The use case seems quite different, but my solar watch uses a (super?)capacitor to buffer the energy delivered by the solar cells. When fully charged, it can r

The use case seems quite different, but my solar watch uses a (super?)capacitor to buffer the energy delivered by the solar cells. When fully charged, it can run for more than a year in the dark (i.e, you can go beyond the polar circle without worrying that your watch will go belly-up in the middle of the polar night). Which is to say, existing capacitors are already darned good at retaining charge. I'd assume the tech described in the article has charge retention “good enough” for the intended use case.