Midgap States in PbS Quantum Dots Induced by Cd and Zn Enhance Photon Upconversion

Exciton exchange between semiconductor quantum dots (QDs) and organic chromophores may find application in next-generation photovoltaics, probes, or photocatalysts. We study the effect of shell composition and thickness on triplet energy transfer (TET) from PbS QDs to rubrene in a hybrid organic–inorganic photon upconverison system and show that defect states introduced by surface adsorbed Zn and Cd result in up to 700 and 325 fold enhancements in the photon upconverison quantum yield (QY). Time-correlated single-photon counting, photoluminescence (PL), and photon upconversion QY measurements on PbS QD light absorbers with submonolayer shells of 0.37–1.60 Å of CdS and 0.04–0.22 Å of ZnS reveal these core–shell QDs have reduced radiative and nonradiative rates relative to the core. Their broadened absorption and PL line widths suggest TET may occur from thermally accessible surface defects, in addition to the dark excitonic states at the band-edge.