Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals.

Photon upconversion (UC) based on sensitized triplet-triplet annihilation (TTA), TTA-UC, can potentially alleviate the transmission loss of below-band-gap photons in solar energy conversion. TTA-UC across various spectral windows has been demonstrated, but efficient visible-to-ultraviolet (UV) UC remains a big challenge primarily due to the lack of suitable triplet sensitizers. Here we report a TTA-UC system sensitized by quantum-confined CsPbBr3 perovskite nanocrystals (NCs) that simultaneously achieves a high photon energy gain of up to 0.7 eV (443-355 nm) and a high UC efficiency up to 10.2%. Time-resolved spectroscopy studies reveal that the performance is mainly enabled by ultrafast and efficient triplet energy transfer from the strongly confined NC sensitizers to triplet acceptors.