Field of Expertise: Advanced Material Science

Bright blue solution processed triple-layer polymer light emitting diodes realized by thermal layer stabilization and orthogonal solvents
Roman Trattnig, Leonid Pevzner, Monika Jäger, Manuel Auer, Raphael Schlesinger, Marco Vittorio Nardi, Giovanni Ligorio, Christos Christodoulou, Norbert Koch, Ma
NanoTecCenter Weiz / Institut für Festkörperphysik
15:20 - 17:30 Thursday 24 October 2013 Foyer Alte Technik

The realization of fully solution processed multilayer polymer light emitting diodes (PLED) constitutes the pivotal point to push PLED technology to its full potential. In this contribution we present a fully solution processed triple layer PLED realized by a combination of two different deposition strategies. The demonstrated approach allows for a successive deposition of more than two polymeric layers without extensively redissolving an already present layer. For that purpose a hole transporting poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB) layer is stabilized by a hard-bake process as bottom 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 a toluene solution. To complete the triple-layer 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 is acting as the electron transporting layer, is deposited in an orthogonal solvent approach from methanol. 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 16540 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.