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.

Organic solar cells (OSCs) based on small molecular donor/ polymer acceptor (M D /P A ) blends have the advantage of excellent thermal stability, but they suffer from low photovoltaic efficiency because of the largesize phase separation morphology of the active layer. Here, we systemically studied the effects of molecular weight on the active layer morphology and device performance of M D /P A -type OSCs. When the molecular weight is low, the polymer acceptor shows weak aggregation in solution and poor crystallinity in film, which allows the small molecular donor form large crystalline domains in M D /P A blends. In contrast, high molecular weight leads to strong aggregation of polymer chains in solution, which not only enhances crystallinity of the polymer acceptor but also suppresses crystallization of the small molecular donor. As a result, the M D /P A -type OSC exhibits small-size phase separation in the active layer and shows much enhanced photovoltaic performance with a power conversion efficiency of 6.4%, which is among the highest reported for M D /P A -type OSCs. These results disclose the great importance of high molecular weight for efficient M D /P A -type OSCs.

(2 of 10)www.advopticalmat.de Scheme 1. Strategies for constructing TADF small molecules and polymers. TADF small molecules: a) redshifted emission with extended conjugation of acceptor. TADF polymers: b) In our previous work, the polymers with BDPA architecture achieve green and yellow emissions with excellent EL performances; c) In this work, the polymers with BAPD architecture are expected to produce a relatively redshifted emission such as orange to red emissions, based on extensively conjugated acceptor backbone.