32 results on '"Karin Zojer"'
Search Results
2. Elementary steps in electrical doping of organic semiconductors
- Author
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Max L. Tietze, Johannes Benduhn, Paul Pahner, Bernhard Nell, Martin Schwarze, Hans Kleemann, Markus Krammer, Karin Zojer, Koen Vandewal, and Karl Leo
- Subjects
Science - Abstract
Molecular doping is routinely used in organic semiconductor devices nowadays, but the physics at play remains unclarified. Tietze et al. describe it as a two-step process and show it costs little, energetically, to dissociate charge transfer complexes due to energetic disorder of organic semiconductors.
- Published
- 2018
- Full Text
- View/download PDF
3. Critical Evaluation of Organic Thin-Film Transistor Models
- Author
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Markus Krammer, James W. Borchert, Andreas Petritz, Esther Karner-Petritz, Gerburg Schider, Barbara Stadlober, Hagen Klauk, and Karin Zojer
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organic thin-film transistor ,transistor model evaluation ,channel-length dependence ,contact resistances ,modeling contact effects ,equivalent circuit ,charge-carrier-mobility extraction ,Crystallography ,QD901-999 - Abstract
The thin-film transistor (TFT) is a popular tool for determining the charge-carrier mobility in semiconductors, as the mobility (and other transistor parameters, such as the contact resistances) can be conveniently extracted from its measured current-voltage characteristics. However, the accuracy of the extracted parameters is quite limited, because their values depend on the extraction technique and on the validity of the underlying transistor model. We propose here a new approach for validating to what extent a chosen transistor model is able to predict correctly the transistor operation. In the two-step fitting approach we have developed, we analyze the measured current-voltage characteristics of a series of TFTs with different channel lengths. In the first step, the transistor parameters are extracted from each individual transistor by fitting the output and transfer characteristics to the transistor model. In the second step, we check whether the channel-length dependence of the extracted parameters is consistent with the underlying model. We present results obtained from organic TFTs fabricated in two different laboratories using two different device architectures, three different organic semiconductors and five different materials combinations for the source and drain contacts. For each set of TFTs, our approach reveals that the state-of-the-art transistor models fail to reproduce correctly the channel-length-dependence of the transistor parameters. Our approach suggests that conventional transistor models require improvements in terms of the charge-carrier-density dependence of the mobility and/or in terms of the consideration of uncompensated charges in the carrier-accumulation channel.
- Published
- 2019
- Full Text
- View/download PDF
4. Publisher Correction: Elementary steps in electrical doping of organic semiconductors
- Author
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Max L. Tietze, Johannes Benduhn, Paul Pahner, Bernhard Nell, Martin Schwarze, Hans Kleemann, Markus Krammer, Karin Zojer, Koen Vandewal, and Karl Leo
- Subjects
Science - Abstract
The original version of this Article contained an error in Equation 1. A factor of ‘c’ was included in the right-hand term. This has been corrected in the PDF and HTML versions of the Article.
- Published
- 2018
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5. Joint Distributions of Local Pore Space Properties Quantitatively Explain Simulated Air Flow Variations in Paper
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Peter Leitl, Eduardo Machado Charry, Ekaterina Baikova, Matthias Neumann, Ulrich Hirn, Volker Schmidt, and Karin Zojer
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General Chemical Engineering ,Catalysis - Abstract
The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy’s law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations.
- Published
- 2023
6. Sample Layout Aiding Efficient Scans of Heterogeneous Sheet-Like Materials
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Michael Stonig, Eduardo Machado Charry, Andre Hilger, Ingo Manke, Matthias Neumann, Volker Schmidt, and Karin Zojer
- Published
- 2023
7. Transport of organic volatiles through paper: physics-informed neural networks for solving inverse and forward problems
- Author
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Alexandra Serebrennikova, Raimund Teubler, Lisa Hoffellner, Erich Leitner, Ulrich Hirn, and Karin Zojer
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General Chemical Engineering ,Catalysis - Abstract
Transport of volatile organic compounds (VOCs) through porous media with active surfaces takes place in many important applications, such as in cellulose-based materials for packaging. Generally, it is a complex process that combines diffusion with sorption at any time. To date, the data needed to use and validate the mathematical models proposed in literature to describe the mentioned processes are scarce and have not been systematically compiled. As an extension of the model of Ramarao et al. (Dry Technol 21(10):2007–2056, 2003) for the water vapor transport through paper, we propose to describe the transport of VOCs by a nonlinear Fisher–Kolmogorov–Petrovsky–Piskunov equation coupled to a partial differential equation (PDE) for the sorption process. The proposed PDE system contains specific material parameters such as diffusion coefficients and adsorption rates as multiplication factors. Although these parameters are essential for solving the PDEs at a given time scale, not all of the required parameters can be directly deduced from experiments, particularly diffusion coefficients and sorption constants. Therefore, we propose to use experimental concentration data, obtained for the migration of dimethyl sulfoxide (DMSO) through a stack of paper sheets, to infer the sorption constant. These concentrations are considered as the outcome of a model prediction and are inserted into an inverse boundary problem. We employ Physics-Informed Neural Networks (PINNs) to find the underlying sorption constant of DMSO on paper from this inverse problem. We illustrate how to practically combine PINN-based calculations with experimental data to obtain trustworthy transport-related material parameters. Finally we verify the obtained parameter by solving the forward migration problem via PINNs and finite element methods on the relevant time scale and show the satisfactory correspondence between the simulation and experimental results.
