Jun 5, 2003
Photonic crystal boosts OLED output
By texturing the glass substrate of an organic light-emitting diode (OLED) with a photonic-crystal (PC) pattern, researchers in Korea claim to have increased the device's extraction efficiency by over 50% over a wide viewing angle. Having theoretically predicted an 80% increase, the team says there is still room for improvement. (Applied Physics Letters 82 3779)
To enhance the extraction, the team deposited a 200-nm thick layer of SiO2 onto the OLED's glass substrate. Then using holographic lithography and reactive ion etching, the researchers created a square array of 200-nm diameter rods with a lattice constant of 600 nm. This was topped off with an 800-nm thick buffer layer of SiNx and the OLED's electrode and light-emitting active layer.
"Experimentally, a 50% enhancement of the extraction efficiency was realized for the viewing angle range 90° ± 40° compared with a conventional OLED," report Yong-Jae Lee and colleagues from Korea's Advanced Institute of Science and Technology and Samsung.
Photons emitted from the active region of an OLED are coupled into one of three modes: direct transmission into the air, which accounts for 20% of the emitted photons; total internal reflection, which accounts for 30% of the emitted photons; and a high-index guided mode, which accounts for 50% of the emitted photons.
Lee and colleagues concentrated on maximising the extraction of photons from the high-index guided mode. "It is important to extract the photons from the high-index layer that is close to the light-emitting region if one wishes to maintain the image quality required in display devices," report the authors.
About the author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.