Red to orange thermally activated delayed fluorescence polymers based on 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide for efficient solution-processed OLEDs

Abstract

Most highly efficient thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) are multi-layer devices fabricated by thermal vacuum evaporation techniques, which are unfavorable for real applications. However, there are only a few reported examples of efficient solution-processed TADF OLEDs, in particular TADF polymer OLEDs. Herein, a series of solution-processable TADF conjugated polymers (PCTXO/PCTXO-Fx (x = 25, 50 and 75)) were designed and synthesized by copolymerization of 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide (TXO-TPA) as a red/orange emissive TADF unit, 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) as host/hole-transporting unit and 2,7-N-(heptadecan-9-yl)carbazole as a conjugated linker and solubilizing group. They possessed a conjugated backbone with donor TPA-carbazole/fluorene moieties and a pendent acceptor 9H-thioxanthen-9-one-10,10-dioxide (TXO) forming a twisted donor–acceptor structure. These polymers in neat films displayed red/orange color emissions (601–655 nm) with TADF properties, proved by theory calculations and transient PL decay measurements. Their hole-transporting capability was improved when the content of 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) within the polymers increased. All polymers were successfully employed as emitters in solution-processed OLEDs. In particular, the doped OLED fabricated with PCTXO exhibited an intense deep orange emission at 603 nm with the best electroluminescence performance (a maximum external quantum efficiency 10.44%, a maximum current efficiency of 14.97 cd A⁻¹ and a turn-on voltage of 4.2 V).