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- 2022
- Full Text
- View/download PDF
8. On Variability and Interdependence of Local Porosity and Local Tortuosity in Porous Materials: a Case Study for Sack Paper
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Matthias Neumann, Eduardo Machado Charry, Volker Schmidt, and Karin Zojer
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Statistics and Probability ,Geodesic ,General Mathematics ,010102 general mathematics ,Mathematical analysis ,Copula (linguistics) ,Sinuosity ,01 natural sciences ,Tortuosity ,010104 statistics & probability ,Goodness of fit ,Gumbel distribution ,Joint probability distribution ,0101 mathematics ,Porosity ,Mathematics - Abstract
The variability and interdependence of local porosity and local mean geodesic tortuosity, which is a measure for the sinuosity of shortest transportation paths, is investigated at the example of the microstructure in sack paper. By means of statistical image analysis, these two morphological characteristics are computed for several cutouts of 3D image data obtained by X-ray microcomputed tomography. Considering cutouts of different sizes allows us to study the influence of the sample size on the local variability of the considered characteristics. Moreover, the interdependence between local porosity and local mean geodesic tortuosity is quantified by modeling their joint distribution parametrically using Archimedean copulas. It turns out that the family of Gumbel copulas is an appropriate model type, which is formally validated by a goodness of fit test. Besides mean geodesic tortuosity, we consider further related morphological characteristics, describing the sinuosity of those shortest transportation paths, whose minimum diameter exceeds a predefined threshold. Moreover, we show that the copula approach investigated in this paper can also be used to quantify the negative correlation between local porosity and these modified versions of local mean geodesic tortuosity. Our results elucidate the impact of local porosity on various kinds of morphological characteristics, which are not experimentally accessible and which are important for local air permeance – a key property of sack paper.
- Published
- 2020
9. Capturing Centimeter Scale Local Variations in Paper Pore Space via mu CT A Benchmark Study Using Calendered Paper
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Ulrich Hirn, Ingo Manke, Eduardo Machado Charry, Matthias Neumann, Ekaterina Baikova, Karin Zojer, Robert Schennach, André Hilger, and Volker Schmidt
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X-Ray Microcomputed Tomography ,Centimeter ,Scale (ratio) ,Computer science ,Benchmark (computing) ,experimental microstructure characterization ,hard nip calendering of paper material ,local microstructure variation ,statistical image analysis ,X ray microcomputed tomography ,Geometry ,Characterisation of pore space in soil ,Instrumentation - Abstract
A two-step framework to analyze local microstructure variations of paper sheets based on 3D image data is presented. First, a multi-stage workflow efficiently acquires a large set of highly resolved tomographic image data, which enables—in combination with statistical image analysis—the quantification of local variations and pairwise correlations of morphological microstructure characteristics on length scales ranging from micrometers to centimeters. Secondly, the microstructure is analyzed in terms of the local behavior of porosity, thickness, and further descriptors related to transportation paths. The power of the presented framework is demonstrated, showing that it allows one (i) to quantitatively reveal the difference in terms of local structural variations between a model paper before and after unidirectional compression via hard-nip calendering and that (ii) the field of view which is required to reliably compute the probability distributions of the considered local microstructure characteristics is at least 20 mm$^{2}$. The results elucidate structural differences related to local densification. In particular, it is shown how calendering transforms local variations in sheet thickness into marked local mass density variations. The obtained results are in line with experimental measurements of macroscopic properties (basis weight, Bekk smoothness parameters, thickness, and Gurley retention times).
- Published
- 2021
10. Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors
- Author
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Karin Zojer, Boyu Peng, Hagen Klauk, Paddy K. L. Chan, James W. Borchert, Joachim N. Burghartz, Sabine Ludwigs, and Florian Letzkus
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0301 basic medicine ,Interface layer ,Materials science ,Science ,Gate dielectric ,General Physics and Astronomy ,02 engineering and technology ,General Biochemistry, Genetics and Molecular Biology ,Article ,law.invention ,03 medical and health sciences ,law ,lcsh:Science ,Multidisciplinary ,business.industry ,Transistor ,Contact resistance ,General Chemistry ,021001 nanoscience & nanotechnology ,Organic semiconductor ,030104 developmental biology ,Subthreshold swing ,Optoelectronics ,lcsh:Q ,0210 nano-technology ,business ,Low voltage ,Voltage - Abstract
The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V., The widespread adoption of organic thin-film transistors (TFTs) in low-voltage high-frequency device applications is impeded by the contact resistance in the TFTs. Here, the authors report record-low contact resistance in bottom-gate, bottom-contact organic TFTs with an ultrathin gate dielectric.
