Lead sulfide quantum dots (PbS QDs) have been a topic of intense study for over a decade due to their excellent optoelectronic properties and their large versatility in such applications as infrared sensors,1–4 infrared photon sources,5 transistors,6–8 and solar cells.9–12 Their versatility stems mainly from the size. We have fabricated highly efficient PBS QD solar cells and explored their temperature dependent properties. The VOC is found to be governed solely by the reverse saturation current, which can be explained using the PN. M. J. Speirs and M. A. Loi acknowledge the financial support of the Alumnikring Den Haag/Rotterdam through the Ubbo Emmius Fund of the University of Groningen. M. A. L. acknowledges also the support of the ERC.