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  • 151.
    Sharma, A.
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, N. M. A.
    Eindhoven University of Technology, Netherlands.
    Mathijssen, S. G. J.
    Eindhoven University of Technology, Netherlands; Philips Research Labs Eindhoven, Netherlands.
    de Leeuw, D. M.
    Philips Research Labs Eindhoven, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Bobbert, P. A.
    Eindhoven University of Technology, Netherlands.
    Effect of Coulomb scattering from trapped charges on the mobility in an organic field-effect transistor2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 12, article id 125310Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of Coulomb scattering from trapped charges on the mobility in the two-dimensional channel of an organic field-effect transistor. The number of trapped charges can be tuned by applying a prolonged gate bias. Surprisingly, after increasing the number of trapped charges to a level where strong Coulomb scattering is expected, the mobility has decreased only slightly. Simulations show that this can be explained by assuming that the trapped charges are located in the gate dielectric at a significant distance from the channel instead of in or very close to the channel. The effect of Coulomb scattering is then strongly reduced.

  • 152.
    Sharma, A.
    et al.
    Technical University of Eindhoven, Netherlands.
    Mathijssen, S. G. J.
    Technical University of Eindhoven, Netherlands; Philips Research Labs, Netherlands.
    Cramer, T.
    University of Bologna, Italy.
    Kemerink, M.
    Technical University of Eindhoven, Netherlands.
    de Leeuw, D. M.
    Philips Research Labs, Netherlands.
    Bobbert, P. A.
    Technical University of Eindhoven, Netherlands.
    Anomalous current transients in organic field-effect transistors2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 10, article id 103306Article in journal (Refereed)
    Abstract [en]

    Here we study the origin of the gate bias-stress effect in organic p-type transistors. Based on water-mediated exchange between holes in the semiconductor and protons in the gate dielectric, we predict anomalous current transients for a non-constant gate bias, while ensuring accumulation. When applying a strongly negative gate bias followed by a less negative bias a back-transfer of protons to holes and an increase of the current is expected. We verify this counterintuitive behavior experimentally and can quantitatively model the transients with the same parameters as used to describe the threshold voltage shift. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3339879]

  • 153.
    Sharma, A.
    et al.
    Technical University of Eindhoven, Netherlands.
    Mathijssen, S. G. J.
    Technical University of Eindhoven, Netherlands; Philips Research Labs, Netherlands.
    Kemerink, M.
    Technical University of Eindhoven, Netherlands.
    de Leeuw, D. M.
    Philips Research Labs, Netherlands.
    Bobbert, P. A.
    Technical University of Eindhoven, Netherlands.
    Proton migration mechanism for the instability of organic field-effect transistors2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 25, article id 253305Article in journal (Refereed)
    Abstract [en]

    During prolonged application of a gate bias, organic field-effect transistors show an instability involving a gradual shift of the threshold voltage toward the applied gate bias voltage. We propose a model for this instability in p-type transistors with a silicon-dioxide gate dielectric, based on hole-assisted production of protons in the accumulation layer and their subsequent migration into the gate dielectric. This model explains the much debated role of water and several other hitherto unexplained aspects of the instability of these transistors. (C) 2009 American Institute of Physics. [doi:10.1063/1.3275807]

  • 154.
    Sharma, A.
    et al.
    Eindhoven University of Technology, Netherlands.
    Mathijssen, S. G. J.
    Eindhoven University of Technology, Netherlands; Philips Research Labs Eindhoven, Netherlands.
    Smits, E. C. P.
    Philips Research Labs Eindhoven, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    de Leeuw, D. M.
    Philips Research Labs Eindhoven, Netherlands.
    Bobbert, P. A.
    Eindhoven University of Technology, Netherlands.
    Proton migration mechanism for operational instabilities in organic field-effect transistors2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 7, article id 075322Article in journal (Refereed)
    Abstract [en]

    Organic field-effect transistors exhibit operational instabilities involving a shift of the threshold gate voltage when a gate bias is applied. For a constant gate bias the threshold voltage shifts toward the applied gate bias voltage, an effect known as the bias-stress effect. Here, we report on a detailed experimental and theoretical study of operational instabilities in p-type transistors with silicon-dioxide gate dielectric both for a constant as well as for a dynamic gate bias. We associate the instabilities with a reversible reaction in the organic semiconductor in which holes are converted into protons in the presence of water and a reversible migration of these protons into the gate dielectric. We show how redistribution of charge between holes in the semiconductor and protons in the gate dielectric can consistently explain the experimental observations. Furthermore, we show how a shorter period of application of a gate bias leads to a faster backward shift of the threshold voltage when the gate bias is removed. The proposed mechanism is consistent with the observed acceleration of the bias-stress effect with increasing humidity, increasing temperature, and increasing energy of the highest molecular orbital of the organic semiconductor.

  • 155.
    Sharma, A.
    et al.
    Eindhoven University of Technology, Netherlands.
    Mathijssen, Simon G. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    de Leeuw, Dago M.
    [Sharma, Netherlands.
    Bobbert, Peter A.
    Eindhoven University of Technology, Netherlands.
    Bias-stress effect and recovery in organic field effect transistors: Proton migration mechanism2010In: ORGANIC FIELD-EFFECT TRANSISTORS IX, Society of Photo-optical Instrumentation Engineers (SPIE) , 2010, Vol. 7778, article id 77780QConference paper (Refereed)
    Abstract [en]

    Organic field-effect transistors exhibit operational instabilities when a gate bias is applied. For a constant gate bias the threshold voltage shifts towards the applied gate bias voltage, an effect known as the bias-stress effect. We have performed a detailed experimental and theoretical study of operational instabilities in p-type transistors with silicon-dioxide gate dielectric. We propose a mechanism in which holes in the semiconductor are converted into protons in the presence of water and a reversible migration of these protons into the gate dielectric to explain the instabilities in organic transistors. We show how redistribution of charge between holes in the semiconductor and protons in the gate dielectric can consistently explain the experimental observations. Furthermore, we explain in detail the recovery of a pres-stressed transistor on applying zero gate bias. We show that recovery dynamics depends strongly on the extent of stressing. Our mechanism is consistent with the known aspects of bias-stress effect like acceleration due to humidity, constant activation energy and reversibility.

