Bright Blue Solution Processed Triple-Layer Polymer Light- Emitting Diodes Realized by Thermal Layer Stabilization and Orthogonal Solvents

The realization of fully solution processed multilayer polymer light-emitting diodes (PLEDs) constitutes the pivotal point to push PLED technology to its full potential. Herein, a fully solution processed triple-layer PLED realized by combining two different deposition strategies is presented. The approach allows a successive deposition of more than two polymeric layers without extensively redissolving already present layers. For that purpose, a poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB) layer is stabilized by a hard-bake process as hole transport layer on top of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). As emitting layer, a deep blue emitting pyrene-triphenylamine copolymer is deposited from toluene solution. To complete the device assembly 9,9-bis(3-(5′,6′-bis(4-(polyethylene glycol)phenyl)-[1,1′:4′,1″-terphenyl]-2′-yl)propyl)-9′,9′-dioctyl-2,7-polyfluorene (PEGPF), a novel polyfluorene-type polymer with polar sidechains, which acts as the electron transport layer, is deposited from methanol in an orthogonal solvent approach. Atomic force microscopy verifies that all deposited layers stay perfectly intact with respect to morphology and layer thickness upon multiple solvent treatments. Photoelectron spectroscopy reveals that the offsets of the respective frontier energy levels at the individual polymer interfaces lead to a charge carrier confinement in the emitting layer, thus enhancing the exciton formation probability in the device stack. The solution processed PLED-stack exhibits bright blue light emission with a maximum luminance of 16 540 cd m−2 and a maximum device efficiency of 1.42 cd A−1, which denotes a five-fold increase compared to corresponding single-layer devices and demonstrates the potential of the presented concept.