Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems. Ongoing detailed mechanistic studies of the TET1 process shows that both QD bright and dark exciton states contribute to the TET process and higher lying bridge states play significant role in enhancing TET from core/shell QDs.