  • 156.
    Sharma, A.
    et al.
    Technical University of Eindhoven, Netherlands.
    van Oost, F. W. A.
    Technical University of Eindhoven, Netherlands.
    Kemerink, M.
    Technical University of Eindhoven, Netherlands.
    Bobbert, P. A.
    Technical University of Eindhoven, Netherlands.
    Dimensionality of charge transport in organic field-effect transistors2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 23, article id 235302Article in journal (Refereed)
    Abstract [en]

    Application of a gate bias to an organic field-effect transistor leads to accumulation of charges in the organic semiconductor within a thin region near the gate dielectric. An important question is whether the charge transport in this region can be considered two-dimensional, or whether the possibility of charge motion in the third dimension, perpendicular to the accumulation layer, plays a crucial role. In order to answer this question we have performed Monte Carlo simulations of charge transport in organic field-effect transistor structures with varying thickness of the organic layer, taking into account all effects of energetic disorder and Coulomb interactions. We show that with increasing thickness of the semiconductor layer the source-drain current monotonically increases for weak disorder, whereas for strong disorder the current first increases and then decreases. Similarly, for a fixed layer thickness the mobility may either increase or decrease with increasing gate bias. We explain these results by the enhanced effect of state filling on the current for strong disorder, which competes with the effects of Coulomb interactions and charge motion in the third dimension. Our conclusion is that apart from the situation of a single monolayer, charge transport in an organic semiconductor layer should be considered three-dimensional, even at high gate bias.

  • 157.
    Smits, Edsger C. P.
    et al.
    University of Groningen, Netherlands; Philips Research Labs, Netherlands; Dutch Polymer Institute, Netherlands.
    Mathijssen, Simon G. J.
    Philips Research Labs, Netherlands; Eindhoven University of Technology, Netherlands.
    van Hal, Paul A.
    Philips Research Labs, Netherlands.
    Setayesh, Sepas
    Philips Research Labs, Netherlands.
    Geuns, Thomas C. T.
    Philips Research Labs, Netherlands.
    Mutsaers, Kees A. H. A.
    Philips Research Labs, Netherlands.
    Cantatore, Eugenio
    Eindhoven University of Technology, Netherlands.
    Wondergem, Harry J.
    Philips Research Labs, Netherlands.
    Werzer, Oliver
    Graz University of Technology, Austria.
    Resel, Roland
    Graz University of Technology, Austria.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Kirchmeyer, Stephan
    HC Starck GmbH, Germany.
    Muzafarov, Aziz M.
    Russian Academic Science, Russia.
    Ponomarenko, Sergei A.
    Russian Academic Science, Russia.
    de Boer, Bert
    University of Groningen, Netherlands.
    Blom, Paul W. M.
    University of Groningen, Netherlands.
    de Leeuw, Dago M.
    University of Groningen, Netherlands; Philips Research Labs, Netherlands.
    Bottom-up organic integrated circuits2008In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 455, no 7215, p. 956-959Article in journal (Refereed)
    Abstract [en]

    Self- assembly - the autonomous organization of components into patterns and structures(1) - is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom- up approach involving self- assembling molecules was proposed(2) in the 1970s. The basic building block of such an integrated circuit is the self- assembled- monolayer field- effect transistor ( SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels(3-5), the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular pi-pi coupling between the molecules in the self-assembled monolayers. Low field- effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom- up integrated circuits. Here we demonstrate SAMFETs with long- range intermolecular pi - pi coupling in the monolayer. We achieve dense packing by using liquid- crystalline molecules consisting of a pi- conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk- like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15- bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom- up electronics in a new perspective.

  • 158.
    Spijkman, M.
    et al.
    Philips Research Labs, Netherlands; University of Groningen, Netherlands.
    Mathijssen, S. G. J.
    Philips Research Labs, Netherlands; Eindhoven University of Technology, Netherlands.
    Smits, E. C. P.
    TNO, Netherlands.
    Kemerink, M.
    [Spijkman, Netherlands.
    Blom, P. W. M.
    University of Groningen, Netherlands; Eindhoven University of Technology, Netherlands; TNO, Netherlands.
    de Leeuw, D. M.
    Philips Research Labs, Netherlands; University of Groningen, Netherlands.
    Monolayer dual gate transistors with a single charge transport layer2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 14, article id 143304Article in journal (Refereed)
    Abstract [en]

    A dual gate transistor was fabricated using a self-assembled monolayer as the semiconductor. We show the possibility of processing a dielectric on top of the self-assembled monolayer without deteriorating the device performance. The two gates of the transistor accumulate charges in the monomolecular transport layer and artifacts caused by the semiconductor thickness are negated. We investigate the electrical transport in a dual gate self-assembled monolayer field-effect transistor and present a detailed analysis of the importance of the contact geometry in monolayer field-effect transistors.

  • 159.
    Tang, Shi
    et al.
    Umeå University, Sweden; LunaLEC AB, Sweden.
    Sandstrom, Andreas
    Umeå University, Sweden; LunaLEC AB, Sweden.
    Lundberg, Petter
    Umeå University, Sweden.
    Lanz, Thomas
    Umeå University, Sweden.
    Larsen, Christian
    Umeå University, Sweden; LunaLEC AB, Sweden.
    van Reenen, Stephan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Edman, Ludvig
    Umeå University, Sweden; LunaLEC AB, Sweden.
    Design rules for light-emitting electrochemical cells delivering bright luminance at 27.5 percent external quantum efficiency2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 1190Article in journal (Refereed)
    Abstract [en]

    The light-emitting electrochemical cell promises cost-efficient, large-area emissive applications, as its characteristic in-situ doping enables use of air-stabile electrodes and a solution-processed single-layer active material. However, mutual exclusion of high efficiency and high brightness has proven a seemingly fundamental problem. Here we present a generic approach that overcomes this critical issue, and report on devices equipped with air-stabile electrodes and outcoupling structure that deliver a record-high efficiency of 99.2 cd A(-1) at a bright luminance of 1910 cd m(-2). This device significantly outperforms the corresponding optimized organic light-emitting diode despite the latter employing calcium as the cathode. The key to this achievement is the design of the host-guest active material, in which tailored traps suppress exciton diffusion and quenching in the central recombination zone, allowing efficient triplet emission. Simultaneously, the traps do not significantly hamper electron and hole transport, as essentially all traps in the transport regions are filled by doping.

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  • 160.
    Timpanaro, S
    et al.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Touwslager, FJ
    Eindhoven University of Technology, Netherlands.
    De Kok, MM
    Eindhoven University of Technology, Netherlands.
    Schrader, S
    Eindhoven University of Technology, Netherlands.
    Morphology and conductivity of PEDOT/PSS films studied by scanning-tunneling microscopy2004In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 394, no 4-6, p. 339-343Article in journal (Refereed)
    Abstract [en]

    The influence of sorbitol on the nanometer-scale morphology of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) is investigated by scanning-tunneling microscopy. In all investigated films relatively well-conducting PEDOT particles are observed. with typical sizes of 10-50 nm, that are embedded in a less conductive PSS matrix. Addition of sorbitol to the casting solution is found to enhance the clustering of the PEDOT particles into larger domains. The observed morphologies are correlated to the macroscopic conductivity of the films, using an intuitive model. In addition, the morphology in the top layer of the films was found to differ substantially from the bulk morphology. (C) 2004 Elsevier B.V. All rights reserved.

