Opt. Express 20, A13–A19 (2012)

Credit: © 2011 OSA

Indium tin oxide (ITO) is widely used as the transparent conductive material in LEDs because of its excellent conductivity and high transparency to visible light. However, ITO is expensive and many improvements can still be made to the transparent layer. Arthur Reading and co-workers from the University of California at Santa Barbara in the USA have found that ZnO — a cheap, highly transparent and highly conductive alternative material — offers improved series resistance, external quantum efficiency and luminous efficacy over its ITO counterpart. The team deposited heteroepitaxial ZnO transparent current-spreading layers with low sheet resistances on GaN-based LEDs using aqueous solution phase epitaxy at temperatures below 90 °C. They measured luminous efficacies of 157 lm W−1 at 0.5 A cm−2 and 84.8 lm W−1 at 35 A cm−2, which are 24% and 50%, respectively, higher than LEDs that exploit ITO. The researchers attribute these improvements to the lower sheet resistance and lower optical absorption of the ZnO layer, which together minimize current crowding and allow more light to escape from the LED dye.