- Published
- 2019
11. Area dependent behavior of bathocuproine (BCP) as cathode interfacial layers in organic photovoltaic cells
- Author
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Talha Qamar, Golnaz Sherafatipour, Horst-Günter Rubahn, Karin Zojer, Bhushan Patil, Mehrad Ahmadpour, Anton Fernandez Fernandez, and Morten Madsen
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0301 basic medicine ,Materials science ,Exciton ,lcsh:Medicine ,02 engineering and technology ,7. Clean energy ,Article ,law.invention ,03 medical and health sciences ,law ,lcsh:Science ,Device failure ,Multidisciplinary ,business.industry ,Photovoltaic system ,lcsh:R ,Ag electrode ,021001 nanoscience & nanotechnology ,Cathode ,Blocking layer ,030104 developmental biology ,Optoelectronics ,lcsh:Q ,0210 nano-technology ,business ,Layer (electronics) - Abstract
Standard and inverted configuration small molecule OPV cells incorporating bathocuproine (BCP) as electron transport and exciton blocking layer is investigated, demonstrating that 2 mm2 standard and inverted cells display a maximum performance for BCP thicknesses of 10 nm and 1.5 nm, respectively. The reason for the different optimum BCP thicknesses for the two device configurations is the BCP-metal complex formed between the Ag electrode and the BCP layer in the standard configuration OPV devices. Interestingly, at optimum BCP thicknesses, the inverted OPV cells outperform the standard devices. Upon up-scaling of the device area of the cells from 2 mm2 to 10 and 100 mm2, device failure becomes prominent for the inverted OPV cells, due to aggregation of the evaporated BCP layer on the ITO surface. This demonstrates that although BCP can be adopted for efficient ETL in inverted configuration OPV devices on small scale, it is not suitable for device up-scaling due to severely decreasing device yields. In this work, a possible solution where an ultrathin layer of C70 is evaporated between the ITO and BCP layer is proposed. It is demonstrated that the proposed solution holds a strong potential to minimize the device failures of the BCP based inverted OPV cells to a significant extent, while maintaining good device performances.
- Published
- 2018
12. Pore space extraction and characterization of sack paper using μ-CT
- Author
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Volker Schmidt, Matthias Neumann, J. Lahti, E. Machado Charry, Karin Zojer, and Robert Schennach
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Histology ,Materials science ,Sample (material) ,02 engineering and technology ,021001 nanoscience & nanotechnology ,computer.software_genre ,01 natural sciences ,Tortuosity ,Pathology and Forensic Medicine ,Characterization (materials science) ,010104 statistics & probability ,Sample size determination ,Voxel ,Kriging ,Region of interest ,0101 mathematics ,0210 nano-technology ,Porosity ,Biological system ,computer - Abstract
We show that attenuation X-ray microcomputed tomography (μ-CT) offers a route to extract the three-dimensional pore space of paper reliably enough to distinguish samples of the same kind of paper. Here, we consider two sack kraft papers for cement bags with different basis weights and thicknesses. Sample areas of approximately 5 mm2 with a resolution of 1.5 μm are considered, i.e. sizes that exceed sample areas of 2 mm2 for which the pore structure was previously studied in the literature. The image segmentation is based on indicator kriging as a local method that removes ambiguities in assigning voxels as pore or as fibre. The microstructures of the two samples are statistically compared in terms of descriptors such as sheet thickness, porosity, fractions of externally accessible pores and mean geodesic tortuosity. We demonstrate that a quantitative comparison of samples in terms of porosity and thickness requires a common definition of the sheet surfaces. Finally, the statistical pore space analysis based on the μ-CT scans reliably reveals structural differences between the two paper samples, but only when several descriptors are used. LAY DESCRIPTION This paper is a seemingly abundant material. Its intrinsic porosity enables a vast number of commercial applications. Particularly packing products, e.g. cement bags, often incorporate sack kraft paper due to its high porosity and its additional mechanical strength. A direct quantification of the porosity of sack kraft papers is, hence, particularly desirable. However, experimental quantification of paper porosity or its pore network properties is difficult and often highly indirect. A nondestructive statistical analysis of the 3D microstructure holds the promise to directly assess the pores. In particular, X-ray microcomputed tomography (μ-CT), frequently with sub-μm resolution, has been established as a method to study the fibre and pore structure of paper. The question arises, whether statistical analysis of the microstructure based on μ-CT imaging is sufficient to reliably distinguish between different sack kraft papers. Here, we explore whether the pore structure of paper can be extracted and statistically analysed for larger sample areas despite the fact that a larger sample size directly translates into a lower resolution of the μ-CT scan. We expect that a large sample size increases the region of interest on the basis of which samples can be better distinguished. A lowered resolution poses a severe challenge for the reliable identification of voxel data as pores or as fibres, because the contrast between paper fibres (made of cellulose) and air, which is established due to X-ray absorption, is weak. We show that we can reliably assign each voxel by using an indicator kriging as a two-step method. This method performs an initial voxel identification based on the overall distribution of measured grey values and refines the identification by inspecting the local environment of each voxel. For the pore space extracted in such a way, we can then compute quantities that are related to the geometry and connectivity properties of the pores. Furthermore, we address a paper-born challenge for such an analysis, i.e. we cannot always unambiguously tell whether a pore is located inside the paper sheet or at the surface of the paper. The way the paper surfaces are extracted from the microstructure decisively determines the final specifications of the predicted properties. A significant distinction of the samples is only possible when comparing the properties of the pore network.
