Semi-orthogonal solution-processed polyfluorene derivative for multilayer blue polymer light-emitting diodes

Semi-orthogonal solution processing can circumvent interface intermixing issue of construction of multilayer polymer light-emitting diode devices (PLEDs). In this manuscript, we developed a new strategy to construct multilayer PLEDs by the semi-orthogonal solvents based on the difference between the solubilities of a poly(9,9-dioctylfluorene) that end-capped with a triphenylamine derivative (PFO-TF) (processed with p-xylene) and the light-emitting polymer PPF-SO15 or PPF-SO15-BT1 (processed with 1,4-dioxane). As the highest occupied molecular orbital energy level of PFO-TF (−5.70 eV) lies in between PEDOT:PSS (−5.20 eV) and the light-emitting copolymer PPF-SO15 (−5.94 eV), it can serve as the hole transport layer (HTL) to facilitate hole injection. The blue PLED based on PFO-TF as the HTL exhibits lower turn-on voltage and nearly identical electroluminescent spectra as that of the control device without HTL. Of particular importance is that the blue device with PFO-TF as the HTL exhibits the luminous efficiency over 5.0 cd A−1, which is much higher than that observed from a device without an HTL (3.51 cd A−1) or one with traditional poly(9-vinylcarbazole) as the HTL (4.74 cd A−1). Similar trend is also realized for PLEDs based on green-light-emitting polymer PPF-SO15-BT1. These observations indicate that processing of a multilayer device with this specific semi-orthogonal system may be a promising strategy for the construction of efficient PLEDs.