This section will cover p-n junction diodes, which are essential to solar cell operation. p-n junction diodes are simple semiconductor devices that permit the flow of current in one direction only; p-n junction diodes do this by using strategically placed atomic impurities to create an electric field within the semiconducting material. These diodes are important in photovoltaic physics because they separate negatively-charged electrons and positively-charged holes into an orderly direct current. The strong electric field also quickly pulls these carrier pairs (electrons and holes) apart after their generation, thereby limiting their ability to recombine. Without a p-n junction diode, electrons and holes move in random directions, no coherent current is generated, and solar cells become useless.