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Feusisberg, Switzerland

Knapp E.,University of Zurich | Ruhstaller B.,University of Zurich | Ruhstaller B.,Fluxim AG
Applied Physics Letters | Year: 2011

We present a comprehensive numerical impedance spectroscopy analysis of an organic semiconductor device. A physical model that considers localized states is combined with a space- and frequency-resolved numerical framework. We study the details of the frequency-dependent capacitance of an electron-only device and distinguish different trapping regimes depending on the parameters. Depending on the choice of the trapping parameters, a capacitance rise at low frequency is observed. The extraction of the characteristic temperature of the exponential of the trap density of states (DOS) by a simplified method by T. Okachi [Appl. Phys. Lett. 94, 043301(2009)] is investigated. © 2011 American Institute of Physics. Source


Knapp E.,University of Zurich | Ruhstaller B.,University of Zurich | Ruhstaller B.,Fluxim AG
Journal of Applied Physics | Year: 2012

We present an analysis of charge mobility determination methods for the steady as well as the transient state and investigate shallow charge traps with respect to their dynamic behavior. We distinguish between fast and slow trap states in our numerical model corresponding to two characteristic regimes. The two regimes manifest themselves in both impedance spectroscopy and dark injection transient currents (DITC). Further we investigate the charge mobility obtained from dynamic simulations and relate it to the extracted charge mobility from steady-state current-voltage curves. To demonstrate the practical impact of these regimes, we apply our numerical model to the DITC that have commonly been used to determine the charge mobility in organic semiconductor devices. The obtained results from DITC studies strongly depend on the measurement conditions. Therefore we analyze the measurements of reference [Esward, J. Appl. Phys. 109, 093707 (2011)] and reproduce the effects of varying pulse off-times on the transient current qualitatively. Thus, our simulations are able to explain the experimental observations with the help of relaxation effects due to shallow traps. © 2012 American Institute of Physics. Source


Knapp E.,ZHAW Zurich University of Applied Sciences | Ruhstaller B.,Fluxim AG
Digest of Technical Papers - SID International Symposium | Year: 2015

A numerical model for charge transport in organic semiconductor devices that accounts for self-heating is presented. In admittance spectroscopy this model reproduces the negative capacitance in bipolar, and more importantly, in single carrier devices. We show that self-heating is crucial not only in large-area OLEDs, but also in small-area devices. © 2015 SID. Source


Knapp E.,University of Zurich | Hausermann R.,University of Zurich | Schwarzenbach H.U.,University of Zurich | Ruhstaller B.,University of Zurich | Ruhstaller B.,Fluxim AG
Journal of Applied Physics | Year: 2010

For the design of organic semiconductor devices such as organic light-emitting devices and solar cells, it is of crucial importance to solve the underlying charge transport equations efficiently and accurately. Only a fast and robust solver allows the use of fitting algorithms for parameter extraction and variation. Introducing appropriate models for organic semiconductors that account for the disordered nature of hopping transport leads to increasingly nonlinear and more strongly coupled equations. The solution procedures we present in this study offer a versatile, robust, and efficient means of simulating organic semiconductor devices. They allow for the direct solution of the steady-state drift-diffusion problem. We demonstrate that the numerical methods perform well in combination with advanced physical transport models such as energetic Gaussian disorder, density-dependent and field-dependent mobilities, the generalized Einstein diffusion, traps, and its consistent charge injection model. © 2010 American Institute of Physics. Source


Lanz T.,University of Zurich | Fang L.,Korea Advanced Institute of Science and Technology | Baik S.J.,Hankyong National University | Lim K.S.,Korea Advanced Institute of Science and Technology | And 2 more authors.
Solar Energy Materials and Solar Cells | Year: 2012

Thin layers of lithium fluoride (LiF), together with aluminum, can be used as rear electrodes in high-efficiency amorphous silicon (a-Si:H) solar cells on rough substrates, as recently demonstrated by Fang et al. (IEEE Transactions on Electron Devices 58(9)(2011) 3048-3052). We employ numerical modeling to evaluate the optical losses and charge generation profiles in these thin film solar cells. We find that the increase in rear electrode reflectivity by inserting LiF, leading to increased photocurrent, is the dominant factor in device performance improvement, accounting for 7% gain in photocurrent. The simulations are in good agreement with measurements of pin a-Si:H solar cells with varied rear electrodes. © 2012 Elsevier B.V. All rights reserved. Source

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