My guess is that the process is fairly slow, and doesn’t produce great volumes of ethanol. If the voltage requirement isn’t too high, it might be viable to do this using a two-layer solar panel: the top layer is the photo cells producing the voltage, feeding a layer just underneath of the carbon-copper catalyst (maximize surface area) that runs more or less continually producing a small but steady stream of ethanol. Get a field of these going and you might have something.
I doubt any process we invent is ever going to be more efficient than just letting plants convert the sunlight into sugar and then letting yeast convert the sugars into ethanol.