Experimental Physics VI



    Our collaboration on

    HOMO–HOMO Electron Transfer: An Elegant Strategy for p‐Type Doping of Polymer Semiconductors toward Thermoelectric Applications

    by Goel M, Siegert M, Krauss G, Mohanraj J, Hochgesang A, Heinrich D C, Fried M, Pflaum J and Thelakkat M

    has just been published in Advanced Materials.

    Semiconducting polymers are gaining huge interest as lightweight, flexible and printable materials for thermoelectric applications. However, they rely on heavily doping to enhance their low intrinsic electrical conductivities and to achieve efficient energy conversion. In this paper a novel concept for p-type doping is presented, exploiting electron transfer from the HOMO of the diketopyrrolopyrrole-based polymer PDPP[T]2-EDOT to the HOMO of the oxidized p-type semiconductor Spiro-OMeTAD(TFSI)2. As a key result, a considerable increase in electrical conductivity by four orders of magnitude is achieved yielding a maximum power factor of 0.07 μW m−1 K−2 and a Seebeck coefficient of 140 μV K−1 at 4 mol% Spiro-OMeTAD(TFSI)2. The doped polymer films exhibit high thermal and ambient stability and render the presented concept a promising strategy towards improved thermoelectrics based on conjugated polymers.


    The study on

    Role of Intermolecular Interactions in the Excited-State Photophysics of Tetracene and 2,2′-Ditetracene

    by Müller U, Roos L, Frank M, Deutsch M, Hammer S, Krumrein M, Friedrich A, Marder T B, Engels B, Krueger A and Pflaum J

    has just been published in The Journal of Physical Chemistry C.

    The process of singlet fission reduces the luminescence of many materials with otherwise promising optical properties, such as tetracene. Our study highlights a possible molecular design, namely, the covalent linking of two tetracenes to form a ditetracene molecule in order to preserve the optical properties of the individual chromophores while eliminating possible loss mechanisms caused by intermolecular interaction in the crystalline aggregate. We demonstrate that tetracene chromophores maintain their spectral properties in ditetracene when observed at the single molecule level. In the crystalline aggregate, however, ditetracene exceeds the photoluminescence of tetracene by more than an order of magnitude due to the absence of singlet fission, which will be beneficial for future light-emitting diode or solid-state laser applications.


    Congratulations to Marie Siegert who was awarded one of the prestigious PhD fellowships of the Deutsche Bundesstiftung Umwelt (DBU). Based on this three-year funding which officially will start in July 2020 Marie will continue her successful work on polymer-based composite films for thermoelectric applications initiated during her master’s thesis in our group. All the best for your scientific activities on this subject being not merely of physical interest but also of high relevance for technological advancement.


    The paper on

    Seed crystal free growth of high-quality double cation – double halide perovskite single crystals for optoelectronic applications

    by Höcker J, Ozcan M, Hammer S, Fischer M, Bichler B, Armer M, Rieder P, Drach V, Pflaum J, Nickel B and Dyakonov V

    has just been published in Journal of Materials Chemistry C.

    Thin films of Organo-lead trihalide perovskite (OLTP) define an interesting material system for future photovoltaics and optoelectronics. However, to analyse the intrinsic material properties and thus, the technological potential of this class of semiconductors the investigation of high-quality single crystals is inevitable. This challenge has been coped with in a recent publication reporting the growth of seed-free double cation – double halide perovskite single crystals composed of (FAPbI3)0.9(MAPbBr3)0.1 and with macroscopic dimensions of several millimeters in length. The superior quality of crystals prepared by the reported method is confirmed by comparative structural, optical as well as electronic characterizations.


    The study on

    Ordered Donor–Acceptor Complex Formation and Electron Transfer in Co-deposited Films of Structurally Dissimilar Molecules

    by Opitz A, Peter C, Wegner B, Ramakrishna Matte H S S, Röttger A, Florian T, Xu X, Beyer P, Grubert L, Hecht S, Belova V, Hinderhofer A, Schreiber F, Kasper C, Pflaum J, Zhang Y, Barlow S, Marder S R and Koch N

    has just been published in The Journal of Physical Chemistry C.