  • 161.
    Upreti, Tanvi
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Wang, Yuming
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Zhang, Huotian
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Scheunemann, Dorothea
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Gao, Feng
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors2019In: Physical Review Applied, E-ISSN 2331-7019, Vol. 12, no 6, article id 064039Article in journal (Refereed)
    Abstract [en]

    Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy-dependent density of states. Extracting disorder and hopping parameters from experimental data, such as temperature-dependent current-voltage characteristics, typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) is extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable-range hopping (VRH) model that is integrated in a freeware drift-diffusion solver. The mobility model is calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space-charge-limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p- and n-type devices. We find a critical ratio of a(NN)/alpha (mean intersite distance:localization radius) of about three, below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance, when the same active layer is used in an organic solar cell.

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  • 162.
    Urbanaviciute, Indre
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Bhattacharjee, Subham
    Eindhoven Univ Technol, Netherlands.
    Biler, Michal
    KTH Royal Inst Technol, Sweden.
    Lugger, Jody A. M.
    Eindhoven Univ Technol, Netherlands.
    Cornelissen, Tim
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Norman, Patrick
    KTH Royal Inst Technol, Sweden.
    Linares, Mathieu
    KTH Royal Inst Technol, Sweden; KTH Royal Inst Technol, Sweden.
    Sijbesma, Rint P.
    Eindhoven Univ Technol, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric2019In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 4, p. 2069-2079Article in journal (Refereed)
    Abstract [en]

    Despite being very well established in the field of electro-optics, ferroelectric liquid crystals so far lacked interest from a ferroelectric device perspective due to a typically high operating temperature, a modest remnant polarization and/or poor polarization retention. Here, we experimentally demonstrate how simple structural modification of a prototypical ferroelectric liquid-crystal benzene-1,3,5-trisamide (BTA) - introduction of branched-tail substituents - results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices at room temperature. The observed differences between linear- and branched-tail compounds are analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. We conclude that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear side-chains can be used to further improve the materials characteristics.

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  • 163.
    Urbanaviciute, Indre
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Cornelissen, Tim
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Meng, Xiao
    Eindhoven Univ Technol, Netherlands.
    Sijbesma, Rint P.
    Eindhoven Univ Technol, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Physical reality of the Preisach model for organic ferroelectrics2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 4409Article in journal (Refereed)
    Abstract [en]

    The Preisach model has been a cornerstone in the fields of ferromagnetism and ferroelectricity since its inception. It describes a real, non-ideal, ferroic material as the sum of a distribution of ideal hysterons. However, the physical reality of the model in ferroelectrics has been hard to establish. Here, we experimentally determine the Preisach (hysteron) distribution for two ferroelectric systems and show how its broadening directly relates to the materials morphology. We connect the Preisach distribution to measured microscopic switching kinetics that underlay the macroscopic dispersive switching kinetics as commonly observed for practical ferroelectrics. The presented results reveal that the in principle mathematical construct of the Preisach model has a strong physical basis and is a powerful tool to explain polarization switching at all time scales in different types of ferroelectrics. These insights lead to guidelines for further advancement of the ferroelectric materials both for conventional and multi-bit data storage applications.

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  • 164.
    Urbanaviciute, Indre
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Meng, Xiao
    Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands .
    Biler, Michal
    Department of Chemistry – BMC, Uppsala University, Uppsala, Sweden.
    Wei, Yingfen
    Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands .
    Cornelissen, Tim D.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Bhattacharjee, Subham
    Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands.
    Linares, Mathieu
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering. Swedish e-Science Research Centre (SeRC), Stockholm, Sweden.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Negative piezoelectric effect in an organic supramolecular ferroelectric2019In: Materials Horizons, ISSN 2051-6347, E-ISSN 2051-6355, Vol. 6, p. 1688-1698Article in journal (Refereed)
    Abstract [en]

    The vast majority of ferroelectric materials demonstrate a positive piezoelectric effect. Theoretically, the negative piezoelectric coefficient d33 could be found in certain classes of ferroelectrics, yet in practice, the number of materials showing linear longitudinal contraction with increasing applied field (d33 < 0) is limited to few ferroelectric polymers. Here, we measure a pronounced negative piezoelectric effect in the family of organic ferroelectric small-molecular BTAs (trialkylbenzene-1,3,5-tricarboxamides), which can be tuned by mesogenic tail substitution and structural disorder. While the large- and small-signal strain in highly-ordered thin-film BTA capacitor devices are dominated by intrinsic contributions and originates from piezostriction, rising disorder introduces additional extrinsic factors that boost the large-signal d33 up to −20 pm V’1 in short-tailed molecules. Interestingly, homologues with longer mesogenic tails show a large-signal electromechanical response that is dominated by the quadratic Maxwell strain with significant mechanical softening upon polarization switching, whereas the small-signal strain remains piezostrictive. Molecular dynamics and DFT calculations both predict a positive d33 for defect-free BTA stacks. Hence, the measured negative macroscopic d33 is attributed to the presence of structural defects that enable the dimensional effect to dominate the piezoelectric response of BTA thin films.

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    Negative piezoelectric effect in an organic supramolecular ferroelectric
  • 165.
    Urbanaviciute, Indre
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Meng, Xiao
    Eindhoven University of Technology, Netherlands.
    Cornelissen, Tim
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Gorbunov, Andrey V.
    Eindhoven University of Technology, Netherlands.
    Bhattacharjee, Subham
    Eindhoven University of Technology, Netherlands.
    Sijbesma, Rint P.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Tuning the Ferroelectric Properties of Trialkylbenzene-1,3,5-tricarboxamide (BTA)2017In: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 3, no 7, article id 1600530Article in journal (Refereed)
    Abstract [en]

    This study demonstrates how simple structural modification of a prototypical organic ferroelectric molecule can be used to tune its key ferroelectric properties. In particular, it is found that shortening the alkyl chain length of trialkylbenzene-1,3,5-tricarboxamide (BTA) from C18H37 to C6H13 causes an increase in depolarization activation energy (approximate to 1.1-1.55 eV), coercive field (approximate to 25-40 V mu m(-1)), and remnant polarization (approximate to 20-70 mC m(-2)). As the polarization enhancement far exceeds the geometrically expected factor, these observations are attributed to an increase in the intercolumnar interaction. The combination of the mentioned characteristics results in a record polarization retention time of close to three months at room temperature for capacitor devices of the material having the shortest alkyl chain. The long retention and the remnant polarization that is as high as that of P(VDF:TrFE) distinguish the BTA-C6 material from other small molecular organic ferroelectrics and make it a perspective choice for applications that require cheap, flexible, and lightweight ferroelectrics.

