Laser Focus World

Despite a nearly 100% internal quantum efficiency (IQE) for commercially available organic light-emitting diodes (OLEDs), luminous efficacy radiation (LER) values for current devices are only 20% to 40%. Recognizing the opportunity for significantimprovements in OLED output, Pixelligent (Baltimore, MD) has developed PixClear, a nanocrystalline synthesis and surface-modification technology that can be incorporated into many polymer and monomer OLED and LED fabrication systems (from acrylics and epoxies to silicones and spin-on glasses) to improve light-extraction efficiency.

High nanocrystal loading
The overall external quantum efficiency for a typical OLED device structure can be described as ηeqe = γηexcχpηcoupling, where γ is the electron–hole charge balance factor; ηexc is the ratio of radiative to nonradiative recombination of excitons (ηexc = 1/4 and 1 for fluorescence- and phosphorescence-based OLED materials, respectively); χp is the intrinsic quantum efficiency for radiative recombination (including both fluorescence and phosphorescence); and ηcoupling is the light-extraction efficiency of the device.

The root cause of the light loss in an OLED device is the fact that the light is produced in a high-refractive-index material and has to be transmitted to air, which has a low index (n = 1). If the incident angle is larger than a critical angle, the light experiences total internal reflection (TIR) and escapes from the edge of the device or is turned into heat, reducing the reliability and lifetime of the device (see figure). Due to the large index mismatch and the number of interfaces in OLED structures, total light-extraction efficiency peaks at around 40%.

 

A schematic comparison describes an OLED a) without any ILE scheme showing TIR and Fresnel reflection and b) with ILE using nanocrystalline scatterers and a high-index smoothing layer that scatters indiscriminately at all incident angles. The blue rays represent the light that travels within the escape cone, with the thin blue rays representing light reflected due to Fresnel reflection and the red rays representing the light that travels outside the device
A schematic comparison describes an OLED a) without any ILE scheme showing TIR and Fresnel reflection and b) with ILE using nanocrystalline scatterers and a high-index smoothing layer that scatters indiscriminately at all incident angles. The blue rays represent the light that travels within the escape cone, with the thin blue rays representing light reflected due to Fresnel reflection and the red rays representing the light that travels outside the device.

Light-extraction methods for OLEDs typically fall into three categories: (1) external light extraction via surface modification at the glass/air interface including substrate shaping, scattering layers, and microlens patterns; (2) internal light extraction (ILE) through modification of the effective index of refraction through photonic crystals, buckling structures, or scattering layers within the OLED device; and/or (3) cathode refinement by patterning, topography, and microstructures to inhibit plasmonic losses at the metal cathode surface or to move the metal cathode away from the emitting layer by increasing the thickness of the electrontransport layer (ETL).

Among these options, ILE through modification of the indium tin oxide (ITO)/substrate interface is the most efficient place in an OLED device to create light-extraction structures for several reasons. First, depending on the device design (mainly the ETL thickness), 80 to 90% of light emitted travels to the ITO/substrate interface; consequently, light extraction at this interface would significantly improve light output. Second, ILE does not negatively impact the performance of the device. And third, ILE is compatible with other mechanisms for light extraction.

Because PixClear dispersions enable formulations with nanocrystal loading around 80 wt% to reach refractive indices as high as 1.85, placing PixClear layers at the ITO/substrate interface significantly improves light extraction. PixClear’s optical properties allow it to be used in a wide range of advanced electronics applications. In solid-state lighting (specifically high-brightness or HB LEDs), the nanocrystals can be used to form high-refractive-index encapsulants and films to improve light extraction from the device and the phosphor-containing layer. In OLEDs, the nanocrystal-based high-refractive-index layer can dramatically improve the internal light extraction, accelerating the market adoption of OLED as an economical lighting option.

With a shelf life of more than three months, PixClear’s surface chemistry can be specifically tailored to achieve refractive-index tuning of commonly used polymer systems including acrylates, siloxanes, and silicones. In addition, unlike many other nanocrystals and nanocomposites that only exist in lab-scale quantities, the new product family is available in commercial quantities.

By Gail Overton, Senior Editor, on July 8, 2014