High-Performance Hole-Transport Layers for Polymer Light-Emitting Diodes. Implementation of Organosiloxane Cross-Linking Chemistry in Polymeric Electroluminescent Devices

This contribution describes an organosiloxane cross-linking approach to robust, efficient, adherent hole-transport layers (HTLs) for polymer light-emitting diodes (PLEDs). An example is 4,4‘-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2), which combines the hole-transporting efficiency of N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl)-4,4-diamine) (TPD, prototypical small-molecule HTL material) and the strong cross-linking/densification tendencies of organosilanol groups. Covalent chemical bonding of TPDSi2 to PLED anodes (e.g., indium tin oxide, ITO) and its self-cross-linking enable fabrication of three generations of insoluble PLED HTLs: (1) self-assembled monolayers (SAMs) of TPDSi2 on ITO; (2) cross-linked blend networks consisting of TPDSi2 + a hole transporting polymer (e.g., poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine), TFB) on ITO; (3) TPDSi2 + TFB blends on ITO substrates precoated with a conventional PLED HTL, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS). PLED devices fabricated using these new HTLs exhibit comparable or superior performance vs comparable devices based on PEDOT-PSS alone. With these new HTLs, current efficiencies as high as ∼17 cd/A and luminances as high as ∼140,000 cd/m2 have been achieved. Further experiments demonstrate that not only do these HTLs enhance PLED anode hole injection but they also exhibit significantly greater electron-blocking capacity than PEDOT-PSS. The present organosiloxane HTL approach offers many other attractions such as convenience of fabrication, flexibility in choosing HTL components, and reduced HTL-induced luminescence quenching, and can be applied as a general strategy to enhance PLED performance.