All solution processed blue multi-layer light emitting diodes realized by thermal layer stabilization and orthogonal solvent processing

Abstract

Herein we report on the fabrication and the properties of two highly efficient blue light emitting multilayer polymer light emitting diodes (PLEDs). The first device structure combines a thermally stabilized polymer with a material processed from an orthogonal solvent, allowing for the fabrication of a triple layer structure from solution. The well known poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), which can be stabilized in a bake-out procedure, was used as a hole transporting layer. A novel pyrene – triphenylamine (PPyrTPA) copolymer was used as emissive layer. The stack was finalized by a poly(fluorene) - derivative with polar side-chains, therefore being soluble in a polar solvent which allows for the deposition onto PPyrTPA without redissolving. The resulting PLED showed bright-blue electroluminescence (CIE1931 coordinates x=0.163; y=0.216) with a high efficiency of 1.42 cd/A and a peak luminescence of 16500 cd/m². The second presented device configuration comprises a thermally stabilized indenofluorene – triphenylamine copolymer acting as hole transporter, and an emissive copolymer with building blocks specifically designed for blue light emission, effective charge carrier injection and transport as well as for exciton generation. This multilayer PLED led to deep-blue emission (CIE1931 x=0.144; y=0.129) with a remarkably high device efficiency of 9.7 cd/A. Additionally, atomic force microscopy was carried out to investigate the film morphology of the components of the stack and x-ray photoemission spectroscopy was performed to ensure a full coverage of the materials on top of each other. Ultraviolet photoemission spectroscopy confirmed the desired type-II band level offsets on the individual interfaces.