Ph.D Thesis

Ph.D StudentRoichman Yohai
SubjectCharge Transport in Conjugated Polymers
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Nir Tessler


The electro-optical properties of conjugated polymers that are based on -conjugated carbon atoms raised a significant scientific and commercial interest in the last three decades. Although we witnessed in the recent years the appearance of new commercial applications that are based on these materials, the basic scientific understanding of the fundamental electro-optical processes in organic semiconductors is far from a full understanding. In particular, the gap between the theoretical description of charge transport, and the experimental results sets a barrier on the description and design of new electro-optical devices based on polymer semiconductors. In this research we proposed that the charge transport in conjugated polymers is a non-linear process that strongly depends on the charge concentration. We developed a unified calculation for a wide range of charge concentrations and electric fields. Using this model we were able to explain a long lasting conflict between the results of high concentration devices (as field effect transistors) and low concentration devices (as light emitting diodes), we demonstrated that the diffusion is stronger than expected from the classical Einstein-relation prediction, and we proposed new methods to determine intrinsic properties of amorphous organic semi-conductors, as polaronic binding energy and density of states. These theoretical predictions were examined and confirmed by examining conjugated polymer field effect transistors that were made of poly-[2-methoxy-5-(2 '-ethyl-hexiloxy)-p-phenylenevinylene]. We demonstrated that the mobility in this material depends strongly on charge concentration and on micro-morphology (e.g. molecular weight), in contrast to the polaronic binding energy that was found to be intrinsic to the material.