Density and Temperature Dependent of Charge Carriers Mobility in Disordered Organic Semiconductors Monte Carlo Simulation Method

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Addis Ababa University


We begin the analysis of transport of charge carriers in organic semiconductors, focusing on the depen- dence of charge carriers mobility on density, external electric _eld and temperature. We start with the formulation of the main relationships that provides the current density as a function of conductivity and electrical _eld. Conjugated polymer _lms are composed of carbon-based molecular chains, which are held together by weak van der Waals interactions. Such type of weak intermolecular interaction gives rise to a soft and exible molecular system. This exibility character and its other properties, which include electronic properties, have made the material attractive for scienti_c investigations since it has shown possible applications in electronic devices such as light emitting diodes, _eld-e_ect transistors (FETs), and solar cells. The electronic states in disordered semiconductors are localized and the transport of charge carriers occurs by hopping between the localized states. The hopping transitions between localized states and the exchange of electrons between localized and extended states form a variety of complex phenomena that determine the charge carrier transport. we discuss a variety of transport phenomena known as hopping transport mechanisms. The description of the relationship between current density and charge carrier density in disordered organic-semiconductors is complicated by the e_ects of di_usion and a charge- carrier mobility that depends on the disorder nature of the material, charge concentration, electric _eld and temperature. With the help of computer simulations, we study the mobility of charge carriers and their corresponding di_usive character at di_erent temperatures. The ratio between mobility and di_usion parameters is derived for a Gaussian like density of states. This steady-state analysis is expected to be applicable to a wide range of organic materials (polymers or small molecules) as it relies on the existence of quasi-equilibrium only. Our analysis shows that there is an inherent dependence of the transport in disordered organic-materials on the charge carrier density. We investigate the e_ects of disorder on the external electric _eld and charge carrier density de- pendence of the mobility in disordered organic semiconductor with a Gaussian shape of the density of states. The results we found are compared with recently published numerically exact expressions for the dependence of the charge-carrier mobility on the carrier density, temperature and the electric _eld in such organic semiconductor materials. We consider the MillerAbrahams expression to describe the hopping rate of charge carriers and employ kinetic Monte Carlo simulation methods to generate data from which we can analyze a charge carrier mobility as a function of applied electric _eld, temperature, localization length, and energetic iii disorder parameters. Based on our results, we discuss the e_ects of these parameters on charge carrier mobility. Our results show the importance of the energetic disorder parameter and localization length on the e_ects of temperature dependence on charge carrier mobility, and we also evaluate the value of localization length that has been mostly considered as 0.1a, where a is the lattice parameter. Finally, the temperature dependence is markedly di_erent in single crystals and in disordered materials. In single crystals, the hole and electron mobilities generally decrease with temperature.



Density, Temperature Dependent, Charge Carriers Mobility, Disordered Organic, Semiconductors, Monte Carlo Simulation Method