Beat Ruhstaller - Academia.edu (original) (raw)

Papers by Beat Ruhstaller

Research paper thumbnail of 38.1: Quantitative Analysis of Charge Transport in Single‐Carrier Devices and OLEDs Combining DC and AC Data

Sid's Digest Of Technical Papers, Sep 1, 2019

Single-carrier devices are perfect model systems to extract material parameters for more complex ... more Single-carrier devices are perfect model systems to extract material parameters for more complex multilayer organic lightemitting diodes (OLEDs) and to learn about charge transport and injection properties of the device. By combining simulation and measurements in steady-state and frequency domain, we obtain a set of reliable material parameters which can be used to model the multilayer OLED structure. Moreover, we can also evaluate the contact behavior and conclude that the OLED operation is limited by electron transport.

Research paper thumbnail of On the role of polar molecules and the barrier for charge injection in OLEDs

Proceedings of SPIE, Sep 23, 2016

Many electron transport layer materials (ETL) employed in state of the art organic light emitting... more Many electron transport layer materials (ETL) employed in state of the art organic light emitting diodes (OLEDs) are known to be polar. We combine for the first time simulations and electrical characterization of OLEDs based on polar ETL, in order to understand the impact of such materials on the device operation. Depending on the orientation of the dipole orientation, simulations predict either a benefit or a disadvantage of the polar ETL for the device performance. We also show that OLEDs featuring a polar material are perfectly suited for extracting mobility activation energy and Injection barrier from the anode to the ETL.

Research paper thumbnail of Detailed electro-optical modeling of thermally-activated delayed fluorescent OLEDs with different host-guest concentrations

Organic Electronics, Aug 1, 2022

Research paper thumbnail of Scrutinizing thermally stimulated current transients originating from trapped charges in organic semiconductors: A drift-diffusion study

Journal of Applied Physics, May 28, 2022

This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Optoelectronic and ... more This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Optoelectronic and mechanical properties of the orthogonal and tetragonal Cu 2 CdGe(S x Se 1−x) 4 semiconducting system via first principles methods

Research paper thumbnail of Assessing the validity of analytical models to determine trap state parameters from thermally stimulated current responses by means of drift-diffusion simulations

Proceedings of the International Conference on Hybrid and Organic Photovoltaics, Apr 20, 2022

Research paper thumbnail of Comprehensive analysis of third-generation solar cells supported by drift-diffusion simulations

Research paper thumbnail of Accurate Characterization of Organic Solar Cells by Combining Steady-State and Transient Techniques

World Conference on Photovoltaic Energy Conversion, Oct 26, 2012

A method to extract material and device parameters of organic bulk heterojunction solar cells fro... more A method to extract material and device parameters of organic bulk heterojunction solar cells from experimental data is presented. Accurate device characterization is an essential part in research and development of solar cells. Obtaining reliable material and device parameters is an important step in this characterization, since it paves the way to a better understanding of device operation and the quantification of loss mechanisms. Parameter extraction is an error-prone task and the obtained values often lack accuracy. Using inaccurate material or device parameters can lead to misleading interpretation of results. We present a method to extract material and device parameters more accurately. To reduce the correlation among the extracted parameters we simultaneously analyze the current-voltage characteristics, the transient photo-CELIV current and the dynamic response to a light pulse. We show that with numerical modeling based on one set of parameters a systematic series of measurements can be reproduced. The full electrical behavior can be described using a basic drift-diffusion model with constant mobilities and direct photon-to-charge conversion.

Research paper thumbnail of P‐184: Distinguished Poster: Coupled 3D Master Equation and 1D Drift‐Diffusion Approach for Advanced OLED Modeling

Sid's Digest Of Technical Papers, Aug 1, 2020

A hybrid drift‐diffusion (DD) master equation (ME) approach for OLED simulation is established, s... more A hybrid drift‐diffusion (DD) master equation (ME) approach for OLED simulation is established, simultaneously benefiting from the efficiency of the DD approach (charge dynamics) and the physical accuracy of the ME approach (excitonics). We show that such a hybrid model can efficiently describe steady‐state and transient behavior of multilayer OLEDs.

Research paper thumbnail of 67‐4: Modeling Electrical and Optical Cross‐Talk between Adjacent Pixels in Organic Light‐Emitting Diode Displays

Sid's Digest Of Technical Papers, May 29, 2019

We achieved a quantitative understanding of both electrical and optical coupling in OLED displays... more We achieved a quantitative understanding of both electrical and optical coupling in OLED displays, by employing the simulation software LAOSS. For an AMOLED of around 300 PPI we found that the relative luminance between the addressed pixel and the neighbors can be as high as 40%, hampering significantly the quality of the final image. Similarly, in WOLEDs/CFs the light leakage to the neighboring pixels is clearly detectable, especially for wavelengths in the blue region of the spectrum.