    Mixed films comprising two or more molecular species are of high technological relevance for their implementation in photovoltaics or photodetectors operating in the near infrared. But what exactly drives the resulting morphology of a blend and how is it affected by a possible charge transfer between the donor and acceptor components?
    This topic has been addresses in a recently published collaborative paper analyzing the structural and optical properties of different binary mixtures always based on the same acceptor compound but three different molecular donor materials. A key result of the reported work is the identification of the delicate balance between binding energy of the neat single-component phases and the coulombic attraction of the formed ions upon ground-state charge transfer and its importance on the final phase separation or mixed-crystal formation.


    Our collaboration on

    Cyclic (amino)(aryl)carbenes (CAArCs) enter the field of chromophore ligands - Expanded π system leads to unusually deep red emitting CuI compounds

    by Gernert M, Balles-Wolf L, Kerner F, Müller U, Schmiedel A, Holzapfel M, Marian C M, Pflaum J, Lambert C, and Steffen A

    has just been published in Journal of the American Chemical Society.

    Thermally activated delayed fluorescence materials are promising candidates for next generation of OLED applications. For the first time we characterized a series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries. As a peculiarity of this class, the excited states are of large metal ligand charge transfer or ligand ligand charge transfer character.
    Beside an exceptional deep red emission, we found fast radiative rate constants, which exceed those of commercially employed IrIII- or PtII-based emitters. Temperature dependent lifetime measurement revealed a singlet-triplet gap of only 65 meV. Furthermore the complexes were found to be sufficiently bright and stable such that they are potentially useful as non-classical light sources for quantum communication or quantum cryptography applications. We demonstrated this non-classical behavior by probing photon antibunching in the emission of single copper(I) complex molecules.


    Based on very fruitful collaboration between various research groups a review on

    Organic-based thermoelectrics

    by Lindorf M,  Mazzio K A, Pflaum J, Nielsch K, Brütting W, and Albrecht M

    has been recently published in Journal of Materials Chemistry A.

    By their abundance, sustainability as well as mechanical properties organic-based thermoelectrics constitute an interesting alternative to inorganic materials and offer new fields of application for the recuperation of waste heat into electric power. This review, which results from an intense collaboration between different research groups working on this subject, provides a general overview of the advantages and challenges of polymeric as well as molecular compounds for thermoelectric applications, their underlying physics, achievements with respect to efficiency and performance and possible future strategies for implementation in thermoelectric generators.

    Enjoy reading!


    Our collaboration on

    Effect of the Degree of the Gate-Dielectric Surface Roughness on the Performance of Bottom-Gate Organic Thin-Film Transistors

    by Geiger M, Acharya R, Reutter E, Ferschke T, Zschieschang U, Weis J, Pflaum J, Klauk H, and Weitz R T

    has just been published in Advanced Materials Interfaces.

    In order to pave the way for fully integrated devices such as active-matrix displays on flexible substrates based on organic semiconductors (OSCs) it is neccessary to further understand and improve organic thin-film transistor performance.

    In this paper this requirement is addressed by investigating the influence of the gate-dielectric roughness on key transistor performance parameters like carrier mobility and substhreshold swing (see figure). Gate dielectric roughness is varied via different substrate temperatures during film deposition. Furthermore the influence of the grain density within the active OSC layer (DNTT) is disentangled from the influence of the (for transistor operation crucial) dielectric-OSC interface by preparation of a comparative set of devices with specifically engineered grain densities within the OSC layer.



    After completion of her master's thesis titled "Photoinduzierter Metall-Isolator-Übergang: Untersuchung von Cu(DCNQI-d6)2 Einkristallen" we welcome Lisa Schraut-May who will stay with our group for her PhD. We wish you all the best for your future scientific work with us.


