Decades ago this technology was like a dream, but today there are good options at a reasonable price. Producing solar panels is day after day less expensive and with better results in efficiency. Companies as Sunpower present solar cells with about 22% efficiency, so multiple portable chargers are based in these cells.
Even with a bit less, the necessary space to recharge usual mobile devices like smartphones, tablets or even laptops or netbooks, is right now reasonable. So, it's possible to carry it with you in a backpack, a little bag, or in the own case of the device. Then, the user places the photovoltaic charger in the Sun and voila, free and clean energy comes to your device. If the cell and the electronics are efficient, and the Sun hits well, the recharging time could even be less than with a normal AC-DC charger.
Some portable chargers have the option to charge two devices at the same time, so it's perfect for scenarios with other people having similar needs. And remember, with a life per solar cell of decades, this investment is one of the best. In any place, any situation with available light, free energy!