- Published
- 2018
13. Simulation of Charge Carriers in Organic Electronic Devices: Methods with their Fundamentals and Applications
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Karin Zojer
- Subjects
Materials science ,Organic devices ,business.industry ,Master equation ,Optoelectronics ,Charge carrier ,Electronics ,Kinetic Monte Carlo ,business ,Circuit modeling ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials - Published
- 2021
14. Modelling Organic Devices — Foundation, Implementation, and Merit of the Kinetic Monte Carlo Method
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Markus Krammer and Karin Zojer
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Materials science ,Organic devices ,Nuclear engineering ,Foundation (engineering) ,Kinetic Monte Carlo - Published
- 2019
15. Publisher Correction: Elementary steps in electrical doping of organic semiconductors
- Author
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Karin Zojer, Johannes Benduhn, Paul Pahner, Koen Vandewal, Max L. Tietze, Markus Krammer, Bernhard Nell, Karl Leo, Hans Kleemann, and Martin Schwarze
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Multidisciplinary ,Materials science ,Science ,Doping ,General Physics and Astronomy ,General Chemistry ,Engineering physics ,General Biochemistry, Genetics and Molecular Biology ,Article ,Term (time) ,Organic semiconductor ,Condensed Matter::Materials Science ,ComputingMethodologies_DOCUMENTANDTEXTPROCESSING ,lcsh:Q ,lcsh:Science - Abstract
Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations., Molecular doping is routinely used in organic semiconductor devices nowadays, but the physics at play remains unclarified. Tietze et al. describe it as a two-step process and show it costs little, energetically, to dissociate charge transfer complexes due to energetic disorder of organic semiconductors.
- Published
- 2018
16. 2D Semiconductors: Interfacial Band Engineering of MoS 2 /Gold Interfaces Using Pyrimidine‐Containing Self‐Assembled Monolayers: Toward Contact‐Resistance‐Free Bottom‐Contacts (Adv. Electron. Mater. 5/2020)
- Author
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Alexander Fian, Esther Karner-Petritz, Michael Gärtner, Andreas Terfort, Aleksandar Matković, Markus Krammer, Christian Teichert, Barbara Stadlober, Markus Kratzer, Herbert Gold, Egbert Zojer, Gerburg Schider, Karin Zojer, and Andreas Petritz
- Subjects
chemistry.chemical_compound ,Materials science ,Semiconductor ,Pyrimidine ,chemistry ,business.industry ,Band engineering ,Contact resistance ,Nanotechnology ,Self-assembled monolayer ,Electron ,business ,Electronic, Optical and Magnetic Materials - Published
- 2020
17. Interfacial Band Engineering of MoS 2 /Gold Interfaces Using Pyrimidine‐Containing Self‐Assembled Monolayers: Toward Contact‐Resistance‐Free Bottom‐Contacts
- Author
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Aleksandar Matković, Andreas Terfort, Egbert Zojer, Andreas Petritz, Gerburg Schider, Barbara Stadlober, Christian Teichert, Herbert Gold, Karin Zojer, Markus Kratzer, Alexander Fian, Michael Gärtner, Esther Karner-Petritz, and Markus Krammer
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chemistry.chemical_compound ,Materials science ,Pyrimidine ,chemistry ,Thin-film transistor ,Schottky barrier ,Contact resistance ,Work function engineering ,Band engineering ,Nanotechnology ,Self-assembled monolayer ,Electronic, Optical and Magnetic Materials - Published
- 2020
18. Impact of thermal transport parameters on the operating temperature of organic light emitting diodes
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Karin Zojer and Georgii Krikun
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010302 applied physics ,Materials science ,business.industry ,General Physics and Astronomy ,02 engineering and technology ,Heat transfer coefficient ,021001 nanoscience & nanotechnology ,01 natural sciences ,Thermal conductivity ,Operating temperature ,0103 physical sciences ,Thermal ,Heat transfer ,Optoelectronics ,Electric current ,0210 nano-technology ,Joule heating ,business ,Thermal analysis - Abstract
Excess heat in organic light emitting diodes (OLEDs) that is produced during their operation may accelerate their degradation and may cause an inhomogeneous brightness distribution, in particular in large area OLEDs. Assessing the quantitative impact of heat excess is difficult, because all decisive processes related to charge transport and emission via charge recombination are thermally activated. For example, electric currents that are elevated due to larger temperatures cause additional Joule heating and, hence, increase the device temperature even further. Here, we establish how parameters responsible for heat transport, i.e., the thermal conductivity of the organic layers and the heat transfer coefficient between the device surface and the environment, govern the temperature inside the OLED. Relying on three-dimensional drift-diffusion simulations that self-consistently couple thermally-activated charge transport and heat transport, we establish that the thermal conductivity of organic layers is not a bottleneck for heat transport, because the encountered layer thicknesses in realistic device geometries prevent heat accumulation. The heat transfer to the ambient environment is the key parameter to dissipate excess heat from the device. Intentionally elevated operating temperatures, which may improve the OLEDs’ electric performance, are not necessarily beneficial, as any increase in operating temperature reduces the device stability. The thermal effects, being decisive for the OLED temperature, occur in device layers beyond the electrically active region. We propose analytical expressions that relate the temperature in the device for a given point of operation to the heat transfer to the environment and the substrate.
