Efficient solid-state photon upconversion enabled by triplet formation at an organic semiconductor interface

The energy of photons, that is, the wavelength of light, can be upgraded through interactions with materials in a process called photon upconversion1. Although upconversion in organic solids is important for various applications, such as photovoltaics and bioimaging, conventional upconversion systems, based on intersystem crossing (ISC), suffer from low efficiency2–6. Here we report a novel upconversion system with heterojunctions of organic semiconductors. The upconversion occurs through charge separation and recombination, which mediate charge transfer states at the interface. This process can efficiently convert the incident photons to triplets without relying on ISC, which is typically facilitated by the heavy-atom effect1. As a result, a solid-state upconversion system is achieved with an external efficiency that is two orders of magnitude higher than those demonstrated by conventional systems6. Using this result, efficient upconversion, from near-infrared to visible light, can be realized on flexible organic thin films under a weak light-emitting-diode-induced excitation, observable by naked eyes. Highly efficient upconversion of light by organic semiconductor heterojunction interfaces is demonstrated. This process is enabled by charge separation- and recombination-mediated charge transfer states at the interface.