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

Abstract

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(FA90-co-BFA10) 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(FA90-co-BFA10) were measured to be −5.14 and −2.31 eV, respectively. After heating at 150 °C for 40 min, the spin-cast poly(FA90-co-BFA10) 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(FA90-co-BFA10). Finally, a solution-processed red phosphorescent OLED device with the structure of indium tin oxide/poly(FA90-co-BFA10)/poly-TPD/emissive layer/TPBi/LiF/Al was fabricated and characterized. The device showed a low turn-on voltage (Von) 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(FA90-co-BFA10) HIL demonstrated a longer lifetime than that fabricated using PEDOT:PSS.