Sandwiching the solar cells inside the device would block some light out and knock down power conversion efficiency even further, Filler cautioned.

FORBES: Magazine Article

With this combination, 4% power-conversion efficiency for solution-processed and visibly transparent polymer solar cells has been achieved.

ENGADGET: UCLA creates transparent solar cell, dreams of current generating windows

If, on the one hand, you've got biofuels which represent only a half a percent conversion of sunlight, and let's say you've got photovoltaic cells and concentrated solar power systems, the efficiency of solar in terms of pure conversion is 50-100 times greater.

CNN: Biofuels: What do the experts think?