- Published
- 2019
19. Embedded Dipole Self-Assembled Monolayers for Contact Resistance Tuning in p-Type and n-Type Organic Thin Film Transistors and Flexible Electronic Circuits
- Author
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Michael Zharnikov, Markus Krammer, Andreea Cojocaru, Giulia Nascimbeni, Karin Zojer, Herbert Gold, Andreas Petritz, Esther Karner-Petritz, Andreas Terfort, Benedikt Schrode, Roland Resel, Egbert Zojer, Alexander Fian, Michael Gärtner, Eric Sauter, and Barbara Stadlober
- Subjects
Organic electronics ,Materials science ,business.industry ,Contact resistance ,Self-assembled monolayer ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Dipole ,Thin-film transistor ,Electrochemistry ,Optoelectronics ,0210 nano-technology ,business ,Electronic circuit - Published
- 2018
20. Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors
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Karin Zojer, Manfred Gruber, Ferdinand Schürrer, and Egbert Zojer
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Electron mobility ,Materials science ,business.industry ,Transistor ,Contact resistance ,Nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,law.invention ,Relative significance ,Characterization (materials science) ,Biomaterials ,Organic semiconductor ,Thin-film transistor ,law ,Electrochemistry ,Optoelectronics ,business ,Order of magnitude - Abstract
The contact resistance is known to severely hamper the performance of organic thin-film transistors, especially when dealing with large injection barriers, high mobility organic semiconductors, or short channel lengths. Here, the relative significance of how it is affected by materials-parameters (mobility and interfacial level-offsets) and geometric factors (bottom-contact vs top-contact geometries) is assessed. This is done using drift-diffusion-based simulations on idealized device structures aiming at a characterization of the “intrinsic” situation in the absence of traps, differences in the film morphology, or metal-atoms diffusing into the organic semiconductor. It is found that, in contrast to common wisdom, in such a situation the top-contact devices do not always outperform the bottom-contact ones. In fact, the observed ratio between the contact resistances of the two device structures changes by up to two orders of magnitude depending on the assumed materials parameters. The contact resistance is also shown to be strongly dependent on the hole mobility in the organic semiconductor and influenced by the chosen point of operation of the device.
- Published
- 2013
21. Relation between injection barrier and contact resistance in top-contact organic thin-film transistors
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Karin Zojer, Manfred Gruber, and Ferdinand Schürrer
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Work (thermodynamics) ,Materials science ,Condensed matter physics ,Contact resistance ,Transfer line ,Analytical chemistry ,Thermionic emission ,General Chemistry ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Operating temperature ,Thin-film transistor ,Materials Chemistry ,Electrical and Electronic Engineering ,Quantum tunnelling ,Communication channel - Abstract
We theoretically investigate the carrier injection into top-contact bottom-gate organic thin film transistors. By means of a two-dimensional drift–diffusion model, we explicitly consider thermionic and tunneling injection in combination with subsequent carrier transport into the device. Based on numerical simulations with this model, we determine the contact resistance as a function of the nominal hole injection barrier height and temperature. Depending on the barrier height or the operating temperature, we find three distinct injection regimes. Our work reveals that in all three regimes self-regulating processes exist due to which the influx of current is adjusted according to the needs of the channel at the given point of operation. We explain why the transmission/transfer line method (TLM) for the determination of the contact resistance, R c , quantitatively fails for non-quasi-ohmic injection. Self-regulation links the contact resistance to the channel resistance and the contact resistance becomes dependent on the channel length. For larger channel lengths, R c is underestimated by TLM; the method yields overestimated values for small channel devices.
- Published
- 2012
22. Impact of the Capacitance of the Dielectric on the Contact Resistance of Organic Thin-Film Transistors
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Egbert Zojer, Karin Zojer, Manfred Gruber, and Anton Fernandez Fernandez
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Materials science ,business.industry ,Schottky barrier ,Contact resistance ,Transistor ,General Physics and Astronomy ,Insulator (electricity) ,Hardware_PERFORMANCEANDRELIABILITY ,Dielectric ,Capacitance ,law.invention ,Thin-film transistor ,law ,Hardware_INTEGRATEDCIRCUITS ,Optoelectronics ,Charge injection ,business ,Hardware_LOGICDESIGN - Abstract
The operation of organic thin-film transistors (OTFTs) relies crucially on charge injection. The authors discuss the impact of insulator capacitance on injection across a Schottky barrier, showing that it strongly depends on device architecture. This understanding is important for implementing OTFTs in high-speed applications such as RFID tags and active-matrix displays.
- Published
- 2015
23. Role of the Charge-Transfer State in Reduced Langevin Recombination in Organic Solar Cells: A Theoretical Study
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Yiming, Liu, Karin, Zojer, Benny, Lassen, Jakob, Kjelstrup-Hansen, Horst-Günter, Rubahn, and Morten, Madsen
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Article - Abstract
Reduced Langevin recombination has been observed in organic solar cells (OSCs) for many years, but its origin is still unclear. A recent work by Burke et al. (Adv. Energy Mater.2015, 5, 1500123-1) was inspired by this reduced Langevin recombination, and they proposed an equilibrium model of charge-transfer (CT) states that correlates the open-circuit voltage of OSCs with experimentally available device parameters. In this work, we extend Burke et al.’s CT model further and for the first time directly correlate the reduced Langevin recombination with the energetic and dynamic behavior of the CT state. Recombination through CT states leads in a straightforward manner to a decrease in the Langevin reduction factor with increasing temperature, without explicit consideration of the temperature dependence of the mobility. To verify the correlation between the CT states and reduced Langevin recombination, we incorporated this CT model and the reduced Langevin model into drift-diffusion simulations of a bilayer OSC. The simulations not only successfully reproduced realistic current–voltage (J–V) characteristics of the bilayer OSC, but also demonstrate that the two models consistently lead to same value of the apparent Langevin reduction factor.
