Showing papers by "Johannes Frisch published in 2011"

Band bending in conjugated polymer layers.

Abstract: We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers.

Tuning hole-injection barriers at organic/metal interfaces exploiting the orientation of a molecular acceptor interlayer

Abstract: Ultraviolet photoelectron spectroscopy was used to demonstrate organic/metal-contact charge injection barrier tuning by exploiting the orientation-dependent work function \ensuremath{\phi} of a molecular acceptor [hexaazatriphenylene-hexanitrile (HATCN)] interlayer on Ag(111). The work function \ensuremath{\phi} of a flat-lying HATCN monolayer on Ag was 4.6 eV (similar to a pristine Ag electrode), whereas a layer of edge-on HATCN on Ag exhibited \ensuremath{\phi} of 5.5 eV (comparable to a pristine Au electrode). The hole-injection barriers (HIBs) between HATCN-modified electrodes and the organic semiconductors tris(8-hydroxyquinoline)aluminum (Alq${}_{3}$) and N,N${}^{\ensuremath{'}}$-bis(1-naphtyhl)-N,N${}^{\ensuremath{'}}$-diphenyl-1,1${}^{\ensuremath{'}}$-biphenyl-4.4${}^{\ensuremath{'}}$-diamine (\ensuremath{\alpha}-NPD) were reduced by more than 1 eV compared to pristine Ag and Au electrodes. Noteworthy, the HIBs determined with the flat-lying HATCN interlayer were lower than those obtained for pristine Ag substrates (\ensuremath{\phi} of both electrodes is 4.6 eV), and the HIBs with the edge-on HATCN on Ag were lower than those found for pristine Au (\ensuremath{\phi} of both electrodes ca. 5.4 eV). This shows that acceptor interlayers are beneficial for charge injection in electronic devices even when the molecularly modified electrode \ensuremath{\phi} is comparable to that of a pristine metal surface. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to Ag and Au surfaces.

Electric fields induced by energy level pinning at organic heterojunctions

Abstract: 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.

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

Abstract: 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...

Metal-to-Acceptor Charge Transfer through a Molecular Spacer Layer

Abstract: We investigate how the thermodynamic equilibrium is reached when a strong electron acceptor molecule (hexaaza-triphenylene-hexacarbonitrile, HATCN) is deposited on Ag(111) precovered with a physisorbed tris(8-hydroxyquinolinato)aluminum (Alq3) monolayer. Photoemission measurements reveal that equilibrium is achieved via a charge transfer from the metal to the HATCN layer through the Alq3 monolayer. The work function increase observed upon HATCN adsorption is explained by the formation of an interface dipole resulting from the metal-to-acceptor charge transfer and an electric field induced reorientation of the Alq3 molecule dipoles within the interlayer.

Electronic Properties of Interfaces between PCPDTBT and Prototypical Electrodes Studied by Photoemission Spectroscopy

Abstract: We report on the electronic structure of poly[2,6-(4,4-bis- (2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), a promising low-band-gap donor material for efficient bulk heterojunction organic solar cells. Electronic properties of interfaces formed between PCPDTBT and prototypical electrodes [Au, indium-tin-oxide and poly(ethylene-dioxythiophene): poly(styrenesulfonate)], obtained from X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy, are evaluated. The formation of interface dipoles is observed, and their consequences for device performance are discussed. For the system PCPDTBT/Au chemical interactions occur, which may affect in particular the charge extraction at the corresponding interface.

The Scopaeus paliferus species group (Staphylinidae, Paederinae, Scopaeina) from the eastern Palaearctic, with exclusion of Scopaeus anhuiensis Li, 1993 from the Scopaeina Mulsant & Rey, 1878

Abstract: Scopaeus paliferus sp. n. (Japan, Russia) and S. chinensis sp. n. (China) (Coleoptera, Staphylinidae, Paederinae) are described and their primary and secondary sexual characters are figured. Both species constitute the monophyletic S. paliferus species group in the eastern Palaearctic. Nazeris anhuiensis (Li, 1993), comb. nov, described from the Chinese Anhui Province, is transferred from Scopaeus Erichson, 1839. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)