Experimental demonstration of the universal energy level alignment rule at oxide/organic semiconductor interfaces

In this work, our experimental demonstration of the universal energy level alignment rule at oxide/organic semiconductor interfaces is reported. Photoemission spectroscopy is used to show the three different regimes of the energy level alignment: the lowest unoccupied molecular orbital (LUMO) is pinned to the substrate Fermi level at the extreme low end of work functions; the energy offset of the highest occupied molecular orbital (HOMO) follows the Schottky-Mott rule; the HOMO is pinned to the substrate Fermi level at the extreme high end of work functions. To demonstrate this, fullerene C${}_{60}$ was deposited on eight different types of transition-metal oxides, ZrO${}_2$, TiO${}_2$, NiO, Co${}_3$O${}_4$, CuO, V${}_2$O${}_5$, MoO${}_3$, and WO${}_3$, followed by in situ ultraviolet photoemission spectroscopy.

Figure 1

Valence band edge, conduction band edge, and work function of ZrO${}_2$, TiO${}_2$, NiO, Co${}_3$O${}_4$, CuO, V${}_2$O${}_5$, MoO${}_3$, and WO${}_3$, arranged in the order of increasing work function from left to right. Superimposed onto this chart is C${}_{60}$'s HOMO and LUMO levels. Band gap values are deduced or taken directly from [16, 17, 18, 19, 20].

Figure 2

UPS spectrum of ZrO${}_2$ (left) and C${}_{60}$ (right), showing the relationship between vacuum energy level ($E_{\mathrm{vac}}$), Fermi energy level ($E_F$), secondary electron (SE) cutoff peak, HOMO peak, and HOMO edge position.

Figure 3

Energy level alignment at the interface of TMO and C${}_{60}$. HOMO edge offset ($\Delta E_H$) of C${}_{60}$ is plotted against the work function (${\Phi }_{\mathrm{sub}}$) of underlying TMOs. Three regimes can be identified. The energy positions of key parameters in each scenario are shown in the schematics beside each region. Note that these inset schematics are not drawn to scale.

Figure 4

(a) UPS valence spectra of C${}_{60}$ on ZrO${}_2$, CuO, Co${}_3$O${}_4$, and MoO${}_3$. (b) XPS C 1$s$ core level peak of C${}_{60}$ on ZrO${}_2$, CuO, and MoO${}_3$. All spectra are arranged in the order of increasing TMO work function from bottom up.

Figure 5

Interception points solving the self-consistency of (5). These points are the $\Delta \Phi$ then used in (7).

Figure 6

The full plot of Eq. (7) and the HOMO equivalent in blue and red, respectively. Points beyond a HOMO offset of 2.3 eV are not physically relevant since this goes beyond the band gap energy of C${}_{60}$.

Figure 7

A plot of Eqs. (8) and (9) as the red and blue curves, respectively, showing the equivalent behavior of the HOMO and LUMO in the weak interaction regime as well as the pinning position of each, which is in good agreement with other experimental data [18].