    Our current work on

    Application of Fluorescent Molecules as Noninvasive Sensors for Optoelectronic Characterization on Nanometer Length Scales

    by Ferschke T, Hofmann A, Brütting W, and Pflaum J

    has been published in ACS Applied Electronic Materials.

    The non-invasive, in operando characterization of opto-electronic properties of organic semiconductors is of special interest as it enables information on microscopic processes governing the performance of a device and sheds light on its real lifecycle.

    In this paper we present a new approach to cope with this challenge by utilizing fluorescent molecules as optically addressable sensors, deterministically positioned at very low concentration in organic light-emitting diodes (OLEDs) as model system. We demonstrate experimentally and complemented by simulations, that the molecular fluorescence measured as function of applied voltage provides information on charge carrier distribution and transfer processes, both in operando and with nanometer spatial resolution.


    Our newest paper on

    Phase transition induced spectral tuning of dual luminescent crystalline zinc-phthalocyanine thin films and OLEDs

    by Hammer S, Ferschke T, v Eyb G, and Pflaum J

    has just been published in Applied Physics Letters.

    The development of multi luminescent organic light emitting diodes (OLEDs) in the near IR constitutes a crucial step towards new photonic devices for future optical data transmission.
    In this paper, we address these demands by preparing a dual luminescent OLED from just a single active organic material, zinc phthalocyanine, utilizing its thermally induced phase transition. We disclose the fundamental physical process underlying the respective luminescence and demonstrate the application in prototypical devices.

    This contribution has also been featured as a “Scilight”, showcasing the research to the broader interested public.


    A new paper on insight into

    Thermoelectric properties improvement in quasi-one-dimensional organic crystals

    by Sanduleac I, Pflaum J, and Casian A

    has just been published by the Journal of Applied Physics.

    In collaboration with our colleagues at the Technical University of Moldova, a theoretical study on the electronic transport characteristics of quasi-1D-organic metals has been carried-out, the latter considered potential candidates for future thermoelectrics. Explicitly taking into account electron-phonon interactions and their partial compensation by the narrow density of states we were able to calculate the distribution of charge carrier relaxation times (s. figure). With reference to p-type tetrathiotetracene-iodide (TTT2I3) and n-type tetrathiotetracene-tetracyanoquinodimethane (TTT(TCNQ)2) single crystals of sufficient purity and optimized energy level alignment thermoelectric figures-of-merit ZT of 5 and ∼ 1.5 are predicted, respectively.


    A new paper about our collaborative work with the chemistry department on

    Luminescent Mono‐, Di‐, and Triradicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes

    by Hattori Y, Michail E, Schmiedel A, Moos M, Holzapfel M, Krummenacher I, Braunschweig H, Müller U, Pflaum J, and Lambert C

    has been published in Chemistry - A European Journal.

    Molecular radicals and biradicals offer very interesting possibilities to harvest photons not only from excited singlet states but also states with triplet spin character. In this publication, triarylmethyl-based radicals have been investigated in detail showing a rich variety of electronic, magnetic as well as optical properties as function of the respective molecular linkage. Besides complementary theoretical modelling the electronic structure by density functional theory, first proof-of-concept OLEDs emitting in the NIR at about 900 nm have been successfully demonstrated (s. figure) and confirm the technological potential of this fairly stable class of open-shell radical compounds.


    A new paper on

    Ultrafast Pathways of the Photoinduced Insulator–Metal Transition in a Low‐Dimensional Organic Conductor

    by Smit B, Hüwe F, Payne N, Olaoye O, Bauer I, Pflaum J, Schwoerer M, and Schwoerer H

    has been published in Advanced Materials.

