Photovoltaic cells convert light energy into electricity at the atomic level. Although first discovered in 1839, the process of producing electric current in a solid material with the aid of sunlight wasn't truly understood for more than a hundred years. Throughout the second half of the 20th century, the science has been refined and the process has been more fully explained. As a result, the cost of these devices has put them into the mainstream of modern energy producers. This was caused in part by advances in the technology, where PV conversion efficiencies have improved considerably.