  Low power photon upconversion (UC) based on sensitized triplet-triplet
annihilation (sTTA) is considered as the most promising upwards
wavelength-shifting technique to enhance the light harvesting capability of
solar devices by recovering the low-energy tail of the solar spectrum.
Semiconductor nanocrystals (NCs) with conjugated organic ligands have been
proposed as broadband sensitizers for extending the light-harvesting capability
of sTTA-UC molecular absorbers. Key to their functioning is efficient energy
transfer (ET) from the NC to the triplet state of the ligands that sensitizes
the triplet state of free emitters, whose annihilation generates the
upconverted emission. To date, the triplet sensitization efficiency in such
systems is limited by parasitic processes, such as charge transfer (typically
of the photohole) to the organic ligand due to the disadvantageous band
alignment of typical NCs and organic moieties. Available strategies only
partially mitigate such losses and intrinsically limit the ET yield. Here we
demonstrate a new exciton-manipulation approach that enables loss-free ET
without detrimental side-effects. Specifically, we use CdSe NCs doped with gold
atoms featuring a hole-accepting state in the NC bandgap at a higher energy
than the HOMO level of the ligand 9-anthracene acid. Upon photoexcitation, the
NC photoholes are routed to the Au-state faster than their transfer to the
ligand, producing a long-lived bound exciton in perfect resonance with its
triplet state. This hinders hole-transfer losses and results in ~100% efficient
ET, over 50-fold higher than in standard NCs. By combining our hybrid
sensitizers with an annihilator moiety, we achieved an sTTA-UC efficiency of
~12% (~24% in the normalized definition), which is the highest value for hybrid
upconverters based on sTTA reported to date and approaches optimized organic
systems.
