Impact of structural imperfections on the energy-level alignment in organic films

This paper reports that structural imperfection in an organic thin film modulates the electronic structure to result in a serious band bending and change in the energy-level alignment (ELA) at the organic-conductor interface. Ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) were adopted to investigate thickness dependences of the electronic structure of polar phthalocyanine (chlorogallium phthalocyanine) thin films grown on graphite with respect to the film structure. We observed a large band-bending–like shift of occupied molecular-orbital bands toward the Fermi level and a continuous increase in the vacuum level for the as-grown film, whereas these phenomena were considerably suppressed by annealing the film. Both the as-grown and annealed films were characterized as essentially the same stacked bilayer film structure; however, high-resolution UPS and MAES measurements evidenced that there are structural defects in the as-grown film but not clearly in the annealed film, indicating that the defects are the origin of the modulation of the ELA and the band bending. Controlling the structural imperfections is a key issue for the desired ELA in organic devices.

Figure 1

(a) Chemical structure and molecular conformation of ClGaPc and the direction of the dipole moment $\text{P}$. (b) Schematic of the structural rearrangement of a stacked bilayer domain into a well-ordered monolayer film (dipole layer). The assignment of different electron emissions from the first ($A_1$) and second ($A_2$) layers in the UPS measurements is shown.

Figure 2

Maps of UPS intensities for the vacuum level (VL) and HOMO band regions of the AG (a) and AN (b) films. The vertical axes represent the binding energy ($E_B$) relative to the Fermi level, and the horizontal axes represent the deposition amount in nm (top) and MLE ($1\hspace{0.5em}\text{MLE}=0.35$ nm) (bottom). The black and white filled circles indicate the $E_B$ positions of peaks $A_1$ and $A_2$ in the HOMO band [see the spectra in Fig. 3 (left)], respectively. A staircase pattern in the maps originates simply from a larger deposition step.

Figure 3

Selected UPS spectra for the VL and HOMO band regions of the AG (0.6, 1.9, and 10.3 MLE; red color) and AN (1.0, 2.2, and 8.4 MLE; blue color) films. * is the vibration satellite in the well-oriented monolayer.[10] The origin of a lower-lying feature ($▽$) of the main peak seen in the AN films is unclear at the current stage.

Figure 4

$\delta$-dependent (a) IP (peak $A_2$) of the AG and AN films and (b${}_1$) a shift in the VL and the position of peak $A_2$ of the AG (top) and (b${}_2$) AN (bottom) films above 2 MLE. In (b${}_1$) and (b${}_2$), $\Delta E|$represents the VL ($△$) and the $E_B$ of $A_2$ peak ($▽$) relative to the corresponding values at 2 MLE film.

Figure 5

MAES spectra of the AN (a) and AG (b) films at selected thicknesses ($\delta$). (c) The UPS spectrum of AG multilayer film ($\delta =10.3$ MLE) of ClGaPc/HOPG and the calculated DOS of a single ClGaPc molecule obtained by convoluting calculated molecular orbitals with a Gaussian function ($\text{width}=0.5$ eV). (d) Schematic of spatial distributions of representative MOs for MAES band $A^{\text{'}}$–$F^{\text{'}}$.

Figure 6

(a) Relative intensity area of band $C^{\text{'}}$ to band $D^{\text{'}}+E^{\text{'}}$ as a function of the deposition amount ($\delta$). The areas are estimated by fitting of those bands with Gaussian curves and linear background [see Fig. 5a]. A sketch of the expected structure of the AG (b) and AN (c) films.