Research paper thumbnail of Modelling crosstalk through common semiconductor layers in AMOLED displays

Journal of The Society for Information Display, May 21, 2018

High pixel density displays are demanded for active matrix organic light-emitting diode displays ... more High pixel density displays are demanded for active matrix organic light-emitting diode displays (AMOLED) in applications such as virtual reality headsets, micro-displays, and high-end smartphones. Parasitic emission from non-addressed neighboring pixels (crosstalk) is a common problem in such high pixel density AMOLED, and this crosstalk becomes more severe as the pixel density and fill ratio of the display increases. One of the causes of crosstalk is parasitic currents that travel through common organic semiconductor layers. In this paper, we model and quantify the pixel crosstalk using a 2 + 1D finite element model that is based on the conductivity of the common layer and the luminance-current-voltage curves of the subpixels as measured input parameters. We assess the effect of crosstalk on the pixel current, observed color, and luminance. The 2 + 1D model limits the number of degrees of freedom so that calculations on a standard personal computer are feasible.

Research paper thumbnail of Organic Salt Semiconductor with High Photoconductivity and Long Carrier Lifetime

Advanced Functional Materials, Feb 7, 2018

Intrinsic photo generation of charge carriers in organic semiconductors is generally attributed t... more Intrinsic photo generation of charge carriers in organic semiconductors is generally attributed to high energy ionization or exciton dissociation by a strong electric field. Here we report on high bulk photoconductivity in pristine pentamethine cyanine films with photocurrent onset at the band-edge of the organic semiconductor. Single layer cyanine diodes with selective hole and electron contacts show linear dependence of photocurrent with reverse voltage and light intensity. Numerical driftdiffusion simulations reveal that the linear resistor behavior stems from low and unbalanced carrier mobilities giving rise to negative space charge. Slow bimolecular recombination kinetics of photoinduced charges obtained by time delayed charge extraction measurements show strongly reduced Langevin recombination with long carrier lifetime of the order of a millisecond. Such reduced charge carrier recombination puts forward a materials concept to be exploited in photodiodes and more generally in optoelectronic devices.

Research paper thumbnail of Simulation of OLEDs with a polar electron transport layer

Organic Electronics, Dec 1, 2016

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energ... more Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device. Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.

Research paper thumbnail of Electronic, Ionic and Optical Perovskite Solar Cell Modeling and Experimental Validation

Research paper thumbnail of Determination of charge transport activation energy and injection barrier in organic semiconductor devices

Journal of Applied Physics, Sep 19, 2017

Research paper thumbnail of Ageing of high-bandgap perovskite for all thin-film tandem flexible solar cell devices

Research paper thumbnail of Measuring frontier orbital energy levels of OLED materials using cyclic voltammetry in solution

Organic Electronics, Nov 1, 2023

Research paper thumbnail of 3.3: Modeling Electrical and Optical Cross‐Talk between Adjacent Pixels in Organic Light‐Emitting Diode Displays

Sid's Digest Of Technical Papers, Sep 1, 2019

We achieved a quantitative understanding of both electrical and optical coupling in OLED displays... more We achieved a quantitative understanding of both electrical and optical coupling in OLED displays, by employing the simulation software LAOSS. For an AMOLED of around 300 PPI we found that the relative luminance between the addressed pixel and the neighbors can be as high as 40%, hampering significantly the quality of the final image. Similarly, in WOLEDs/CFs the light leakage to the neighboring pixels is clearly detectable, especially for wavelengths in the blue region of the spectrum.

Research paper thumbnail of 27‐3: Optimizing Charge Transport and Reducing Excitonic Loss Channels in Fluorescent Doped TADF OLEDs

Sid's Digest Of Technical Papers, May 1, 2021

A combined experimental and simulation study on a set of 4 OLEDs with 2 systematically varied gue... more A combined experimental and simulation study on a set of 4 OLEDs with 2 systematically varied guest dopants in the emission layer is presented. A numerical electro‐optical model is employed, demonstrating one consistent parameter set that describes both steady‐state and ac characteristics of all devices. The model accounts for charge transport and trapping, polar layers, and excitonic loss processes inducing an efficiency rolloff that can be correlated with the shape of the emission zone and the guest dopant concentrations. Furthermore, starting from the resulting parameter set, we show routes for optimized devices with a reduced roll‐off.

Research paper thumbnail of Accumulation of ionic charge carriers and the influence of steric potential in perovskite solar cells

Research paper thumbnail of Impact of non-stoichiometry on ion migration and photovoltaic performance of formamidinium-based perovskite solar cells

Journal of materials chemistry. A, Materials for energy and sustainability, 2022

Ion migration mechanisms are often behind degradation pathways in perovskite solar cells under op... more Ion migration mechanisms are often behind degradation pathways in perovskite solar cells under operating conditions.

