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  • 1.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Charge and Energy Transport in Disordered Organic Semiconductors2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Improvement of the performance of organic disordered semiconductors (OSC) is driven by the understanding   of the underlying charge transport mechanisms and systematic exploitation thereof. There exists a multitude of materials and material systems based on polymers and small molecules with promising performance for use in organic light emitting diodes, photovoltaics, organic field-effect transistors and thermoelectrics. However, universal understanding of many classes of these materials has eluded researchers, due to their broad   spectrum of morphologies, molecular structures and electrical properties. Building on the large body of existing models, this thesis deals with charge transport phenomena from the perspective of transport energetics, by studying the interplay between a few but important concepts commonly accepted to play a crucial role in all  OSC materials; energetic disorder, charge carrier hopping and Coulomb interactions. The influence of these concepts on the energetic landscape through which charge carriers move and how this translates to experimentally observed transport phenomena are studied by a combination of experimental work, kinetic Monte Carlo (MC) simulations and empirical and analytical models.

    The universal scaling and collapse of the temperature and electric field dependence of the conductivity of PEDOT:PSS to a single curve is shown to be functionally equivalent to the scaling of the effective temperature, which describes the effect of field heating as a broadening of the charge carrier distribution. From numerical investigation of the energy relaxation, an empirical model is developed that relates the physical meaning   behind both concepts to the heat balance between Joule heating of the carrier distribution via the effective temperature and energy loss to the lattice. For this universal description to be applicable a strongly energy- dependent density of states (DOS) as well as Coulomb interactions and large carrier concentrations are needed.

    Chemical doping is a common way of improving charge transport in OSC and is also beneficial for energy transport, which combined leads to an increased thermoelectric power factor. The ensuing thermoelectric investigations not only showed the potential of these materials for use in thermoelectric generators, but are  also helpful in unraveling charge transport mechanism as they give direct insight into the energetics of a material. Interestingly, doped OSC exhibit the same universal power-law relationship between thermopower and conductivity, independent of material system or doping method, pointing towards a common energy and charge transport mechanism. In this thesis an analytical model is presented, which reproduces said universal power-law behavior and is able to attribute it to Variable Range Hopping (VRH) or a transition between Nearest Neighbour Hopping (NNH) and VRH at higher concentrations. This model builds on an existing three- dimensional hopping formalism that includes the effect of the attractive Coulomb potential of ionized dopants that leads to a broadening of the DOS. Here, this model is extended by including the energy offset between   host and dopant material and is positively tested against MC simulations and a set of thermoelectric measurements covering different material groups and doping mechanisms.

    Organic field effect transistors (OFETs) have become increasingly comparable in electrical mobility to their inorganic (silicon) counterparts. The spatial extent of charge transport in OFETs has been subject to debate since their inception with many experimental, numerical and analytical studies having been undertaken. Here it is shown that the common way of analyzing the dimensionality of charge transport in OFETs may be prone to misinterpretations. Instead, the results in this thesis suggest that charge transport in OFETs is, in fact, quasi- two-dimensional (2D) due to the confinement of the gate field in addition to a morphology-induced preferred in-plane direction of the transport. The inherently large charge carrier concentrations in OFETs in addition to   the quasi-2D confinement leads to increased Coulomb interaction between charge carriers as compared to bulk material, leading to a thermoelectric behavior that deviates from doped organic systems. At very large concentrations interesting charge transport phenomena are observed, including an unexpected simultaneous increase of the concentration dependence and the magnitude of the mobility, the appearance of a negative transconductance, indicating a transition to an insulating Mott-Hubbard phase. The experimental and   numerical results in this thesis relate these phenomena the intricacies of the interplay between Coulomb interactions, energetic disorder and charge carrier hopping.

    List of papers
    1. Effective Temperature and Universal Conductivity Scaling in Organic Semiconductors
    Open this publication in new window or tab >>Effective Temperature and Universal Conductivity Scaling in Organic Semiconductors
    2015 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 16870Article in journal (Refereed) Published
    Abstract [en]

    We investigate the scalability of the temperature-and electric field-dependence of the conductivity of disordered organic semiconductors to universal curves by two different but commonly employed methods; by so-called universal scaling and by using the effective temperature concept. Experimentally both scaling methods were found to be equally applicable to the out-of-plane charge transport in PEDOT: PSS thin films of various compositions. Both methods are shown to be equivalent in terms of functional dependence and to have identical limiting behavior. The experimentally observed scaling behavior can be reproduced by a numerical nearest-neighbor hopping model, accounting for the Coulomb interaction, the high charge carrier concentration and the energetic disorder. The underlying physics can be captured in a simple empirical model, describing the effective temperature of the charge carrier distribution as the outcome of a heat balance between Joule heating and (effective) temperature-dependent energy loss to the lattice.

    Place, publisher, year, edition, pages
    Nature Publishing Group, 2015
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-123329 (URN)10.1038/srep16870 (DOI)000364933800002 ()26581975 (PubMedID)
    Available from: 2015-12-14 Created: 2015-12-11 Last updated: 2018-10-08
    2. Impact of doping on the density of states and the mobility in organic semiconductors
    Open this publication in new window or tab >>Impact of doping on the density of states and the mobility in organic semiconductors
    2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 23, p. 235203-Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2016
    National Category
    Other Physics Topics
    Identifiers
    urn:nbn:se:liu:diva-130276 (URN)10.1103/PhysRevB.93.235203 (DOI)000378813800009 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC)

    Available from: 2016-08-01 Created: 2016-07-28 Last updated: 2018-08-29
    3. Range and energetics of charge hopping in organic semiconductors
    Open this publication in new window or tab >>Range and energetics of charge hopping in organic semiconductors
    2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 241202Article in journal (Refereed) Published
    Abstract [en]

    The recent upswing in attention for the thermoelectric properties of organic semiconductors (OSCs) adds urgency to the need for a quantitative description of the range and energetics of hopping transport in organic semiconductors under relevant circumstances, i.e., around room temperature (RT). In particular, the degree to which hops beyond the nearest neighbor must be accounted for at RT is still largely unknown. Here, measurements of charge and energy transport in doped OSCs are combined with analytical modeling to reach the univocal conclusion that variable-range hopping is the proper description in a large class of disordered OSC at RT. To obtain quantitative agreement with experiment, one needs to account for the modification of the density of states by ionized dopants. These Coulomb interactions give rise to a deep tail of trap states that is independent of the materials initial energetic disorder. Insertion of this effect into a classical Mott-type variable-range hopping model allows one to give a quantitative description of temperature-dependent conductivity and thermopower measurements on a wide range of disordered OSCs. In particular, the model explains the commonly observed quasiuniversal power-law relation between the Seebeck coefficient and the conductivity.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2017
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-144143 (URN)10.1103/PhysRevB.96.241202 (DOI)000418616700001 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC); Knut och Alice Wallenberg stiftelse

    Available from: 2018-01-10 Created: 2018-01-10 Last updated: 2018-08-29
    4. Investigation of the dimensionality of charge transport in organic field effect transistors
    Open this publication in new window or tab >>Investigation of the dimensionality of charge transport in organic field effect transistors
    2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 8, article id 85301Article in journal (Refereed) Published
    Abstract [en]

    Ever since the first experimental investigations of organic field effect transistors (OFETs) the dimensionality of charge transport has alternately been described as two dimensional (2D) and three dimensional (3D). More recently, researchers have turned to an analytical analysis of the temperature-dependent transfer characteristics to classify the dimensionality as either 2D or 3D as well as to determine the disorder of the system, thereby greatly simplifying dimensionality investigations. We applied said analytical analysis to the experimental results of our OFETs comprising molecularly well-defined polymeric layers as the active material as well as to results obtained from kinetic Monte Carlo simulations and found that it was not able to correctly distinguish between 2D and 3D transports or give meaningful values for the disorder and should only be used for quasiquantitative and comparative analysis. We conclude to show that the dimensionality of charge transport in OFETs is a function of the interplay between transistor physics and morphology of the organic material.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2017
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-138929 (URN)10.1103/PhysRevB.95.085301 (DOI)000402194500006 ()
    Note

    Funding Agencies|Knut och Alice Wallenbergs stiftelse; Advanced Functional Materials Center at Linkoping University [2009-00971]; VINNOVA [2015-04859]

    Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2018-03-14
  • 2.
    Abdalla, Hassan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Fabiano, Simone
    Linköping University, Department of Science and Technology, Physics and Electronics. 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.
    Investigation of the dimensionality of charge transport in organic field effect transistors2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 8, article id 85301Article in journal (Refereed)
    Abstract [en]

    Ever since the first experimental investigations of organic field effect transistors (OFETs) the dimensionality of charge transport has alternately been described as two dimensional (2D) and three dimensional (3D). More recently, researchers have turned to an analytical analysis of the temperature-dependent transfer characteristics to classify the dimensionality as either 2D or 3D as well as to determine the disorder of the system, thereby greatly simplifying dimensionality investigations. We applied said analytical analysis to the experimental results of our OFETs comprising molecularly well-defined polymeric layers as the active material as well as to results obtained from kinetic Monte Carlo simulations and found that it was not able to correctly distinguish between 2D and 3D transports or give meaningful values for the disorder and should only be used for quasiquantitative and comparative analysis. We conclude to show that the dimensionality of charge transport in OFETs is a function of the interplay between transistor physics and morphology of the organic material.

