Radiative Limits in Quantum Well Solar Cells

Quantum wells enable solar cell bandgaps to be tailored to the theoretical optimum, under the assumption of unity absorption. In practice however, quantum well regions are thinner and hence absorb less strongly than bulk material. We predict detailed balance efficiency limits for varying levels of absorptivity in singleand double-junction quantum-well solar cells. These calculations allow us to find the absorption levels required for quantum wells to increase cell performance. It is found that the ideal band edge under concentration for a conventional single-junction cell for all bulk absorptivities lies near 1100nm, whilst for quantum well solar cells it is pinned at approximately 925nm for quantum well absorptivities up to ~90%. In the case of tandem cells our model indicates that bottom cell QWs should be advantageous for low absorptivities whilst additional top cell QWs improve performance for absorptivities above 50%. The effect of increased bottom cell photocurrent due to radiative losses in the top cell is also quantified.