The organic light emitting diode (OLED) array used in the OROM reader is made from organic compounds composed of lots of polymer chains with carbon, hydrogen, nitrogen, oxygen, and phosphorus atoms. They tend to be wickedly complex and large molecules, often with benzene rings and side chains of polymers attached to the main backbone.
For OLEDs, the side chains' energy absorption is what defines the properties of the light emitter. You adjust the material making up the polymer, tuning the emission wavelength. This material behaves like a semiconductor, which generally have material-defined, atomic energy bands to produce their distinct emission wavelengths when electrons change energy levels.
The polymers can be mixed up and spun onto a substrate at room temperatures. No clean room jazz, no extensive, and expensive, processing, implanting, nor semiconductor crystal growth environments. Just mix and paint. You can make 'em as small or as large as you want, or put them on flexible substrates. Serve and light.