A circular film of glow-in-the-dark organic materials is shown in weak ambient light (top) and in the dark after exposure to ultraviolet light (bottom). Ultraviolet light was used to quickly accumulate energy and produce a strong glow, but the glow-in-the-dark effect can also be achieved by exposure with an ordinary white LED light. The film employs a mixture of donor and acceptor molecules to achieve this effect for the first time with organic materials.
The process starts when an acceptor absorbs incident light energy, leading to the transfer of a positive charge, or hole, from the electron acceptor to an electron donor (1). The additional negative charge, or electron, on the acceptor then separates from the hole by hopping among other acceptors (2). The energy is now stored across a spatially separated electron and hole (3). The electron eventually moves back toward the hole (4), and light is emitted when the two come together (5). Some charges recombine quickly, but many can remain stored in the charge separated state for a long time (3), which leads to the glowing emission long after the excitation light is turned off.