Solar panels on your roof won't do much good if you're off grid and need to charge your phone. Just get a small, folding solar panel that you can carry in your backpack. A 10W panel will only need a couple hours of direct sunlight to fully charge a phone.

There was a very simple phone from Samsung a few years back that had a solar cell on the back.

Since the battery lasted over a week anyway, you could easily double the battery life by just having it in indirect light.

Modern phones are guzzling so much power that it's hardly useful there.

yeah I was thinking if this was so useful we would have panels on the back of the phones already.

Great points.

Regular solar cells with better efficiency are already are thing, even in a compact travel format or as a novelty part of some electric cars. Those are cheap to produce, but still aren't practical at all, unless we're talking about something like a 2m² solar panel to charge a phone in a somewhat reasonable time on a very sunny day in an off-grid situation.

Using transparent solar cells additionally to regular ones in buildings instead of windows is pretty much the only reasonable application I can think of right now, but with a visible transmittance of 20% that's kinda farfetched as well.

You could look at it another way- a solar power screen on a phone won't charge it all the way, but as a secondary source of power that is essentially always on when the sun is up or other light sources are on, it reduces the battery drain over the day, meaning less charging needed at night.