Browsing by Author "Demeyu, Lemi (PhD)"

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Effects of Concentration and Localization Length on Charge Carrier Mobility in Organic Disordered Semiconductors
    (Addis Ababa University, 2018-08-04) Melaku, Seid; Demeyu, Lemi (PhD)
    Using computer simulations, we show that the dependence of the hopping carrier mobility on the electric field _(F) in a disordered organic semiconducting polymer of random energy sites described by a Gaussian density of states is determined by the localization length _ and not by the concentration of sites N. This result is in drastic contrast to what is usually assumed in the literature for a theoretical description of experimental data and for device modeling, where N􀀀1=3 is considered as the decisive length scale for _(F). In the proposed it is assumed that the localized states are randomly distributed in energy and space coordinates. The expression for hopping conductivity is obtained for the Gaussian density of states and the analysis of electric field dependent hopping mobility is shown by the Poole-Frenkel behavior is only valid in medium electric field.
  • No Thumbnail Available
    Item
    Electron Transport in Silicon Carbide System
    (Addis Ababa University, 2020-01-01) Chukalo, Roba; Nemera, Kenate (PhD); Demeyu, Lemi (PhD)
    In this work electron transport in silicon carbide (SiC) system were considered. The calculations are done based on Density Functional Theory (DFT) which adopt a use of the generalized gradient approximation (GGA) in PBE. An abinit code based on the DFT is applied. From the calculations, we obtained optimum values of lattice constant (parameters), bulk modulus, cut-o energy, and di erent energies (surface energy, cohesive energy, vacancy energy) of silicon carbide. These results are reported using a step-by-step approach and compared with other exprimental values. Analysis based on bandstructure, density of state, projected density of state, work-function, and optical properties are also presented. The optical property has a direct relationship with the distribution of crystal bandgap and electronic density of state. The Monte Carlo method takes into account band structure model to investigate electron transport.
  • No Thumbnail Available
    Item
    Monte Carlo Simulation of Diffusion Coefficient and Average Energy of Charge Carriers in a Disordered Organic Semiconductors
    (Addis Ababa University, 2017-08) Legesse, Fekadu; Demeyu, Lemi (PhD)
    One of the most remarkable results known for energy relaxation of charge carriers in a Gaussian DOS is the existence of the so called equilibration energy. With the help of computer simulations, we study the relaxation of charge carriers and their corresponding diffusive character in a zero electric field region. The relaxation time is of key importance for the analysis of analytical and numerical results in this study because it determines the transition from dispersive to non dispersive (Gaussian) transport behavior. So this project reviews the strong dependence of relaxation time on temperature. We propose also a Gaussian disordered model (GDM) based on the hopping transport theory of charge carriers (non-interacting particles) to study the relaxation phenomena, the equilibration energy ⟨"∞⟩ and the effect of diffusion in thermal equilibrium in the absence of electric field.In addition we reproduced numerical results of the mean energy and dispersion of charged carriers using the model we have designed and compared our results with those in the literature, found using kinetic Monte-carlo simulation techniques for different values of temperature. Through our simulation, we observed that the mean energy of the particles and the carrier diffusion are both depend on temperature and our analytical discussion also approves the same phenomena
  • No Thumbnail Available
    Item
    Monte Carlo Simulation of Diffusion Coefficient and Average Energy of Charge Carriers in a Disordered Organic Semiconductors
    (Addis Ababa University, 2017-08) Legesse, Fekadu; Demeyu, Lemi (PhD)
    One of the most remarkable results known for energy relaxation of charge carriers in a Gaussian DOS is the existence of the so called equilibration energy. With the help of computer simulations, we study the relaxation of charge carriers and their corresponding diffusive character in a zero electric field region. The relaxation time is of key importance for the analysis of analytical and numerical results in this study because it determines the transition from dispersive to non dispersive (Gaussian) transport behavior. So this project reviews the strong dependence of relaxation time on temperature. We propose also a Gaussian disordered model (GDM) based on the hopping transport theory of charge carriers (non-interacting particles) to study the relaxation phenomena, the equilibration energy ⟨"∞⟩ and the effect of diffusion in thermal equilibrium in the absence of electric field.In addition we reproduced numerical results of the mean energy and dispersion of charged carriers using the model we have designed and compared our results with those in the literature, found using kinetic Monte-carlo simulation techniques for different values of temperature. Through our simulation, we observed that the mean energy of the particles and the carrier diffusion are both depend on temperature and our analytical discussion also approves the same phenomena
  • No Thumbnail Available
    Item
    Transport Properties of Charge Carriers in Organic Polymer Films Used in Field Effect Transistor
    (Addis Ababa University, 2018-10-03) Tadesse, Begashaw; Demeyu, Lemi (PhD)
    Application of a gate bias to an organic field-effect transistor leads to accumulation of the charge carriers in an organic semiconductor within a thin region near an interface of the gate dielectric. An important question raised by this study is whether the charge transport in the region can be affected by thickness layer of semiconductor, the morphology of active material and the gate bias are considered as the factor for varying charge carrier mobility for this particular study. In order to answer this question, we have performed Montecarlo simulation of charge transport in organic field-effect transistor structure with a varying thickness of the organic layer, the gate bias, and the morphology of active material taking into account Coulomb interactions. We explain the charge carrier mobility as a function of field strength for disorder parameter ^_ = 4 and ^_ = 3 to draw a conclusion for strong disorder charge carrier mobility is good enough than weak disorder, For gate bias since the number of particles on metallic contact depend on the gate bias therefore when we increase the gate bias charge carrier mobility increases and for different organic layer thickness specifically up to the third layer the charge carrier mobility increase which leads us to a conclusion that charge transport in organic semiconductor layer should be considered three dimensional.