OLED outcoupling

Problem Description

The efficiency of organic light-emitting devices (OLEDs) has increased steadily since Ching Tang's landmark paper in 1987. The internal quantum and voltage efficiencies now both approach 100%. However, large efficiency gains still remain to be made by increasing the efficiency at which photons are extracted from the device structure, i.e. the outcoupling efficiency.

Numerous approaches to improve the outcoupling efficiency have been proposed and demonstrated. However, none of these methods are applicable to displays either because they introduce angular color/intensity dependence, or because they rely on scattering which leads to pixel blurring. The method we developed has none of these disadvantages.

Approach

We have developed a method that modifies only the substrate with a 2-component dielectric stack that adjust the local photon density of states in the emissive layer to suppress emission into unwanted modes and enhance emission into desired modes. As a result, a given OLED can appear up to 2.5 times brighter using this approach, for the same power consumption, as shown in Fig. 1.

Fig. 1: (a) Relation between achievable apparent outcoupling efficiency and viewing cone for which a Lambertian response is desired. For a 60-degree viewing cone (sufficient for many small displays), an effective brightness that is 2.5 times that of a conventional OLED can be achieved. If a Lambertian response is not required, overall outcoupling efficiencies of 73% (polymer OLED) and 45% (small-molecule OLED) can be achieved. (b) CIE coordinates vs angle for a 60-degree cone and 120-degree cone design, compared to a standard OLED.

Publications

Journal Publications

• M. Agrawal, Y. Sun, S.R. Forrest and P. Peumans, "Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors," Applied Physics Letters 90, 241112 (2007). Highlight: Most downloaded paper from the Applied Physics Letters website 3 months in a row! [June ( link), July ( link) and August ( link) 2007]
• M. Agrawal and P. Peumans, "Design of non-periodic dielectric stacks for tailoring the enission of organic light-emitting diodes," Optics Express 15, 9715-9721 (2007).
  

Conference talks

• M. Agrawal, Y. Sun, S.R. Forrest and P. Peumans, "Efficient organic light-emitting diodes using tapered periodic and aperiodic dielectric mirrors," MRS Fall 2006, Boston, MA, November 2006.
• [INVITED] P. Peumans, "Solving the OLED outcoupling problem using aperiodic dielectric stacks," AIChE, San Francisco, CA, November 2006.
• M. Agrawal and P. Peumans, “Molding the emission of OLEDs using non-periodic dielectric photonic structures,” MRS Fall Meeting 2005, Boston, MA, November 2005.

Patents

• “Aperiodic dielectric multilayer stack, ” P. Peumans and S.R. Forrest, US Patent #7,196,835, March 27, 2007.

Project info

• People: Mukul Agrawal and Peter Peumans
• Collaborators: General Electric, Steve Forrest (University of Michigan)
  
• Current sponsor: DOE. Former sponsors: SRC and UDC.
  
• Contact: Peter Peumans
  

Resources