Clearer and more efficient transparent OLEDs

Quasi-random nanostructures could improve the efficiency of transparent OLEDs without haziness.

Article  |  Spring 2014

Transparent Organic Light Emitting Diodes (OLEDs) are considered promising candidates for next-generation displays such as transparent smart windows, transparent TV screens, and monitors, as shown in science fiction movies. However, transparent OLEDs have the disadvantage that the amount of light seen by the observer is much less than that of conventional OLEDs since transparent OLEDs emit light both forward and backward.

In order to avoid this disadvantage and achieve higher image quality, there have been several efforts to improve the efficiency of conventional OLEDs by incorporating a periodic nanostructure, but it is expected to be an angular dependency and a limitation on optical clarity for transparent OLEDs.

Therefore, Professor Kyung Cheol Choi’s research group has proposed a method to improve the efficiency of transparent OLEDs without haziness or angular dependency by introducing a quasi-random nano-structure obtained by colloidal lithography. The quasi random nano-structure is inserted into transparent OLEDs as a Hole Injection Layer (HIL) made of WO₃. Professor Choi’s group proved that the nano-structures can reduce the surface plasmon and waveguide losses, which are known to account for a large portion of energy loss in OLEDs. Owing to the reduction of surface plasmon and waveguide losses, the nanostructured transparent OLEDs achieve a relative enhancement of external quantum efficiency (34% for the bottom, 79% for the top) of emission. More importantly, in spite of the existence of the embedded light-extracting nanostructure, angular dependency and diffuse transmission that would lead to optical blur or haze do not occur. This is due to a random orientation of the nanostructures over a scale of tens of micrometers. This randomized characteristic of the nanostructures reduces the angular dependency of emission properties while the periodicity within each domain could still enhance the out-coupling of the lights emitted from OLEDs.

This research article was published as the frontispiece paper in Advanced Optical Materials on October, 2013, and it is expected that this research would be a core technology for transparent OLEDs in the near future.

This image shows the schematics of the quasi-random nanostructured transparent OLEDs. The quasi-random nanostructured WO₃ layer was fabricated on the ITO layer using colloidal lithography. The nanostructures make possible to extract the trapped waveguide modes and surface plasmon modes in transparent OLEDs.

[Journal of Advanced Optical Materials]

http://onlinelibrary.wiley.com/doi/10.1002/adom.201300189/abstract

by Professor Kyung Cheol Choi (Department of Electrical Engineering)