(Invited) Photon Upconversion through Dynamic Exciton at Organic Semiconductor Interface

Abstract

Energy of photons, i.e. the wavelength of light, can be upgraded through interactions with materials—a process called photon upconversion (UC). Although UC in organic solids is important for various applications, such as in photovoltaics and bioimaging, conventional UC systems, based on intersystem crossing (ISC), suffer from low efficiency. Herein, we report a novel UC mechanism at heterojunctions of organic semiconductors in bilayer structures. The UC occurs by triplet formation during the charge separation and recombination through dynamic exciton: charge transfer (CT) state at the interface. This can efficiently convert the incident photons to triplets without relying on the ISC, whose rate is typically accelerated by the heavy-atom effect. As a result, a solid-state UC system is achieved with an external efficiency of two orders of magnitude higher than those of the conventional systems. Using this result, efficient UC, from near-infrared to visible light, cab be realized on flexible organic thin films under a weak light-emitting diode-induced excitation, observable by naked eyes.Reference: S. Izawa and M. Hiramoto, Nat. Photon., DOI: org/10.1038/s41566-021-00904-w.