Publications by authors named "Dominique Lungwitz"

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics.

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Strong molecular dopants for organic semiconductors that are stable against diffusion are in demand, enhancing the performance of organic optoelectronic devices. The conventionally used p-dopants based on 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives "FTCN(N)Q", such as 2,3,4,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ), feature limited oxidation strength, especially for modern polymer semiconductors with high ionization energy (IE). These small molecular dopants also exhibit pronounced diffusion in the polymer hosts.

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The modern picture of negative charge carriers on conjugated polymers invokes the formation of a singly occupied (spin-up/spin-down) level within the polymer gap and a corresponding unoccupied level above the polymer conduction band edge. The energy splitting between these sublevels is related to on-site Coulomb interactions between electrons, commonly termed Hubbard . However, spectral evidence for both sublevels and experimental access to the value is still missing.

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Article Synopsis
  • The text discusses challenges in creating multi-layer optoelectronic devices using solution-processed doped polymer layers, particularly the issue of the first layer dissolving when applying a second layer.
  • It highlights two mechanisms to improve solvent resistance while keeping the dopants intact: one involves pre-aggregating the polymer in solution, and the other uses a photo-reactive agent for cross-linking.
  • The study finds that pre-aggregation enhances solvent resistance but limits the effectiveness of the cross-linking agent, and that suppressing this pre-aggregation can lead to stronger solvent resistance and better electrical conductivity in doped polymer layers.
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The ability to insolubilize doped semiconducting polymer layers can help enable the fabrication of efficient multilayer solution-processed electronic and optoelectronic devices. Here, we present a promising approach to simultaneously n-dope and largely insolubilize conjugated polymer films using tetrakis[{4-(1,3-dimethyl-2,3-dihydro-1-benzo[]imidazol-2-yl)phenoxy}methyl]methane (tetrakis-O-DMBI-H), which consists of four 2,3-dihydro-1-benzoimidazole (DMBI-H) n-dopant moieties covalently linked to one another. Doping a thiophene-fused benzodifurandione-based oligo(-phenylenevinylene)--thiophene polymer (TBDOPV-T) with tetrakis-O-DMBI-H results in a highly n-doped film with bulk conductivity of 15 S cm.

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Molecular doping is a key process to increase the density of charge carriers in organic semiconductors. Doping-induced charges in polymer semiconductors result in the formation of polarons and/or bipolarons due to the strong electron-vibron coupling in conjugated organic materials. Identifying the nature of charge carriers in doped polymers is essential to optimize the doping process for applications.

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Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto-) electronics. Beyond single-component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited.

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The use of polyethylenimine (PEI) as a thin interlayer between cathodes and organic semiconductors in order to reduce interfacial Ohmic losses has become an important approach in organic electronics. It has also been shown that such interlayers can form spontaneously because of vertical phase separation when spin-coating a blended solution of PEI and the semiconductor. Furthermore, bulk doping of semiconducting polymers by PEI has been claimed.

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Precise doping of organic semiconductors allows control over the conductivity of these materials, an essential parameter in electronic applications. Although Lewis acids have recently shown promise as dopants for solution-processed polymers, their doping mechanism is not yet fully understood. In this study, we found that B(CF) is a superior dopant to the other Lewis acids investigated (BF, BBr and AlCl).

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