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 convent ional solar cells by the single, fixed band-gap. In this paper we revi ew recent studies made by our group of the fundamental physics of quan tum wells in the forward bias, reverse current regime of p-i-n solar c ell 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 well s in a strained system. We also report on recent theoretical and exper imental work on the dark currents in the AlGaAs/GaAs system. We discus s a new experimental result that the open-circuit voltage is enhanced over that of a comparable conventional cell formed from the well mater ial, by more than the change in absorption edge. The theoretical studi es show that the addition of quantum wells increases the dark current but this increase is significantly less than expected from a simple mo del in which the quantum wells are in quasi-equilibrium with the barri er 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 domi nates.