Theoretical and experimental studies since the 1980s have pointed to the existence of organic molecules that violate Hund's rule of maximum multiplicity, with the lowest singlet excited state having lower energy than the lowest triplet excited state. With the rising prevalence of organic light-emitting diodes (OLEDs) in display technology, these types of molecules are being investigated as a new class of organic emitters. The singlet–triplet inversion implies that thermal activation is not necessary to achieve fast triplet harvesting, providing potential benefits over conventional thermally activated delayed fluorescence emitters. Here, we overview prominent studies regarding inverted singlet and triplet excited states in the context of OLEDs.

This review summarizes recent prominent examples of substituent engineering on small-molecule organic semiconductors for photovoltaic applications, focusing on flexible substituents that regulate the active-layer morphology.

A solid/ionic gel dual-gate transistor is demonstrated using a bilayer single-crystal organic semiconductor. Hole mobilities in the bottom layer are accelerated by simultaneous charge-carrier modulation by electrolyte-gating to the top layer.