Abstract
Quantum well cells (QWCs) are p–i–n photovoltaic devices with multiple quantum wells (MQWs) inserted in the intrinsic region. They show the desirable characteristic for thermophotovoltaics (TPV) of an easily tuneable bandgap. This is highly relevant because the bandgaps of homostructures are restricted by the range of lattice matched substrates available. As a consequence bulk structures cannot be tuned to desirable narrow band emission spectra. QWCs, however, display tuneable bandgaps from several points of view. Firstly, the MQW bandedge is inherently tuneable by changing well depth and dimension. More significantly, strain compensated MQW systems allow lattice mismatched layers to be deposited without the formation of dislocations. This permits a much greater range of compositions and hence bandgaps than are obtainable with lattice matched systems. We first review the fundamental understanding of spectral response and dark current of the QWC and describe the initial proposal of QWCs for TPV before examining subsequent work on lattice matched InGaAsP/InP. Design limitations of the lattice matched materials can be eliminated by applying strain compensation techniques to strained materials. We show dark current measurements for lattice matched and strained systems with dark currents lower than other promising TPV designs despite longer wavelength absorption edges. This work has recently produced a QWC capable of reaching the Holmia narrow band emission peak at wavelength 1950 nm which shows the promise of this approach for TPV.