QUANTUM-WELL SOLAR-CELLS

Quantum well solar cells are an alternative to tandem or multiband-gap approaches to the problem of the efficiency limits imposed on conventional solar cells by the single, fixed band-gap. In this paper we review recent studies made by our group of the fundamental physics of quantum wells in the forward bias, reverse current regime of p-i-n solar cell operation and the optimisation of the design of quantum well solar cells for higher efficiency. We discuss the first results obtained by adding InGaAs wells to a high efficiency GaAs conventional solar cell. Here the main problem is the incorporation of a large number of wells in a strained system. We also report on recent theoretical and experimental work on the dark currents in the AlGaAs/GaAs system. We discuss a new experimental result that the open-circuit voltage is enhanced over that of a comparable conventional cell formed from the well material, by more than the change in absorption edge. The theoretical studies show that the addition of quantum wells increases the dark current but this increase is significantly less than expected from a simple model in which the quantum wells are in quasi-equilibrium with the barrier material. We discuss the significance of these results in the light of recent theoretical predictions about the behaviour of quantum well solar cells in the ideal situation where radiative recombination dominates.