George Cove seems to have stumbled into a workable PV panel circa 1905. Sure, the efficiency stinks compared with modern stuff – about 5% vs. modern SOTA around 20% – but if you can crank these out for an order of magnitude lower price using a simple, relatively environmentally benign manufacturing process, who cares? What's more, we now have a decent theoretical understanding of how this fortuitous accident works, and so can probably tweak it to much higher efficiencies.
Well I'm no solid state physicist, so I'll just refer you to the article at Low<––Tech Magazine. Also of note, that site runs off a used laptop and is powered by a tiny panel and a none-too-new battery, so you might have to wait until the Sun comes back up in Spain. Yes, they practice what they preach at Low<––Tech.