Quantitative analysis of the electrostatic and electronic polarization energies in molecularly mixed films of organic semiconductors

The energy levels of organic semiconductors are primarily determined by the molecular orbital energies of constituent molecules. Recent studies have, however, shown that the energy levels can be changed by the mixing ratio of two molecules which have different permanent quadrupole moments. From the good correlation between the magnitude of the mixed film's energy shift and the constituent molecules' permanent quadrupole moment, it was noted that the molecular quadrupole plays an important role in the energy shift. In this study, ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoemission spectroscopy (LEIPS) are applied to the mixed films of zinc phthalocyanine (ZnPc) and perfluorinated ZnPc (${\mathrm{F}}₁₆\mathrm{ZnPc}$), which have permanent quadrupole moments with opposite directions. From the precisely determined ionization energies and electron affinities, we directly determine the electronic polarization energy $D$ and electrostatic energy $S$ as a function of mixing ratio. Furthermore, we examined the molecular orientation dependence of $S$ and $D$ values. $D$ is almost independent of the mixing ratio (the difference is less than 0.2 eV over the range of mixing ratio) whereas $S$ differs by as much as 1.6 eV. The result clearly shows that the energy levels' continuous shift by the mixing ratio originates in the electrostatic interaction, whose leading term is the charge-permanent quadrupole interaction.