Batteries did power Galileo's atmospheric probe, but it only had to run its instruments and transmitter for a little more than an hour-an appropriate task for batteries. As the name suggests, photovoltaic materials have the wonderful, seemingly magic ability to convert light to electricity. You might have a desktop calculator that runs on ambient light by using a photovoltaic cell. Crystalline silicon and crystalline gallium arsenide are two typical choices of photovoltaic materials for deep space applications. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are mass-produced for the microelectronics industry and are less expensive.
Gallium arsenide is notably tougher and more efficient.
Connect one wire to each metal surface, and you have an electrical power source-a photovoltaic cell. These cells are trimmed into appropriate shapes and sizes and cemented onto a substrate. Electrical connections are then made among all the cells to provide an output suitable to power a spacecraft. These assemblies are called solar arrays. In flight, the cells absorb sunlight and get quite warm, so a solar array's cement and substrate have to be thermally conductive to help dissipate the heat.