Ph.D Thesis


Ph.D StudentNikiforov Daniel
SubjectMagnetic Field Effects on Electrical and Optical
Properties of Organic Semiconductors
DepartmentDepartment of Physics
Supervisor PROFESSOR EMERITUS Eitan Ehrenfreund


Abstract

In this research we studied magnetic field effects in devises based on organic semiconductors. In addition, we investigated magneto-conductance of thin films composed of high-conductivity (~200 S/cm) doped organic semiconductor, poly(3 4-ethylene dioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS).

Weak magnetic fields (~10 mT) can change the organic semiconductor device performance by as much as ~10%. These effects are related to the electron spin degrees of freedom of the electron-hole spin pairs (termed polaron pairs) within the organic semiconductor layer. Since in organic semiconductor the spin-orbit coupling is weak, the charge carriers exhibit long spin coherence time (~1 µs) and precise spin selectivity. As a consequence, magnetic field effects occur even at room temperature despite the large thermal fluctuations in comparison with the spin interactions.

We studied two types of devices exhibiting magnetic field effects. The first device is a homopolymer organic light emitting diode device. We investigated: magneto-conductance, magneto-electroluminescence, magneto-photocurrent, and magneto-photoluminescence. We have found that the electron g-factor anisotropy has a significant effect on the magneto-conductance and magneto-electroluminescence responses at the magnetic fields of the order of B ~ 1 T. By fitting the experimental data to the polaron-direction dependent polaron pair model, we were able to examine the preferable orientation between the polaron pairs participating in magneto-conductance and magneto-electroluminescence. We also have found that the magneto-photocurrent and magneto-conductance responses are very different; in particular, the low-field magneto-photocurrent response is narrower by a factor of  from that of magneto-conductance response. We attribute this difference to a unique feature of charge transfer excimers that are responsible for magneto-photocurrent:  sub-nanosecond fast fusion back to singlet excitons and slow (ns to µs) dissociation to free charge carriers. In contrast, the magneto-conductance and magneto-electroluminescence responses are determined by long lived (~1 µs) polaron pairs having singlet and triplet decay rates of the same order. The second device, is a photodiode based on organic donor-acceptor planar heterojunction. In this system we have shown that the low-field magneto-photocurrent response is dependent, in the temperature range 120 − 240 K, on the charge transfer exciton lifetime. To describe the time evolution of the spin state, we considered three spin mixing mechanisms: the hyperfine interaction, the effect of g-factor anisotropy, and the thermal spin polarization.

The important portion of the thesis deals with magneto-conductance in high-conductivity grade PEDOT:PSS thin-films. We distinguish between two main contributions to the magneto-conductance response. The first positive and anisotropic contribution is accounted for the effect of weak localization that takes place within the PEDOT rich regions. The second negative and isotropic contribution is explained by the effect of magnetic field on the hopping probability between the PEDOT rich areas. Particularly, we explain the magneto-conductance response saturation, at low temperatures, by the effect of spin polarization on the hopping between sites with intra-site Coulomb correlation energy, U. We determined that U has to be at least 2 meV in order to explain the experimental saturating magneto-conductance response.