Ph.D Thesis


Ph.D StudentBagrat Khachatryan
SubjectSpin Related Magnetic Field Effects in Organic
Semiconductor Devices
DepartmentDepartment of Physics
Supervisor Professor Emeritus Ehrenfreund Eitan
Full Thesis textFull thesis text - English Version


Abstract

A key role in magnetic field effects in organic semiconductors belongs to temporarily existing polaron pairs (PP - a pair of two spin ? oppositely charged bound particles). The PP exists in either singlet or triplet spin configuration states. These spin sub-levels may be mixed by spin related mechanisms.  By applying an external magnetic field the PP singlet and triplet state weights of the mixed energy levels vary with the applied field rendering magnetic field dependent response. The magnetic field dependent response is a direct consequence of the spin dependent rate of polaron pair dissociation (or recombination) to free charges giving rise to magneto-conductance (or excitons giving rise to electro-luminescence). In addition, at low temperatures and high magnetic fields, the magnetic field dependent thermal (Boltzmann) population of the singlet and triplet energy levels has a significant impact on the magneto-current response .


The Polaron Pair model is based on the time evolution of polaron pair spin sub-level population in a magnetic field. Within the framework described in this work we discuss the following spin related mechanisms: (a) hyperfine (HF) interaction - an interaction between the atomic nucleus and one of a polaron pair constituent polaron; (b) Δg mechanism - based on different g-factors of polarons residing on different molecules; (c) thermal spin polarization - temperature dependent Boltzmann population of the energy levels.  These mechanisms are meticulously described in this work and the predictions of the above three mechanisms, compared to our experimental results, are supporting the validity of the PP model .


In this work, magnetic field effects have been investigated under various experimental conditions in organic photovoltaic cells and newly developed vertical organic field effect transistors. High magnetic field (up to ±9T) was applied to the devices under test in a wide operating temperature range (4-320K). Several experimental systems were developed and built with an ability to detect and record extremely small changes in the measured signals, provided a sensitivity to measured signal variation of ~10-5 (or 1m%). This sensitivity enabled us to measure the magnetic field response in low current and low electro-luminescence signals, especially at low temperature conditions (e.g.,~10pA variation of the ~100nA measured electrical current amplitude) .


Three major research projects, presented in this work, emphasize a key role of PP on the devices electro-optical characteristics such as photo- and electrical current under an external magnetic field :

         We have studied the hyperfine interaction generated magneto-current response in the novel vertical organic field effect transistors and found that the PP consists of an electron in the active organic layer and a positive defect in the proximate a-SiO2 insulating layer inorganic layer.

         The investigated organic photovoltaic cells magneto-photocurrent has a positive non-saturating behavior in high magnetic fields at room temperature. We relate this response to Δg mechanism of short-lived (sub-ns) charge transfer excitons

         At low temperatures and high fields, the organic photovoltaic cells magneto-photocurrent response increases in magnitude and reverses sign. Thermal spin polarization is found to be the underlying mechanism for this behavior.