- Published
- 2015
24. Mechanism of surface proton transfer doping in pentacene based organic thin-film transistors
- Author
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Lucas Hauser, Lukas Ladinig, Anja Haase, Gregor Trimmel, Heinz-Georg Flesch, Reinhold Hetzel, Ferdinand Schürrer, Simon J. Ausserlechner, Roland Resel, Egbert Zojer, Barbara Stadlober, Michael Buchner, Karin Zojer, and Manfred Gruber
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Materials science ,business.industry ,Doping ,Nanotechnology ,Surfaces and Interfaces ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Threshold voltage ,X-ray reflectivity ,Pentacene ,Contact angle ,Organic semiconductor ,chemistry.chemical_compound ,chemistry ,Thin-film transistor ,Gate oxide ,Materials Chemistry ,Optoelectronics ,Electrical and Electronic Engineering ,business - Abstract
A number of studies show that chemical modification of the semiconductor–dielectric interface can be used to control the threshold voltage (Vth) of organic thin film transistors (OTFTs). A promising chemical functionality to achieve that are acidic groups, which – at the semiconductor–dielectric interface – have been used to realize chemically responsive OTFTs and easy to fabricate inverter structures. Especially for pentacene based OTFTs, the underlying chemical and physical mechanisms behind the acid-induced Vth shifts are not yet fully understood. Their clarification is the topic of the present paper. To distinguish between space-charge and dipole-induced effects, we study the impact of the thickness of the gate oxide on the device characteristics achieving maximum Vth-shifts around 100 V. To elucidate the role of the acid, we compare the doping of pentacene by acidic interfacial layers with the impact of hydrochloric acid vapour and investigate the consequences of exposing devices to ammonia. Complementary experiments using 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) as active layer hint toward the central (6 and 13) carbon atoms being subject to the electrophilic attack by the acidic protons. They also prove that the observed Vth shifts in pentacene devices are indeed a consequence of the interaction between the acidic groups and the active material. The experimental device characterization is supported by drift-diffusion based device modelling, by quantum chemically simulations, as well as by contact angle, atomic force microscopy (AFM) and X-ray reflectivity (XRR). The combination of the obtained results leads us to suggest proton transfer doping at the semiconductor–dielectric interface as the process responsible for the observed shift of Vth.
- Published
- 2011
25. Influence of transport-related material parameters on the I–V characteristic of inorganic–organic hybrid solar cells
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Gregor Trimmel, Karin Zojer, Manfred Gruber, Ferdinand Schürrer, and Benjamin A. Stickler
- Subjects
Organic solar cell ,Band gap ,business.industry ,Chemistry ,Exciton ,Heterojunction ,General Chemistry ,Hybrid solar cell ,Condensed Matter Physics ,Acceptor ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Semiconductor ,Chemical physics ,V curve ,Materials Chemistry ,Electrical and Electronic Engineering ,business - Abstract
The carrier transport in inorganic–organic hybrid cells based on small band gap inorganic semiconductors is theoretically studied. In such cells, photo-carriers are generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material due to direct generation in the small band gap inorganic material. By means of a two-dimensional drift–diffusion model, we demonstrate that material properties directly related to transport, i.e., (i) the carrier mobilities, (ii) the static dielectric constants, and (iii) transport level offsets give, essentially, rise to marked variations in the open circuit voltage, V oc , and the shape of the I – V curve. As the most striking consequence of the differing generation mechanisms, the role of the electron and hole level offsets entirely differ. A lack of a hole transport level offset represents the most serious loss factor for the power conversion efficiency (PCE) due to a pronounced sensitivity of the shape of the I – V curve towards the carrier mobilities and the dielectric constant. On the other hand, degenerate electron transport levels give rise to (i) appreciably large fill factors and (ii) values of V oc , j sc , and PCE that are insensitive towards the ratio of mobility and ϵ r values. However, a non-vanishing electron transport level offset is necessary to achieve a V oc beyond V built-in at all. Then, V oc is further (i) increased by an amount that corresponds to the smaller value of the transport level offsets and (ii) reduced due to a diffusion term that depends on the ratio of the mobility values in the organic and inorganic phase. The latter detrimental term is the smaller, the less the mobility ratio differs from unity, i.e., the smaller the mismatch in the mobility values becomes. Moreover, any mismatch in the carrier mobilities causes an effective mobility-induced diffusion of holes from the organic to the inorganic layer that gives rise to a s-shaped section in the I – V curve.
- Published
- 2011
26. Utilizing Schottky barriers to suppress short-channel effects in organic transistors
- Author
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Karin Zojer and Anton Fernandez Fernandez
- Subjects
010302 applied physics ,Materials science ,Physics and Astronomy (miscellaneous) ,business.industry ,Schottky barrier ,Transistor ,Schottky diode ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,law.invention ,law ,0103 physical sciences ,Hardware_INTEGRATEDCIRCUITS ,Optoelectronics ,Current (fluid) ,0210 nano-technology ,business ,Hardware_LOGICDESIGN ,Communication channel - Abstract
Transistors with short channel lengths exhibit profound deviations from the ideally expected behavior. One of the undesired short-channel effects is an enlarged OFF current that is associated with a premature turn on of the transistor. We present an efficient approach to suppress the OFF current, defined as the current at zero gate source bias, in short-channel organic transistors. We employ two-dimensional device simulations based on the drift-diffusion model to demonstrate that intentionally incorporating a Schottky barrier for injection enhances the ON-OFF ratio in both staggered and coplanar transistor architectures. The Schottky barrier is identified to directly counteract the origin of enlarged OFF currents: Short channels promote a drain-induced barrier lowering. The latter permits unhindered injection of charges even at reverse gate-source bias. An additional Schottky barrier hampers injection for such points of operations. We explain how it is possible to find the Schottky barrier of the smallest...
