Light Extraction and Stability of Fluorescent and Phosphorescent Organic Light-Emitting Diodes Incorporating Plasmonic Ag Nanoparticles

<br/>In recent years, organic light-emitting diodes (OLEDs) have gained great popularity in the display industry due to their thin form factor, high color quality and the ability to be fabricated onto flexible substrates. However, OLED stability and efficiency, particularly at blue wavelengths, need significant improvement for future, more demanding display applications, such as micro displays. Several methods have been employed to increase the light-extraction efficiency (LEE) and stability in OLEDs, including the development of new active-layer materials, interfacial layer modifications, addition of nanoparticles or nanopatterns into various layers of the devices, optimization of the structural design and the application of microlens arrays. The LEE in an OLED is typically ~20% with the rest of the 80% of photon energy being lost to substrate, surface plasmon and thin-film waveguide modes. While these approaches are effective for improving LEE of red and green OLEDs, very few of these approaches have significantly improved the efficiency of blue OLEDs. We have shown in previous studies that plasmonic Ag enhances the quantum yield in fluorescent materials through the Purcell effect and improves the photostability of phosphorescent OLED materials by a factor of 3.6 by reducing the triplet emission lifetime. Here, we present a study of the use of silver nanoparticles introduced at different device interfaces and within different layers from the anode to the cathode to combat the various optical losses that reduce LEE. The results show improvements in blue OLED stability and luminescence lifetime in addition to improved LEE We have chosen a blend of PFO: F8BT to study fluorescent OLEDs and a blend of PVK:FIrpic to study blue phosphorescent OLEDs as a function of different nanoparticle additions. The devices are built on ITO on glass substrates with different formulations of PEDOT:PSS to get the highest conductivity. We vary the Ag nanoparticle size from 15-50 nm and introduce them into different layers of the OLED including: (i) on ITO directly, through spin coating or de-wetting methods, (ii) doped into the PEDOT:PSS layer; (iii) grown on the PEDOT:PSS layer; (iv) within and on top of active layer; and finally (v) deposited on top of the active layer before depositing the Ca/Al cathode. We present current-density measurements, luminance, electroluminescence measurements, external quantum efficiency and stability measurements in addition to lifetime and quantum yield measurements to investigate how positioning of the plasmonic structures effects the stability and lifetime of the OLEDs.We are hoping to contribute to solving the issue of low efficient blue OLEDS by improving the lifetime and stability of the devices as well as achieving the required “true blue” color that is required by the display industry by manipulating the interactions between the plasmonic Ag particles and the emissive layers.