  • 3.
    Abdalla, Hassan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    van de Ruit, Kevin
    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. Eindhoven University of Technology, Netherlands.
    Effective Temperature and Universal Conductivity Scaling in Organic Semiconductors2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 16870Article in journal (Refereed)
    Abstract [en]

    We investigate the scalability of the temperature-and electric field-dependence of the conductivity of disordered organic semiconductors to universal curves by two different but commonly employed methods; by so-called universal scaling and by using the effective temperature concept. Experimentally both scaling methods were found to be equally applicable to the out-of-plane charge transport in PEDOT: PSS thin films of various compositions. Both methods are shown to be equivalent in terms of functional dependence and to have identical limiting behavior. The experimentally observed scaling behavior can be reproduced by a numerical nearest-neighbor hopping model, accounting for the Coulomb interaction, the high charge carrier concentration and the energetic disorder. The underlying physics can be captured in a simple empirical model, describing the effective temperature of the charge carrier distribution as the outcome of a heat balance between Joule heating and (effective) temperature-dependent energy loss to the lattice.

  • 4.
    Abdalla, Hassan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. 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.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Range and energetics of charge hopping in organic semiconductors2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 241202Article in journal (Refereed)
    Abstract [en]

    The recent upswing in attention for the thermoelectric properties of organic semiconductors (OSCs) adds urgency to the need for a quantitative description of the range and energetics of hopping transport in organic semiconductors under relevant circumstances, i.e., around room temperature (RT). In particular, the degree to which hops beyond the nearest neighbor must be accounted for at RT is still largely unknown. Here, measurements of charge and energy transport in doped OSCs are combined with analytical modeling to reach the univocal conclusion that variable-range hopping is the proper description in a large class of disordered OSC at RT. To obtain quantitative agreement with experiment, one needs to account for the modification of the density of states by ionized dopants. These Coulomb interactions give rise to a deep tail of trap states that is independent of the materials initial energetic disorder. Insertion of this effect into a classical Mott-type variable-range hopping model allows one to give a quantitative description of temperature-dependent conductivity and thermopower measurements on a wide range of disordered OSCs. In particular, the model explains the commonly observed quasiuniversal power-law relation between the Seebeck coefficient and the conductivity.

  • 5.
    Bernard, L.
    et al.
    Empa, Switzerland; Eindhoven University of Technology, Netherlands.
    Khikhlovskyi, V.
    TNO Dutch Org Appl Science Research, Netherlands.
    van Breemen, A.
    TNO Dutch Org Appl Science Research, Netherlands.
    Michels, J. J.
    Max Planck Institute Polymer Research, Germany.
    Janssen, R.
    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. Eindhoven University of Technology, Netherlands.
    Gelinck, G.
    Eindhoven University of Technology, Netherlands; TNO Dutch Org Appl Science Research, Netherlands.
    Pilet, N.
    Paul Scherrer Institute, Switzerland.
    Study of the morphology of organic ferroelectric diodes with combined scanning force and scanning transmission X-ray microscopy2018In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 53, p. 242-248Article in journal (Refereed)
    Abstract [en]

    Organic ferroelectric diodes attract increasing interest as they combine non-destructive data read-out and low cost fabrication, two requirements in the development of novel non-volatile memory elements. The macroscopic electrical characteristics and performances of such devices strongly depend on their structural properties. Various studies of their global microscopic morphology have already been reported. Here, a multi-technique approach including different scanning force and X-ray microscopies permitted to reveal and locally study nanometer-scale unexpected sub-structures within a P(VDF-TrFE):F8BT ferroelectric diode. The strong impact of these structures on the local polarizability of the ferroelectric is shown. Two alternative fabrication methods are proposed that prevent the formation of these structures and demonstrate improved macroscopic device performances such as endurance and ON/OFF ratio.

    The full text will be freely available from 2019-12-05 16:28
  • 6.
    Booker, Ian Don
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology.
    Lilja, L.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    ul-Hassan, Jawad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Bergman, Peder
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Sveinbjörnsson, Einar
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Oxidation induced ON1, ON2a/b defects in 4H-SiC characterized by DLTS2014In: SILICON CARBIDE AND RELATED MATERIALS 2013, PTS 1 AND 2, Trans Tech Publications , 2014, Vol. 778-780, p. 281-284Conference paper (Refereed)
    Abstract [en]

    The deep levels ON1 and ON2a/b introduced by oxidation into 4H-SiC are characterized via standard DLTS and via filling pulse dependent DLTS measurements. Separation of the closely spaced ON2a/b defect is achieved by using a higher resolution correlation function (Gaver-Stehfest 4) and apparent energy level, apparent electron capture cross section and filling pulse measurement derived capture cross sections are given.

  • 7.
    DInnocenzo, V.
    et al.
    Ist Italian Tecnol, Italy; Politecn Milan, Italy.
    Luzio, A.
    Ist Italian Tecnol, Italy.
    Abdalla, Hassan
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Fabiano, Simone
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering. University of Groningen, Netherlands.
    Loi, M. A.
    University of Groningen, Netherlands.
    Natali, D.
    Ist Italian Tecnol, Italy; Politecn Milan, Italy.
    Petrozza, A.
    Ist Italian Tecnol, Italy.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Caironi, M.
    Ist Italian Tecnol, Italy.
    Two-dimensional charge transport in molecularly ordered polymer field-effect transistors2016In: JOURNAL OF MATERIALS CHEMISTRY C, ISSN 2050-7526, Vol. 4, no 47, p. 11135-11142Article in journal (Refereed)
    Abstract [en]

    Nanometer-thick Langmuir-Schafer monolayers of an electron transporting polymer display charge transport, optical and electro-optical properties that do not depend on the number of layers deposited one above the other. This phenomenon can be rationalized with the micro-structure of the specific multi-layers, which introduces an interlayer hopping penalty confining transport to a neat 2D regime, with a channel not extending beyond a single similar to 3 nm thick polymer strand, as confirmed by kinetic Monte Carlo simulations. Such findings are critical to establish a quantitative structure-property nexus in high mobility polymer semiconductors and in the control of charge transport at a molecular scale.

  • 8.
    Felekidis, Nikolaos
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Effects of Energetic Disorder on the Optoelectronic Properties of Organic Solar Cells2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Organic photovoltaics (OPVs) is a promising low-cost and environmental-friendly technology currently achieving 12-14% power conversion efficiency. Despite the extensive focus of the research community over the last years, critical mechanisms defining the performance of OPVs are still topics of debate. While energetic disorder is known to be characteristic of organic semiconductors in general, its potential role in OPV has received surprisingly little attention. In this thesis we investigate some aspects of the relation between energetic disorder and several optoelectronic properties of OPV.

    Charge carrier mobility is a key parameter in characterizing the performance of organic semiconductors. Analyzing the temperature dependence of the mobility is also an oftenused method to obtain (estimates for) the energetic disorder in the HOMO and LUMO levels of an organic semiconductor material. Different formalisms to extract and analyze mobilities from space charge limited conductivity (SCLC) experiments are reviewed. Surprisingly, the Murgatroyd-Gill analytical model in combination with the Gaussian disorder model in the Boltzmann limit yields similar mobilities and energetic disorders as a more elaborate drift-diffusion model with parametrized mobility functionals. Common analysis and measurement errors are discussed. All the models are incorporated in an automated analysis freeware tool.

    The open circuit voltage (Voc) has attracted considerable interest as the large difference between Voc and the bandgap is the main loss mechanism in bulk heterojunction OPVs. Surprisingly, in ternary devices composed of two donors and one acceptor, the Voc is not pinned to the shallowest HOMO but demonstrates a continuous tunability between the binary extremities. We show that this phenomenon can be explained with an equilibrium model where Voc is defined as the splitting of the quasi-Fermi levels of the photo-created holes and electrons in a common density of states accounting for the stoichiometry, i.e. the ratio of the donor materials and the broadening by Gaussian disorder. Evaluating the PCE, it is found that ternary devices do not offer advantages over binary unless the fill factor (FF) is increased at intermediate compositions, as a result of improved transport/recombination upon material blending.

