Triazolobenzothiadiazole-Based Copolymers for Polymer Light-Emitting Diodes: Pure Near-Infrared Emission via Optimized Energy and Charge Transfer

A series of new near-infrared (NIR) emitting copolymers, based on a low band gap 6-(2-butyloctyl)-4,8-di(thiophen-2-yl)-[1,2,3] triazolo[4',5':4,5]benzo[1,2-c]-[ 1,2,5] thiadiazole (TBTTT) fluorophore copolymerized into a high band gap poly[3,3'-ditetradecyl-2,2'-bithiophene-5,5'-diyl-alt-5-(2-ethylhexyl)-4H-thieno[ 3,4-c] pyrrole-4,6(5H)-dione-1,3-diyl] (P2TTPD) host backbone, for polymer light-emitting diode (PLED) applications is reported. PLEDs fabricated from the host polymer (P2TTPD-0) show external quantum efficiencies (EQEs) up to 0.49% at 690 nm, with turn-on voltage (V-on) at only 2.4 V. By incorporating the TBTTT segments into the host polymer backbone, pure NIR emission peaking at ca. 900 nm is obtained with V-on remaining below 5 V. This work demonstrates that such a low V-on can be attributed to efficient intrachain energy and/ or charge transfer to the TBTTT sites. When the NIR emitting copolymer (P2TTPD-10) is blended with P2TTPD-0, the TBTTT are confined to well-separated polymer chains. As a result, the EQE from the blend is lower and the V-on higher than that obtained from the pure copolymer (P2TTPD-1.0) with equal content of TBTTT. An analogous copolymer (P4T-1.0), consisting of poly[3,3'-ditetradecyl-2,2':5',2'':5'',2'''-quaterthiophene-5,5'''- diyl] (P4T) as the host and 1% TBTTT as the NIR emitter, further demonstrates that pure NIR emission can be obtained only through optimized molecular orbital energy levels, as in P2TTPD-1.0, which minimizes chances for either charge trapping or exciton splitting.