We present an experimental characterization study of high-efficiency ultra-small multi-junction concentrator solar cells. Particular emphasis is placed on the sensitivity of photovoltaic performance to the intensity and distribution of the concentrated sunlight delivered to the cell. This type of information is important in the design and optimization of the most recent generations of high-concentration photovoltaic systems. Miniaturizing the solar cell can give rise to an increase in the concentration value at which cell efficiency peaks, facilitates passive heat rejection and permits the use of all-glass optics. However, few measurements have been published for ultra-small cells. We report and analyze extensive measurements, up to approximately 5000 suns, on the 1.0 mm<sup>2</sup> active region within the busbars of such commercial concentrator solar cells.
Essentially loss-less all-dielectric micro-fabricated optics can be tailored to completely eliminate the shadowing losses metallization grids create on the surface of concentrator solar cells. The nonimaging micro-concentrator exploits total internal reflection to redistribute the elevated flux from available macro-concentrators, rather than increasing overall concentration. The optical designs permit widening the metal fingers toward lessening series resistance losses, which can also finesse the need for the intricate metallization patterns of some high-flux cells. Realistic net efficiency gains of ~15% (relative) are achievable in a wide variety of concentrator cells.