  White organic light-emitting diodes (OLEDs) are ultra-thin, large-area light
sources made from organic semiconductor materials. Over the last decades, much
research has been spent on finding the suitable materials to realize highly
efficient monochrome and white OLEDs. With their high efficiency,
color-tunability, and color-quality, white OLEDs are emerging to become one of
the next generation light sources. In this review, we discuss the physics of a
variety of device concepts that are introduced to realize white OLEDs based on
both polymer and small molecule organic materi als. Owing to the fact that
about 80 % of the internally generated photons are trapped within the thin-film
layer structure, we put a second focus on reviewing promising concepts for
improved light outcoupling.
