They are also very small and expensive. There is no reason one cannot use diode lasers (or LEDs?) at a wavelength suited to silicon solar cells. Likewise the fibers that carry the light from laser to solar cell. If one uses two solar cells facing one another in a V with fiber emitting into the groove, reflections off the solar cell faces won't matter, and the entire output of the fiber will be absorbed.
To carry power, step-index glass fiber is suitable. Here is one example, available in small quantity:
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Joe