    Stressing the importance of material intermixing to improve the performance, we found that the presence of an acceptor may drastically alter the mobility and energetic disorder of the donor and vice versa. The effect of different acceptors was studied in a ternary onedonor- two-acceptors system, where the unpredictable variability with composition of the energetic disorder in the HOMO and the LUMO explained the almost linear tunability of Voc. Designing binary OPVs based on the design rule that the energetic disorder can be reduced upon material blending, as we observed, can yield a relative PCE improvement of at least 20%.

    CT states currently play a key role in evaluating the performance of OPVs and CTelectroluminescence (CT-EL) is assumed to stem from the recombination of thermalized electron-hole pairs. The varying width of the CT-EL peak for different material combinations is intuitively expected to reflect the energetic disorder of the effective HOMO and LUMO. We employ kinetic Monte Carlo (kMC) CT-EL simulations, using independently measured disorder parameters as input, to calculate the ground-to-ground state (0-0) transition spectrum. Including the vibronic broadening according to the Franck Condon principle, we reproduce the width and current dependence of the measured CT-EL peak for a large number of donor-acceptor combinations. The fitted dominant phonon modes compare well with the values measured using the spectral line narrowing technique. Importantly, the calculations show that CT-EL originates from a narrow, non-thermalized subset of all available CT states, which can be understood by considering the kinetic microscopic process with which electron-hole pairs meet and recombine.

    Despite electron-hole pairs being strongly bound in organic materials, the charge separation process following photo-excitation is found to be extremely efficient and independent of the excitation energy. However, at low photon energies where the charges are excited deep in the tail of the DOS, it is intuitively expected for the extraction yield to be quenched. Internal Quantum Efficiency (IQE) experiments for different material systems show both inefficient and efficient charge dissociation for excitation close to the CT energy. This finding is explained by kinetic Monte Carlo simulations accounting for a varying degree of e-h delocalization, where strongly bound localized CT pairs (< 2nm distance) are doomed to recombine at low excitation energies while extended delocalization over 3-5nm yields an increased and energy-independent IQE. Using a single material parameter set, the experimental CT electroluminescence and absorption spectra are reproduced by the same kMC model by accounting for the vibronic progression of the calculated 0-0 transition. In contrast to CT-EL, CT-absorption probes the complete CT manifold.

    Charge transport in organic solar cells is currently modelled as either an equilibrium or a non-equilibrium process. The former is described by drift-diffusion (DD) equations, which can be calculated quickly but assume local thermal equilibrium of the charge carriers with the lattice. The latter is described by kMC models, that are time-consuming but treat the charge carriers individually and can probe all relevant time and energy scales. A hybrid model that makes use of the multiple trap and release (MTR) concept in combination with the DD equations is shown to describe both steady-state space charge limited conductivity experiments and non-equilibrium time-resolved transport experiments using a single parameter set. For the investigated simulations, the DD-MTR model is in good agreement with kMC and ~10 times faster.

    Steady-state mobilities from DD equations have been argued to be exclusively relevant for operating OPVs while charge carrier thermalization and non-equilibrium time-dependent mobilities (although acknowledged) can be disregarded. This conclusion, based on transient photocurrent experiments with μs time resolution, is not complete. We show that non-equilibrium kMC simulations can describe the extraction of charge carriers from subps to 100 μs timescales with a single parameter set. The majority of the fast charge carriers, mostly non-thermalized electrons, are extracted at time scales below the resolution of the experiment. In other words, the experiment resolves only the slower fraction of the charges, predominantly holes.

    List of papers
    1. Automated open-source software for charge transport analysis in single-carrier organic semiconductor diodes
    Open this publication in new window or tab >>Automated open-source software for charge transport analysis in single-carrier organic semiconductor diodes
    2018 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 61, p. 318-328Article, review/survey (Refereed) Published
    Abstract [en]

    Organic electronics is an emerging technology with numerous applications in which the active layer is composed of an organic semiconductor (OSC) or blends of multiple OSC. One of the key performance parameters for such devices is the charge carrier mobility which can be evaluated by different measurement techniques. Here, we review different formalisms for extraction and analysis of hole mobilities from temperature-dependent space-charge limited conductivity (SCLC) measurements for pristine OSC as well as for binary and ternary blends as used in e.g. photovoltaic applications. The model is also applicable to n-type materials. Possible sources of measurement errors, such as the presence of traps and series resistance, are discussed. We show that by a simple method of selecting a proper experimental data range these errors can be avoided. The Murgatroyd-Gill analytical model in combination with the Gaussian Disorder Model is used to extract zero-field hole mobilities as well as estimates of the Gaussian energetic disorder in the HOMO level from experimental data. The resulting mobilities are in excellent agreement with those found from more elaborate fits to a full drift-diffusion model that includes a temperature, field and density dependent charge carrier mobility; the same holds for the Gaussian disorder of pure materials and blends with low fullerene concentration. The zero-field mobilities are also analyzed according to an Arrhenius model that was previously argued to reveal a universal mobility law; for most -but not all- material systems in the present work this framework gave an equally good fit to the experimental data as the other models. An automated fitting freeware, incorporating the different models, is made openly available for download and minimizes error, user input and SCLC data analysis time; e.g. SCLC current-voltage curves at several different temperatures can be globally fitted in a few seconds.

    Place, publisher, year, edition, pages
    ELSEVIER SCIENCE BV, 2018
    National Category
    Other Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:liu:diva-150200 (URN)10.1016/j.orgel.2018.06.010 (DOI)000438894600040 ()
    Note

    Funding Agencies|Knut and Alice Wallenberg Foundation [KAW 2016.0494]

    Available from: 2018-08-22 Created: 2018-08-22 Last updated: 2018-09-19
    2. Open circuit voltage and efficiency in ternary organic photovoltaic blends
    Open this publication in new window or tab >>Open circuit voltage and efficiency in ternary organic photovoltaic blends
    2016 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 9, no 1, p. 257-266Article in journal (Refereed) Published
    Abstract [en]

    Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one acceptor are investigated by experiments and modeling. The commonly observed continuous tunability of the open circuit voltage V-OC with the donor1 : donor2 ratio can quantitatively be explained as quasi-Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts for the composition-dependent absorption and the shape of the current-voltage characteristic curve is developed. When all other parameters, most notably the fill factor, are constant, we find that for state-of-the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with complementary absorption spectra, offer modest improvements over binary ones. In contrast, when, upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor, ternaries may offer significant advantages over binaries.

    Place, publisher, year, edition, pages
    Royal Society of Chemistry, 2016
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-124474 (URN)10.1039/c5ee03095a (DOI)000367622700030 ()
    Note

    Funding Agencies|Swedish Research Council

    Available from: 2016-02-02 Created: 2016-02-01 Last updated: 2018-09-10
    3. Design Rule for Improved Open-Circuit Voltage in Binary and Ternary Organic Solar Cells
    Open this publication in new window or tab >>Design Rule for Improved Open-Circuit Voltage in Binary and Ternary Organic Solar Cells
    2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 42, p. 37070-37077Article in journal (Refereed) Published
    Abstract [en]

    Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells, hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (V-oc) of ternary Donor:Acceptor(l):Acceptor(2) (D:A(1):A(2)) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the Voc with the A(1):A(2) weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11 to 13.5% for a hypothetical state-ofthe-art organic solar cell, highlighting the importance of this design rule.

