Bookeen and solar innovator SunPartner are developing an e-reader with a built-in solar panel. SunPartner's technology (called Wysips - What You See is Photovoltaic Surface) enables a transparent solar panel to be embedded in the display. This means that the device will charge when light is present - even while reading and you do not have to turn it over.
Most mobile devices use too much power to be actually solar charged, but with an e-reader this may work very well. Bookeen aims to release the solar-powered e-reader in 2016.