A material from the perovskite family of semiconductors emits light much more efficiently than expected. The explanation for this anomalous behaviour could lead to improvements in light-emitting technology.
When a semiconductor absorbs light, a particle-like entity called an exciton can be produced. Excitons comprise an electron and a hole (the absence of an electron), and have two possible states: singlet and triplet. Triplet states were thought to be poor emitters of light, but, Becker et al. report that semiconductors known as lead halide perovskites have bright triplet excitons. The results could signify a breakthrough in optoelectronics because triplet states are three times more abundant than singlet states and currently limit the efficiency of organic light-emitting diodes.
Conventional wisdom holds that triplet states are dark because of the spin selection rule, which forbids electrons from changing their intrinsic angular momentum (spin) during an optical transition — the process in which an atom or molecule switches from one energy state to another by emitting or absorbing light. The rule is taught in quantum-mechanics classes when atomic transitions are first introduced, and is so general that one might think that it is written in stone. Fortunately, there are loopholes that can be exploited.
Lead halide perovskites seem to dispose of all conventional wisdom in materials science. Like organic semiconductors, they are relatively easy to fabricate, and their bandgap (a property that determines their conductivity and optical properties) can be tuned by varying their composition. Yet, like thin-layer (epitaxial) inorganic semiconductors, they are highly crystalline and exhibit efficient charge transport. It is as if their properties were selected from a materials scientist’s wish list, combining the best aspects of organic molecules, nanocrystals and epitaxial inorganic semiconductors.