    Place, publisher, year, edition, pages
    AMER CHEMICAL SOC, 2017
    Keywords
    organic solar cells; ternary blends; bulk heterojunctions; open-circuit voltage; energetic disorder
    National Category
    Textile, Rubber and Polymeric Materials
    Identifiers
    urn:nbn:se:liu:diva-143091 (URN)10.1021/acsami.7b08276 (DOI)000414115700064 ()28967245 (PubMedID)
    Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2018-09-10
    4. Nonequilibrium drift-diffusion model for organic semiconductor devices
    Open this publication in new window or tab >>Nonequilibrium drift-diffusion model for organic semiconductor devices
    2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 3, article id 035205Article in journal (Refereed) Published
    Abstract [en]

    Two prevailing formalisms are currently used to model charge transport in organic semiconductor devices. Drift-diffusion calculations, on the one hand, are time effective but assume local thermodynamic equilibrium, which is not always realistic. Kinetic Monte Carlo models, on the other hand, do not require this assumption but are computationally expensive. Here, we present a nonequilibrium drift-diffusion model that bridges this gap by fusing the established multiple trap and release formalism with the drift-diffusion transport equation. For a prototypical photovoltaic system the model is shown to quantitatively describe, with a single set of parameters, experiments probing (1) temperature-dependent steady-state charge transport-space-charge limited currents, and (2) time-resolved charge transport and relaxation of nonequilibrated photocreated charges. Moreover, the outputs of the developed kinetic drift-diffusion model are an order of magnitude, or more, faster to compute and in good agreement with kinetic Monte Carlo calculations.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2016
    National Category
    Other Physics Topics
    Identifiers
    urn:nbn:se:liu:diva-131516 (URN)10.1103/PhysRevB.94.035205 (DOI)000381482700003 ()
    Available from: 2016-09-26 Created: 2016-09-23 Last updated: 2018-09-10
    The full text will be freely available from 2019-03-01 00:01
  • 9.
    Felekidis, Nikolaos
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Melianas, Armantas
    Stanford Univ, CA 94305 USA.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Automated open-source software for charge transport analysis in single-carrier organic semiconductor diodes2018In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 61, p. 318-328Article, review/survey (Refereed)
    Abstract [en]

    Organic electronics is an emerging technology with numerous applications in which the active layer is composed of an organic semiconductor (OSC) or blends of multiple OSC. One of the key performance parameters for such devices is the charge carrier mobility which can be evaluated by different measurement techniques. Here, we review different formalisms for extraction and analysis of hole mobilities from temperature-dependent space-charge limited conductivity (SCLC) measurements for pristine OSC as well as for binary and ternary blends as used in e.g. photovoltaic applications. The model is also applicable to n-type materials. Possible sources of measurement errors, such as the presence of traps and series resistance, are discussed. We show that by a simple method of selecting a proper experimental data range these errors can be avoided. The Murgatroyd-Gill analytical model in combination with the Gaussian Disorder Model is used to extract zero-field hole mobilities as well as estimates of the Gaussian energetic disorder in the HOMO level from experimental data. The resulting mobilities are in excellent agreement with those found from more elaborate fits to a full drift-diffusion model that includes a temperature, field and density dependent charge carrier mobility; the same holds for the Gaussian disorder of pure materials and blends with low fullerene concentration. The zero-field mobilities are also analyzed according to an Arrhenius model that was previously argued to reveal a universal mobility law; for most -but not all- material systems in the present work this framework gave an equally good fit to the experimental data as the other models. An automated fitting freeware, incorporating the different models, is made openly available for download and minimizes error, user input and SCLC data analysis time; e.g. SCLC current-voltage curves at several different temperatures can be globally fitted in a few seconds.

    The full text will be freely available from 2020-06-15 17:03
  • 10.
    Felekidis, Nikolaos
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Melianas, Armantas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Stanford University, CA 94305 USA.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Design Rule for Improved Open-Circuit Voltage in Binary and Ternary Organic Solar Cells2017In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 42, p. 37070-37077Article in journal (Refereed)
    Abstract [en]

    Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells, hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (V-oc) of ternary Donor:Acceptor(l):Acceptor(2) (D:A(1):A(2)) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the Voc with the A(1):A(2) weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11 to 13.5% for a hypothetical state-ofthe-art organic solar cell, highlighting the importance of this design rule.

  • 11.
    Felekidis, Nikolaos
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Melianas, Armantas
    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, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Nonequilibrium drift-diffusion model for organic semiconductor devices2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 3, article id 035205Article in journal (Refereed)
    Abstract [en]

    Two prevailing formalisms are currently used to model charge transport in organic semiconductor devices. Drift-diffusion calculations, on the one hand, are time effective but assume local thermodynamic equilibrium, which is not always realistic. Kinetic Monte Carlo models, on the other hand, do not require this assumption but are computationally expensive. Here, we present a nonequilibrium drift-diffusion model that bridges this gap by fusing the established multiple trap and release formalism with the drift-diffusion transport equation. For a prototypical photovoltaic system the model is shown to quantitatively describe, with a single set of parameters, experiments probing (1) temperature-dependent steady-state charge transport-space-charge limited currents, and (2) time-resolved charge transport and relaxation of nonequilibrated photocreated charges. Moreover, the outputs of the developed kinetic drift-diffusion model are an order of magnitude, or more, faster to compute and in good agreement with kinetic Monte Carlo calculations.

  • 12.
    Felekidis, Nikolaos
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Wang, E.
    Chalmers University of Technology, Göteborg, Sweden.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Open circuit voltage and efficiency in ternary organic photovoltaic blends2016In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 9, no 1, p. 257-266Article in journal (Refereed)
    Abstract [en]

    Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one acceptor are investigated by experiments and modeling. The commonly observed continuous tunability of the open circuit voltage V-OC with the donor1 : donor2 ratio can quantitatively be explained as quasi-Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts for the composition-dependent absorption and the shape of the current-voltage characteristic curve is developed. When all other parameters, most notably the fill factor, are constant, we find that for state-of-the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with complementary absorption spectra, offer modest improvements over binary ones. In contrast, when, upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor, ternaries may offer significant advantages over binaries.

  • 13.
    Garcia-Iglesias, Miguel
    et al.
    Eindhoven University of Technology, Netherlands.
    de Waal, Bas F. M.
    Eindhoven University of Technology, Netherlands.
    Gorbunov, Andrey V.
    Eindhoven University of Technology, Netherlands.
    Palmans, Anja R. A.
    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. Eindhoven University of Technology, Netherlands.
    Meijer, E. W.
    Eindhoven University of Technology, Netherlands.
    A Versatile Method for the Preparation of Ferroelectric Supramolecular Materials via Radical End-Functionalization of Vinylidene Fluoride Oligomers2016In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 138, no 19, p. 6217-6223Article in journal (Refereed)
    Abstract [en]

    A synthetic method for the end-functionalization of vinylidene fluoride oligomers (OVDF) via a radical reaction between terminal olefins and I-OVDF is described. The method shows a wide substrate scope and excellent conversions, and permits the preparation of different disc-shaped cores such as benzene-1,3,5-tricarboxamides (BTAs), perylenes bisimide and phthalocyanines (Pc) bearing three to eight ferroelectric oligomers at their periphery. The formation, purity, OVDF conformation, and morphology of the final adducts has been assessed by a combination of techniques, such as NMR, size exclusion chromatography, differential scanning calorimetry, polarized optical microscopy, and atomic force microscopy. Finally, PBI-OVDF and Pc-OVDF materials show ferroelectric hysteresis behavior together with high remnant polarizations, with values as high as P-r approximate to 37 mC/m(2) for Pc-OVDF. This work demonstrates the potential of preparing a new set of ferroelectric materials simply by attaching OVDF oligomers to different small molecules. The use of carefully chosen small molecules paves the way to new functional materials in which ferroelectricity and electrical conductivity or light-harvesting properties coexist in a single compound.

  • 14.
    Gorbunov, A. V.
    et al.
    Eindhoven University of Technology, Netherlands.
    Putzeys, T.
    Katholieke University of Leuven, Belgium.
    Urbanaviciute, Indre
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Janssen, R. A. J.
    Eindhoven University of Technology, Netherlands.
    Wubbenhorst, M.
    Katholieke University of Leuven, Belgium.
    Sijbesma, R. 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. Eindhoven University of Technology, Netherlands.
    True ferroelectric switching in thin films of trialkylbenzene-1,3,5-tricarboxamide (BTA)2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 34, p. 23663-23672Article in journal (Refereed)
    Abstract [en]

    We have investigated the ferroelectric polarization switching properties of trialkylbenzene-1,3,5-tricarboxamide (BTA), which is a model system for a large class of novel organic ferroelectric materials. In the solid state BTAs form a liquid crystalline columnar hexagonal phase that provides long range order that was previously shown to give rise to hysteretic dipolar switching. In this work the nature of the polar switching process is investigated by a combination of dielectric relaxation spectroscopy, depth-resolved pyroelectric response measurements, and classical frequency- and time-dependent electrical switching. We show that BTAs, when brought in a homeotropically aligned hexagonal liquid crystalline phase, are truly ferroelectric. Analysis of the transient switching behavior suggests that the ferroelectric switching is limited by a highly dispersive nucleation process, giving rise to a wide distribution of switching times.