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  • 166.
    van Breemen, Albert J. J. M.
    et al.
    Holst Centre TNO, Netherlands.
    van der Steen, Jan-Laurens
    Holst Centre TNO, Netherlands.
    van Heck, Geri
    Holst Centre TNO, Netherlands.
    Wang, Rui
    Holst Centre TNO, Netherlands; Eindhoven University of Technology, Netherlands.
    Khikhlovskyi, Vsevolod
    Holst Centre TNO, Netherlands; Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Eindhoven, The Netherlands.
    Gelinck, Gerwin H.
    Holst Centre TNO, Netherlands.
    Crossbar arrays of nonvolatile, rewritable polymer ferroelectric diode memories on plastic substrates2014In: APPLIED PHYSICS EXPRESS, ISSN 1882-0778, Vol. 7, no 3, article id 031602Article in journal (Refereed)
    Abstract [en]

    In this paper, we demonstrate a scalable and low-cost memory technology using a phase separated blend of a ferroelectric polymer and a semiconducting polymer as data storage medium on thin, flexible polyester foils of only 25 mu m thickness. By sandwiching this polymer blend film between rows and columns of metal electrode lines where each intersection makes up one memory cell, we obtained 1 kbit cross bar arrays with bit densities of up to 10 kbit/cm(2). (C) 2014 The Japan Society of Applied Physics

  • 167.
    van Breemen, Albert
    et al.
    Holst Centre/TNO, Eindhoven, The Netherlands.
    Zaba, Tomasz
    Holst Centre/TNO, Eindhoven, The Netherlands.
    Khikhlovskyi, Vsevolod
    Holst Centre/TNO, Eindhoven, The Netherlands; Eindhoven University of Technology, MB, Eindhoven, The Netherlands.
    Michels, Jasper
    Holst Centre/TNO, Eindhoven, The Netherlands.
    Janssen, Rene
    Eindhoven University of Technology, MB, Eindhoven, The Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, MB, Eindhoven, The Netherlands.
    Gelinck, Gerwin
    Holst Centre/TNO, Eindhoven, The Netherlands.
    Surface Directed Phase Separation of Semiconductor Ferroelectric Polymer Blends and their Use in Non-Volatile Memories2015In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 2, p. 278-286Article in journal (Refereed)
    Abstract [en]

    The polymer phase separation of P(VDF-TrFE):F8BT blends is studied in detail. Its morphology is key to the operation and performance of memory diodes. In this study, it is demonstrated that it is possible to direct the semiconducting domains of a phase-separating mixture of P(VDF-TrFE) and F8BT in a thin film into a highly ordered 2D lattice by means of surface directed phase separation. Numerical simulation of the surface-controlled de-mixing process provides insight in the ability of the substrate pattern to direct the phase separation, and hence the regularity of the domain pattern in the final dry blend layer. By optimizing the ratio of the blend components, the number of electrically active semiconductor domains is maximized. Pattern replication on a cm-scale is achieved, and improved functional device performance is demonstrated in the form of a 10-fold increase of the ON-current and a sixfold increase in current modulation. This approach therefore provides a simple and scalable means to higher density integration, the ultimate target being a single semiconducting domain per memory cell.

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  • 168.
    van de Ruit, Kevin
    et al.
    Eindhoven University of Technology, Netherlands.
    Itzhak Cohen, Racheli
    Ben Gurion University of Negev, Israel.
    Bollen, Dirk
    Agfa Gevaert NV, Belgium.
    van Mol, Ton
    Holst Centre TNO, Netherlands.
    Yerushalmi-Rozen, Rachel
    Ben Gurion University of Negev, Israel; Ben Gurion University of Negev, Israel.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Quasi-One Dimensional in-Plane Conductivity in Filamentary Films of PEDOT:PSS2013In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 23, no 46, p. 5778-5786Article in journal (Refereed)
    Abstract [en]

    The mechanism and magnitude of the in-plane conductivity of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) thin fi lms is determined using temperature dependent conductivity measurements for various PEDOT: PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in-plane conductivity of PEDOT: PSS is lower and for all studied weight ratios well described by the relation s = s0exp[-T0 T 0.5] with T 0 a characteristic temperature. The exponent 0.5 indicates quasi-one dimensional (quasi-1D) variable range hopping (VRH). The conductivity prefactor s 0 varies over three orders of magnitudes and follows a power law s 0. c 3.5 PEDOT with c PEDOT the weight fraction of PEDOT in PEDOT: PSS. The fi eld dependent conductivity is consistent with quasi-1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi-1D fi laments. Using transmission electron microscopy (TEM) fi lamentary structures are observed in vitrifi ed dispersions and dried fi lms. For PEDOT: PSS fi lms with HBS, the conductivity also exhibits quasi-1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T 0 indicates that HBStreated fi lms are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with c PEDOT, indicating the conduction is no longer limited by a percolation of fi laments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual fi laments, or a higher probability for neighboring fi laments to be connected rather than being caused by major morphological modifi cation of the material.

  • 169.
    van de Ruit, Kevin
    et al.
    Eindhoven University of Technology, Netherlands.
    Katsouras, Ilias
    University of Groningen, Netherlands.
    Bollen, Dirk
    Agfa Gevaert NV, Belgium.
    van Mol, Ton
    Holst Centre TNO, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    de Leeuw, Dago M.
    Max Planck Institute Polymer Research, Germany.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    The Curious Out-of-Plane Conductivity of PEDOT:PSS2013In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 23, no 46, p. 5787-5793Article in journal (Refereed)
    Abstract [en]

    For its application as transparent conductor in light-emitting diodes and photovoltaic cells, both the in-plane and out-of-plane conductivity of PEDOT:PSS are important. However, studies into the conductivity of PEDOT:PSS rarely address the out-of-plane conductivity and those that do, report widely varying results. Here a systematic study of the out-of-plane charge transport in thin films of PEDOT:PSS with varying PSS content is presented. To this end, the PEDOT:PSS is enclosed in small interconnects between metallic contacts. An unexpected, but strong dependence of the conductivity on interconnect diameter is observed for PEDOT:PSS formulations without high boiling solvent. The change in conductivity correlates with a diameter dependent change in PEDOT:PSS layer thickness. It is suggested that the order of magnitude variation in out-of-plane conductivity with only a 3-4-fold layer thickness variation can quantitatively be explained on basis of a percolating cluster model.

  • 170.
    van der Hofstad, Tom G. J.
    et al.
    Eindhoven University of Technology, Netherlands.
    Di Nuzzo, Daniele
    Eindhoven University of Technology, Netherlands.
    van Reenen, Stephan
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Meskers, Stefan C. J.
    Eindhoven University of Technology, Netherlands.
    Carrier Recombination in Polymer Fullerene Solar Cells Probed by Reversible Exchange of Charge between the Active Layer and Electrodes Induced by a Linearly Varying Voltage2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 7, p. 3210-3220Article in journal (Refereed)
    Abstract [en]

    The use of a voltage pulse that varies linearly with time and that is symmetric in time around t = 0 allows for simultaneous determination of (photo)capacitance and (photo)conductance of polymer solar cells. From the measured capacitance, an average density of reversibly extractable carriers is determined, and the result is compared to numerical drift-diffusion simulations. Results are in agreement with large charge densities near the contacts that can be exchanged with the electrode in a thermodynamically reversible manner upon changing the voltage. The combined determination of capacitance and conductance yields a relaxation time tau(rel) for photogenerated charge carriers. Results on thermally annealed poly(3-hexylthiopene):fullerene bulk heterojunction solar cells indicate tau(rel) similar to 2 mu s, limited by extraction and not significantly affected by bimolecular recombination under intensities up to 1 sun. In contrast, for small bandgap poly(diketopyrrolopyrrole-alt-quinquethiophene)-fullerene solar cells with similar to 5% power conversion efficiency, tau(rel) is limited by bimolecular recombination. This illustrates the need for very fast charge transport rates to avoid losses due to bimolecular recombination in solar cells with high charge generation rates. Conclusions from the charge exchange experiments are confirmed by time domain measurements using pulsed illumination.

