M.Sc Thesis

M.Sc StudentNikiforov Daniel
SubjectMagnetic Field Effects in Organic Semicounductor Devices
in High Magnetic Fields
DepartmentDepartment of Physics
Supervisor PROFESSOR EMERITUS Eitan Ehrenfreund
Full Thesis textFull thesis text - English Version


Organic semiconductors are composed of π-conjugated polymers and molecules. There is a large variety of devices in which the active layers are organic semiconductors; among them are organic light emitting diodes and organic photovoltaic cells. The devices may contain a single organic semiconductor or a blend of donor and acceptor molecular units. When devices based on organic semiconductors are exposed to external magnetic field their performance changes. These changes in performance are called magnetic field effects. In this work we are investigating magnetic field effects in low and high fields (up to 8 Tesla but with high resolution of 10-4 Tesla at low fields) in organic light emitting diodes made up of homo-polymer organic semiconductors. In these devices we measured magnetic field effects, such as magneto-conductance, magneto-electroluminescence, magneto-photocurrent and magneto-photoluminescence. In these low mobility substances magnetic field effects are attributed to the spin degrees of freedom. We show that it is possible to explain the experimental results by taking one pair of charges with their spin Hamiltonian. The pair of charges is called polaron pair (or radical pair). In the low field regime the magnetic field effects are caused by the hyperfine interaction between the spins of the polaron pairs and spins of the many protons that are present in all organic substances. In the intermediate field region we consider a mechanism that was not discussed in detail previously and is caused by the anisotropy of the g-factor. In low symmetry organic semiconductors the g-factor of the electron (hole) is anisotropic. Given a polaron pair, for which the anisotropy axes of the two polarons are not parallel, that is placed in a magnetic field in an arbitrary direction then effectively each polaron has a different g-factor. Thus, the anisotropic g-tensor is equivalent to the well known Δg mechanism that was used to explain the magnetic field effects in devices based on a blend of donor and acceptor molecular units. In addition to the anisotropic g-tensor mechanism we consider partial thermal spin polarization as the mechanism responsible for the magnetic field effects at fields higher than 1 Tesla. We developed a simplified quantum mechanical approach taking into account the above interactions. With this model we could fit our experimental data and draw the following conclusions. The exchange interaction between polarons that compose the polaron pairs is approximately 6neV while for charge transfer excitons is around 0.1μeV because of the close distance between the polarons. The orientation of the g-tensors within the polaron pair was found to depend on the organic semiconductor. The life time of a polaron pair in these organic light emitting diodes was determined to be of the order of ~μs.