You’ve touched on a great point. The power provided is so low that solar can effectively provide equivalent power in nearly every application except one where the continuous operating environment is pitch black. 15x15mm for 0.0001w is small. For comparison, that’s about 1/6 of the power that falls on a 15x15mm patch in an indoor office (300lux environment with led lighting), out about the same as could be harvested by an efficient solar panel off the same size. You could collect a full days power from this battery (and store it in a 2mm thick li cell behind the panel) in roughly three minutes of sunshine or ten to fifteen minutes on an overcast day.

There certainly are applications where it would be useful, but most could just as easily be served by a small solar patch and lithium cell or super capacitor.