Organic Electroluminescent Metasurfaces

Organic materials have been used in optoelectronic devices such as light emitting diodes (LEDs) and solar cells due to their great tunability and flexibility. The patterning of organic materials has been challenging, especially when pushing the feature size down to the sub-micron level, as organic materials are incompatible with conventional lithography-based patterning techniques. Here we present the scalable fabrication of nano organic LED (nano-OLED) devices that achieve an array density of up to 100,000 pixels per inch (ppi) with sub-micron-sized pixels. Direct nanopatterning of organic emissive material was realized through 'nanostencil lithography.' Nanostencil lithography is a resistless patterning technique where a free-standing thin silicon nitride membrane is utilized as a shadow mask, on which sub-micron-sized pores are located. We optimized the device fabrication process by evaporating hole transport and emission layer through nanostencil shadow mask, whereas other layers in the OLED stack were deposited as bulk film. The average external quantum efficiencies (EQEs) extracted from a nano-OLED device record up to 10%. As array periodicity and feature size lie in subwavelength scale, individual pixels of a nano-OLED device act as electroluminescent meta-atoms forming metasurfaces that convert electric energy directly into modulated light. The diffractive coupling between the light-emitting nanopixels enables control over far-field emission properties. We have examined a variety of periodic arrays such as 1D gratings, 2D square and hexagonal arrays, and circularly symmetric bullseye arrays to investigate if the emission directionality or polarizability can be modulated.