A research team from the Ulsan National Institute of Science and Technology (UNIST) in South Korea has developed transparent solar cell technology capable of directly charging a battery from a glass surface. This innovation offers numerous applications, allowing for direct energy generation from sources like smartphone screens, car windows, and building facades.
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.