Characterization of ITO/CuPc/AI and ITO/ZnPc/Al structures using optical and capacitance spectroscopy

Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J–V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations.