Thermally crosslinkable hole-transporting poly(fluorene-co-triphenylamine) for multilayer polymer light-emitting diodes

This article reports the synthesis and characterization of a novel thermally crosslinkable hole-transporting poly (fluorene-co-triphenylamine) (PFO-TPA) by Suzuki coupling reaction, followed with its application in the fabrication of multilayer light-emitting diodes by wet processes. The thermal, photophysical, and electrochemical properties of PFO-TPA were investigated by differential scanning calorimeter, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry, respectively. Thermally crosslinked PFO-TPA, through pendant styryl groups, demonstrates excellent thermal stability (T_d > 400 °C, T_g = 152 °C), solvent resistance, and film homogeneity. Its highest occupied molecular orbital level (-5.30 eV) lies between those of PEDOT:PSS (-5.0 ∼ -5.2 eV) and poly(9,9-dioctylfluorene) (PFO: -5.70 eV), forming a stepwise energy ladder to facilitate hole injection. Multilayer device with crosslinked PFO-TPA as hole-injection layer (HIL) (ITO/PEDOT:PSS/HIL/PFO/LiF/Ca/Al) was readily fabricated by successive spin-coating processes, its maximum luminance efficiency (3.16 cd/A) were about six times higher than those without PFO-TPA layer (0.50 cd/A). The result of hole-only device also confirmed hole-injection and hole-transport abilities of crosslinked PFO-TPA layer. Consequently, the device performance enhancement is attributed to more balanced charges injection in the presence of crosslinked PFO-TPA layer. The thermally crosslinkable PFO-TPA is a promising material for the fabrication of efficient multilayer polymer light-emitting diodes because it is not only a hole-transporting polymer but also thermally crosslinkable.