  • 15.
    Gorbunov, Andrey V.
    et al.
    Eindhoven University of Technology, Netherlands.
    Haedler, Andreas T.
    Eindhoven University of Technology, Netherlands.
    Putzeys, Tristan
    Katholieke University of Leuven, Belgium.
    Zha, R. Helen
    Eindhoven University of Technology, Netherlands.
    Schmidt, Hans-Werner
    University of Bayreuth, Germany.
    Kivala, Milan
    University of Erlangen Nurnberg, Germany.
    Urbanaviciute, Indre
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Wubbenhorst, Michael
    Katholieke University of Leuven, Belgium.
    Meijer, E. W.
    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. Eindhoven University of Technology, Netherlands.
    Switchable Charge Injection Barrier in an Organic Supramolecular Semiconductor2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 24, p. 15535-15542Article in journal (Refereed)
    Abstract [en]

    We disclose a supramolecular material that combines semiconducting and dipolar functionalities. The material consists of a discotic semiconducting carbonyl-bridged triarylamine core, which is surrounded by three dipolar amide groups. In thin films, the material self-organizes in a hexagonal columnar fashion through Jr-stacking of the molecular core and hydrogen bonding between the amide groups. Alignment by an electrical field in a simple metal/semiconductor/metal geometry induces a polar order in the interface layers near the metal contacts that can be reversibly switched, while the bulk material remains nonpolarized. On suitably chosen electrodes, the presence of an interfacial polarization field leads to a modulation of the barrier for charge injection into the semiconductor. Consequently, a reversible switching is possible between a high-resistance, injection-limited off-state and a low-resistance, space-charge-limited on-state. The resulting memory diode shows switchable rectification with on/off ratios of up to two orders of magnitude. This demonstrated multifunctionality of a single material is a promising concept toward possible application in lowcost, large-area, nonvolatile organic memories.

  • 16.
    Gorbunov, Andrey V.
    et al.
    Eindhoven Univ Technol, Netherlands.
    Iglesias, Miguel Garcia
    Eindhoven Univ Technol, Netherlands.
    Guilleme, Julia
    Univ Autonoma Madrid, Spain.
    Cornelissen, Tim
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Roelofs, W. S. Christian
    Eindhoven Univ Technol, Netherlands.
    Torres, Tomas
    Univ Autonoma Madrid, Spain; IM DEA Nanociencia, Spain.
    Gonzalez-Rodriguez, David
    Univ Autonoma Madrid, Spain.
    Meijer, E. W.
    Eindhoven Univ Technol, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven Univ Technol, Netherlands.
    Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity2017In: Science Advances, ISSN 0036-8156, E-ISSN 2375-2548, Vol. 3, no 9, article id e1701017Article in journal (Refereed)
    Abstract [en]

    Advanced molecular materials that combine two or more physical properties are typically constructed by combining different molecules, each being responsible for one of the properties required. Ideally, single molecules could take care of this combined functionality, provided they are self-assembled correctly and endowed with different functional subunits whose strong electronic coupling may lead to the emergence of unprecedented and exciting properties. We present a class of disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups. Supramolecular organization of these materials provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. The ferroelectric polarization in these supramolecular polymers is found to couple to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. An intuitive model is developed and found to quantitatively reproduce the experimental observations. In a larger perspective, these results highlight the possibility of modulating material properties using the large electric fields associated with ferroelectric polarization.

  • 17.
    Gorbunov, AV
    et al.
    Eindhoven University of Technology, the Netherlands.
    Meng, X
    Eindhoven University of Technology, the Netherlands.
    Urbanaviciute, Indre
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Putzeys, T
    KU Leuven, Heverlee, Belgium.
    Wübbenhorst, M
    KU Leuven, Heverlee, Belgium.
    Sijbesma, RP
    Eindhoven University of Technology, the Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, the Netherlands.
    Polarization loss in the organic ferroelectrictrialkylbenzene-1,3,5-tricarboxamide (BTA)2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 4, p. 3192-3200Article in journal (Refereed)
    Abstract [en]

    We investigate the polarization loss in the archetypical molecular organic ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA). We prove that the polarization loss is due to thermally activated R-relaxation,which is a collective reversal of the amide dipole moments in ferroelectric domains. By applying a weakelectrostatic field both the polarization loss and the R-relaxation are suppressed, leading to anenhancement of the retention time by at least several orders of magnitude. Alternative loss mechanismsare discussed and ruled out. By operating the thin-film devices slightly above the crystalline to liquidcrystalline phase transition temperature the retention time of one compound becomes more than12 hours even in absence of supportive bias, which is among the longest reported so far for organicferroelectric materials.

  • 18.
    Howard, Ian A.
    et al.
    Max Planck Institute Polymer Research, Germany .
    Etzold, Fabian
    Max Planck Institute Polymer Research, Mainz, Germany.
    Laquai, Frederic
    Max Planck Institute Polymer Research, Mainz, Germany.
    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, Netherlands.
    Nonequilibrium Charge Dynamics in Organic Solar Cells2014In: Advanced Energy Materials, ISSN 1614-6832, Vol. 4, no 9, article id 1301743Article in journal (Refereed)
    Abstract [en]

    The dynamics of charge carriers after their creation at, or near, an interface play a critical role in determining the efficiency of organic solar cells as they dictate, via mechanisms that are not yet fully understood, the pathways for charge separation and recombination. Here, a combination of ultrafast transient spectroscopy and kinetic Monte Carlo simulations based on a minimalistic model are used to examine various aspects of these charge dynamics in a typical donor-acceptor copolymer:methanofullerene blend. The observed rates of charge carrier energetic relaxation and recombination for a sequence of charge densities can be all consistently described in terms of the extended Gaussian disorder model. The physical picture that arises is a) that initial charge motion is highly diffusive and boosted by energetic relaxation in the disordered density of states and b) that mobile charge carriers dissociate from and re-associate into Coulombically associated pairs faster than they recombine, especially at early times. A simple analytical calculation confirms this picture and can be used to identify sub-Langevin recombination as the cause for quantitative deviations between the Monte Carlo calculations and the measured concentration dependence of the charge recombination.

  • 19.
    Hynynen, Jonna
    et al.
    Chalmers University of Technology, Sweden.
    Kiefer, David
    Chalmers University of Technology, Sweden.
    Yu, Liyang
    Chalmers University of Technology, Sweden.
    Kroon, Renee
    Chalmers University of Technology, Sweden.
    Munir, Rahim
    King Abdullah University of Science and Technology KAUST, Saudi Arabia; King Abdullah University of Science and Technology KAUST, Saudi Arabia.
    Amassian, Aram
    King Abdullah University of Science and Technology KAUST, Saudi Arabia; King Abdullah University of Science and Technology KAUST, Saudi Arabia.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Mueller, Christian
    Chalmers University of Technology, Sweden.
    Enhanced Electrical Conductivity of Molecularly p-Doped Poly(3-hexylthiophene) through Understanding the Correlation with Solid-State Order2017In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 50, no 20, p. 8140-8148Article in journal (Refereed)
    Abstract [en]

    Molecular p-doping of the conjugated polymer poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) is a widely studied model system. Underlying structure property relationships are poorly understood because processing and doping are often carried out simultaneously. Here, we exploit doping from the vapor phase, which allows us to disentangle the influence of processing and doping. Through this approach, we are able to establish how the electrical conductivity varies with regard to a series of predefined structural parameters. We demonstrate that improving the degree of solid-state order, which we control through the choice of processing solvent and regioregularity, strongly increases the electrical conductivity. As a result, we achieve a value of up to 12.7 S cm(-2) for P3HT:F4TCNQ, We determine the F4TCNQ anion concentration and find that the number of (bound + mobile) charge carriers of about 10(-4) mol cm(-3) is not influenced by the degree of solid-state order. Thus, the observed increase in electrical conductivity by almost 2 orders of magnitude can be attributed to an increase in charge-carrier mobility to more than 10(-1) cm(2) V-1 s(-1). Surprisingly, in contrast to charge transport in undoped P3HT, we find that the molecular weight of the polymer does not strongly influence the electrical conductivity, which highlights the need for studies that elucidate structure property relationships of strongly doped conjugated polymers.

  • 20.
    Jasiunas, Rokas
    et al.
    Ctr Phys Sci and Technol, Lithuania.
    Melianas, Armantas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Stanford Univ, CA 94305 USA.
    Xia, Yuxin
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Felekidis, Nikolaos
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Gulbinas, Vidmantas
    Ctr Phys Sci and Technol, Lithuania.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Dead Ends Limit Charge Carrier Extraction from All-Polymer Bulk Heterojunction Solar Cells2018In: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 4, no 8, article id 1800144Article in journal (Refereed)
    Abstract [en]

    Extraction of photocreated charge carriers from a prototypical all-polymer organic solar cell is investigated by combining transient photocurrent and time-delayed collection field experiments with numerical simulations. It is found that extraction is significantly hampered by charges getting trapped in spatial traps that are tentatively attributed to dead ends in the intermixed polymer networkin photovoltaic devices based on the same donor polymer and a fullerene acceptor this effect is much weaker. The slow-down in charge extraction leads to enhanced recombination and associated performance losses. These effects are observed in addition to the dispersive behavior that is characteristic of charge motion in energetically disordered media. Upon annealing the effects of spatial traps diminish, rationalizing the doubling in device power conversion efficiency after annealing.

