Synthesis and characterization of thermally cross-linkable poly(iminoarylene)-based hole injection layer for solution-processed organic light-emitting diodes.

[Display omitted] • Poly(iminoarylene)s through Buchwald-Hartwig reaction. • Hole injection polymers with thermally cross-linkable benzocyclobutene groups. • Excellent solvent resistance over 99 % after annealing at 150 °C for 40 min. • Better hole injection properties in the fabricated OLEDs than PEDOT:PSS. A cross-linkable poly(iminoarylene), poly[(9,9-dioctyl-N-(p-tolyl)-9H-fluoren-2-amine) –co- (9,9-bis(4-(bicyclo[4.2.0]octa-1(6),2,4-trien-7-yloxy)phenyl)-N-(p-tolyl)-9H-fluoren-2-amine)] [poly(FA -co- BFA)], was synthesized as a hole injection layer (HIL) for solution-processed organic light-emitting diodes (OLEDs). Poly(FA 90 - co -BFA 10) containing 10 mol% of benzocyclobutene (BCB) which is a thermally curable functional group was used in this study. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of poly(FA 90 -co- BFA 10) were measured to be −5.14 and −2.31 eV, respectively. After heating at 150 °C for 40 min, the spin-cast poly(FA 90 -co- BFA 10) film became completely crosslinked without requiring the use of additional initiators. The crosslinked thin film showed good film-forming properties and excellent solvent resistance (up to 99 %). A photo-crosslinked poly[bis(4-butypheny)-bis(phenyl)benzidine] (poly-TPD) was used as the hole-transporting layer (HTL) on top of the crosslinked poly(FA 90 -co- BFA 10). Finally, a solution-processed red phosphorescent OLED device with the structure of indium tin oxide/poly(FA 90 -co- BFA 10)/poly-TPD/emissive layer/TPBi/LiF/Al was fabricated and characterized. The device showed a low turn-on voltage (V on) of 3.5 V, a reasonably high current efficiency (CE) of 16.6 cd/A, and an external quantum efficiency (EQE) of 12.5 %, which are better than those of the corresponding reference device using PEDOT:PSS as the HIL. Moreover, the device fabricated using the poly(FA 90 -co- BFA 10) HIL demonstrated a longer lifetime than that fabricated using PEDOT:PSS. [ABSTRACT FROM AUTHOR]