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.
Carrier escape from InP/AlGaAs single quantum well structures is studied by means of simultaneous steady state photocurrent and photoluminescence measurements. The activation energy for escape is measured for the first time in this system. The photoluminescence from the InGaAs wells indicates that a significant number of carriers do not escape at room temperature thus affecting the temperature dependence of the cell. An estimate of the nonradiative efficiency of the device studied is given as a function of bias and temperature. The relevance to new applications is discussed.
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