2018 年 67 巻 11 号 p. 647-651
In organic semiconductor materials, investigating the energy levels is very important. For example, the combination of the donor’s and acceptor’s energy levels affects the performance of organic solar cells (OSCs). The HOMO and LUMO levels are generally determined by cyclic voltammetry or differential plus voltammetry, which require the solution state. However, these methods are influenced by the purity, solvent, solubility, vibration, and temperature. In this work, we employed photoemission yield spectroscopy and UV-vis-NIR spectroscopy in the thin-film state to measure the energy levels of magnesium porphyrin derivatives for OSCs. The results explained that the energy levels were correlated with a substituent introduced in the porphyrin core. In the case of electron-withdrawing substituent porphyrin, the HOMO and LUMO levels were lowest and the HOMO-LUMO gap was narrowest. On the other hand, electron-donating substituent porphyrin showed high-energy levels and a wide energy band gap. It is noteworthy that the energy levels were lower and the band gaps were narrower in the thin-film state compared with the solution state. This result explains that strong π-stacking derived from intermolecular interaction in thin films. We concluded that measurements of the energy levels in thin film state by PYS and UV-vis-NIR spectroscopy are beneficial for investigating organic semiconductor materials.