The article is really funny, because they talk about how this company's innovation could be used in pacemakers. When they had betavoltaic pacemakers in the 1970s. https://en.wikipedia.org/wiki/Betavoltaic_device
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.
Here's the summary for the wikipedia article you mentioned in your comment:
A betavoltaic device (betavoltaic cell or betavoltaic battery) is a type of nuclear battery which generates electric current from beta particles (electrons) emitted from a radioactive source, using semiconductor junctions. A common source used is the hydrogen isotope tritium. Unlike most nuclear power sources which use nuclear radiation to generate heat which then is used to generate electricity, betavoltaic devices use a non-thermal conversion process, converting the electron-hole pairs produced by the ionization trail of beta particles traversing a semiconductor.Betavoltaic power sources (and the related technology of alphavoltaic power sources) are particularly well-suited to low-power electrical applications where long life of the energy source is needed, such as implantable medical devices or military and space applications.