Andreas Wilke

Charged and metallic molecular monolayers through surface-induced aromatic stabilization

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

Correlation between interface energetics and open circuit voltage in organic photovoltaic cells

We have used ultraviolet and inverse photoemission spectroscopy to determine the transport gaps (Et) of C60 and diindenoperylene (DIP), and the photovoltaic gap (EPVG) of five prototypical donor/acceptor interfaces used in organic photovoltaic cells (OPVCs). The transport gap of C60 (2.5 ± 0.1) eV and DIP (2.55 ± 0.1) eV at the interface is the same as in pristine films. We find nearly the same energy loss of ca 0.5 eV for all material pairs when comparing the open circuit voltage measured for corresponding OPVCs and EPVG.

Identification of different origins for s-shaped current voltage characteristics in planar heterojunction organic solar cells

We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection laye...

Electric fields induced by energy level pinning at organic heterojunctions

We investigated the energy levels at organic heterojunctions comprising the donor diindenoperylene (DIP) on top of the acceptor C60 with photoelectron spectroscopy. The intermolecular interaction is weak as evidenced on a moderate work function electrode by a small interface dipole of 0.15 eV and flat energy levels on both sides of the junction. When a high work function electrode is used, the DIP levels become Fermi-level pinned and an electric field drops within the C60 layer underneath. The electric field distribution within an organic opto-electronic device may thus be adjusted by employing interfacial energy level pinning, even at physisorptive organic/organic interfaces.

Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution.

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 (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-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.

Electronic Properties of Cu-Phthalocyanine/Fullerene Planar and Bulk Hetereojunctions on PEDOT:PSS

In this paper, we report UV and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phtalocyanine (CuPc) and C60, a prototypical material pair for organic photovoltaic cells (OPVCs). The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates in order to achieve morphologies comparable to those in actual OPVCs. As a result of a CuPc-to-substrate electron transfer, the work function of pristine PEDOT:PSS is reduced from 5.4 to 4.4 eV. The deposition of C60 onto CuPc, however, leads to a work function increase of 0.2 eV. The codeposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. The energy offset between the highest occupied levels of CuPc and C60 was determined as 1.3 eV for both the layered planar and mixed bulk heterojunction. With reported values of the charge transport gap of C60, we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy level offsets are independent of particular interface morphology in C60/CuPc heterojunctions grown on PEDOT:PSS, and that differences in device efficiency are due to other effects.

Interlayer molecular diffusion and thermodynamic equilibrium in organic heterostructures on a metal electrode

In this paper, we investigate by photoemission the electronic properties of an organic/organic interface consisting in a strong electron acceptor hexaazatriphenylene-hexacarbonitrile (HATCN) deposited on a physisorbed N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) monolayer on Ag(111). At the first HATCN deposition steps (∼1 monolayer), the sample work function increases by 1.05 eV and the hole injection barrier (HIB) in the pre-adsorbed α-NPD monolayer is lowered by 0.65 eV. This results from HATCN diffusion to the silver surface through the α-NPD monolayer. Furthermore, this HATCN monolayer is proposed to form a compact chemisorbed monolayer, with a different structural arrangement than that observed on pristine Ag(111). In a second step, the additional deposited HATCN start growing on top of the α-NPD layer, and Fermi-level pinning, associated with the formation of HATCN negative polarons, is identified at the HATCN/α-NPD interface. Finally, HATCN is deposited on a α-NPD multilay...

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Planar and bulk heterojunctions of organic donor and acceptor molecules are used to understand elementary processes in photovoltaic cells. The electronic structure, interface and film morphology, excitonic behavior, device characteristics, and correlations between these properties are reviewed here using a wide range of material combinations.