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AlGaAs/GaAs heteroface solar concentrator cells which exhibit 26% efficiency at 753 sun (AM1.5, 100 mW/cm2) have been fabricated using metalorganic chemical vapor deposition. Magnesium was used as the dopant in the p-type emitter and selenium was used as the n-type dopant. The design parameters of the solar cell were determined with the aid of a computer program which realistically models the performance of such cells.

High-efficiency Ga0.75In0.25As/GaAs concentrator solar cells (Eg=1.15 eV, area=0.32 cm2) have been fabricated and tested at 21.4% efficiency under 380 AM2.4 sun. The cell performance parameters of open-circuit voltage, fill factor, and efficiency are presented as functions of concentration up to 987 sun. The spectral response, and current-voltage characteristics under concentration, of cells grown by organometallic vapor phase epitaxy on graded and ungraded lattice-constant buffer layers are compared. It is shown that a graded lattice-constant buffer layer is necessary for high-efficiency performance. In general, Ga0.75In0.25As cells have higher efficiencies and open circuit voltages than Si cells (Eg=1.11 eV) for concentrations in excess of about 200 sun.

A 27.6% efficiency measured under 1 sun, air mass 1.5 illumination has been achieved in a two-terminal, monolithic two-junction cascade solar cell consisting of an Al0.37Ga0.63As (Eg=1.93 eV) upper cell and a GaAs lower cell. The component cells were electrically connected together using a metal-interconnect contact fabricated during post-growth processing. Also, a prismatic cover glass was bonded to the front surface of the cascade structure to minimize the obscuration effect introduced by the grid lines and metal-interconnect contacts. As the cascade cell was operated under air mass 0 illumination, an efficiency of 23.0% was obtained. Both results represent the highest 1 sun efficiencies ever reported. The implications of achieving this high efficiency in a two-junction cascade solar cell for terrestrial and space applications are also described.

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