[mchannon]

Concentrator cells make a number of tradeoffs in order to make their performance as high as possible (and keep from melting, etc.). I would guess around 22% efficient in one-sun AM1.0; that's about what other concentrator cells in this upper tier do.

If it's slightly cloudy, I could see efficiencies dropping to the single digits, as each of these cells is actually four different cells all wired in series, each sensitive to different wavelengths of light. If the blue drops out, the corresponding cell on the top of the stack acts as a resistor (and it can even burn out, being so thin).

Solar cell efficiency records (particularly for concentrator applications) are like discovering new transuranic elements- extremely useful and interesting at first, then significantly diminishing returns.