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A sub‐monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor,5‐CT. While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the5‐CTtriplet (with rates of 5.91±0.60 ns⁻¹and 1.03±0.09 ns⁻¹respectively), ultrafast hole transfer occurs only from PbS QDs to5‐CT. Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the5‐CTtriplet and thus enhances TET from5‐CTto the rubrene emitter, further bolstering the upconverison QY.

A sub‐monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor,5‐CT. While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the5‐CTtriplet (with rates of 5.91±0.60 ns⁻¹and 1.03±0.09 ns⁻¹respectively), ultrafast hole transfer occurs only from PbS QDs to5‐CT. Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the5‐CTtriplet and thus enhances TET from5‐CTto the rubrene emitter, further bolstering the upconverison QY.