    Low dimensional molecular crystals represent an intriguing class of solid states due to their tunable electronic, magnetic and structural ground states. In this contribution we investigated Cu(Me,Br-Dicyanoquinonediimine)2 single crystals which exhibits a Peierls insulator-to-metal transition at around 155 K. By ultrafast electron diffraction experiments (s. figure) we were able to disclose for the first time the underlying atomic motions accompanying and stabilizing the photo-induced phase transition which happens within 2 picoseconds across the entire crystal volume. Identifying the microscopic pathways that drive the Peierls transition into the transient metallic state highlights the tailored response to external stimuli possible in this class of low dimensional electronic systems.


    After successfull completion of their master's theses titled "Aufbau und Optimierung eines Messplatzes zur zeitaufgelösten Fluoreszenzspektroskopie an Tetracen und Ditetracen" and "Aufbau eines Oberflächenplasmonresonanz-Spektrometers zur Bestimmung der Dispersionsrelation von Metall/F16ZnPc Hybridstrukturen" we welcome Maximilian Frank and Maximilian Rödel who will stay with our group for their PhD projects. We wish you all the best for your continued work within our group.


    New paper on

    Quantitative Analysis of the Density of Trap States in Semiconductors by Electrical Transport Measurements on Low-Voltage Field-Effect Transistors

    by M. Geiger, L Schwarz, U. Zschieschang, D. Manske, J. Pflaum, J. Weis, H. Klauk, and R.T. Weitz

    has just been published in PHYSICAL REVIEW APPLIED.

    Trap states and their energetic distribution play a key role in the electrical performance of thin film transistors based on either inorganic or organic semiconductors. Whereas for inorganic semiconductors elaborated models exist since many years, the specific characteristics of organic semiconductors are not fully accounted for by those. In the present study, which results from a collaboration between groups in Stuttgart, München and Würzburg, the well-established Grünewald model (Phys. Stat. Sol. B 100, K139 ) has been extended to the low-voltage regime, i.e. for thin gate dielectrics, being relevant for the operation of organic thin film transistors. As proven by the good agreement between the extended model and the experimental data on various transistor geometries, the Grünewald description holds true not only for the extended voltage regime but, in addition, offers an unique approach to extract the relevant trap density of states for this kind of devices.



    ‚Ein Laser-Mikroskop für nur 50 Euro‘ – Neuer Beitrag im aktuellen Campus Magazin

    Wie kann man aufwendige Versuchsaufbauten, die in der aktuellen Forschung Anwendung finden, für die Ausbildung und Lehre kostengünstig und unter Erhalt der wichtigsten funktionalen Eigenschaften realisieren?

    Mit dieser Fragestellung haben wir uns im Rahmen zweier Zulassungsarbeiten befasst und ein Laser-Scanning Mikroskop entwickelt und in Betrieb genommen, welches auf zwei handelsüblichen DVD-Spielern und einem einfachen Mikrokontroller basiert. Die aktuelle Version des Gerätes besitzt ein laterales Auflösungsvermögen von 17 µm und erlaubt die Oberflächenuntersuchung von Proben mit Abmessungen von bis zu 5 cm. Der Gesamtpreis des Gerätes beläuft sich dabei nur auf etwa 50 €. Über das Projekt berichtet das Campus Magazin in seiner neusten Ausgabe vom September 2018.

    Klick zum Beitrag im CAMPUS Magazin


    On July 6th 2018, Verena Kolb successfully defended her PhD thesis on the topic of 'Impact of metallic nanostructures on the opto-electronic properties of organic semiconductors'. Congratulations to Verena and all the best for her future.


    "Coherent two-dimensional fluorescence micro-spectroscopy"

    by S. Götz, D. Li, V. Kolb, J. Pflaum, and T. Brixner

    just been published in Optics Express.