    The full text will be freely available from 2019-06-26 10:22
  • 21.
    Karuthedath, Safakath
    et al.
    KAUST, Saudi Arabia.
    Melianas, Armantas
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Stanford Univ, CA 94305 USA.
    Kan, Zhipeng
    KAUST, Saudi Arabia.
    Pranculis, Vytenis
    Ctr Phys Sci and Technol, Lithuania.
    Wohlfahrt, Markus
    KAUST, Saudi Arabia.
    Khan, Jafar I.
    KAUST, Saudi Arabia.
    Gorenflot, Julien
    KAUST, Saudi Arabia.
    Xia, Yuxin
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. 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.
    Gulbinas, Vidmantas
    Ctr Phys Sci and Technol, Lithuania.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Laquai, Frederic
    KAUST, Saudi Arabia.
    Thermal annealing reduces geminate recombination in TQ1:N2200 all-polymer solar cells2018In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 6, no 17, p. 7428-7438Article in journal (Refereed)
    Abstract [en]

    combination of steady-state and time-resolved spectroscopic measurements is used to investigate the photophysics of the all-polymer bulk heterojunction system TQ1:N2200. Upon thermal annealing a doubling of the external quantum efficiency and an improved fill factor (FF) is observed, resulting in an increase in the power conversion efficiency. Carrier extraction is similar for both blends, as demonstrated by time-resolved electric-field-induced second harmonic generation experiments in conjunction with transient photocurrent studies, spanning the ps-mu s time range. Complementary transient absorption spectroscopy measurements reveal that the different quantum efficiencies originate from differences in charge carrier separation and recombination at the polymer-polymer interface: in as-spun samples similar to 35% of the charges are bound in interfacial charge-transfer states and recombine geminately, while this pool is reduced to similar to 7% in thermally-annealed samples, resulting in higher short-circuit currents. Time-delayed collection field experiments demonstrate a field-dependent charge generation process in as-spun samples, which reduces the FF. In contrast, field-dependence of charge generation is weak in annealed films. While both devices exhibit significant non-geminate recombination competing with charge extraction, causing low FFs, our results demonstrate that the donor/acceptor interface in all-polymer solar cells can be favourably altered to enhance charge separation, without compromising charge transport and extraction.

    The full text will be freely available from 2019-03-27 15:52
  • 22.
    Khikhlovskyi, Vsevolod
    et al.
    Eindhoven University of Technology, Netherlands.
    van Breemen, Albert J. J. M.
    Holst Centre, TNO, Eindhoven, The Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Gelinck, Gerwin H.
    Eindhoven University of Technology, Netherlands; TNO, Eindhoven, Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Data retention in organic ferroelectric resistive switches2016In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 31, p. 56-62Article in journal (Refereed)
    Abstract [en]

    Solution-processed organic ferroelectric resistive switches could become the long-missing non-volatile memory elements in organic electronic devices. To this end, data retention in these devices should be characterized, understood and controlled. First, it is shown that the measurement protocol can strongly affect the apparent retention time and a suitable protocol is identified. Second, it is shown by experimental and theoretical methods that partial depolarization of the ferroelectric is the major mechanism responsible for imperfect data retention. This depolarization occurs in close vicinity to the semiconductor-ferroelectric interface, is driven by energy minimization and is inherently present in this type of phase-separated polymer blends. Third, a direct relation between data retention and the charge injection barrier height of the resistive switch is demonstrated experimentally and numerically. Tuning the injection barrier height allows to improve retention by many orders of magnitude in time, albeit at the cost of a reduced on/off ratio. (c) 2016 Elsevier B.V. All rights reserved.

  • 23.
    Khikhlovskyi, Vsevolod
    et al.
    Eindhoven University of Technology, Netherlands; TNO, Netherlands.
    van Breemen, Albert J. J. M.
    Holst Centre, TNO-The Dutch Organization for Applied Scientific Research, The Netherlands.
    Michels, Jasper J.
    Max Planck Institute for Polymer Research (MPI), Germany.
    Janssen, Rene A. J.
    Department of Applied Physics, Eindhoven University of Technology, The Netherlands.
    Gelinck, Gerwin H.
    Department of Applied Physics, Eindhoven University of Technology, The Netherlands; Holst Centre, TNO-The Dutch Organization for Applied Scientific Research, The Netherlands.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Department of Applied Physics, Eindhoven University of Technology, The Netherlands.
    3D-Morphology Reconstruction of Nanoscale Phase-Separation in Polymer Memory Blends2015In: Journal of Polymer Science Part B: Polymer Physics, ISSN 0887-6266, E-ISSN 1099-0488, Vol. 53, no 17, p. 1231-1237Article in journal (Refereed)
    Abstract [en]

    In many organic electronic devices functionality is achieved by blending two or more materials, typically polymers or molecules, with distinctly different optical or electrical properties in a single film. The local scale morphology of such blends is vital for the device performance. Here, a simple approach to study the full 3D morphology of phase-separated blends, taking advantage of the possibility to selectively dissolve the different components is introduced. This method is applied in combination with AFM to investigate a blend of a semiconducting and ferroelectric polymer typically used as active layer in organic ferroelectric resistive switches. It is found that the blend consists of a ferroelectric matrix with three types of embedded semiconductor domains and a thin wetting layer at the bottom electrode. Statistical analysis of the obtained images excludes the presence of a fourth type of domains. The criteria for the applicability of the presented technique are discussed. (c) 2015 Wiley Periodicals, Inc.

  • 24.
    Lee, Jiyoul
    et al.
    Holst Centre TNO, Netherlands; Pukyong National University, South Korea.
    van Breemen, Albert J. J. M.
    Holst Centre TNO, Netherlands.
    Khikhlovskyi, Vsevolod
    Holst Centre TNO, Netherlands; 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. Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    Gelinck, Gerwin H.
    Holst Centre TNO, Netherlands; Eindhoven University of Technology, Netherlands.
    Pulse-modulated multilevel data storage in an organic ferroelectric resistive memory diode2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, no 24407Article in journal (Refereed)
    Abstract [en]

    We demonstrate multilevel data storage in organic ferroelectric resistive memory diodes consisting of a phase-separated blend of P(VDF-TrFE) and a semiconducting polymer. The dynamic behaviour of the organic ferroelectric memory diode can be described in terms of the inhomogeneous field mechanism (IFM) model where the ferroelectric components are regarded as an assembly of randomly distributed regions with independent polarisation kinetics governed by a time-dependent local field. This allows us to write and non-destructively read stable multilevel polarisation states in the organic memory diode using controlled programming pulses. The resulting 2-bit data storage per memory element doubles the storage density of the organic ferroelectric resistive memory diode without increasing its technological complexity, thus reducing the cost per bit.

  • 25.
    Lenz, Thomas
    et al.
    Max Planck Institute Polymer Research, Germany; Grad School Mat Science Mainz, Germany.
    Zhao, Dong
    Max Planck Institute Polymer Research, Germany.
    Richardson, George
    University of London Imperial Coll Science Technology and Med, England; University of London Imperial Coll Science Technology and Med, England.
    Katsouras, Ilias
    Max Planck Institute Polymer Research, Germany; Holst Centre, Netherlands.
    Asadi, Kamal
    Max Planck Institute Polymer Research, Germany; Max Planck Grad Centre, Germany.
    Glasser, Gunnar
    Max Planck Institute Polymer Research, Germany.
    Zimmermann, Samuel T.
    University of London Imperial Coll Science Technology and Med, England; University of London Imperial Coll Science Technology and Med, England.
    Stingelin, Natalie
    University of London Imperial Coll Science Technology and Med, England; University of London Imperial Coll Science Technology and Med, England.
    Christian Roelofs, W. S.
    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. Eindhoven University of Technology, Netherlands.
    Blom, Paul W. M.
    Max Planck Institute Polymer Research, Germany; Grad School Mat Science Mainz, Germany.
    de Leeuw, Dago M.
    Max Planck Institute Polymer Research, Germany; King Abdulaziz University, Saudi Arabia.
    Microstructured organic ferroelectric thin film capacitors by solution micromolding2015In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 212, no 10, p. 2124-2132Article in journal (Refereed)
    Abstract [en]

    Ferroelectric nanostructures offer a promising route for novel integrated electronic devices such as non-volatile memories. Here we present a facile fabrication route for ferroelectric capacitors comprising a linear array of the ferroelectric random copolymer of vinylidenefluoride and trifluoroethylene (P(VDF-TrFE)) interdigitated with the electrically insulating polymer polyvinyl alcohol (PVA). Micrometer size line gratings of both polymers were fabricated over large area by solution micromolding, a soft lithography method. The binary linear arrays were realized by backfilling with the second polymer. We investigated in detail the device physics of the patterned capacitors. The electrical equivalent circuit is a linear capacitor of PVA in parallel with a ferroelectric capacitor of P(VDF-TrFE). The binary arrays are electrically characterized by both conventional Sawyer-Tower and shunt measurements. The dependence of the remanent polarization on the array topography is explained by numerical simulation of the electric field distribution.

