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

Abstract

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. An in-situ crosslinkable composite ink of poly(N-vinyl carbazole) (PVK) and phosphomolybdic acid (PMA) was developed to prevent interlayer mixing as inkjet printing multilayer. After instant crosslinking of PVK through oxidative coupling by PMA, the luminous efficiency of the multilayer printed OLEDs increased from 10.6 to 21.7 cd/A with a 2-fold enhancement. This work provides a facile and effective strategy to print multilayer OLEDs without ultra-violet irradiation or high temperature annealing treatment. ● Hole transport polymer poly(N-vinylcarbazole) (PVK) and oxidative coupling agent phosphomolybdic acid (PMA) are used to prepare in-situ crosslinkable ink. ● The modified PVK:PMA ink leads to a crosslinked hole-transporting layer with an excellent solvent resistance and temperature-independent property. ● This approach enhances the luminous efficiency from 10.6 to 21.7 cd/A for multilayer printed light-emitting diodes.