  • 171.
    van Eersel, Harm
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Mechanism for Efficient Photoinduced Charge Separation at Disordered Organic Heterointerfaces2012In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 22, no 13, p. 2700-2708Article in journal (Refereed)
    Abstract [en]

    Despite the poor screening of the Coulomb potential in organic semiconductors, excitons can dissociate efficiently into free charges at a donoracceptor heterojunction, leading to application in organic solar cells. A kinetic Monte Carlo model that explains this high efficiency as a two-step process is presented. Driven by the band offset between donor and acceptor, one of the carriers first hops across the interface, forming a charge transfer (CT) complex. Since the electron and hole forming the CT complex have typically not relaxed within the disorder-broadened density of states (DOS), their remaining binding energy can be overcome by further relaxation in the DOS. The model only contains parameters that are determined from independent measurements and predicts dissociation yields in excess of 90% for a prototypical heterojunction. Field, temperature, and band offset dependencies are investigated and found to be in agreement with earlier experiments. Whereas the investigated heterojunctions have substantial energy losses associated with the dissociation process, these results suggest that it is possible to reach high dissociation yields at low energy loss.

  • 172.
    van Reenen, S.
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, R. A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Doping dynamics in light-emitting electrochemical cells2011In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 12, no 10, p. 1746-1753Article in journal (Refereed)
    Abstract [en]

    A major drawback of light-emitting electrochemical cells (LECs) is the long time scale associated with switching, during which ions redistribute in the active layer. We present a numerical modeling study that gives fundamental insight in the dynamics during turn-on. The characteristic response of LECs to an applied bias is the electrochemical doping of the active layer by doping fronts moving across the active layer. Formation and motion of such doping fronts are shown to be intimately related to both the electronic and ionic mobility and therefore provide useful information regarding these two quantities in LECs. In particular, it is shown that the switch-on time in LECs is directly related to the time an ion needs to cross approximately half the device, enabling the extraction of the ion mobility from the switch-on time. (C) 2011 Elsevier B.V. All rights reserved.

  • 173.
    van Reenen, S.
    et al.
    Eindhoven University of Technology, Netherlands.
    Kersten, S. P.
    Eindhoven University of Technology, Netherlands.
    Wouters, S. H. W.
    Eindhoven University of Technology, Netherlands.
    Cox, M.
    Eindhoven University of Technology, Netherlands.
    Janssen, P.
    Eindhoven University of Technology, Netherlands.
    Koopmans, B.
    Eindhoven University of Technology, Netherlands.
    Bobbert, P. A.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Large magnetic field effects in electrochemically doped organic light-emitting diodes2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 12, article id 125203Article in journal (Refereed)
    Abstract [en]

    Large negative magnetoconductance (MC) of similar to 12% is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (V-bias). Simultaneously, a positive magnetoefficiency (M eta) of 9% is observed at V-bias = 2 V. At higher bias voltages, both the MC and M eta diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive M eta effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet: triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

  • 174.
    van Reenen, S.
    et al.
    Eindhoven University of Technology, Netherlands.
    Scheepers, M.
    Eindhoven University of Technology, Netherlands.
    van de Ruit, K.
    Eindhoven University of Technology, Netherlands.
    Bollen, D.
    Agfa Gevaert NV, Belgium.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology.
    Explaining the effects of processing on the electrical properties of PEDOT:PSS2014In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 15, no 12, p. 3710-3714Article in journal (Refereed)
    Abstract [en]

    By simultaneously measuring the Seebeck coefficient and the conductivity in differently processed PEDOT:PSS films, fundamental understanding is gained on how commonly used processing methods improve the conductivity of PEDOT:PSS. Use of a high boiling solvent (HBS) enhances the conductivity by 3 orders of magnitude, as is well-known. Simultaneously, the Seebeck coefficient S remains largely unaffected, which is shown to imply that the conductivity is improved by enhanced connectivity between PEDOT-rich filaments within the film, rather than by improved conductivity of the separate PEDOT filaments. Post-treatment of PEDOT: PSS films by washing with H2SO4 leads to a similarly enhanced conductivity and a significant reduction in the layer thickness. This reduction strikingly corresponds to the initial PSS ratio in the PEDOT:PSS films, which suggests removal and replacement of PSS in PEDOT:PSS by HSO4- or SO42- after washing. Like for the HBS treatment, this improves the connectivity between PEDOT filaments. Depending on whether the H2SO4 treatment is or is not preceded by an HBS treatment also the intra-filament transport is affected. We show that by characterization of S and sigma it is possible to obtain more fundamental understanding of the effects of processing on the (thermo) electrical characteristics of PEDOT:PSS.

  • 175.
    van Reenen, S.
    et al.
    Eindhoven University of Technology, The Netherlands.
    Vitorino, M.V.
    Eindhoven University of Technology, The Netherlands; University of Lisbon, Portugal .
    Meskers, S.C.J.
    Eindhoven University of Technology, The Netherlands.
    Janssen, R.A.J.
    Eindhoven University of Technology, The Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, The Netherlands.
    Photoluminescence quenching in films of conjugated polymers by electrochemical doping2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 20, p. 205206-Article in journal (Refereed)
    Abstract [en]

    An important loss mechanism in organic electroluminescent devices is exciton quenching by polarons. Gradual electrochemical doping of various conjugated polymer films enabled the determination of the doping density dependence of photoluminescence quenching. Electrochemical doping was achieved by contacting the film with a solid electrochemical gate and an injecting contact. A sharp reduction in photoluminescence was observed for doping densities between 1018 and 1019 cm(-3). The doping density dependence is quantitatively modeled by exciton diffusion in a homogeneous density of polarons followed by either F "orster resonance energy transfer or charge transfer. Both mechanisms need to be considered to describe polaron-induced exciton quenching. Thus, to reduce exciton-polaron quenching in organic optoelectronic devices, both mechanisms must be prevented by reducing the exciton diffusion, the spectral overlap, the doping density, or a combination thereof.