- Published
- 2017
27. Impact of position of electron withdrawing cyano groups on nonlinear optical properties of centrosymmetric donor-π-acceptor system
- Author
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Kwang-Sup Lee, Karin Zojer, Ran Hee Kim, Jin Sun Park, and Jean-Luc Brédas
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Chemistry ,Charge density ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Ring (chemistry) ,Photochemistry ,01 natural sciences ,Acceptor ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Crystallography ,Excited state ,Intramolecular force ,Vinylene group ,Polar effect ,CN-group ,Physical and Theoretical Chemistry ,0210 nano-technology - Abstract
Two symmetrically substituted phenylenevinylene D-A-D′-A-D type siblings, (2Z,2′Z)-2,2′-(2,5-dimethoxy-1,4-phenylene)bis(3-(4-(dimethylamino)phenyl)acrylonitrile) (↑-dscn) and (2Z,2′Z)-3,3′-(2,5-dimethoxy-1,4-phenylene)bis(2-(4-(dimethylamino)phenyl)acrylonitrile) (↓-dscn), are prepared. We investigate the effect of different but symmetrical location of these cyano groups in vinylene bridges on the 1-photon and 2-photon absorption behaviors. We report that the closeness of CN group on the vinyl group to the central phenyl ring in ↑-dscn induces an intramolecular geometric distortion between the central phenyl ring and vinylene group, preventing the effective π-conjugation length in ground and excited states. Thus, the transition energy that is observed in 1-photon absorption and fluorescence is larger in ↑-dscn than in ↓-dscn. This leads to a different intramolecular charge distribution, as a result of which the linear and nonlinear optical properties strongly depend on the position of acceptors. These results are theoretically unraveled in terms of charge transfer pathways, charge distribution, and charge distribution differences on transition.
- Published
- 2017
28. Impact of energy alignment and morphology on the efficiency in inorganic–organic hybrid solar cells
- Author
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Ferdinand Schürrer, Karin Zojer, Benjamin A. Stickler, Gregor Trimmel, and Manfred Gruber
- Subjects
Organic solar cell ,Chemistry ,Band gap ,business.industry ,Exciton ,Heterojunction ,General Chemistry ,Hybrid solar cell ,Condensed Matter Physics ,Acceptor ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Optics ,Chemical physics ,Materials Chemistry ,Charge carrier ,Electrical and Electronic Engineering ,business ,Ohmic contact - Abstract
Carrier transport in hybrid inorganic–organic solar cells has been studied by means of a two-dimensional drift-diffusion-based model including the generation and motion of excitons. The devices consist of a polymer serving as donor material and a semiconducting small-band gap inorganic component as acceptor material. For the first time it is taken into account that, in strong contrast to purely organic or inorganic cells, charge carriers can be generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material (due to band to band generation in the inorganic material). The efficiencies of devices were investigated dependent on (i) the donor–acceptor interface geometry, (ii) the transport level offsets at the heterojunction, and (iii) the energy barriers formed at the contacts. For each case, a detailed analysis of the behavior is given. We demonstrate that, depending on the particular scenario, each of these three factors can be responsible for profoundly reduced efficiencies and pronounced s-shaped sections in the I – V curves. Moreover, we show that each of the investigated factors may give rise to equally serious efficiency losses. However, it is not possible to identify a dominant effect. Depending on the particular combination, the efficiency can vary by two orders of magnitudes. In order to avoid such losses, our theoretical assessment reveals that suitable material combinations are required to form (i) ohmic contacts, (ii) preclude formation of isolated islands or nanoparticles during growth, and (iii) possess a hole-blocking offset in the transport levels at the heterojunction.
- Published
- 2010
29. Threshold Voltage Shifts in Organic Thin-Film Transistors Due to Self-Assembled Monolayers at the Dielectric Surface
- Author
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Egbert Zojer, Karin Zojer, Stefan Possanner, Peter Pacher, and Ferdinand Schürrer
- Subjects
Materials science ,business.industry ,Nanotechnology ,Self-assembled monolayer ,Dielectric ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Threshold voltage ,Biomaterials ,Organic semiconductor ,Dipole ,Depletion region ,Thin-film transistor ,Electrochemistry ,Optoelectronics ,Thin film ,business - Abstract
Recently, it has been shown by several groups that the electrical characteristics of organic thin-film transistors (OTFTs) can be significantly influenced by depositing self-assembled monolayers (SAMs) at the organic semiconductor/dielectric interface. In this work, the effect of such SAMs on the transfer characteristics and especially on the threshold voltage of OTFTs is investigated by means of two-dimensional drift-diffusion simulations. The impact of the SAM is modeled either by a permanent space charge layer that can result from chemical reactions with the active material, or by a dipole layer representing an array of ordered dipolar molecules. It is demonstrated that, in both model cases, the presence of the SAM significantly changes the transfer characteristics. In particular, it gives rise to a modified, effective gate voltage V eff that results in a rigid shift of the threshold voltage, ΔV th , relative to a SAM-free OTFT. The achievable amount of threshold voltage shift, however, strongly depends on the actual role of the SAM. While for the investigated device dimensions, an organic SAM acting as a dipole layer can realistically shift the threshold voltage only by a few volts, the changes in the threshold voltage can be more than an order of magnitude larger when the SAM leads to charges at the interface. Based on the analysis of the different cases, a route to experimentally discriminate between SAM-induced space charges and interface dipoles is proposed. The developed model allows for qualitative description of the behavior of organic transistors containing reactive interfacial layers; when incorporating rechargeable carrier trap states and a carrier density-dependent mobility, even a quantitative agreement between theory and recent experiments can be achieved.
