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Low power photon upconversion (UC) based on sensitized triplet-tripletannihilation (sTTA) is considered as the most promising upwardswavelength-shifting technique to enhance the light harvesting capability ofsolar devices by recovering the low-energy tail of the solar spectrum.Semiconductor nanocrystals (NCs) with conjugated organic ligands have beenproposed as broadband sensitizers for extending the light-harvesting capabilityof sTTA-UC molecular absorbers. Key to their functioning is efficient energytransfer (ET) from the NC to the triplet state of the ligands that sensitizesthe triplet state of free emitters, whose annihilation generates theupconverted emission. To date, the triplet sensitization efficiency in suchsystems is limited by parasitic processes, such as charge transfer (typicallyof the photohole) to the organic ligand due to the disadvantageous bandalignment of typical NCs and organic moieties. Available strategies onlypartially mitigate such losses and intrinsically limit the ET yield. Here wedemonstrate a new exciton-manipulation approach that enables loss-free ETwithout detrimental side-effects. Specifically, we use CdSe NCs doped with goldatoms featuring a hole-accepting state in the NC bandgap at a higher energythan the HOMO level of the ligand 9-anthracene acid. Upon photoexcitation, theNC photoholes are routed to the Au-state faster than their transfer to theligand, producing a long-lived bound exciton in perfect resonance with itstriplet state. This hinders hole-transfer losses and results in ~100% efficientET, over 50-fold higher than in standard NCs. By combining our hybridsensitizers with an annihilator moiety, we achieved an sTTA-UC efficiency of~12% (~24% in the normalized definition), which is the highest value for hybridupconverters based on sTTA reported to date and approaches optimized organicsystems.

High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole-Routing in Electronic-Doped Nanocrystals