Visible Imaging of Incoherent 1200-nm Light via Triplet--Triplet Annihilation Upconversion

Upconversion of low-energy photons to higher-energy photons provides an opportunity to surpass traditional limitations in fields such as 3D printing, photovoltaics, and photocatalysis. Triplet--triplet annihilation upconversion (TTA-UC) is particularly appealing for such applications as it can efficiently upconvert low-intensity, incoherent light. However, previously demonstrated thin-film TTA systems are simultaneously constrained by modest efficiencies and limited reach into the near infrared (NIR). Here, we design a single-layer thin-film bulk heterojunction that integrates PbS quantum dots (QDs) as tunable NIR absorbers within an organic semiconductor matrix of TES-ADT, achieving large anti-Stokes shifts up to 500 nm and internal quantum efficiencies that surpass previous reports across the NIR-I and NIR-II windows (800-1200 nm). UC performance is improved 15-fold through the incorporation of 5-tetracene carboxylic acid ligands on the PbS QD surface, which served to increase the yield of sensitized triplets, as confirmed by transient absorption and time-resolved photoluminescence measurements. We demonstrate visible imaging of incoherent 1200 nm light via thin-film TTA-UC at incident intensities at the imaging mask as low as 20 mWcm$^2$, marking a significant advance toward practical implementation of solid-state NIR-to-visible upconversion.