This schematic shows the basic structure of an exciplex-based OLED with emission color and efficiency that can be controlled simply by changing the spacer thickness. An exciplex forms when a hole in the highest occupied molecular orbital (HOMO) of a donor molecule is attracted to an electron in the lowest unoccupied molecular orbital (LUMO) of an acceptor molecule. Light is emitted when the electron releases energy as light and transfers across the spacer layer to the donor molecule, thus replacing the missing electron represented by the hole. The thickness of the spacer can be used to modify the attraction between the hole and electron and tune the exciplex energy. Without changing any of the molecules, emission color could be varied from orange to yellowish green and the efficiency enhanced eight fold by increasing the spacer thickness to 5 nm. This device was first reported in the paper titled 'Long-range coupling of electron-hole pairs in spatially separated organic donor-acceptor layers' written by H. Nakanotani at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) in Fukuoka, Japan, and colleagues and published online Feb. 26, 2016 in the journal Science Advances.