Voltage enhancement in quantum well solar cells

Abstract

It is known that quantum well solar cells (QWSCs) can enhance short circuit current and power conversion efficiency in comparison with similar, conventional solar cells made from the quantum well (QW) barrier material alone. In this article we report measurements of the dark-current and open-circuit voltage (Voc) of a number of quantum well cells in three different lattice-matched material systems, namely, Al0.35Ga0.65As/GaAs, GaInP/GaAs, and InP/InGaAs. We also present the results obtained from comparable control cells without wells formed either from the material of the barriers or the well material alone. Our results clearly demonstrate in all three cases that, at fixed voltage, QWSC dark currents are systematically lower than would be expected from control cells with the same effective absorption edge. Measurements of Voc in a white-light source show that the open-circuit voltages of the QWSCs are higher than those of control cells formed from the well material. Furthermore, this enhancement is more than is expected from the shift in the absorption edge due to the effect of confinement in the wells. We discuss these results in the light of recent theoretical speculation about the upper limit to the efficiency of an ideal quantum well solar cell. We report on a 50 well QWSC with open-circuit voltage higher than the world record conventional cell formed from the well material, namely, GaAs.