersystem cross from triplet to singlet excited state, as well as the record external quantum efficiencies of 21.2 % and 18.4 % with the EL emission peaks over 620 nm. In contrast, the PL and EL performances eventually deteriorate with further increasing M n s due to the sparser chain stacking and larger phase separation. The results show that the lower M n s enable the rigid polymers to form perfect amorphous films and render the luminescent unit more uniform distribution, similar to the classical doped systems, improving the EL behaviors of the polymers. [ABSTRACT FROM AUTHOR]