    Locally resolved non-linear spectroscopy on nanostructured surfaces reveals important information on the microscopic excitation processes as well as on the effects of structural homogeneity on the emission characteristics. In this publication, a newly developed coherent 2D fluorescence micro-spectroscopy setup with sub-micron resolution is presented and has been successfully tested on hexagonally patterned perfluorinated zinc-phthalocyanine (F16ZnPc) nanopillars of about 200 nm in size and 500 nm in lattice constant. As it was demonstrated, the rephasing, non-rephasing and absorptive 2D spectra could be reconstructed and related to the local morphology of the structured surface and its lateral disturbances. The technique reported is thus capable of detecting the nonlinear response in chromophoric systems and their couplings to the close proximity.


    By a successful and scientifically highly exciting oral defence, Florian Hüwe finished his PhD in our group on July 26th. In his thesis titled: 'Electrothermal Investigation of Charge and Heat Transport in the Low-Dimensional Organic Conductor (DCNQI)2Cu' he analyses the electrical and thermal properties of archetypical low-dimensional organic metals and demonstrates their potential for future thermoelectric applications.

    Congratulations and all the best for your future at Zeiss Semiconductor Manufactoring Technology!


    A new publication on "QM/MM calculations combined with the dimer approach on the static disorder at organic-organic interfaces of thin-film organic solar cells composed of small molecules"

    by C. Brückner. M. Stolte, F. Würthner, J. Pflaum, B. Engels

    has just been accepted for publication in the Journal of Physical Organic Chemistry.

    In this work, masterminded by the colleagues from theoretical chemistry, the effect of energetic disorder occuring in the vicinity of interfaces in amorphous organic heterostructures has been analyzed in great detail for various molecular semiconductors by extended calculations on the QM/MM level. Variations of ground-, excited-, and cationic-state energies as well as of ionization potentials and excitation energies unambiguosly corroborate the importance of this physical quantity on the charge and exciton transport and thus, on the opto-electronic performance of organic thin film devices.


    Congratulations to Michael Brendel. On July 7, he successfully defended his doctoral thesis on the topic of "Correlation between Interface Energetics of Molecular Semiconductors and Opto-Electronic Properties of Planar Organic Solar Cells". We wish him best luck for the future and hope that he can benefit from his knowledge gained on interface and thin film physics also at his new job at Zeiss Semiconductor Manufactoring Technology.


    New publication on "Energy Losses in Small-Molecule Organic Photovoltaics"
    by T. Linderl, T. Zechel, M. Brendel, D. Moseguí González, P. Müller-Buschbaum, J. Pflaum, W. Brütting
    in Advanced Energy Materials

    Energy losses play an important role in the performance of organic photovoltaic cells, but up-to-date, are not completely understood on microscopic scales. In this contribution, which emerged from a collaborative work of the universities in Augsburg, Munich and Wuerzburg within the "Solar Technologies Go Hybrid (SolTech)" project of the Bavarian Government, we address this fundamental question by analyzing the impact of charge-transfer states generated at molecular donor/acceptor interfaces on the resulting device parameters, in particular, the open-circuit voltage.


    We congratulate Benedikt Stender on his successful PhD thesis defense on the topic of "Single photon emitters and their interactions with charge carriers in organic light emitting diodes".


    Our latest manuscript on the topic "Hybrid metal-organic nanocavity arrays for efficient light out-coupling"
    by V. Kolb and J. Pflaum

    has been published in Optics Express.

    We investigated the light outcoupling of periodically ordered hybrid metal/organic nanocavities by means of confocal photoluminescence microscopy. We were able to distinguish between plasmonic and geometrical contributions to the overall emission enhancement compared to unstructured thin films. Supporting TCSPC measurements revealed an exciton lifetime reduction of at least one order of magnitude, consistent with the plasmonic contribution to an areal emission enhancement up to 700.


    Our latest manuscript with the topic "Low-Cost and Sustainable Organic Thermoelectrics Based on Low-Dimensional Molecular Metals"
    by F. Huewe, A. Steeger, K. Kostova, L. Burroughs, I. Bauer, P. Strohriegl, V. Dimitrov, S. Woodward, and J. Pflaum

    has been published in Advanced Materials.

