Ph.D Thesis

Ph.D StudentRappaport Noam
SubjectResearch of Physical Processes of Optical Excitations and
Electric Conduction in Light Detection Polymeric
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Nir Tessler
Full Thesis textFull thesis text - English Version


The discovery of semiconduction in the polyacetelyne polymer by Shirakawa Heeger and McDiarmid back in 1977 has opened the road for new types of electronic and opto-electronic devices made of what is essentially plastic. This new field of scientific research has been showing signs of technological maturity with the introduction of prototype flat screen displays by leading electronics companies and increasing sun power harvesting capabilities of organic based solar cell devices. The great achievements in organic chemistry synthesis of the 20th century and the beginning of the 21st are now at the disposal of scientists and engineers seeking to exploit this versatility in the evolving frontier of organic-polymeric electro-optical devices.

Although there are many parallels between conventional inorganic crystalline-based devices and amorphous organic based devices, there are major differences between the two. Therefore, straightforward application of conventional transport equations and concepts is not always suitable for describing electro-optical phenomena in organic devices. Using the unique electrical characteristics of organic materials, we show in the first part of our work the relation between the power dependence of the photocurrent quantum efficiency and the slower-carrier mobility in organic photocell and light detector devices. This dependence was explored both experimentally and theoretically and we provide a simple technique for extracting the slower-carrier mobility from the quantum efficiency measurements. 

Following photocurrent transient experiments we conducted on thin organic film photocell devices that showed intriguing and puzzling characteristics, we developed a completely new description of transport in organic and amorphous thin semiconducting layers that bears both physical and engineering significance. The core idea behind this description is that thin amorphous semiconducting layers cannot be described appropriately by attributing a single mobilty value for charge carriers in the layer but rather should be described as constituting a manifold of parallel pathways of different mobilities. The basis for our claim, we argue, is that due to the short distance a carrier travels from one side of the thin amorphous film to the opposite side, there is a lack of statistical convergence of the characteristic properties of the environments. We therefore suggest and show experimentally that a mobility distribution function is the right way to characterize such thin amorphous films. We also present the mathematical tools for extracting such a distribution function from a transient measurement. Theoretical analysis of thin film disordered environments using direct solutions of the master equation provided additional basic-principle support to our approach.