Electro-optically Efficient and Thermally Stable Multilayer Semitransparent Pristine Ag Cathode Structure for Top-Emission Organic Light-Emitting Diodes

To attain high efficiency and stability in a top-emission organic light-emitting diode (TEOLED), the semitransparent metal cathode must have high transmittance, low absorbance, and uniform surface properties under continuous driving conditions. However, conventional semitransparent metal electrodes have numerous fundamental issues. Here, we report an electro-optically efficient and stable semitransparent pristine thin Ag cathode configuration for the high performances of TEOLED. A multilayer semitransparent cathode structure is fabricated by sandwiching a thin silver (Ag) layer between new organic wetting inducer and capping layer, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB). A thin Ag layer in the presence of p-bPPhenB exhibits superior optical and electrical properties, including a high transmittance of 83.8% at 550 nm, low absorbance of 7.65% at 550 nm, and the low sheet resistance of 2.1 Ω/sq, respectively, at room temperature and even under 110 °C for 100 h. It is observed that our organic wetting inducer and capping layer, p-bPPhenB has considerable influence on the stabilization of surface properties of the thin Ag film by forming coordination bond between the Ag atoms and nitrogen atoms of p-bPPhenB through donating unshared electrons to the Ag atom. An optically optimized phosphorescent green TEOLED with this pure Ag cathode configuration enables a low driving voltage of 5.0 V at 10 mA/cm2 and remarkably high maximum current efficiency of about 167 cd/A compared with the Mg:Ag (10:1) cathode (5.4 V and 109 cd/A).