Heiko Peisert

Full characterization of the interface between the organic semiconductor copper phthalocyanine and gold

We present a study of the interface properties of the molecular organic semiconductor copper phthalocyanine (CuPC) on single crystalline as well as polycrystalline Au using photoemission spectroscopy. Despite the different orientation of the molecules on the two substrates, the observed energy level alignment is identical. We observe the formation of an interface dipole while band bending is very small. In addition, we have carried out complementary studies of the CuPC/GeS(001) interface to pin down photoemission final state effect contributions to the observed energy level shifts.

Order on disorder: Copper phthalocyanine thin films on technical substrates

We have studied the molecular orientation of the commonly used organic semiconductor copper phthalocyanine (CuPC) grown as thin films on the technically relevant substrates indium tin oxide, oxidized Si, and polycrystalline gold using polarization-dependent x-ray absorption spectroscopy, and compare the results with those obtained from single crystalline substrates [Au(110) and GeS(001)]. Surprisingly, the 20–50 nm thick CuPC films on the technical substrates are as highly ordered as on the single crystals. Importantly, however, the molecular orientation in the two cases is radically different: the CuPC molecules stand on the technical substrates and lie on the single crystalline substrates. The reasons for this and its consequences for our understanding of the behavior of CuPC films in devices are discussed.

Energy level alignment at organic/metal interfaces: Dipole and ionization potential

We present a systematic study of the energy level alignment at the interfaces between gold and organic semiconductors. It is shown that there are at least two leading contributions to the potential drop (dipole) across the metal/organic interface: A modification of the metal work function due to the adsorption of the organic molecules and a potential change in the organic semiconductor.

Fluorination of copper phthalocyanines: Electronic structure and interface properties

We compare the electronic structure of differently fluorinated copper phthalocyanines (CuPC, CuPCF4, and CuPCF16) using x-ray photoemission spectroscopy and valence-band ultraviolet photoemission spectroscopy. Whereas the ionization potential (IP) is increased by more than 1 eV as a function of the degree of fluorination, further electronic properties such as the optical gap or the composition of the highest occupied molecular orbital and lowest unoccupied molecular orbital remain nearly unchanged. This fact renders these compounds an ideal tool for the investigation of the influence of the IP on the interface properties. At the interface to gold, besides interface dipoles we observe both downward and upward band bending. These phenomena depend clearly on the IP of the phthalocyanines.

Relaxation energies in XPS and XAES of solid sulfur compounds

Abstract Solid sulfur compounds have been investigated by XPS/XAES chemical shift measurements of the S 1s, S 2p, and S KLL peaks. Auger parameters and relaxation energies have been derived. The relaxation energy was discussed in terms of initial state charge and dielectric screening models. A local screening model was used to calculate relaxation energy shifts semiquantitatively. The effect of crystal water has been illustrated. A comparison of trends in the relaxation energy of the third row elements silicon, phosphorus, and sulfur shows striking similarities.

Influence of temperature on HSQ electron-beam lithography

The authors present a study of the influence of temperature on hydrogen silsesquioxane (HSQ) e-beam lithography during drying, developing, and postdevelopment baking. In accordance with the observation that tempering at relatively low temperatures can already lead to noticeable cross-linking, comparable to the effect of e-beam exposure, the authors find that decreasing the prebake temperature below 90°C and drying the HSQ resist at room temperature in vacuum yields better resolution compared with resist that was dried in a furnace or on a hotplate at 90°C or above. Developing the exposed resist not at room temperature (23°C) but at 60°C results in significant contrast enhancement. Further solidification of the developed resist is obtained by baking the material above 300°C. Correlations between these findings and IR data are presented.

Interface properties of organic/indium–tin oxide and organic/GeS(001) studied using photoemission spectroscopy

We present an investigation of interface properties of the hole transport material N,N′-diphenyl-N,N′-bis (3-methylphenyl)–(1,1′)–biphenyl-4,4′diamine (TPD) on indium–tin oxide (ITO) and on GeS(001) using x-ray and ultraviolet photoelectron spectroscopy. On both the TPD/ITO interface and the TPD/GeS(001) heterojunction we found shifts of the vacuum level indicating a formation of an interfacial dipole barrier. We obtained a barrier for the hole injection for TPD on Ar ion sputter-cleaned ITO of 1.5 eV which is significantly higher than the commonly cited value of 0.5 eV for ozone-oxidized ITO, whereas the interface dipole barrier of 0.3 eV is in good agreement with reported data. At low coverages of TPD on ITO additional features in the C 1s core level spectra were found, which can be ascribed to a partially oxidized TPD species at the interface.

Charge transfer and doping at organic/organic interfaces

We studied the electronic properties of technically relevant organic/organic interfaces using photoemission spectroscopy. Representatives of organic semiconductors from the family of the phthalocyanines were evaporated onto PEDOT:PSS [mixture of poly-3,4-ethylenedioxy-thiophene and polystyrenesulfonate] thin films, which are often applied as electrode material in organic semiconductor devices. Besides the formation of interface dipoles we observe energetic shifts of the electronic levels of the semiconductors, which are explained by a doping of the organic semiconductor near the interface due to a mixing of the two organic materials within the interface region.

Electronic properties of interfaces between different sexithiophenes and gold

We present a photoemission study of the interface between sexithiophene and polycrystalline gold. Two different sexithiophenes have been investigated—α‐6T and a derivative of α‐6T(ββ′-DH6T) with additionally attached alkane chains at thiophene rings, which increases its solubility and allows application in solution-based processes. We find an interface dipole of 1.2 eV and we observe chemical interactions for both sexithiophenes. Special attention has been paid to the possible influence of the chemical modification on the electronic properties of the material itself and its influence on the interface properties with gold. In addition, we discuss the differences in the electronic structure of vacuum-sublimated and spin-coated films of ββ′‐DH6T. We find indications for better ordered films applying the solution-based process and the spin-coating procedure leads to oxidation of the organic film.

Strong chemical interaction between indium tin oxide and phthalocyanines

We present a study of the interface properties between indium tin oxide (ITO) and the well-established molecular organic semiconductor copper phthalocyanine (CuPC) and its fluorinated relative copper tetraflourophthalocyanine (CuPCF4) using photoemission spectroscopy. It is shown that a strong chemical interaction occurs between the PC molecules and ITO which indicates the presence of interface states at these technically relevant interfaces.