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.