and photodynamic therapy using near-infrared (NIR) light, which offers the advantage of high penetration in living tissues. Among the numerous UC mechanisms, triplet-triplet annihilation (TTA) has attracted substantial attention because it can be realized by non-coherent photoexcitation such as sunlight. The common steps of TTA UC are as follows: Firstly, a low-energy photon is absorbed and a triplet exciton is formed by intersystem crossing inside a sensitizer molecule. Subsequently, triplet energy is transferred from the sensitizer to an emitter molecule, and the TTA forms one singlet exciton with high energy. Finally, UC emission occurs in the emitter molecule. TTA UC has been studied intensively to date; however, the greatest challenge remaining is that solid-state UC, which is necessary for real device applications, is inefficient, with an external quantum efficiency (EQE) of less than 0.1%. Moreover, other problems such as the necessity of strong laser excitation and the use of minor metals, rare-earth or toxic elements must be overcome simultaneously. In this study, we realized efficient solid-state UC with 100 times higher EQE than a conventional system by discovering a novel phenomenon in which UC could be observed in bilayer organic semiconductor heterojunctions. The UC occurred through triplet formation from photogenerated freecharge recombination (Figure ). The key to this mechanism was spin inversion from a singlet to a triplet during the charge separation and recombination at the interface. As a result of this novel mechanism, efficient UC from near-infrared to visible light on flexible thin films without minor metals, rare-earth or toxic elements by LED excitation was made possible.