Here we report on heterojunction PbS quantum dot (QD) solar cells using RF magnetron sputtered CdS as the n-type window layer. These solar cells generate large open circuit voltage compared to previously reported PbS-QD solar cells. Our investigations of this device design show an optimized CdS film thickness of 70 nm and an optimized PbS QD diameter of similar to 2.7 nm, corresponding to a bandgap energy of similar to 1.57 eV. Under simulated AM 1.5 G illumination, we attain short circuit current as high as 12 mA-cm(-2), an open circuit voltage of 0.65 V and efficiency as high as 3.3%. (C) 2013 Elsevier B.V. All rights reserved.