This paper reports on significantly improved efficiency of InAs/GaAs quantum dot (QD) solar cells by directly doping Si into InAs QDs during the QD growth. The devices which contain five stacked QDs in their i-regions were grown using molecular beam epitaxy. It is shown that using appropriate Si-doing, the open-circuit voltage of the device can be increased to 0.84 V. This is dramatically higher than the value of 0.67 V obtained in undoped device using the same structure. Moreover, the efficiency of corresponding device is improved from 11.3% to 17.0%. This improvement in efficiency is attributed to greatly reduced energy loss in the devices that results from the reduction of the defect density in the stacked InAs/GaAs QD layers due to the doping. (C) 2013 Elsevier B.V. All rights reserved.