Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic applications. Currently, device efficiencies are limited by the high density of trap states caused by lattice imperfections on the QD surface. Introducing a thin shell of wide bandgap semiconductor to the QD surface is a promising method to passivate these trap states. Here we demonstrate solar cells made from PbS-CdS core-shell QDs, yielding a 147 mV increase in VOC compared to core only PbS QDs. We explore the physical reason for this enhancement and demonstrate that it is indeed caused by improved passivation of the PbS surface by the CdS shell, leading to a lower electron trap density.