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  • 176.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Akatsuka, Takeo
    University of Valencia, Spain; Nippon Shokubai Co Ltd, Japan.
    Tordera, Daniel
    University of Valencia, Spain.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Bolink, Henk J.
    University of Valencia, Spain.
    Universal Transients in Polymer and Ionic Transition Metal Complex Light-Emitting Electrochemical Cells2013In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 135, no 2, p. 886-891Article in journal (Refereed)
    Abstract [en]

    Two types of light-emitting electrochemical cells (LECs) are commonly distinguished, the polymer-based LEC (p-LEC) and the ionic transition metal complex-based LEC (iTMC-LEC). Apart from marked differences in the active layer constituents, these LEC types typically show operational time scales that can differ by many orders of magnitude at room temperature. Here, we demonstrate that despite these differences p-LECs and iTMC-LECs show current, light output, and efficacy transients that follow a universal shape. Moreover, we conclude that the turn-on time of both LEC types is dominated by the ion conductivity because the turn-on time exhibits the same activation energy as the ion conductivity in the off-state. These results demonstrate that both types of LECs are really two extremes of one class of electroluminescent devices. They also implicate that no fundamental difference exists between charge transport in small molecular weight or polymeric mixed ionic and electronic conductive materials. Additionally, it follows that the ionic conductivity is responsible for the dynamic properties of devices and systems using them. This likely extends to mixed ionic and electronic conductive materials used in organic solar cells and in a variety of biological systems.

  • 177.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Dynamic Processes in Sandwich Polymer Light-Emitting Electrochemical Cells2012In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 22, no 21, p. 4547-4556Article in journal (Refereed)
    Abstract [en]

    The operational mechanism of polymer light-emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, pn junction formation gives rise to an increase in capacitance at intermediate frequencies (approximate to 10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low-conductivity region that can be significantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light-emitting pn junction has formed. This implies that in order to significantly decrease the response-time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.

  • 178.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands; Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, Netherlands.
    Fundamental Tradeoff between Emission Intensity and Efficiency in Light-Emitting Electrochemical Cells2015In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 20, p. 3066-3073Article in journal (Refereed)
    Abstract [en]

    The characteristic doping process in polymer light-emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that, on the one hand, carrier injection and transport benefit from electrochemical doping, leading to increased electron-hole recombination. On the other hand, the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently, the quasi-steady-state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll-off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron-quenching via diffusion-assisted Forster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high-brightness, low-production cost devices, rather than in high-efficiency devices.

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  • 179.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, The Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, The Netherlands.
    Correcting for contact geometry in Seebeck coefficient measurements of thin film devices2014In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 15, no 10, p. 2250-2255Article in journal (Refereed)
    Abstract [en]

    Driven by promising recent results, there has been a revived interest in the thermoelectric properties of organic (semi) conductors. Concomitantly, there is a need to probe the Seebeck coefficient S of modestly conducting materials in thin film geometry. Here we show that geometries that seem desirable from a signal-to-noise perspective may induce systematic errors in the measured value of S, S-m, by a factor 3 or more. The enhancement of S-m by the device geometry is related to competing conduction paths outside the region between the electrodes. We derive a universal scaling curve that allows correcting for this and show that structuring the semiconductor is not needed for the optimal electrode configuration, being a set of narrow, parallel strips.

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  • 180.
    van Reenen, Stephan
    et al.
    University of Oxford, England.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Snaith, Henry J.
    University of Oxford, England.
    Modeling Anomalous Hysteresis in Perovskite Solar Cells2015In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 6, no 19, p. 3808-3814Article in journal (Refereed)
    Abstract [en]

    Organic inorganic lead halide perovskites are distinct from most other semiconductors because they exhibit characteristics of both electronic and ionic motion. Accurate understanding of the optoelectronic impact of such properties is important to fully optimize devices and be aware of any limitations of perovskite solar cells and broader optoelectronic devices. Here we use a numerical drift-diffusion model to describe device operation of perovskite solar cells. To achieve hysteresis in the modeled current voltage characteristics, we must include both ion migration and electronic charge traps, serving as recombination centers. Trapped electronic charges recombine with oppositely charged free electronic carriers, of which the density depends on the bias-dependent ion distribution in the perovskite. Our results therefore show that reduction of either the density of mobile ionic species or carrier trapping at the perovskite interface will remove the adverse hysteresis in perovskite solar cells. This gives a clear target for ongoing research effort and unifies previously conflicting experimental observations and theories.

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  • 181.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Kouijzer, Sandra
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands; Eindhoven University of Technology, Netherlands.
    Wienk, Martijn M.
    Eindhoven University of Technology, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, Netherlands.
    Origin of Work Function Modification by Ionic and Amine-Based Interface Layers2014In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 1, no 8, p. 1400189-Article in journal (Refereed)
    Abstract [en]

    Work function modification by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. Data supporting this model is from scanning Kelvin probe microscopy, used to determine the work function modification by thin layers of such materials on different substrates. Both reductions and increases in work function by different materials are found to follow a general mechanism. Work function modification is found to only take place when the work function modification layer (WML) is deposited on conductors or semiconductors. On insulators no effect is observed. Additionally, the work function modification is independent of the WML thickness or the substrate work function in the range of 3 to 5 eV. Based on these results charge transfer, doping, and spontaneous dipole orientation are excluded as possible mechanisms. This understanding of the work function modification by polyelectrolytes and amines facilitates design of new air-stable and solution-processable WMLs for organic electronics.

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  • 182.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Matyba, Piotr
    Umeå University, Sweden.
    Dzwilewski, Andrzej
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Edman, Ludvig
    Umeå University, Sweden.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    A Unifying Model for the Operation of Light-Emitting Electrochemical Cells2010In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 39, p. 13776-13781Article in journal (Refereed)
    Abstract [en]

    The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p-n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device.

  • 183.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology, Netherlands.
    Matyba, Piotr
    Umeå University, Sweden.
    Dzwilewski, Andrzej
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Edman, Ludvig
    Umeå University, Sweden.
    Kemerink, Martijn
    Eindhoven University of Technology, Netherlands.
    Salt Concentration Effects in Planar Light-Emitting Electrochemical Cells2011In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 21, no 10, p. 1795-1802Article in journal (Refereed)
    Abstract [en]

    Incorporation of ions in the active layer of organic semiconductor devices may lead to attractive device properties like enhanced injection and improved carrier transport. In this paper, we investigate the effect of the salt concentration on the operation of light-emitting electrochemical cells, using experiments and numerical calculations. The current density and light emission are shown to increase linearly with increasing ion concentration over a wide range of concentrations. The increasing current is accompanied by an ion redistribution, leading to a narrowing of the recombination zone. Hence, in absence of detrimental side reactions and doping-related luminescence quenching, the ion concentration should be as high as possible.

