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.
Study: https://news.unist.ac.kr/new-study-unveils-all-back-contact-neutral-colored-transparent-crystalline-silicon-solar-cells-enabling-seamless-modularization/
Even having a panel covering your backpack would be a good idea.