This has so many errors. Copper is a far better conductor than people. Set up a multimeter for resistance across your skin if you’re dubious, it’ll be in the kΩ per cm. Current will flow if a potential difference is present, regardless of whether there is a less resistive path available. Also the material in question is a metal oxide, not a metal. It’s brittle. So making it into a cable in the first place will be incredibly difficult and expensive. And even in their own paper they showed a limiting current of something like 400 mA, which is not suitable for high power applications.
All the energy from the sun becomes heat in the environment eventually.