Why GFP? For one, it's inexpensive. It doesn't require expensive additives or costly processing, but can go directly onto the substrate where it starts cranking out juice. Further, it can be integrated into a self-contained fuel cell that requires no outside light source. Photons would instead be generated within the fuel cell by enzymes like the ones found in natural light-producers, like fireflies or sea pansies. Such a power source could be miniaturized to power tiny nano-devices.