Electroluminescence is the emission of light by a solid when an electric field is imposed. This effect is the basis for building light-emitting diodes (LED). If organic compounds are used as the electroluminescent materials, these are referred to as organic light-emitting diodes (OLED). Electroluminescence (EL) of an organic semiconductor was first reported by Pope et al., in 1963. They observed emission from single crystals of anthracene, using silver paste as the electrode. The efficiency of the EL was however very low and high voltage was required to achieve the emission. Today, modern OLEDs are used to build digital displays in devices such as TV screens, tablets or mobile phones.