  • 184.
    Wagemans, W.
    et al.
    Eindhoven University of Technology, Netherlands.
    Janssen, P.
    Eindhoven University of Technology, Netherlands.
    van der Heijden, E. H. M.
    Eindhoven University of Technology, Netherlands.
    Kemerink, M.
    Eindhoven University of Technology, Netherlands.
    Koopmans, B.
    Eindhoven University of Technology, Netherlands.
    Frequency dependence of organic magnetoresistance2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 12, article id 123301Article in journal (Refereed)
    Abstract [en]

    To identify the microscopic mechanisms of organic magnetoresistance (OMAR), the dependency on the frequency of the applied magnetic field is explored, which consists of a dc and ac component. The measured magnetoconductance decreases when the frequency is increased. The decrease is stronger for lower voltages, which is shown to be linked to the presence of a negative capacitance, as measured with admittance spectroscopy. The negative capacitance disappears when the frequency becomes comparable to the inverse transit time of the minority carriers. These results are in agreement with recent interpretations that magnetic field effects on minority carrier mobility dominate OMAR. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3491217]

  • 185.
    Wanzhu, Cai
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Jinan Univ, Peoples R China.
    Österberg, Thomas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Jafari, Mohammad Javad
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, Faculty of Science & Engineering.
    Musumeci, Chiara
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Wang, Chuan Fei
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Zuo, Guangzheng
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Yin, Xiaolong
    Jinan Univ, Peoples R China.
    Luo, Xuhao
    Jinan Univ, Peoples R China.
    Johansson, Jim
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Ouyang, Liangqi
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Ederth, Thomas
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Physics. Linköping University, Faculty of Science & Engineering.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Dedoping-induced interfacial instability of poly(ethylene imine)s-treated PEDOT:PSS as a low-work-function electrode2020In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 8, no 1, p. 328-336Article in journal (Refereed)
    Abstract [en]

    Transparent organic electrodes printed from high-conductivity PEDOT:PSS have become essential for upscaling all-carbon based, low-cost optoelectronic devices. In the printing process, low-work-function PEDOT:PSS electrodes (cathode) are achieved by coating an ultra-thin, non-conjugated polyelectrolyte that is rich in amine groups, such as poly(ethylene imine) (PEI) or its ethoxylated derivative (PEIE), onto PEDOT:PSS surfaces. Here, we mapped the physical and chemical processes that occur at the interface between thin PEIx (indicating either PEI or PEIE) and PEDOT:PSS during printing. We identify that there is a dedoping effect of PEDOT induced by the PEIx. Using infrared spectroscopy, we found that the amine-rich PEIx can form chemical bonds with the dopant, PSS. At lower PSS concentration, PEIx also shows an electron-transfer effect to the charged PEDOT chain. These interface reactions lock the surface morphology of PEDOT:PSS, preventing the redistribution of PSS, and reduce the work function. Subsequent exposure to oxygen during the device fabrication process, on the other hand, can result in redoping of the low-work-function PEDOT:PSS interface, causing problems for printing reproducible devices under ambient conditions.

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  • 186.
    Xu, Kai
    et al.
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Sun, Hengda
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Ruoko, Tero-Petri
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Wang, Gang
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kroon, Renee
    Chalmers Univ Technol, Sweden.
    Kolhe, Nagesh B.
    Univ Washington, WA 98195 USA.
    Puttisong, Yuttapoom
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Liu, Xianjie
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Fazzi, Daniele
    Univ Cologne, Germany.
    Shibata, Koki
    Chiba Univ, Japan.
    Yang, Chiyuan
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Sun, Ning
    Yunnan Univ, Peoples R China.
    Persson, Gustav
    Chalmers Univ Technol, Sweden.
    Yankovich, Andrew B.
    Chalmers Univ Technol, Sweden.
    Olsson, Eva
    Chalmers Univ Technol, Sweden.
    Yoshida, Hiroyuki
    Chiba Univ, Japan; Chiba Univ, Japan.
    Chen, Weimin
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Fahlman, Mats
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Jenekhe, Samson A.
    Univ Washington, WA 98195 USA.
    Mueller, Christian
    Chalmers Univ Technol, Sweden.
    Berggren, Magnus
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Fabiano, Simone
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Ground-state electron transfer in all-polymer donor-acceptor heterojunctions2020In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660Article in journal (Refereed)
    Abstract [en]

    Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor-acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor-acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of similar to 10(13) cm(-2). Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics. Doping through spontaneous electron transfer between donor and acceptor polymers is obtained by selecting organic semiconductors with suitable electron affinity and ionization energy, achieving high conductivity in blends and bilayer configuration.

  • 187.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Conjugated Polymer Blends for Organic Thermoelectrics2019In: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 5, no 11, article id 1800821Article in journal (Refereed)
    Abstract [en]

    A major attraction of organic conjugated semiconductors is that materials with new, emergent functionality can be designed and made by simple blending, as is extensively used in, e.g., bulk heterojunction organic solar cells. Herein doped blends based on organic semiconductors (OSCs) for thermoelectric applications are critically reviewed. Several experimental strategies to improve thermoelectric performance, measured in terms of power factor (PF) or figure-of-merit ZT, have been demonstrated in recent literature. Specifically, density-of-states design in blends of two OSCs can be used to obtain electronic Seebeck coefficients up to approximate to 2000 mu V K-1. Alternatively, blending with (high-dielectric constant) insulating polymers can improve doping efficiency and thereby conductivity, as well as induce more favorable morphologies that improve conductivity while hardly affecting thermopower. In the PEDOT:polystyrene-sulfonate (PEDOT:PSS) blend system, processing schemes to either improve conductivity via morphology or via (partial) removal of the electronically isolating PSS, or both, have been demonstrated. Although a range of experiments have at least quasi-quantitatively been explained by analytical or numerical models, a comprehensive model for organic thermoelectrics is lacking so far.

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  • 188.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Impact of doping on the density of states and the mobility in organic semiconductors2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 23, p. 235203-Article in journal (Refereed)
    Abstract [en]

    We experimentally investigated conductivity and mobility of poly(3-hexylthiophene) (P3HT) doped with tetrafluorotetracyanoquinodimethane (F(4)TCNQ) for various relative doping concentrations ranging from ultralow (10(-5)) to high (10(-1)) and various active layer thicknesses. Although the measured conductivity monotonously increases with increasing doping concentration, the mobilities decrease, in agreement with previously published work. Additionally, we developed a simple yet quantitative model to rationalize the results on basis of a modification of the density of states (DOS) by the Coulomb potentials of ionized dopants. The DOS was integrated in a three-dimensional (3D) hopping formalism in which parameters such as energetic disorder, intersite distance, energy level difference, and temperature were varied. We compared predictions of our model as well as those of a previously developed model to kinetic Monte Carlo (MC) modeling and found that only the former model accurately reproduces the mobility of MC modeling in a large part of the parameter space. Importantly, both our model and MC simulations are in good agreement with experiments; the crucial ingredient to both is the formation of a deep trap tail in the Gaussian DOS with increasing doping concentration.

