The highest efficiency reported for infrared-to-visible photon upconversion at sub-solar excitation densities1 used PbS QDs (quantum dot) light absorbers with CdS shells. Despite these encouraging results, the mechanism of passivation is not very well understood. In this presentation, we demonstrate the importance of synthesis and shell composition in this hybrid photon upconversion system. Using oleylamine to install different shells of ZnS, SnS, CdS, InS, NiS, and BiS on PbS QDs, we find only the ZnS and CdS shells enhance the upconversion quantum yield (QY) to 0.28% and 0.13% for oleic acid capped ZnS-PbS and CdS-PbS core-shell QDs respectively. This is comparable to the previously reported upconversion QY of PbS-CdS core-shell QDs (0.20%)1. Interestingly, here, the enhancement in photon upconversion is accompanied by a decrease in the photoluminescence QY at much thinner shell thickness (less than 0.5 Å) compared to the 2.3 Å CdS shell thickness reported earlier. Time correlated single photon counting (TCPSC) measurements show that the primary reason for the enhancement of the upconversion QY is due to the decrease in non-radiative transition rates. The very thin nature of the shell allows 970 nm near infrared photons to be efficiently converted to visible photons. This is a 120 nm red shift compared to our previous reported. We believe our work provides a new insight on the surface chemistry modification of the QDs that can be applied in solar cells, bio imaging, cancer therapy, and sensors.