  • 26.
    Melianas, Armantas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Etzold, Fabian
    Max Planck Institute Polymer Research, Germany.
    Savenije, Tom J.
    Delft University of Technology, Netherlands.
    Laquai, Frederic
    Max Planck Institute Polymer Research, Germany; King Abdullah University of Science and Technology, Saudi Arabia.
    Inganäs, Olle
    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, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells2015In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, no 8778Article in journal (Refereed)
    Abstract [en]

    In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.

  • 27.
    Melianas, Armantas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Pranculis, Vytenis
    Center for Physical Sciences and Technology, Lithuania.
    Devižis, Andrius
    Center for Physical Sciences and Technology, Lithuania.
    Gulbinas, Vidmantas
    Center for Physical Sciences and Technology, Lithuania.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Department of Applied Physics, Eindhoven University of Technology, MB, Eindhoven, The Netherlands.
    Dispersion-Dominated Photocurrent in Polymer:Fullerene Solar Cells2014In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 24, no 28, p. 4507-4514Article in journal (Refereed)
    Abstract [en]

    Organic bulk heterojunction solar cells are often regarded as near-equilibrium devices, whose kinetics are set by well-defined charge carrier mobilities, and relaxation in the density of states is commonly ignored or included purely phenomenologically. Here, the motion of photocreated charges is studied experimentally with picosecond time resolution by a combination of time-resolved optical probing of electric field and photocurrent measurements, and the data are used to define parameters for kinetic Monte Carlo modelling. The results show that charge carrier motion in a prototypical polymer:fullerene solar cell under operational conditions is orders of magnitude faster than would be expected on the basis of corresponding near-equilibrium mobilities, and is extremely dispersive. There is no unique mobility. The distribution of extraction times of photocreated charges in operating organic solar cells can be experimentally determined from the charge collection transients measured under pulsed excitation. Finally, a remarkable distribution of the photocurrent over energy is found, in which the most relaxed charge carriers in fact counteract the net photocurrent.

  • 28.
    Melianas, Armantas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Pranculis, Vytenis
    Center for Physical Sciences and Technology, Vilnius, Lithuania.
    Spoltore, Donato
    Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, Germany.
    Benduhn, Johannes
    Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, Germany.
    Inganäs, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Gulbinas, Vidmantas
    Center for Physical Sciences and Technology, Vilnius, Lithuania / Department of General Physics and Spectroscopy, Faculty of Physics, Vilnius University, Vilnius, Lithuania.
    Vandewal, Koen
    Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, Germany.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Charge Transport in Pure and Mixed Phases in Organic Solar Cells2017In: Advanced Energy Materials, ISSN 1614-6840, Vol. 7, no 20Article in journal (Refereed)
    Abstract [en]

    In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport.

  • 29.
    Melianas, Armantas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Pranculis, Vytenis
    Center for Physical Sciences and Technology Savanoriu, Lithuania.
    Xia, Yuxin
    Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Felekidis, Nikolaos
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. 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.
    Gulbinas, Vidmantas
    Center for Physical Sciences and Technology Savanoriu, Lithuania.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells2017In: Advanced Energy Materials, ISSN 1614-6840, Vol. 7, no 9, article id 1602143Article in journal (Refereed)
    Abstract [en]

    Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum-power point (short-circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.

  • 30.
    Meng, Xiao
    et al.
    Eindhoven University of Technology, Netherlands.
    Gorbunov, Andrey V.
    Eindhoven University of Technology, Netherlands.
    Christian Roelofs, W. S.
    Eindhoven University of Technology, Netherlands.
    Meskers, Stefan C. J.
    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, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Sijbesma, Rint P.
    Eindhoven University of Technology, Netherlands.
    Ferroelectric Switching and Electrochemistry of Pyrrole Substituted Trialkylbenzene-1,3,5-Tricarboxamides2017In: Journal of Polymer Science Part B: Polymer Physics, ISSN 0887-6266, E-ISSN 1099-0488, Vol. 55, no 8, p. 673-683Article in journal (Refereed)
    Abstract [en]

    We explore a new approach to organic ferroelectric diodes using a benzene-tricarboxamide (BTA) core connected with C10 alkyl chains to pyrrole groups, which can be polymerized to provide a semiconducting ferroelectric material. The compound possesses a columnar hexagonal liquid crystalline (LC) phase and exhibits ferroelectric switching. At low switching frequencies, an additional process occurs, which leads to a high hysteretic charge density of up to similar to 1000 mC/m(2). Based on its slow rate, the formation of gas bubbles, and the emergence of characteristic polypyrrole absorption bands in the UV-Vis-NIR, the additional process is identified as the oxidative polymerization of pyrrole groups, enabled by the presence of amide groups. Polymerization of the pyrrole groups, which is essential to obtain semiconductivity, is limited to thin layers at the electrodes, amounting to similar to 17 nm after cycling for 21 h. (C) 2017 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc.

  • 31.
    Pilet, N.
    et al.
    PSI, Switzerland.
    Khikhlovskyi, V.
    Eindhoven University of Technology, Netherlands.
    van Breemen, A. J. J. M.
    TNO Dutch Org Appl Science Research, Netherlands.
    Michels, J. J.
    TNO Dutch Org Appl Science Research, Netherlands; Max Planck Institute Polymer Research, Germany.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering. Eindhoven University of Technology, Netherlands.
    Gelinck, G.
    Eindhoven University of Technology, Netherlands; TNO Dutch Org Appl Science Research, Netherlands.
    Warnicke, P.
    PSI, Switzerland.
    Bernard, L.
    Swiss Federal Labs Mat Science and Technology, Switzerland.
    Piezoelectricity enhancement of P(VDF/TrFE) by X-ray irradiation2016In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 37, p. 257-262Article in journal (Refereed)
    Abstract [en]

    Organic electronics is becoming more and more important because the low level of fabrication and deposition complexity even at large scale makes it a good candidate for future low cost technological product development. P(VDF-TrFE) is a co-polymer of special interest due its ferroelectric property enabling usage in re-programmable non-volatile organic memory and magnetoelectric sensors. Piezo force microscopy (PFM) provides access to the technologically relevant ferroelectric polarisability and its remanent polarization via imaging of the piezoelectric property. Here we use PFM to show that piezoelectric response of a P(VDF-TrFE) film can be enhanced by up to 260 % after soft X-ray irradiation. This enhancement correlates with morphological change of part of the film, from amorphous to crystalline. An optimal irradiation dose is found above which the film gets eroded and the piezoelectric response gets lowered. (C) 2016 Elsevier B.V. All rights reserved.

  • 32.
    Roelofs, W. S. Christian
    et al.
    Eindhoven University of Technology, Netherlands; Philips Research Labs, Netherlands.
    Li, Weiwei
    Eindhoven University of Technology, Netherlands.
    Janssen, Rene A. J.
    Eindhoven University of Technology, Netherlands.
    de Leeuw, Dago M.
    Max Planck Institute Polymer Research, Germany; King Abdulaziz University, Saudi Arabia.
    Kemerink, Martijn
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, The Institute of Technology. Eindhoven University of Technology, Eindhoven, The Netherlands.
    Contactless charge carrier mobility measurement in organic field-effect transistors2014In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 15, no 11, p. 2855-2861Article in journal (Refereed)
    Abstract [en]

    With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.

  • 33.
    Roelofs, W.S. Christian
    et al.
    Eindhoven University of Technology, The Netherlands; Philips Research Laboratories, Eindhoven, The Netherlands .
    Charrier, Dimitri S. H.
    Eindhoven University of Technology, The Netherlands .
    Dzwilewski, Andrzej
    Eindhoven University of Technology, The Netherlands .
    Janssen, Rene A. J.
    Eindhoven University of Technology, The Netherlands .
    de Leeuw, Dago M.
    Max Planck Institute Polymer Research, Mainz, Germany; King Abdulaziz University, Jeddah, Saudi Arabia .
    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 .
    Scanning tunnelling microscopy on organic field-effect transistors based on intrinsic pentacene2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 26, p. 263301-Article in journal (Refereed)
    Abstract [en]

    The full potential of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy for in-situ characterization of organic semiconductors has so far not been accessible. Here, we demonstrate that the underlying problem, the low intrinsic conductivity, can be overcome by working in a field-effect geometry. We present high resolution surface topographies obtained by STM on pentacene organic field-effect transistors (OFETs). By virtue of the OFET geometry, the hole accumulation layer that is present at sufficiently negative gate bias acts as back contact, collecting the tunnelling current. The presence of a true tunnelling gap is established, as is the need for the presence of an accumulation layer. The tunnelling current vs. tip bias showed rectifying behaviour, which is rationalized in terms of the tip acting as a second gate on the unipolar semiconductor. An explanatory band diagram is presented. The measurements shown indicate that intrinsic organic semiconductors can be in-situ characterized with high spatial and energetic resolution in functional devices.

