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Personal page Hassan Abdalla

My name is Hassan Abdalla, I am currently a PhD student working within the group ‘Complex Materials and Devices’ under Martijn Kemerink on the topic ‘Energetics of Charge Transport in Organic Semiconductors’. A special emphasis is given on thermoelectric properties of doped materials and far-from-equilibrium phenomena induced by external stimuli like electric field or light.
I have obtained my Bachelor’s degree in Technical Physics from the Vienna University of Technology, where I continued to obtain a Master’s degree in Material Science. My final project involved the carrier lifetime increase in 4H-SiC by thermal oxidation [1].

The thermoelectric properties of a material are best described by the Seebeck and Peltier coefficients. The Seebeck coefficient describes a materials ability to create a voltage from a temperature gradient applied along it. This is also related to a materials ability to convert electrical energy into heat (Peltier effect).

A temperature gradient along a slab of semiconductor creates an energetic imbalance of the charge carriers, which they will try to compensate by moving along the material. As shown in figure 1, carriers will be at higher energy levels (higher Fermi energy E
F1) at the hotter end and will thus try to relax by moving to the colder end (lower Fermi energy EF2). These carriers will be electrons in a negatively doped material and holes in a positively doped material. Combining two materials of opposite doping creates an electric generator or heater (thermoelement). Many of these thermoelements may be combined in series for higher output power.

Figure 1 - simple energy diagram for a slab of semiconductor with a temperature gradient

Both principles, Seebeck and Peltier effect, find wide commercials applications such as, temperature sensing, heating, cooling or generating electricity from waste heat and are a mature technology for inorganic materials. Thermoelectrics from organic materials are a highly desirable alternative, due to their structural flexibility, ecological compatibility, ease of manufacturing and low cost compared to inorganis. Furthermore, they generally exhibit low thermal conductivities, high conductivities and relatively high Seebeck coefficients, making them a promising candidate for thermoelectric applications [2].

Figure 2 - Example of a thermogenerator consisting of a n- and p-doped semiconductor in a leg-structure

Apart from real-life applications, the combination of experimental investigation of thermoelectric devices and materials with theoretical models and computer simulations, is a powerful tool in unraveling, the yet still poorly understood mechanisms of charge carrier transport energetics in organic disordered semiconductors (ODSC). The reason lays in the dependence of the efficiency of such devices on crucial material-related parameters like thermal conductivity, electrical conductivity and, of course, Seebeck coefficient.

The current project concerns the investigation of the so-called universal scaling of the current in ODSC [3] and the role that field-induced heating of the charge carrier distribution might play in it [4]. It was found that an electric field, applied for instance at the gate of an organic field effect transistor, has the same effect as increasing the lattice temperature, giving rise to a hot charge carrier distribution in a cold lattice. This effect might be very useful for thermoelectric applications, as a low 
thermal conductivity and high electrical conductivity both aid the Seebeck effect.

[1] I. D. Booker, H. Abdalla, L. Lilja, J. Hassan, J. P. Bergman, E. Ö. Sveinbjörnsson, E. Janzén; Oxidation induced ON1, ON2a/b defects in 4H-SiC characterized by DLTS’ - Accepted for ICSCRM2013, to be published in Materials Science Forum

[2] G-H. Kim, L. Shao, K. Zhang and K. P. Pipe; Engineered doping of organic semiconductors for enhanced thermoelectric efficiency - Nature Materials 12, 719–723 (2013)

[3] K.l Asadi, A. J. Kronemeijer, T. Cramer3, L. J. A. Koster, P. W.M. Blom & D. M. de Leeuw; Polaron hopping mediated by nuclear tunnelling in semiconducting polymers at high carrier density - Nature Communications 4, 1710 (2013)

[4] F. Jansson, S. D. Baranovskii, F. Gebhard, and R. Österbacka; Effective temperature for hopping transport in a Gaussian density of states – Physical Review B 77, 195211 (2008)

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Last updated: 02/09/15