Triplet sensitization via charge recombination at organic heterojunction for efficient near-infrared to visible solid-state photon upconversion

A near-infrared to visible photon upconversion device was previously shown to increase external quantum efficiency by up to 2.3%. Here, the upconversion mechanism is studied by time-resolved spectroscopy, revealing that 67% of incident photons are utilized. Realizing efficient near-infrared to visible photon upconversion in the solid state is pivotal for commercial applications in various fields. We previously reported a solid-state upconversion device which imitated the photovoltaic conversion mechanisms of organic solar cells. This leads to a significant improvement of up to 2.3% in the external quantum efficiency, which is two orders of magnitude higher than that of conventional devices. Here, we investigate the upconversion mechanism of this device. We examine exciton and charge dynamics using transient absorption spectroscopy and find that approximately 67% of incident photons are utilized owing to fast singlet exciton diffusion in the nonfullerene acceptor layer. Strikingly, triplet excitons are accumulated near the donor/acceptor interface, enabling accelerated triplet-triplet annihilation by a factor of more than 10.