  • 34.
    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.

  • 35.
    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.

  • 36.
    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.

  • 37.
    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.

  • 38.
    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.

  • 39.
    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.

  • 40.
    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.

  • 41.
    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.

  • 42.
    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.

  • 43.
    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.

  • 44.
    Zuo, Guangzheng
    Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
    Doping and Density of States Engineering for Organic Thermoelectrics2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Thermoelectric materials can turn temperature differences directly into electricity. To use this to harvest e.g. waste heat with an efficiency that approaches the Carnot efficiency requires a figure of merit ZT larger than 1. Compared with their inorganic counterparts, organic thermoelectrics (OTE) have numerous advantages, such as low cost, large-area compatibility, flexibility, material abundance and an inherently low thermal conductivity. Therefore, organic thermoelectrics are considered by many to be a promising candidate material system to be used in lower cost and higher efficiency thermoelectric energy conversion, despite record ZT values for OTE currently lying around 0.25.

    A complete organic thermoelectric generator (TEG) normally needs both p-type and n-type materials to form its electric circuit. Molecular doping is an effective way to achieve p- and ntype materials using different dopants, and it is necessary to fundamentally understand the doping mechanism. We developed a simple yet quantitative analytical model and compare it with numerical kinetic Monte Carlo simulations to reveal the nature of the doping effect. The results show the formation of a deep tail in the Gaussian density of states (DOS) resulting from the Coulomb potentials of ionized dopants. It is this deep trap tail that negatively influences the charge carrier mobility with increasing doping concentration. The trends in mobilities and conductivities observed from experiments are in good agreement with the modeling results, for a large range of materials and doping concentrations.

    Having a high power factor PF is necessary for efficient TEG. We demonstrate that the doping method can heavily impact the thermoelectric properties of OTE. In comparison to conventional bulk doping, sequential doping can achieve higher conductivity by preserving the morphology, such that the power factor can improve over 100 times. To achieve TEG with high output power, not only a high PF is needed, but also having a significant active layer thickness is very important. We demonstrate a simple way to fabricate multi-layer devices by sequential doping without significantly sacrificing PF.

    In addition to the application discussed above, harvesting large amounts of heat at maximum efficiency, organic thermoelectrics may also find use in low-power applications like autonomous sensors where voltage is more important than power. A large output voltage requires a high Seebeck coefficient. We demonstrate that density of states (DOS) engineering is an effective tool to increase the Seebeck coefficient by tailoring the positions of the Fermi energy and the transport energy in n- and p-type doped blends of conjugated polymers and small molecules.

    In general, morphology heavily impacts the performance of organic electronic devices based on mixtures of two (or more) materials, and organic thermoelectrics are no exception. We experimentally find that the charge and energy transport is distinctly different in well-mixed and phase separated morphologies, which we interpreted in terms of a variable range hopping model. The experimentally observed trends in conductivity and Seebeck coefficient are reproduced by kinetic Monte Carlo simulations in which the morphology is accounted for.  

    List of papers
    1. Impact of doping on the density of states and the mobility in organic semiconductors
    Open this publication in new window or tab >>Impact of doping on the density of states and the mobility in organic semiconductors
    2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 23, p. 235203-Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2016
    National Category
    Other Physics Topics
    Identifiers
    urn:nbn:se:liu:diva-130276 (URN)10.1103/PhysRevB.93.235203 (DOI)000378813800009 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC)

    Available from: 2016-08-01 Created: 2016-07-28 Last updated: 2018-08-29
    2. Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems
    Open this publication in new window or tab >>Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems
    Show others...
    2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 14, p. 7767-7775Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    AMER CHEMICAL SOC, 2017
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-137610 (URN)10.1021/acs.jpcc.7b01758 (DOI)000399629000022 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC); Knut och Alice Wallenbergs stiftelse (project "Tail of the Sun"); Swedish Research Council; Swedish Research Council Formas; Chalmers Area of Advance Energy

    Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2018-05-14
    3. High Seebeck Coefficient in Mixtures of Conjugated Polymers
    Open this publication in new window or tab >>High Seebeck Coefficient in Mixtures of Conjugated Polymers
    2018 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 15, article id 1703280Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    Wiley-Blackwell, 2018
    Keywords
    conjugated polymers, doping, kinetic Monte Carlo simulations, organic thermoelectrics, Seebeck coefficients
    National Category
    Materials Engineering Physical Sciences
    Identifiers
    urn:nbn:se:liu:diva-147779 (URN)10.1002/adfm.201703280 (DOI)000430101100004 ()
    Conference
    2018/05/14
    Note

    Funding Agencies: Chinese Scholarship Council (CSC)

    Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2018-05-31Bibliographically approved
    4. Range and energetics of charge hopping in organic semiconductors
    Open this publication in new window or tab >>Range and energetics of charge hopping in organic semiconductors
    2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 241202Article in journal (Refereed) Published
    Abstract [en]

    The recent upswing in attention for the thermoelectric properties of organic semiconductors (OSCs) adds urgency to the need for a quantitative description of the range and energetics of hopping transport in organic semiconductors under relevant circumstances, i.e., around room temperature (RT). In particular, the degree to which hops beyond the nearest neighbor must be accounted for at RT is still largely unknown. Here, measurements of charge and energy transport in doped OSCs are combined with analytical modeling to reach the univocal conclusion that variable-range hopping is the proper description in a large class of disordered OSC at RT. To obtain quantitative agreement with experiment, one needs to account for the modification of the density of states by ionized dopants. These Coulomb interactions give rise to a deep tail of trap states that is independent of the materials initial energetic disorder. Insertion of this effect into a classical Mott-type variable-range hopping model allows one to give a quantitative description of temperature-dependent conductivity and thermopower measurements on a wide range of disordered OSCs. In particular, the model explains the commonly observed quasiuniversal power-law relation between the Seebeck coefficient and the conductivity.

    Place, publisher, year, edition, pages
    AMER PHYSICAL SOC, 2017
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-144143 (URN)10.1103/PhysRevB.96.241202 (DOI)000418616700001 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC); Knut och Alice Wallenberg stiftelse

    Available from: 2018-01-10 Created: 2018-01-10 Last updated: 2018-08-29
    5. High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics
    Open this publication in new window or tab >>High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics
    2018 (English)In: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 4, no 1, article id 1700501Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    Wiley-VCH Verlagsgesellschaft, 2018
    Keywords
    n-type doping; organic thermoelectrics; power factor; Seebeck coefficient
    National Category
    Other Materials Engineering
    Identifiers
    urn:nbn:se:liu:diva-144566 (URN)10.1002/aelm.201700501 (DOI)000419670400026 ()
    Note

    Funding Agencies|Chinese Scholarship Council (CSC)

    Available from: 2018-01-29 Created: 2018-01-29 Last updated: 2018-05-14
    6. High thermoelectric power factor from multilayer solution-processed organic films
    Open this publication in new window or tab >>High thermoelectric power factor from multilayer solution-processed organic films
    2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 8, article id 083303Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    AMER INST PHYSICS, 2018
    National Category
    Materials Chemistry
    Identifiers
    urn:nbn:se:liu:diva-145764 (URN)10.1063/1.5016908 (DOI)000425977500021 ()
    Note

    Funding Agencies|China Scholarship Council (CSC); Knut och Alice Wallenbergs stiftelse (Project "Tail of the Sun")

    Available from: 2018-03-22 Created: 2018-03-22 Last updated: 2018-05-14
    7. Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends
    Open this publication in new window or tab >>Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends
    2018 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 11, p. 9638-9644Article in journal (Refereed) Published
    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.

    Place, publisher, year, edition, pages
    AMER CHEMICAL SOC, 2018
    Keywords
    Seebeck coefficient; morphology; charge transport; conjugated polymers; kinetic Monte Carlo simulations
    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:liu:diva-147581 (URN)10.1021/acsami.8b00122 (DOI)000428356800052 ()29488380 (PubMedID)
    Note

    Funding Agencies|China Scholarship Council (CSC)

    Available from: 2018-04-26 Created: 2018-04-26 Last updated: 2018-05-18
  • 45.
    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.

  • 46.
    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.