| Description |
The work presented in this dissertation is mainly focused on the quantitative study of the two dominant interactions experienced by charge carrier spins in π-conjugated semiconductors, namely the hyperfine interaction due to adjacent nuclear spins and the spin-orbit interaction that arises due to relativistic effects on the coupling of charge carriers' angular momenta and their respective spins. In the literature, several experimental and theoretical studies have concluded that hyperfine interaction is the dominant spin relaxation mechanism in organic semiconductors and this hypothesis can explain many magneto-optoelectronic behaviors of these materials. The effect of spinorbit coupling on spin relaxation mechanisms has oftentimes been considered negligible as these materials are mainly composed of the light elements (with small atomic numbers). However, several recent experimental and theoretical studies have shown that spin-orbit coupling, while small, cannot sometimes not be neglected for the macroscopic materials behaviors, especially under high magnetic field conditions (> 100mT). Thus, the separate quantitative understanding of each of these two interaction types is crucial for the understanding of electronic, optoelectronic, and magneto-optoelectronic materials' behaviors. For this study, electrically detected magnetic resonance (EDMR) spectroscopy was conducted over a wide range of frequencies. EDMR has been known to be a highly sensitive and versatile technique to probe the spin dynamics of charge carriers in iv condensed matter. Using thin film bipolar injection devices made out of prototypical π- conjugated materials such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV), Super-Yellow poly-phenylenevinylene (SYPPV), Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), multifrequency EDMR spectra were collected in a magnetic field range from 3 mT up to 12 T and then analyzed in order to unambiguously discriminate hyperfine field effects from spin-orbit-induced effects on the measured electric current. The study shows that charge carrier EDMR linewidths at low magnetic fields depend on hyperfine fields only, while EDMR spectra at high magnetic fields are dominated by spin-orbit coupling. |
