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NanoPower Research Labs

Nanostructured III-V Photovoltaics

Quantum Dot Space Solar Cells

Improving the efficiency of solar cells is the main focus of the space photovoltaic community. Design enhancements at the cell and module levels have steadily increased the efficiency over the years. Current state-of-the-art triple-junction space cells exceed 30% efficiency. The ultimate efficiency of high-efficiency cells is constrained by several factors. The available bandgaps for each junction is restricted by the lattice-matching requirement for high quality material. The current use of InGaP/GaAs/Ge, while enabling >30% efficiency, is not the most optimum bandgap combination for highest efficiency.

An additional limitation is the series nature of the multiple junctions with the high-efficiency solar cell. Each cell must be constructed to be current-matched with the others in series. This reduces currents and increases device complexity. The current-limiting junction in a conventional triple-junction cell is the GaAs middle cell. Theoretical calculations have shown that the overall efficiency of the triple-junction stack can be markedly improved by extending the spectral bandwidth of the cell and thus increasing the current. The lattice-matching requirement, however, preclude a straightforward extension of the composition of the middle cell without resorting to buffer layer methodologies.

Current enhancements can be obtained via an extended absorption spectrum of low bandgap, low-dimensional materials. In this approach, quantum-size features with lower bandgap within the current limiting junction absorb extra photons that would otherwise be transmitted. This approach mitigates the lattice-matching constraint by using nanoscale heterostructures. Modeling of an InGaP/GaAs/Ge triple junction cell, in which QDs are used to extend the middle junction absorption spectrum, have indicated that the efficiency could be improved to 47% under one sun AM0.

(a) Lattice-matched triple junction solar cell design and (b) theoretical efficiency contours based on the middle and top cell bandgaps.
Recent Publications:
R.P. Raffaelle, S. Sinharoy, J. Andersen, D.M. Wilt, S.G. Bailey, “Multi-Junction Solar Cell Spectral Tuning with Quantum Dots”, Proc. of the IEEE World Conference on Photovoltaic Energy Conversion, 1, 162 (2006).

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