Syntheses and eletroluminescence properties of red emitting copolymers with different lengths of diketopyrrolopyrrole units

Five diketopyrrolopyrrole (DPP)-containing monomers, named as DPP1–3 and DPP-T1–2 were synthesized. DPP1–3 contained mono-, di- and tri-DPP units respectively; DPP-T1–2 contained one DPP unit with adjacent mono- and di-thiophene units. The five monomers were copolymerized with benzothiadiazole, dioctyloxybenzene and fluorene through fluoride-mediated Suzuki polymerization to give five copolymers (P-DPP1–3, P-DPP-T1–2) with low content of DPP (1 mol%). All copolymers were used as the emitting layers in the light-emitting diodes (ITO/PEDPT:PSS/polymer/CsF/Al). The devices of P-DPP2–3 and P-DPP-T1–2 exhibited red emitting with external quantum efficiency between 0.18% and 0.30%, and with maximum brightness between 422 and 677 cd/m2. The results showed that it is an effective method to keep emitting pure red light and enhance the performance of the devices with DPP-containing copolymers by lengthening DPP units and lowering its content.