The electronic structures of interfaces between metals and Copper phthalocyanine (CuPc) organic films are investigated using the combination of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level, E<sub>F</sub>, in the organic film can be modified by metals through charge transfer or doping. The FERMI level at the Cs/CuPc interface is observed to shift to less than 0.2 eV below the CuPc LUMO. The IPES observation is the first direct confirmation of Fermi level pinning near the LUMO in organic films. The pinning of the Fermi level close to the LUMO can be explained by electron transfer from Cs to CuPc, which is supported by the presence of a gap state in CuPc as observed with UPS. On the other hand, the Au/CuPc interface is characterized by electron transfer from CuPc to Au, resulting in a reduced HOMO intensity shown in the UPS spectra and a new feature below the LUMO shown in the IPES spectra. These observations shed new light onto the understanding of interface formation in organic semiconductor devices.
Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in these devices occurs within the first few molecular layers of the organic at the interfaces between the organic and the dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene, perylene, and sexithiophene on conductors, dielectrics, and charge transfer agents and in some cases vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. There appears to be a linear relation between the interface dipole and metal workfunction with the observed dipoles ranging from a 1 eV dipole at the interface between sexithiophene and gold to a -0.46 eV dipole at the interface between pentacene and calcium. We also observed that more complex material intermixing takes place during metal on organic deposition than during organic deposition onto metal and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.
We have investigated the evolution of the growth front of perylene, an organic semiconductor with high carrier mobility, on glass and Au substrates grown side-by-side by vapor deposition. The films were grown with gradually increasing thickness which allowed us to examine both the spatial and temporal correlation of the surface roughness using atomic force microscopy. Our results show that perylene growth on glass and Au substrates is non-stationary. However, the instability during the growth is shown to depend largely on the substrate. A roughness exponent of 0.82 is obtained for glass and 0.84 for Au. A growth exponent of 0.21 is obtained for glass and 0.74 for Au. The results indicate the strong influence of the substrate on the film morphology and point to possible ways to control and improve it.
We report the characterization of interface formation between Au(Au) and pentacene, an organic material used as an active material in Organic Thin-Film Transistors, using x- ray and ultraviolet photoelectron spectroscopy (XPS and UPS). XPS results indicate that there is no chemical reaction between Au and pentacene regardless of deposition order. UPS results indicate the presence of interface dipoles at both the pentacene/Au and the Au/pentacene interfaces. In addition there are indications of band- bending at the Au/pentacene interface.