T. Schwieger

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.

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.

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.

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.

Electronic structure of pristine and intercalated Sc3N@C80 metallofullerene

We report a study of the electronic structure and charge transfer in the metallofullerene Sc 3N@C80 using photoemission and x-ray absorption spectroscopy. Through a comparison of the x-ray absorption spectrum of Sc3N@C80 at the Sc L2,3 edge with atomic multiplet calculations, the Sc 3 d electron count is determined to be 0.6, thus giving an effective Sc valency of 2.4. With the N atom gaining a full electronic shell by means of covalent bonding with the Sc ~also involving the Sc 3d electron density observed in the x-ray absorption experiments!, the remaining six valence electrons of the Sc3N cluster are then transferred to the carbon cage which stabilizes the C80 cage structure with I h symmetry, a structure which is not energetically favored in neutral C80 . The presence of the highly symmetric I h cage structure is further supported by the observation of distinct fine structure in the valence band photoemission spectra of the endohedral, which results from the high degree of effective degeneracy of the electronic states in the molecule. Finally, the results of investigations of K-doped Sc3N@C80 using photoemission give insight into the KxSc3N@C80 phases that are formed upon intercalation.

Tetra-t-butyl magnesium phthalocyanine on gold: Electronic structure and molecular orientation

In this work we have investigated the electronic structure and the molecular orientation of (t-Bu)(4)PcMg (tetra-t-butyl magnesium phthalocyanine) on polycrystalline and single crystalline gold substrates using photoemission spectroscopy and x-ray absorption spectroscopy, and we compare the results to the unsubstituted PcCu (copper phthalocyanine). The C 1s photoemission spectrum is described similar to unsubstituted relatives with an additional component for the aliphatic substituents. The variation of the excitation energy causes distinct differences in the shape of the C 1s spectrum, which is very useful for the analysis of the molecular orientation in the uppermost layer. It is shown that despite of the sterically demanding substituents, ordered sublimed films of (t-Bu)(4)PcMg are accessible, the orientation of the molecules, however, is different from the orientation of the unsubstituted relatives.

Orientation and electronic properties of pentacene molecules on SiO2 and GeS(0001) studied using x-ray absorption spectroscopy

We present an investigation on the morphology of pentacene molecules, a candidate for organic semiconductors in organic electronic devices. In this case the molecules are deposited on the technologically relevant substrate SiO2. In order to observe possible changes in the morphology of a monolayer of pentacene compared to a much thicker pentacene layer, we have performed x-ray absorption experiments on both films. We did not find significant differences in the orientation of the pentacene molecules as a function of film thickness. The same is true for the orientation of pentacene on the model substrate GeS(0001), however, the molecular orientation is opposite to that of pentacene on SiO2. Whereas the pentacene molecules on GeS(0001) lie flat on the substrate surface, on SiO2 they are standing. These results have important consequences for our understanding of the functionality of organic devices based on pentacene.

Band gap and charge carrier wavefunction in organic semiconductors

The performance of novel organic devices such as organic light-emitting diodes or organic field-effect transistors is intimately connected to the nature and dynamics of the charge carriers in the device components. Carrying out intercalation studies of solid model oligomers, it is experimentally demonstrated that the low lying electronic excitations in p-type doped systems are significantly confined on the individual molecules due to polaronic effects and thus deserve the name polaron excitations. These results allow for a quantitative experimental estimate of the charge carrier (polaron) extension which is of the order of 20 Angstrom . In addition, it is shown that electron correlation effects play an important role in the determination of the band gap of molecular organic semiconductors. The implications of these results for organic devices are discussed.