Enhanced Performance of Blue Light Emitting Polymer Diodes via Anode Modification (22103)
Researchers at Northwestern University invented a process that significantly improves Blue polymer light emitting diode (PLED) performance, efficiency, and stability. Blue PLED devices generally require anode coating with a conductive polymer hole transport layer (HTL), which can adversely impact device fabrication, useful spectral range, performance and stability. This invention replaces the conductive polymer with an ultra-thin siloxane-derivative that enhances the ITO anode / HTL interface, thus simplifying the construction of the device as well as significantly improving PLED performance, efficiency and stability.
ADVANTAGE: New self-assembling anode interlayer materials that eliminate use of spin coated doped conductive polymer hole transport layers, enhance PLED performance and stability. The invention offers simpler device design and fabrication.
SUMMARY: Polymer light emitting diodes are of increasing interest in a wide range of display applications. However few efficient blue PLEDs have been reported. The high ionization potential of most blue emitting polymers is proposed to limit hole injection at the anode of these devices. Conductive polymers, such as poly(3,4-ethylenedioxythiophene)-poly(stryenesulfonate) (PEDOT-PSS), are spin coated between the ITO anode and emissive polymer to overcome hole injection limitations. This additional layer improves PLED performance but increases visible light absorption and adversely impacts ITO interface properties.
This invention affords high-performance blue light-emitting diodes without spin-coated doped conductive polymers as injection/transport layers. An ultra-thin self-assembled monolayer of a siloxane-derivatized hole-transport material (TPD-Si2) is used to modify the ITO anode surface, which is then fabricated with the archetypical poly (9,9-dioctylflourene) (PFO) as the emissive layer. This process dramatically increases the performance of the PFO-based blue PLED device. Thus PLED devices (ITO/TPD-Si2/PFO/Ca/Al) fabricated in this manner exhibit turn on voltage (~5.5 V), maximum luminance (~7000 cd/m2 at ~15 V)) and current efficiency (1.1cd/A) superior to a bare ITO (ITO/PFO/Ca/Al) and equal or greater than PEDOT-PSS (ITO/PEDOT-PSS/PFO/Ca/Al) devices (Figures).
In addition, the thin TPD-Si2 layer (~1.8 nm) has negligible visible region absorption versus PEDOT-PSS and other doped conductive polymers. The technology eliminates the need for a PEDOT-PSS layer (~50 nm) that increases device thickness, intrinsic operating voltage, and reduces ITO interface stability. These advances promise significant improvement in PLED manufacture.
STATUS: A patent application has been allowed, US Patent 6,939,625
Qinglan Huang, He Yan and Tobin J. Marks
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