Origin of the highest occupied band position in pentacene films from ultraviolet photoelectron spectroscopy: Hole stabilization versus band dispersion

The electronic structure of pentacene/graphite, pentacene/Cu-phthalocyanine (CuPc)/graphite, and $\text{pentacene}∕\mathrm{Si}{\mathrm{O}}_2∕\mathrm{Si}\left(100\right)$ was studied by ultraviolet photoelectron spectroscopy as a function of the thickness of the pentacene film. We observed that the binding energy position of the highest occupied molecular orbital (HOMO) of pentacene becomes significantly lower on CuPc and $\mathrm{Si}{\mathrm{O}}_2$ surfaces than that on graphite. Furthermore, a splitting of the UPS band was observed only for the HOMO of thicker crystalline films on CuPc and $\mathrm{Si}{\mathrm{O}}_2$ surfaces, where the molecules are oriented with their long axis nearly perpendicular to the substrate surface. The splitting is ascribed to the intermolecular band dispersion, and the decrease in the threshold ionization energy on CuPc and $\mathrm{Si}{\mathrm{O}}_2$ surfaces originates from the HOMO-band dispersion as well as the increase in the relaxation/polarization energies, which may be caused by the better molecular packing structure with a nearly standing molecular orientation.

Figure 1

He I UPS spectra of pentacene/HOPG system as function of deposition amount of pentacene $\left(\delta \right)$ in secondary region (a) and HOMO region (b). The films are not annealed. All of the spectra are measured at $295\mathrm{K}$ with a $-5\mathrm{V}$ bias applied to the sample to observe the vacuum level. The $E_{\mathrm{B}}$ scale refers to $E_{\mathrm{F}}{}^{\mathrm{sub}}$.

Figure 2

He I UPS spectra of pentacene/CuPc/HOPG system as function of deposition amount of pentacene overlayer $\left(\delta \right)$ in secondary region (a) and HOMO region (b). The films are not annealed except for the CuPc film. All of the spectra are measured at $295\mathrm{K}$ with a $-5\mathrm{V}$ bias applied to the sample to observe the vacuum level. The $E_{\mathrm{B}}$ scale refers to $E_{\mathrm{F}}{}^{\mathrm{sub}}$.

Figure 3

He I UPS spectra of $\text{pentacene}∕\mathrm{Si}{\mathrm{O}}_2∕\mathrm{Si}\left(100\right)$ system as function of deposition amount of pentacene $\left(\delta \right)$ in secondary region (a) and HOMO region (b). The films are not annealed. All of the spectra are measured at $295\mathrm{K}$ with a $-5\mathrm{V}$ bias applied to the sample to observe the vacuum level. The $E_{\mathrm{B}}$ scale refers to $E_{\mathrm{F}}{}^{\mathrm{sub}}$. Panel (c): The wide valence band region of pentacene/HOPG, pentacene/CuPc/HOPG and $\text{pentacene}∕\mathrm{Si}{\mathrm{O}}_2∕\mathrm{Si}\left(100\right)$ systems, compared with that of the gas-phase spectrum (Ref. 35). The energy is relative to the HOMO peak. MO energies calculated by a DFT $(\mathrm{B}3\mathrm{LYP}∕6–31{\mathrm{G}}^{**})$ method are also shown.

Figure 4

(Color online)Schematic diagram of energy levels of various pentacene systems from vacuum level; (i) gas phase, (ii) monolayer film on HOPG, (iii) amorphous film on HOPG, (iv) crystalline film on CuPc/HOPG, and (v) crystalline film on $\mathrm{Si}{\mathrm{O}}_2$. The gas-phase value is cited from Ref. 35. The Fermi level of each system is also shown.