טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMendels Dan
SubjectTheoretical Aspects of Charge Transport and
Thermoelectricity in Disordered Organic
Semiconductors
DepartmentDepartment of Electrical Engineering
Supervisor Professor Nir Tessler
Full Thesis textFull thesis text - English Version


Abstract

Organic electronics in its present form has been a fertile ground for scientific research for over three decades. It has concomitantly constituted a basis for potential breakthrough technologies including solar cells, ambient lighting and bio-compatible electronics. But despite substantial advances and the wide commercialization of organic light emitting diodes (OLEDs), better modeling of the mechanisms underlying the functionality of organic devices is understood to be still required for the field to be able to materialize its potential. The study presented in this dissertation has, thus, focused on understanding and modeling charge transport in organic systems under the premise of the widely employed Gaussian Disorder Model (GDM) framework. The framework is known to often yield agreement with experimental results, but its validity and accuracy are still questioned due to the large number of free parameters it consists. The main objectives of the presented study were, thus, to deepen current understanding of the charge transport process in organic systems and concomitantly promote the refinement of the GDM framework.

The first study presented within the dissertation focused on investigating the validity of the Generalized Einstein Relation (GER) within the GDM framework. Using Monte Carlo simulations and semi-analytical methodologies the study found that while the GER is valid when applied in the context of the Drift Diffusion equation (e.g. for experimental analysis), caution needs be exercised regarding the definitions of the mobility and diffusion coefficient to which it is applied. Namely, the coefficient definitions depend on the context in which they are used, rendering the ratio between them dependent on the context as well.

The dissertation’s second study aimed on providing a detailed and comprehensive description of the transport process as it is manifested in energy space. The study additionally provided new insight regarding the widely employed transport energy concept. Here, it was found that the concept corresponds in effect to two quantitatively different physical entities. The first, the energy through which most of the current in the system flows corresponding to the system thermoelectric properties. The second, the energy to which carriers hop in the rate-limiting-step of the transport process, corresponding to the system conductive properties. Utilizing the knowledge gained by the preceding study, the third and forth studies presented in this dissertation have focused on quantifying the thermoelectric properties arising from the GDM framework under equilibrium and non-equilibrium conditions, respectively. The potential new insight and knowledge of the charge transport process residing in implementing combined transport and thermoelectric studies are explored in both studies.

The dissertation’s final study focused on exploring the influence of explicitly accounting for polymer-chain morphology and rapid on-polymer-chain charge propagation within the GDM framework. The study’s main findings show that a combined presence of several polymer related morphological attributes in concurrence with low energetic disorder can lead to enhancement of the system mobility, thus, corresponding to recently experientially discovered ‘high mobility’ materials. The study concomitantly predicts that such ‘high mobility’ materials, will be prone to exhibit mobility field dependences, more negatively inclined than those exhibited by ‘standard materials’.