Research paper thumbnail of 38.1: Quantitative Analysis of Charge Transport in Single‐Carrier Devices and OLEDs Combining DC and AC Data

Sid's Digest Of Technical Papers, Sep 1, 2019

Single-carrier devices are perfect model systems to extract material parameters for more complex ... more Single-carrier devices are perfect model systems to extract material parameters for more complex multilayer organic lightemitting diodes (OLEDs) and to learn about charge transport and injection properties of the device. By combining simulation and measurements in steady-state and frequency domain, we obtain a set of reliable material parameters which can be used to model the multilayer OLED structure. Moreover, we can also evaluate the contact behavior and conclude that the OLED operation is limited by electron transport.

Research paper thumbnail of On the role of polar molecules and the barrier for charge injection in OLEDs

Proceedings of SPIE, Sep 23, 2016

Many electron transport layer materials (ETL) employed in state of the art organic light emitting... more Many electron transport layer materials (ETL) employed in state of the art organic light emitting diodes (OLEDs) are known to be polar. We combine for the first time simulations and electrical characterization of OLEDs based on polar ETL, in order to understand the impact of such materials on the device operation. Depending on the orientation of the dipole orientation, simulations predict either a benefit or a disadvantage of the polar ETL for the device performance. We also show that OLEDs featuring a polar material are perfectly suited for extracting mobility activation energy and Injection barrier from the anode to the ETL.

Research paper thumbnail of Detailed electro-optical modeling of thermally-activated delayed fluorescent OLEDs with different host-guest concentrations

Organic Electronics, Aug 1, 2022

Research paper thumbnail of Scrutinizing thermally stimulated current transients originating from trapped charges in organic semiconductors: A drift-diffusion study

Journal of Applied Physics, May 28, 2022

This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Optoelectronic and ... more This paper was selected as an Editor's Pick ARTICLES YOU MAY BE INTERESTED IN Optoelectronic and mechanical properties of the orthogonal and tetragonal Cu 2 CdGe(S x Se 1−x) 4 semiconducting system via first principles methods

Research paper thumbnail of Assessing the validity of analytical models to determine trap state parameters from thermally stimulated current responses by means of drift-diffusion simulations

Proceedings of the International Conference on Hybrid and Organic Photovoltaics, Apr 20, 2022

Research paper thumbnail of Comprehensive analysis of third-generation solar cells supported by drift-diffusion simulations

Research paper thumbnail of Accurate Characterization of Organic Solar Cells by Combining Steady-State and Transient Techniques

World Conference on Photovoltaic Energy Conversion, Oct 26, 2012

A method to extract material and device parameters of organic bulk heterojunction solar cells fro... more A method to extract material and device parameters of organic bulk heterojunction solar cells from experimental data is presented. Accurate device characterization is an essential part in research and development of solar cells. Obtaining reliable material and device parameters is an important step in this characterization, since it paves the way to a better understanding of device operation and the quantification of loss mechanisms. Parameter extraction is an error-prone task and the obtained values often lack accuracy. Using inaccurate material or device parameters can lead to misleading interpretation of results. We present a method to extract material and device parameters more accurately. To reduce the correlation among the extracted parameters we simultaneously analyze the current-voltage characteristics, the transient photo-CELIV current and the dynamic response to a light pulse. We show that with numerical modeling based on one set of parameters a systematic series of measurements can be reproduced. The full electrical behavior can be described using a basic drift-diffusion model with constant mobilities and direct photon-to-charge conversion.

Research paper thumbnail of P‐184: Distinguished Poster: Coupled 3D Master Equation and 1D Drift‐Diffusion Approach for Advanced OLED Modeling

Sid's Digest Of Technical Papers, Aug 1, 2020

A hybrid drift‐diffusion (DD) master equation (ME) approach for OLED simulation is established, s... more A hybrid drift‐diffusion (DD) master equation (ME) approach for OLED simulation is established, simultaneously benefiting from the efficiency of the DD approach (charge dynamics) and the physical accuracy of the ME approach (excitonics). We show that such a hybrid model can efficiently describe steady‐state and transient behavior of multilayer OLEDs.

Research paper thumbnail of 67‐4: Modeling Electrical and Optical Cross‐Talk between Adjacent Pixels in Organic Light‐Emitting Diode Displays

Sid's Digest Of Technical Papers, May 29, 2019

We achieved a quantitative understanding of both electrical and optical coupling in OLED displays... more We achieved a quantitative understanding of both electrical and optical coupling in OLED displays, by employing the simulation software LAOSS. For an AMOLED of around 300 PPI we found that the relative luminance between the addressed pixel and the neighbors can be as high as 40%, hampering significantly the quality of the final image. Similarly, in WOLEDs/CFs the light leakage to the neighboring pixels is clearly detectable, especially for wavelengths in the blue region of the spectrum.