- Published
- 2009
30. Origin of the bimodal island size distribution in ultrathin films ofpara-hexaphenyl on mica
- Author
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Karin Zojer, Levent Tumbek, Christoph Gleichweit, and Adolf Winkler
- Subjects
Materials science ,Chemical physics ,Phase (matter) ,Physical vapor deposition ,Nucleation ,Crystallite ,Mica ,Kinetic Monte Carlo ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Wetting layer ,Amorphous solid - Abstract
Ultrathin films of para-hexaphenyl (6$P$) were prepared on freshly cleaved and sputter-amorphized mica(001) by physical vapor deposition. Ex situ atomic force microscopy (AFM) revealed a bimodal island size distribution for the films on both surfaces. On freshly cleaved mica long needlelike islands exist, which are surrounded by small crystallites. On the sputter-amorphized substrates, large dendritic islands exist which are again surrounded by small, compact islands. We could prove by thermal desorption spectroscopy that the small islands are the result of adsorbate-induced subsequent nucleation, when the films were exposed to air. In case of the freshly cleaved mica, islands grow on a wetting layer in vacuum. This layer dewets and forms the small islands upon venting, due to the adsorption of water. In the case of the amorphous mica substrate an equilibrium exists between the islands and a two-dimensional gas phase in the sub-monolayer regime. Again, the latter phase nucleates after venting. In a particular coverage range, islands due to nucleation during deposition and subsequent nucleation coexist on the substrate, leading to the bimodal island size distribution. Kinetic Monte Carlo (KMC) simulations were performed to model the nucleation process after venting on the sputter-modified mica substrate. The density of the subsequently nucleated islands just depends on the initial coverage and the critical island size. A critical cluster size of $i$ $=$ 7 molecules was determined for 6$P$ on amorphized mica, by comparing the KMC results with the AFM images in case of adsorbate-induced nucleation. Furthermore, the experimentally obtained island size distributions could be well reproduced by KMC simulations.
- Published
- 2012
31. Heteroleptic platinum(II) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position
- Author
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Fabian Niedermair, Karin Zojer, Ohyun Kwon, Kurt Mereiter, Gregor Trimmel, Christian Slugovc, and Stefan Kappaun
- Subjects
Inorganic Chemistry ,Crystallography ,chemistry ,Ligand ,Excited state ,chemistry.chemical_element ,Density functional theory ,Nuclear magnetic resonance spectroscopy ,Singlet state ,Luminescence ,Platinum ,Photochemistry ,HOMO/LUMO - Abstract
A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO2 (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H (d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460-480 nm) and emit from fluid solution and in the solid state at room temperature at 600-630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20-30 micros in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group.
- Published
- 2008
32. Influence of morphology and polymer:nanoparticle ratio on device performance of hybrid solar cells—an approach in experiment and simulation
- Author
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Neha Bansal, Ferdinand Hofer, Michael Edler, Saif A. Haque, Wernfried Haas, Gregor Trimmel, Mario Arar, Karin Zojer, Manfred Gruber, L Reynolds, and Thomas Rath
- Subjects
Photocurrent ,chemistry.chemical_classification ,Materials science ,Mechanical Engineering ,Analytical chemistry ,Nanoparticle ,Bioengineering ,General Chemistry ,Polymer ,Hybrid solar cell ,Surface-area-to-volume ratio ,chemistry ,Chemical engineering ,Mechanics of Materials ,Phase (matter) ,Percolation ,General Materials Science ,Charge carrier ,Electrical and Electronic Engineering - Abstract
We present a thorough study on the various impacts of polymer:nanoparticle ratios on morphology, charge generation and device performance in hybrid solar cells, comprising active layers consisting of a conjugated polymer and in situ prepared copper indium sulfide (CIS) nanoparticles. We conducted morphological studies through transmission electron microscopy and transient absorption measurements to study charge generation in absorber layers with polymer:nanoparticle weight ratios ranging from 1:3 to 1:15. These data are correlated to the characteristic parameters of the prepared solar cells. To gain a deeper understanding of our experimental findings, three-dimensional drift-diffusion-based simulations were performed. Based on elaborate descriptions of the contributions of polymer and nanoparticle phase to device performances, our results suggest that a polymer:CIS volume ratio of 1:2 (weight ratio 1:9) is necessary to obtain a balanced hole and electron percolation. Also at higher CIS loadings the photocurrent remains surprisingly high due to the contribution of the CIS phase to the charge carrier generation.
- Published
- 2013
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