Dark currents in double-heterostructure and quantum-well solar cells

Abstract

Numerical modelling shows that the separation of the quasi-Fermi potentials in the lower bandgap region of a double-heterostructure may be less than the terminal voltage, resulting in smaller dark currents than would be expected if flat quasi-Fermi levels were assumed. Quasi-Fermi level variations occur as a response to carrier transport limitation by drift and diffusion within the space-charge region or by thermionic emission. This is a possible explanation for the low dark currents which have been measured in quantum-well p-i-n solar cells. This effect, together with evidence that photogenerated carriers can escape from quantum wells with high efficiency, suggests that the inclusion of low-bandgap regions in the depletion regions of solar cells may lead to high efficiency devices.