Research paper thumbnail of Modelling crosstalk through common semiconductor layers in AMOLED displays

Journal of The Society for Information Display, May 21, 2018

High pixel density displays are demanded for active matrix organic light-emitting diode displays ... more High pixel density displays are demanded for active matrix organic light-emitting diode displays (AMOLED) in applications such as virtual reality headsets, micro-displays, and high-end smartphones. Parasitic emission from non-addressed neighboring pixels (crosstalk) is a common problem in such high pixel density AMOLED, and this crosstalk becomes more severe as the pixel density and fill ratio of the display increases. One of the causes of crosstalk is parasitic currents that travel through common organic semiconductor layers. In this paper, we model and quantify the pixel crosstalk using a 2 + 1D finite element model that is based on the conductivity of the common layer and the luminance-current-voltage curves of the subpixels as measured input parameters. We assess the effect of crosstalk on the pixel current, observed color, and luminance. The 2 + 1D model limits the number of degrees of freedom so that calculations on a standard personal computer are feasible.

Research paper thumbnail of Organic Salt Semiconductor with High Photoconductivity and Long Carrier Lifetime

Advanced Functional Materials, Feb 7, 2018

Intrinsic photo generation of charge carriers in organic semiconductors is generally attributed t... more Intrinsic photo generation of charge carriers in organic semiconductors is generally attributed to high energy ionization or exciton dissociation by a strong electric field. Here we report on high bulk photoconductivity in pristine pentamethine cyanine films with photocurrent onset at the band-edge of the organic semiconductor. Single layer cyanine diodes with selective hole and electron contacts show linear dependence of photocurrent with reverse voltage and light intensity. Numerical driftdiffusion simulations reveal that the linear resistor behavior stems from low and unbalanced carrier mobilities giving rise to negative space charge. Slow bimolecular recombination kinetics of photoinduced charges obtained by time delayed charge extraction measurements show strongly reduced Langevin recombination with long carrier lifetime of the order of a millisecond. Such reduced charge carrier recombination puts forward a materials concept to be exploited in photodiodes and more generally in optoelectronic devices.

Research paper thumbnail of Simulation of OLEDs with a polar electron transport layer

Organic Electronics, Dec 1, 2016

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energ... more Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device. Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.

Research paper thumbnail of Electronic, Ionic and Optical Perovskite Solar Cell Modeling and Experimental Validation

Research paper thumbnail of Determination of charge transport activation energy and injection barrier in organic semiconductor devices

Journal of Applied Physics, Sep 19, 2017

Research paper thumbnail of Ageing of high-bandgap perovskite for all thin-film tandem flexible solar cell devices

Research paper thumbnail of Measuring frontier orbital energy levels of OLED materials using cyclic voltammetry in solution

Organic Electronics, Nov 1, 2023

Research paper thumbnail of 3.3: Modeling Electrical and Optical Cross‐Talk between Adjacent Pixels in Organic Light‐Emitting Diode Displays

Sid's Digest Of Technical Papers, Sep 1, 2019

We achieved a quantitative understanding of both electrical and optical coupling in OLED displays... more We achieved a quantitative understanding of both electrical and optical coupling in OLED displays, by employing the simulation software LAOSS. For an AMOLED of around 300 PPI we found that the relative luminance between the addressed pixel and the neighbors can be as high as 40%, hampering significantly the quality of the final image. Similarly, in WOLEDs/CFs the light leakage to the neighboring pixels is clearly detectable, especially for wavelengths in the blue region of the spectrum.

Research paper thumbnail of 27‐3: Optimizing Charge Transport and Reducing Excitonic Loss Channels in Fluorescent Doped TADF OLEDs

Sid's Digest Of Technical Papers, May 1, 2021

A combined experimental and simulation study on a set of 4 OLEDs with 2 systematically varied gue... more A combined experimental and simulation study on a set of 4 OLEDs with 2 systematically varied guest dopants in the emission layer is presented. A numerical electro‐optical model is employed, demonstrating one consistent parameter set that describes both steady‐state and ac characteristics of all devices. The model accounts for charge transport and trapping, polar layers, and excitonic loss processes inducing an efficiency rolloff that can be correlated with the shape of the emission zone and the guest dopant concentrations. Furthermore, starting from the resulting parameter set, we show routes for optimized devices with a reduced roll‐off.

Research paper thumbnail of Accumulation of ionic charge carriers and the influence of steric potential in perovskite solar cells

Research paper thumbnail of Impact of non-stoichiometry on ion migration and photovoltaic performance of formamidinium-based perovskite solar cells

Journal of materials chemistry. A, Materials for energy and sustainability, 2022

Ion migration mechanisms are often behind degradation pathways in perovskite solar cells under op... more Ion migration mechanisms are often behind degradation pathways in perovskite solar cells under operating conditions.