Inkjet printing a small-molecule binary emitting layer for organic light-emitting diodes

Abstract

For inkjet-printed organic light-emitting diodes, it is critical to obtain a homogeneous small molecule-based emitting film. In contrast to the conventional strategy of utilizing mixed solvents, we have successfully inkjet printed a binary electrophosphorescent emitter with a single solvent. The binary emitter consists of a new host material 3-(3,6-di-t-butylcarbazol-9-yl)-3′-(2-(4-t-butylphenyl)benzoimidazol-1-yl)-[1,1′-biphenyl], and an iridium complex dopant bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)-iridium(III) (Ir(MDQ)₂(acac)). Compatible with Ir(MDQ)₂(acac), the newly developed host is highly soluble and amorphous with a glass transition temperature of 137 °C upon cooling from the melt. The inkjet printability and film morphology of the emitting layer are strongly dependent both on the substrates and the solvents. With o-dichlorobenzene (o-DCB) as the single solvent, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))diphenylamine)] (TFB) as the hole transporting layer, a uniform emitter film morphology without any coffee ring is obtained. The phosphorescent OLED with the inkjet-printed emitter yields a maximum current efficiency of 17.89 cd A⁻¹, comparable to that of Ir(MDQ)₂(acac)-based OLEDs with a spin-cast emitter.