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Available topics for master theses

Information about master projects can be obtained by contacting

  
  • Prof. Ferdinand Schürrer
  •   
  • Dr. Karin Zojer
  • Simulation of charge transport in organic electronic devices

    Organic electronic devices are in the focus of current research as they hold the promise of manifacturing low-cost, large-area and flexible devices. The Master Projects will contribute to an drift-diffusion based simulation tool that will aid to gain a deeper understanding of elementary processes within the devices and to obtain quantitative prediction of the device performance. The projects span different topics from developing new methods, e.g., to rigorously deal with the microscopic structure in the semiconductor, to the sophisticated evaluation of experiments.

    Topics for Master Projects include:


    Common details of the projects: possible start anytime.

    Further information: Dr. Karin Zojer, Institute of Theoretical and Computational Physics, second floor, room number PH 02 084

    Keywords: Organic electronics, numerical solution of PDEs.
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    Quantum transport in solids

    Nowadays, the most common approaches for the simulation of charge transport in semiconductor devices are drift-diffusion and hydrodynamic models. Such macroscopic descriptions of particle transport enable a rather quick computation and therefore, an industrial application. But, these models are only applicable as long as the device dimensions are large enough to justify the definition of macroscopic quantities like mobilities or diffusion coefficients. Depending on the particular application, such macroscopic transport models are applicable to devices with characteristic lengths in the range of micrometers or even several hundred nanometers. For smaller devices a microscopic description is necessary, usually based on the semi-classical Boltzmann transport equation (BTE). The application of the BTE may yield accurate results for the transport characteristics in many small-scaled devices but fails, as soon as quantum mechanical effects dominate and a description of the charge carriers as localized particles becomes invalid. A prototypical example for such a device is a resonant tunneling diode (RTD), which is used as a benchmark problem in this project. Two common approaches for a fully quantum mechanical description of charge transport are the non-equilibrium Green's functions (NEGF) and the Wigner transport equation (WTE). The WTE shares many analogies with the BTE but is based on the von Neumann equation for the density operator instead of classical equations of motion. The NEGF approach is widely used to calculate steady state properties but is numerically too demanding to determine explicit time evolutions, at the moment at least. The great advantage of the WTE approach is, that calculations of steady states as well as time evolutions are feasible and require about the same computational resources. A peculiarity of the Wigner function (solution to the WTE) is the appearance of prominent and short-scaled oscillations in phase space. Different numerical schemes for the solution of the WTE were proposed in literature and compared to NEGF results, but often show rather large quantitative discrepancies. We believe that this is mainly due to a poor resolution of the short-scaled oscillations in phase space and therefore, we propose a new numerical scheme which allows for an adaptive grid refinement. The algorithm was extensively tested against NEGF results for a RTD in the ballistic regime and a quasi-exact agreement could be achieved.

    Topics for Master Projects include:

    • Simulation of charge carrier transport in resonant tunneling diodes

    • Common details of the projects: possible start anytime.

      Further information: Ao.Univ.-Prof. Dr. Ferdinand Schürrer, Institute of Theoretical and Computational Physics, second floor, room number PH 02 108, DI Antonius Dorda, Institute of Theoretical and Computational Physics, third floor, room number PH 03 ...

      Keywords: Quantum Transport, Wigner equation, resonant tunneling diodes.
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    Derivation and implementation of drift-diffusion equations for charge transport in disordered solids

    Within this Master project, for the first time, macroscopic equations for charge transport in organic devices with disordered semiconductors are rigorously derived and implemented.



    scheme


    Potential candidates should be interested in paper-and-pencil-derivations, numerical implementation, and to work in a close feedback loop with other theorical groups.


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    Implementation of a Monte-Carlo solver for the charge transport in disordered solids

    Within this Master Project, a 3D Monte Carlo solver will be implemented that allows to simulate the hopping transport of charge carriers in an organic electronic device under the consideration of the microscopic structure of the semiconductor.



    scheme


    Potential candidates should be interested in developing numerical schemes and to work in a close feedback loop with other theorical groups. This project is will carried out in collaboration with Dr. C. Groves (U. Durham, UK).

    Compensation: Forschungsbeihilfe for 6 month, EUR 440 / month
    starting date: immediately

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    Simulation of the DC operation of organic transistors

    Within this Master Project, the frequency-dependend operation of organic thin-film transistors shall be simulated using a drift-diffusion based model.



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    Potential candidates should be interested in developing numerical schemes and to work in a close feedback loop with experimental groups, i.e., the groups of B. Stadlober (Joanneum Research, NMP Weiz) and E. List (NTC Weiz).

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    Comparing the switch-on behavior of top-contact and bottom-contact organic transistors

    Within this Master Project, the time-dependence of charge transport in organic thin-film transistors shall be simulated using a drift-diffusion based model. It is the goal of this project to characterize the switch-on behavior of organic thin-film transistors under consideration of the device architecture (i.e., arrangement of the electrodes with respect to semiconducting and insulating layers).



    scheme

    Potential candidates should be interested in developing numerical schemes and to work in a close feedback loop with experimental groups, i.e., the groups of B. Stadlober (Joanneum Research, NMP Weiz) and E. List (NTC Weiz).

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    Contact resistance in short-channel organic transistors

    The contact resistance is a quantity known to severly hamper the performance in organic thin-film transistors. Short channel organic transistors, being designed for high frequency operation, suffer particularly from the parasitic influence of the contact resistance. Building on our fundamental understanding of the origin of the contact resistance, the candidate will pursue simulations with a drift-diffusion based model for actually fabricated and characterized devices to establish the influence of the contact resistance and develop strategies for improving the device performance.



    scheme

    Potential candidates should be interested in developing numerical schemes and to work in a close feedback loop with experimental groups, i.e., the group of B. Stadlober (Joanneum Research, NMP Weiz) who is fabricating and characterizing these devices.

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    Tuning the properties of organic solar cells using local electric fields

    It has been realized that the performance of organic solar cells can be considerably altered by introducing heterojunctions and/or local doping at specific positions in the device. All such measures correspond to the establishment of a local electric field that is able to aid or to hamper the charge carrier extraction and, thus, power generation. Yet, the actual impact of such fields depending on the field orientation and the actual position in the device are not understood. Based on preliminary investigations, this Master project continues to explore the impact of such fields by means of simulation using a drift-diffusion based. The project is aiming at formulating general design strategies to employ intentionally introduced local fields to improve the power efficiency of organic solar cells.



    scheme

    Potential candidates should be interested in developing numerical schemes and to work in a close feedback loop with theoretical and experimental groups.

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    Simulation of charge carrier transport in resonant tunneling diodes

    After this thorough convergence and accuracy studies of the Wigner approach to simulate the ballistic charge carrier Transport in resonant tunneling diodes (RTDs) it would now be of great interest for future work to extend the method to more realistic device descriptions by including scattering mechanisms and a self-consistent solution of the Wigner and Poisson equation. Another highly interesting route of investigation would be the calculation of explicitly time-dependent situations. Especially for RTDs such an approach would be of great use since these devices are designed to operate in the THz regime, thus far away from the steady state.



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    Potential candidates should be interested in dealing with problems of quantum mechanics in the phase space and in developing numerical schemes.

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