Browsing by Author "Salzner, Ulrike"

Now showing 1 - 6 of 6

Open Access
Conjugated organic polymers: from bulk to molecular wire
(American Scientific Publishers , 2006) Salzner, Ulrike; Rieth, M.; Schommers, W.
Open Access
Electrochemical polymerization of ambipolar carbonyl-functionalized indenofluorene with memristive properties
(Elsevier, 2019) Figà, V.; Usta, H.; Macaluso, R.; Salzner, Ulrike; Özdemir, M.; Kulyk, B.; Krupka, O.; Bruno, M.
Carbonyl-functionalized indenofluorene was electropolymerized with a high faradaic efficiency of 85% and the solid state properties of the resulting polymeric thin films were investigated. They displayed modular optical properties depending on their oxidation state. The approach used for inorganic semiconductors was applied to polyindeonofluorene derivative. Mott-Schottky analysis evidenced a switching from p-type to n-type electrical conduction, suggesting an ambipolar behaviour of the polymer. As an application, flexible organic memristors were fabricated and resistive switching properties were observed.
Open Access
Electrochemical stability and ambipolar charge transport in diketopyrrolopyrrole-based organic materials
(ACS, 2019) Ray, S.; Panidi, J.; Mukhopadhyay, T.; Salzner, Ulrike; Anthopoulos, T. D.; Patil, S.
An important strategy for realizing flexible complementary circuits with organic semiconductors is to achieve balanced ambipolar charge transport properties with reduced anisotropy. Here, we present a series of star-shaped diketopyrrolopyrrole (DPP)-based organic materials synthesized for improved intermolecular charge transport while retaining the ambipolar charge transport properties of their linear counterparts. Steady-state UV−visible spectroscopic studies confirm that the oligomers are highly aggregated in the thin film as evidenced from appearance of prominent vibronic features and red-shifted absorption bands. Ambipolar transport properties of these materials were verified in organic field-effect transistors (OFETs). The results show that the star-shaped DPP systems have the potential to outperform their linear counterparts in devices
Open Access
Fermi level pinning ınduced by doping in air stable n type organic semiconductor
(American Chemical Society, 2020) Sharma, S.; Ghosh, S.; Ahmed, T.; Ray, S.; Islam, S.; Salzner, Ulrike; Ghosh, A.; Seki, S.; Patil, S.
Doping of organic semiconductors enhances the performance of optoelectronic devices. Although p-type doping is well studied and successfully deployed in optoelectronic devices, air stable ntype doping was still elusive. We succeeded with n-type doping of organic semiconductors using molecular dopant N-DMBI under ambient conditions. Strikingly, n-type doping accounts for a gigantic increase of the photoconductivity of doped thin films. Electrical and optical properties of the n-doped molecular semiconductor were investigated by temperature dependent conductivity, electron paramagnetic resonance (EPR), and flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. A significant reduction and saturation in activation energy with increasing doping level clearly suggests the formation of an impurity band and enhancement in carrier density. Computational studies reveal the formation of a charge transfer complex mediated by hydrogen abstraction as the rate-determining step for the doping mechanism. The colossal enhancement of photoconductivity induced by n-doping is a significant step toward optoelectronic devices made of molecular semiconductors.
Open Access
Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor–acceptor copolymers
(A I P Publishing LLC, 2021-02-04) Salzner, Ulrike
Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor–acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character. Therefore, a theoretical study was carried out to elucidate the properties of the unknown DPP homopolymer. Calculations were done with density functional theory and with the complete active space self-consistent field method plus n-electron valence state perturbation theory for the dynamic correlation. Poly-DPP is predicted to have radical character and an extremely wide low-lying conduction band. If it were possible to produce this material, it should have unprecedented n-type conductivity and might be a synthetic metal. A comparison with various unknown donor–acceptor systems containing vinyl groups and thienyl rings with a higher concentration of DPP than the known copolymers reveals how donor–acceptor substitution reduces bandwidths and decreases electron affinities.
Open Access
Spin density encodes intramolecular singlet exciton fission in pentacene dimers
(Nature Publishing Group, 2019) Krishnapriya, K. C.; Roy, P.; Puttaraju, B.; Salzner, Ulrike; Musser, A. J.; Jain, M.; Dasgupta, J.; Patil, S.
The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.