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

For inkjet-printed organic light-emitting diodes, it is critical to obtain a homogeneous small molecular-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 of 3-(3,6-di-t-butylcarbazol-9-yl)-3'-(2-(4-t-butylphenyl)benzoimidazol-1-yl)-[1,1'-biphenyl], and an iridium complex dopant of 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 oC upon cooling from the melt. The inkjet printability and film morphology of the emitting layer are strongly dependent both on substrates and the solvents. With the 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 OLEDs with the inkjet-printed emitter yields a maximum current efficiency of 17.89 cd/A, comparable to that of the Ir(MDQ)2(acac)based OLEDs with spin-cast emitter.