1.4 Organic Light-Emitting Diodes (OLED)

A major breakthrough in the field of organic semiconductors was the discovery of light emission from an electrically active polymer [25]. The ease of processing, combined by pure colors make it an ideal candidate for lighting applications [26,27]. Especially the display industry is highly interested in organic semiconductors, as these have advantages over liquid crystal displays such as high switching speed, wide viewing angle, pure color, and also over cathode ray tubes such as low energy consumption, flat screen, and light weight. These properties stimulated the research on organic semiconductors strongly. Basically, a OLED consists of a thin layer of a polymer sandwiched between two electrodes on top of glass substrate (see Fig 1.3). On top of this bottom contact, a thin organic semiconductor layer is deposited. Layer thicknesses of this active layer are typically only of the order of 100nm, because of the low carrier mobility.

Figure 1.3: Left: device layout of a typical organic light-emitting diode (OLED). It consists of a glass substrate with an indium-tin-oxide (ITO) coating functioning as anode, a spin-coated layer of an organic semiconductor as the active layer, and an evaporated metal cathode. Right: working principle of an OLED. Four important processes are shown: (1) Charge injection (2) Transport (3) Exciton formation (4) Photon emission. The last two steps form the recombination process.

Under forward bias electrons and holes are injected into the organic semiconductor from the cathode and the anode, respectively. Driven by the applied electric field, the carriers move through the organic semiconductors in opposite direction until recombination takes place. The device operation of an OLED is thus determined by four processes: charge injection, transport, recombination and phonon emission.

The transport and injection properties of holes can be investigated by choosing a special contact material. In these hole-only devices, the workfunction of both electrodes are very close to the HOMOs of the organic semiconductor, preventing electron injection from the cathod.