Cobalt phthalocynine (CoPc), a member of metal phthalocyanines (MPcs), is frequently used in many organic electronic devices such as light-emitting diodes (LEDs), organic photovoltaics (OPVs), organic field-effect transistors (OFETs) and chemical sensors as a p-type semiconducting material. Compared to other hole-injection layer (HIL) materials, most metal phthalocyanines are water and air stable, thermally stable, and nontoxic. They can be sublimed or sputtered with highly uniform, thin films on a variety of substrates. The synthesis of such materials are also relatively inexpensive and easy to prepare. The chemical structure of MPc allows tuning of its ionization potential or HOMO levels by altering the central atom in Pc macrocycles.
Using cobalt phthalocyanine (CoPc) layer as a hole-injection layer (HIL), remarkable improvements in turn-on voltage and luminance have been observed in organic light-emitting diodes (OLED) [1, 2, 3, 4]. The driving voltages of the MPc electroluminance devices are found to decrease in the order of ZnPc < CuPc < FePc < CoPc < PbPc < NiPc < VOPc < SnPc < H2Pc. This is in agreement with the order of HOMO levels of MPcs [1].