Quantum dots (QDs) are very attractive materials for solar cells due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Particularly interesting are PbS QDs owing to their broad spectral absorption until long wavelengths, their easy processability and low cost. Here, we used control of the PbS QD size to understand charge transfer processes at the interfaces of a NiO semiconductor and explain the optimal QD size in photovoltaic devices. Towards this goal, we have synthesized a series of PbS QDs with different diameters (2.8 nm to 4 nm) and investigated charge transfer dynamics by time resolved spectroscopy and their ability to act as sensitizers in nanocrystalline NiO based solar cells using the cobalt tris(4,4'-ditert-butyl-2,2'-bipyridine) complex as a redox mediator. We found that PbS QDs with an average diameter of 3.0 nm show the highest performance in terms of efficient charge transfer and light harvesting efficiency. Our study showed that hole injection from the PbS QDs to the NiO valence band (VB) is an efficient process even with low injection driving force (-0.3 eV) and occurs in 6-10 ns. Furthermore we found that direct electrolyte reduction (photoinduced electron transfer to the cobalt redox mediator) also occurs in parallel to the hole injection with a rate constant of similar magnitude (10-20 ns). In spite of its large driving force, the rate constant of the oxidative quenching of PbS by Co(iii) diminishes more steeply than hole injection on NiO when the diameter of PbS increases. This is understood as the consequence of increasing the trap states that limit electron shift. We believe that our detailed findings will advance the future design of QD sensitized photocathodes.