Characterizing Solution-Processed Small-Molecule Organic Light-Emitting Diodes Depending on Host Composition of Emitting Layer

Abstract

A method for achieving efficient and stable solution-processed small-molecule organic light-emitting diodes (OLEDs) is presented by utilizing a combination of a multifunctional hole injection layer (HIL) and a mixed-host emitting layer (EML). The polymeric HIL facilitates efficient hole injection into the solution-processed EML and blocks electrons from the EML through self-organization of polymer chains in the HIL. In addition to the multifunctional HIL, the optimized mixed-host EML, composed of electron and hole transporting host materials, along with phosphorescent dopant, enables efficient energy transfer, balanced charge transport, and efficient charge carrier recombination in the device. As a result, it improves luminance (~14,000 cd/m2), luminous efficiency (~55 cd/A), and operational lifetime (~180 minutes under constant current emitting initial luminance of 1,000 cd/m2, equivalent to approximately 150 hours at an initial luminance of 100 cd/m2). Notably, this device architecture does not include an additional hole transporting/electron blocking layer. This is because the introduction of a mixed-host composition widens the recombination zone in the EML, effectively preventing triplet-triplet excitons/triplet-polaron annihilation caused by charge carriers and excitons accumulated at the narrow heterointerfaces in OLEDs.