  • 189.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Andersson, Olof
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    High thermoelectric power factor from multilayer solution-processed organic films2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 8, article id 083303Article in journal (Refereed)
    Abstract [en]

    We investigate the suitability of the "sequential doping" method of organic semiconductors for thermoelectric applications. The method consists of depositing a dopant (F4TCNQ) containing solution on a previously cast semiconductor (P3HT) thin film to achieve high conductivity, while preserving the morphology. For very thin films (similar to 25 nm), we achieve a high power factor around 8 mu W/mK(-2) with a conductivity over 500 S/m. For the increasing film thickness, conductivity and power factor show a decreasing trend, which we attribute to the inability to dope the deeper parts of the film. Since thick films are required to extract significant power from thermoelectric generators, we developed a simple additive technique that allows the deposition of an arbitrary number of layers without significant loss in conductivity or power factor that, for 5 subsequent layers, remain at similar to 300 S/m and similar to 5 mu W/mK(-2), respectively, whereas the power output increases almost one order of magnitude as compared to a single layer. The efficient doping in multilayers is further confirmed by an increased intensity of (bi)polaronic features in the UV-Vis spectra. Published by AIP Publishing.

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  • 190.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Li, Zhaojun
    Chalmers, Sweden.
    Andersson, Olof
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Wang, Ergang
    Chalmers, Sweden.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 14, p. 7767-7775Article in journal (Refereed)
    Abstract [en]

    We investigate conductivity and mobility of different hosts mixed with different electron-withdrawing guests in concentrations ranging from ultralow to high. The effect of the guest material on the mobility and conductivity of the host material varies systematically with the guests LUMO energy relative to the host HOMO, in quantitative agreement with a recently developed model. For guests with a LUMO within similar to 0.5 eV of the host HOMO the dominant process governing transport is the competition between the formation of a deep tail in the host DOS and state filling. In other cases, the interaction with the host is dominated by any polar side groups on the guest and changes in the host morphology. For relatively amorphous hosts the latter interaction can lead to a suppression of deep traps, causing a surprising mobility increase by 1-2 orders of magnitude. In order to analyze our data, we developed a simple method to diagnose both the presence and the filling of traps.

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  • 191.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Li, Zhaojun
    Chalmers University of Technology, Sweden.
    Wang, Ergang
    Chalmers University of Technology, Sweden.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics2018In: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 4, no 1, article id 1700501Article in journal (Refereed)
    Abstract [en]

    The n-type thermoelectric properties of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) are investigated for different solution-based doping methods. A novel inverse-sequential doping method where the semiconductor (PCBM) is deposited on a previously cast dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline film to achieve a very high power factor PF approximate to 35 mu W m(-1) K-2 with a conductivity sigma approximate to 40 S m(-1) is introduced. It is also shown that n-type organic semiconductors obey the -1/4 power law relation between Seebeck coefficient S and sigma that are previously found for p-type materials. An analytical model on basis of variable range hopping unifies these results. The power law for n-type materials is shifted toward higher conductivities by two orders of magnitude with respect to that of p-type, suggesting strongly that n-type organic semiconductors can eventually become superior to their p-type counterparts. Adding a small fraction lower lowest unoccupied molecular orbital material (core-cyanated naphthalene diimide) into PCBM leads to a higher S for inverse-sequential doping but not for bulk doping due to different morphologies.

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  • 192.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Linares, Mathieu
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering.
    Upreti, Tanvi
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    General rule for the energy of water-induced traps in organic semiconductors2019In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 18, no 6, p. 588-+Article in journal (Refereed)
    Abstract [en]

    Charge carrier traps are generally highly detrimental for the performance of semiconductor devices. Unlike the situation for inorganic semiconductors, detailed knowledge about the characteristics and causes of traps in organic semiconductors is still very limited. Here, we accurately determine hole and electron trap energies for a wide range of organic semiconductors in thin-film form. We find that electron and hole trap energies follow a similar empirical rule and lie similar to 0.3-0.4 eV above the highest occupied molecular orbital and below the lowest unoccupied molecular orbital, respectively. Combining experimental and theoretical methods, the origin of the traps is shown to be a dielectric effect of water penetrating nanovoids in the organic semiconductor thin film. We also propose a solvent-annealing method to remove water-related traps from the materials investigated, irrespective of their energy levels. These findings represent a step towards the realization of trap-free organic semiconductor thin films.

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  • 193.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Liu, Xianjie
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
    Fahlman, Mats
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    High Seebeck Coefficient in Mixtures of Conjugated Polymers2018In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 15, article id 1703280Article in journal (Refereed)
    Abstract [en]

    A universal method to obtain record?high electronic Seebeck coefficients is demonstrated while preserving reasonable conductivities in doped blends of organic semiconductors through rational design of the density of states (DOSs). A polymer semiconductor with a shallow highest occupied molecular orbital (HOMO) level?poly(3?hexylthiophene) (P3HT) is mixed with materials with a deeper HOMO (PTB7, TQ1) to form binary blends of the type P3HTx:B1?x (0 ≤ x ≤ 1) that is p?type doped by F4TCNQ. For B = PTB7, a Seebeck coefficient S = 1100 µV K?1 with conductivity σ = 0.3 S m?1 at x = 0.10 is achieved, while for B = TQ1, S = 2000 µV K?1 and σ = 0.03 S m?1 at x = 0.05 is found. Kinetic Monte Carlo simulations with parameters based on experiments show good agreement with the experimental results, confirming the intended mechanism. The simulations are used to derive a design rule for parameter tuning. These results can become relevant for low?power, low?cost applications like (providing power to) autonomous sensors, in which a high Seebeck coefficient translates directly to a proportionally reduced number of legs in the thermogenerator, and hence in reduced fabrication cost and complexity.

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  • 194.
    Zuo, Guangzheng
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Liu, Xianjie
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
    Fahlman, Mats
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 11, p. 9638-9644Article in journal (Refereed)
    Abstract [en]

    The impact of nanoscale morphology on conductivity and Seebeck coefficient in p-type doped all-polymer blend systems is investigated. For a strongly phase separated system (P3HT:PTB7), we achieve a Seebeck coefficient that peaks at S similar to 1100 mu V/K with conductivity sigma similar to 3 x 10(-3) S/cm for 90% PTB7. In marked contrast, for well-mixed systems (P3HT:PTB7 with 5% diiodooctane (DIO), P3HT:PCPDTBT), we find an almost constant S similar to 140 mu V/K and sigma similar to 1 S/cm despite the energy levels being (virtually) identical in both cases. The results are interpreted in terms of a variable range hopping (VRH) model where a peak in S and a minimum in a arise when the percolation pathway contains both host and guest sites, in which the latter acts as energetic trap. For well-mixed blends of the investigated compositions, VRH enables percolation pathways that only involve isolated guest sites, whereas the large distance between guest clusters in phase separated blends enforces (energetically unfavorable) hops via the host. The experimentally observed trends are in good agreement with the results of atomistic kinetic Monte Carlo simulations accounting for the differences in nanoscale morphology.

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