Improved EL efficiency of fluorene-thieno[3,2-b] thiophene-based conjugated copolymers with hole-transporting or electron-transporting units in the main chain

New electroluminescent polymers (poly(9,9′-dioctylfluorene-co-thieno[3,2-6]thiophene-co-benzo[2,3,5] thiadiazole) (P1) and poly(9,9′-dioctylfluorene-co-thieno[3, 2-b]thiophene-co-benzo[2,3,5]thiadiazole-co-[4-(2-ethylhexyloxyl)phenyl] diphenylamine (P2)) possess hole-transporting or electron-transporting units or both in the main chains. Electron-deficient benzothiadiazole and electron-rich triphenylamine moieties were incorporated into the polymer backbone to improve the electron-transporting and hole-transporting characteristics, respectively. P1 and P2 show greater solubility than poly(9,9′-dioctylfluorene-co-thieno[3,2-b]thiophene (PFTT), without sacrificing their good thermal stability. Moreover, owing to the incorporation of the electron-deficient benzothiadiazole unit, P1 and P2 exhibit remarkably lower LUMO levels than PFTT, and thus, it should facilitate the electron injection into the polymer layer from the cathode electrode. Consequently, because of the balance of charge mobility, LED devices based on P1 and P2 exhibit greater brightness and efficiency (up to 3000 cd/m² and 1.35 cd/A) than devices that use the pristine PFTT.