    In this contribution we investigate the material class of low-dimensional organic conductors with respect to their thermoelectric application potential. The molecular metals DCNQI2Cu (n-type) and TTT2I3 (p-type) not only reveal appreciable thermoelectric performance at room temperature but even outperform conventional inorganic thermoelectrics at cryogenic temperatures. A thermoelectric generator built of the two materials reveals areal power outputs of unprecendented magnitude in organic thermoelectrics.


    Congratulations to Nis Hauke Hansen on the successful defense of his PhD thesis entitled "Microscopic charge transport mechanisms and exciton annihilation in organic thin films and single crystals"!


    We welcome the new PhD students in our group! Sebastian Hammer joined our group already in July after finishing his graduate thesis in Experimental Physics 5. Thomas Ferschke remains for his PhD thesis in our group after submitting his master thesis on "Photoluminescence Studies on Tetraphenyldibenzoperiflanthene (DBP) Guest Molecules in Tris(8-hydroxyquinolinato)aluminum (Alq3) Based Light Emitting Diodes". We wish you fun and success during your studies here!


    Again we finished the ChemCup 2016 football tournament as best physics team! Special thanks to the organizers as well as every member of our outstanding team!


    We congratulate Anna Katharina Topczak on succesfully defending her PhD thesis on "Mechanisms of the exciton transport and its dynamics in molecular thin films for organic photovoltaic applications".


    Congratulations to Andreas Steindamm on defending his PhD thesis with the title "Excitonic loss mechanisms in organic bilayer solar cells"!


    We published a new article on the "Energy exchange between phononic and electronic subsystems governing the nonlinear conduction in DCNQI2Cu"
    by Florian Huewe, Alexander Steeger, Irene Bauer, Steffen Doerrich, Peter Strohriegl, and Jens Pflaum

    in Physical Review B.
    DOI: 10.1103/PhysRevB.92.155107.

    Our dynamical analysis of the resistive switching in the low-dimensional organic conductor DCNQI2Cu revealed a nonequilibrium optical phonon population by efficient interaction with the charge-carrying entity. Therefore, we extended the electrothermal model by a multiplication channel of charge-carriers via optical phonons leading to the nonlinear conduction in DCNQI2Cu.


    Our new publication on "Diketopyrrolopyrroles with a Distinct Energy Level Cascade for Efficient Charge Carrier Generation in Organic Solar Cells"
    by Christian J. Mueller, Michael Brendel, Pia Ruckdeschel, Jens Pflaum, and Mukundan Thelakkat

    has been published in Advanced Energy Materials.
    DOI: 10.1002/aenm.201500914.

    By tuning the electron deficiency of the terminal aryl unit of low molecular weight diketopyrrolopyrrole compounds a defined modification of their frontier orbital energies is established. Utilizing these compounds in cascade solar cells in combination with the electron acceptor C60 yields a huge increase of the short circuit current densities and power conversion efficiencies in comparison to the corresponding bilayer devices.


    Our new paper on "The Effect of Gradual Fluorination on the Properties of FnZnPc Thin Films and FnZn Pc/C60 Bilayer Photovoltaic Cells"
    by M. Brendel, S. Krause, A. Steindamm, A. K. Topczak, S. Sundarraj, P. Erk, N. Koch, and J. Pflaum

    has been published in Advanced Functional Materials.
    DOI: 10.1002/adfm.201404434.

    The impact of gradually fluorinated zinc phthalocyanine molecules on the opto-electronic properties of FnZnPc/C60 bilayer solar cells is investigated. Upon increasing the degree of fluorination, distinct variations of the cell parameters such as the open circuit voltage and the short circuit current density are detected. In combination with complementary photoluminescence and ultraviolet photoelectron spectroscopy measurements a detailed picture of the relevant donor/acceptor interface energetics and processes evolving on microscopic length scales is developed.