Probing the energy levels in hole-doped molecular semiconductors

Understanding the nature of polarons – the fundamental charge carriers in molecular semiconductors – is indispensable for rational material design that targets superior (opto-) electronic device functionality. The traditionally conceived picture of the corresponding energy levels invokes singly occupied molecular states within the energy gap of the semiconductor. Here, by employing a combined theoretical and multi-technique experimental approach, we show that this picture needs to be revised. Upon introducing an excess electron or hole into the material, the respective frontier molecular level is split by strong on-site Coulomb repulsion into an upper unoccupied and a lower occupied sub-level, only one of which is located within the semiconductor gap. By including also inter-site Coulomb interaction between molecular ions and circumjacent neutral molecules, we provide a complete picture for the electronic structure of molecular semiconductors in the presence of excess charges. With this understanding, a critical re-examination of previous results is called for, and future investigations of the properties and dynamics of polarons in weakly interacting molecular systems are put on sound footing.Peer Reviewe

Organic heterojunctions : Contact-induced molecular reorientation, interface states, and charge redistribution

We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure-in the presence of Fermi-level pinning-at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (E-F) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes E-F-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction's electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices

A disordered layered phase in thin films of sexithiophene

This Letter reports the impact of the evaporation rate on the crystallographic phase formation of vacuum deposited alpha sexithiophene thin films studied by X ray diffraction methods. The experiments reveal the formation of two crystal phases, one of which is a thermodynamically stable phase occurring at low rates, while the second is favored by high rates. This second phase exhibits an increased layer spacing and diffraction features typical for two dimensional crystals which are laterally ordered but without interlayer correlations of the molecular positions. This disordered layered phase comprises molecules of nonuniform conformations, and is kinetically induce

The Impact of Local Work Function Variations on Fermi Level Pinning of Organic Semiconductors

This photoemission study shows that the work function (Φ) of indium–tin-oxide (ITO) can be increased from 4.2 up to 6.5 eV upon the deposition of the molecular electron acceptors tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and hexaazatriphenylene-hexacarbonitrile (HATCN). The evolution of sample Φ and the hole injection barrier upon subsequent deposition of the hole transport material <i>N</i>,<i>N</i>′-bis­(1-naphthyl)-<i>N</i>,<i>N</i>′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) was studied for different acceptor precoverages of ITO, corresponding to different initial Φ values. When Φ of the acceptor covered substrate exceeds a critical value Φ_crit, the highest occupied molecular level of multilayer α-NPD is found to be pinned 0.5 eV below the Fermi level (E_F). Noteworthy, Φ_crit is found at 5.2 eV, which is 0.4 eV higher than expected for α-NPD (4.8 eV), and vacuum level alignment does not apply even before E_F-pinning sets in. An electrostatic model that accounts for nonuniformity of the substrate at acceptor submonolayer coverages and the associated local work function changes explains the origin of “delayed” E_F-pinning

Tuning the Electronic Structure of Graphene by Molecular Dopants Impact of the Substrate

A combination of ultraviolet and X ray photoelectron spectroscopy, X ray absorption spectroscopy and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8 hexafluorotetracyano naphthoquinodimethane F6TCNNQ and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge re distribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support p doping of graphene , the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is ndoped, and remains n doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced 1.3 eV on Cu compared to quartz 1.0 eV because of the larger electrostatic potential drop associated with the long distance graphene mediated Cu F6TCNNQ electron transfer. We thus provide means to realize high work function surfaces for both p and n type doped graphen