We report on an experimental study of the intersubband optical response of an In0.5Ga0.5As/GaAs quantum dot solar cell (QDSC). By calculating the quantum dot absorption cross section, the strength of the intersubband optical transitions is gauged, and their importance and influence on the electrical properties of the solar cell can be compared with those of other physical processes such as thermal intersubband and optical interband transitions. The temperature-dependent photocurrent and dark current characteristics of the QDSC have also been analyzed in detail, leading to an understanding of the specific effects reducing the open-circuit voltage. The deviation of QDSCs from idealized models is discussed, and some conditions required for an improved open-circuit voltage are suggested. Copyright © 2012 John Wiley & Sons, Ltd.