New Hole Transport Layer Materials for Polymer Light Emitting Diodes (23071
Northwestern University researchers invented a new hole transport system for polymer light emitting diodes (PLEDs) with negligible visible light absorption, stability to ITO, solvent resistance and tunable electronic properties.
ADVANTAGE: Superior PLED performance and efficiency is observed versus conventional PEDOT-PSS based hole transport layer (HTL) devices. ITO interface stability promises enhanced device life and solvent resistance accommodates emissive layer (EML) coating.
SUMMARY: Polymer light emitting diodes are of increasing interest in a wide range of display applications. However conventional hole transport layer poly(3,4-ethylenedioxythiophene)-poly(stryenesulfonate) (PEDOT-PSS) devices exhibit significant visible light absorption and ITO interface instability.
A new crosslinkable siloxane-derivatized hole-transport material (TPD-Si2) in combination with hole transporting or insulating polymers such as poly(9,9-dioctyl-fluorene-co-N-(4- butylphenyl)diphenylamine) (TFB) or polystyrene overcome the above limitations. Spin coating a TPD-Si2 / TFB blend on ITO with thermal curing affords a strongly adherent crosslinked network, providing an efficient HTL stable to subsequent solvent coating operations. The system exhibits ITO interface stability and minimal visible light absorption. PLED devices constructed with ITO/TPD-Si2-TFB blend or PEDOT-PSS/ EML/Ca/Al demonstrate approximately 180% greater maximum light out for the TPD-Si2 /TFB system at comparable turn on voltage (Figure 1). A 40% higher maximum quantum efficiency was also observed compared to the corresponding PEDOT based device (Figure 2), and a current efficiency of 17 cd/A was achieved. Xylene solvent treatment of the cured TPD-Si2/TFB layer had no impact on coating stability. The electronic (HOMO) properties of the system can be tuned by blending different hole transporting polymers with TPD-Si2 to match the electronic properties of the emissive layer polymer. These advances promise significant improvement in PLED fabrication and performance.
STATUS: A patent application has been allowed (US 7,449,509)
Qinglan Huang, He Yan and Tobin J. Marks
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