Photonic Coupling in Multi-Junction Quantum Well Concentrator Cells

Quantum well solar cells have a number of advantages over metamorphic multi-junction cells for high concentration applications. This paper discusses the advantages of a novel variant: the dual-quantum well solar cell which has a multi-quantum well system in both the top and middle sub-cells of a triple-junction cell. In such a device the band-gaps of the top and middle cells can be independently optimised for high efficiency and maximum energy harvesting in a given spectrum. We present simulations and experimental results on a further unique feature, which results from the dominance of radiative recombination in quantum well cells at high concentration, namely the efficiency enhancement which results from photon coupling from a quantum well top cell to the middle cell in a triple junction device. We demonstrate that this radiative recycling between sub-cells leads to a reduced sensitivity of the cell efficiency to spectral and temperature variations. We also present new experimental data on top and deep well middle sub-cells and demonstrate that the both sub-cells have high radiative efficiency.