Precision synthesis of thermally crosslinkable hole-transporting block copolymers via living anionic polymerization for solution-processable organic light-emitting diodes

The anionic polymerization of 3-(4-vinylphenyl)bicyclo[4.2.0]octa-1,3,5-triene (A), which incorporates a thermally crosslinkable benzocyclobutene (BCB) group, was initially carried out using sec-butyllithium (sec-BuLi) and potassium naphthalenide as initiators in tetrahydrofuran at - 30 degrees C for 10 min. The resulting poly(A)s possessed predictable molecular weights (Mn = 7.0 - 20.9 kg/mol) and narrow molecular weight distributions (Mw/Mn = 1.12 - 1.24). Based on the living nature of A, sequential block copolymerization with N-[1,1 '-biphenyl]-4-yl-N(4 '-ethenyl[1,1 '-biphenyl]-4-yl)-9,9-dimethyl-9H-fluoren-2-amine (B) was conducted using sec-BuLi to prepare a well-defined poly(A-b-B) block copolymer, which contains hole-transporting triphenylamine and thermally crosslinkable BCB, as hole-transporting layer (HTL) for solution-processable organic light-emitting diodes (OLEDs). Copolymerization achieved the synthesis of controlled poly(A-b-B) with a precise molecular structure. The solvent resistance of the thermally crosslinked poly(A-b-B) film was determined using a rinsing test and surface morphology analysis. Poly(A-b-B) treated at 190 degrees C for 10 min exhibited excellent solvent resistance with respect to the solvent utilized in the emitting layer. The phosphorescent OLEDs fabricated with crosslinked poly (A-b-B) as the HTL showed a higher current efficiency (eta CE,max = 76.6 cd/A) and maximum external quantum efficiency (eta EQE,max = 20.6 %) than the device without HTL (eta CE,max = 42.3 cd/A, eta EQE,max = 11.4 %). This device performance strongly suggests that the precisely synthesized poly(A-b-B) has considerable potential as an HTL in solution-processable OLEDs.