New Methodology of Organic Electrochemical Synthesis - Fundamentals and Applications of Organic Electrochemistry - page 299

7.1.1 Organic Electroluminescence [1,2]
Organic photoluminescent materials generally
emit through the relaxation process of once
photoexcited electrons to their ground state
( Figure 7.1 ). The excited state can also be
formed electrochemically. When enough
voltage is applied to the anode and cathode
sandwiching the organic emitting layer (thin
film), an injection of holes at the anode and an
injection of electrons at the cathode occur. The
charges migrate inside the layer and recombine
to emit light ( Figure 7.2 ). In order to promote
smooth electron injection from the electrode
surface to the LUMO of the emission layer, an
additional thin layer, i.e. the electron-transport
layer, is introduced between the interfaces. In a
similar manner, the hole-transport layer is used
for hole injection from the electrode to the
HOMO of the emission layer. Although these
transport layers are important for injecting
charge carriers into the emission layer, they
should not interrupt the emission of the
emission layer. Each organic layer is formed by
chemical vapour desorption (CVD) of
crystalline molecules or a wet-process of
polymer solution.