Basic device architecture
The typical device architecture consists of a light-emitting polymer layer, an optically transparent anode and metallic cathode. The typical material for the anode is indium/tin oxide (ITO) coated on glass, while the cathode is formed of a low work function metal, such as Ca, Mg or Al. The polymer solution can be deposited on the ITO by spin-coating and the thickness of such prepared film is typically of the order of ca. 100 nm thick. The cathode metal is evaporated onto the polymer film in vacuum. The cathode injects electrons into the conduction band of the polymer (π* state), which corresponds to the lowest unoccupied molecular orbital (LUMO), and the anode injects holes into the valence band (π state), which corresponds to the highest occupied molecular orbital (HOMO).
The injection of charge from most electrode materials requires overcoming the barrier at the electrode and the organic layer interface. The injected charges form polarons which may travel from one electrode to the other. In the organic films used in LEDs the charge carrier transport is usually operated by hopping between localized states. Some of the charge carriers can be trapped and when they approach each other they may recombine. This results in the formation of neutral excited species, called excitons, which can be in singlet or triplet state according to spin statistics. Because the electrons and the holes injected from the opposite electrodes are not correlated, statistically only one singlet is formed for three triplets. And because only singlets can decay radiatively, the maximum quantum efficiency (emitted photons per injected electrons), which is theoretically available is 25%. This limit may be overcome by using phosphorescent materials that can generate emission from triplet excitons.