The effects of quantum dot (QD) size on the optical and electrical properties of InAs/GaAs QD solar cells (QDSCs) were investigated. QDSCs with varying InAs QD size were fabricated by controlling the total InAs deposition thickness (0) from 0 to 3.0 mono-layers (ML). The optical and electrical properties of the QDSCs were investigated using photoluminescence (PL), time-resolved PL (TRPL), photoreflectance (PR) spectroscopy, capacitance-voltage (C-V), and current-voltage (J-V) measurements. The QD size effects on the p-n junction electric fields (F-pn) and the efficiencies (eta) of the QDSCs were revealed. The QDSCs had a maximum of 21.17% for theta=2.0 ML (the efficiency is enhanced by 17.4% over the reference GaAs-SC) and minimized F-pn (113 kV/cm) by an enhanced photovoltaic effect caused by improved carrier generation. We find that these optimal properties result from a balance between carrier generation and exhaustion processes through trapping and re-capturing by defects and relatively large QDs. (C) 2016 Elsevier B.V. All rights reserved.