Ink formulation of in-situ crosslinkable hole-transporting composite for multilayer inkjet-printed organic light-emitting diodes

Inkjet printing is considered to be the most promising technique for high-resolution, large-scale and low-cost organic light-emitting diode (OLED) displays. However, the device performance with multilayer printing is still a challenge that needs to be tackled, because of the serious re-dissolving problem of bottom film into the subsequent ink during printing processes. This work demonstrates an in-situ crosslinkable composite of hole-transporting polymer poly(N-vinylcarbazole) (PVK) and oxidative coupling agent phosphomolybdic acid (PMA) without ultra-violet irradiation or high temperature annealing treatment. The hole-transporting ink is formulated by adding high-boiling point solvents including o-dichlorobenzene, butyl benzoate, cyclohexylbenzene and 1-chloronaphthalene to eliminate the coffee ring effect. Their influence on the ink property is systematically investigated, and the corresponding solidification mechanism is also proposed. The modified PVK:PMA ink leads to a crosslinked hole-transporting layer (HTL) with an excellent solvent resistance and temperature-independent property. Finally, the emitting layer is successfully inkjet-printed onto this HTL, and multilayer inkjet-printed OLEDs are achieved with a current efficiency of 21.7 cd/A. In summary, this in-situ crosslinkable hole-transporting composite ink is facile and effective for multilayer inkjet-printed OLEDs.