Improving film uniformity and interface solvent resistance to realize multilayer printing of OLED devices

Abstract

Inkjet printing has great potential in processing organic light emitting diodes (OLEDs) with the advantages of large-area and pixel-processing panels with high material utilization. However, there are still some key problems in multilayer inkjet printing of OLEDs, such as dissolution between layers and poor uniformity of multilayer films, which results in the low performance of printed OLEDs. In this work, we printed the hole injection layer (HIL) PEDOT:PSS, the hole transport layer (HTL) N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (VNPB), and the emissive layer (EML) G2P2 successively. A binary ink formulation of cyclohexanone (CYC) and cyclohexylbenzene (CHB) was used to print the HTL and EML. The main solvent CYC provides solubility for the materials and the co-solvent CHB ensures slow evaporation of solvents and weakens outwards capillary flow to inhibit the coffee ring for its high boiling point and high viscosity. Besides, cross-linkable small molecules were used as interlayers. Therefore, the binary solvent almost does not have solubility for the crosslinked HTL and a clear interface is achieved between the HTL and EML. The multilayer-printed OLED device with the structure ITO/PEDOT:PSS (50 nm)/VNPB (20 nm)/G2P2 (30 nm)/SPPO13 (55 nm)/LiF (1 nm)/Al (120 nm) has shown a turn-on voltage of around 3.0 V (at 1 cd m⁻²), a maximum luminance of 12233.3 cd m⁻², and a current efficiency of 20.4 cd A⁻¹. This work provides a reference for inkjet printing of patterned multilayer OLED devices.