In the case of a typical solar cell, when sunlight hits its surface, the small particles that make up light, called photons, knock off electrons from the solar cell’s surface material. These electrons flow in a particular direction (Sidebar below). This creates an electric current, which can be used to power machines and devices.
As we all know, smartphones, iPods, and other 21st century technologies often need to be recharged. To run these devices, a solar-powered battery charger could be a better option. This type of battery charger contains solar cells that charge an internal battery, so the charger can sit idly and collect solar energy all day long. When connected via USB, the charger transfers the stored energy from its own battery into a smartphone or iPod battery—no wall socket required!
Some solar battery chargers have been adapted into other products, including a solar backpack. It's an ordinary cloth backpack fitted with thin solar cells that connect to a built-in battery, which can charge external devices, such as smartphones, MP3 players, and even computers. Solar backpacks are useful for hikers, rescue crews, and military personnel who spend long days in the sun without access to electricity.