In SituPhotoelectron Spectroscopy and Theoretical Calculation Study of Thermally Evaporated Copper Naphthalocyanine

Abstract

Recently, organic photovoltaics (OPVs) have attracted attention as a next-generation energy source as their power conversion efficiency (PCE) has significantly improved. To increase the PCE of OPVs further, a fundamental understanding of material properties of a new organic semiconductor is highly important. Copper naphthalocyanine (CuNc) is composed of macrocyclic ligands with extended benzene rings of copper phthalocyanine (CuPc). Thus, it can be considered as a potential candidate for an efficient p-type organic semiconductor, similar to a well-known conventional p-type organic semiconductor CuPc. In this study, we investigated the electronic structure of thermally evaporated CuNc on indium tin oxide (ITO) wit in situ ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and density functional theory (DFT) calculations. DFT calculations predict that CuNc has a lower ionization energy (IE) and band gap than CuPc. However, the IE measured by UPS and the band gap measured by UV-vis absorption of the deposited film were much higher than CuPc. Derived from the measured UPS, XPS spectra and DFT calculation results, we concluded that CuNc could be decomposed with thermal evaporation in a vacuum and the pyrrole-based material might be deposited on ITO. The increased IE and band gap were attributed to the disappearance of the highest occupied molecular orbital originating from the macrocyclic ligands with D4h symmetry. Therefore, the thermal evaporation method would not be a suitable method to obtain the CuNc film for device application, and the alternative solution process at low temperature would be more adequate.