Gajadhar Joshi Friday, June 1, 2018 at 1 PM (334 JFB)
Title: Detection Quantitative Study of Charge Carrier Spin-Orbit Coupling Effects on Electronic Currents in Conducting Polymers
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 which 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 spin-orbit 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, although small, cannot be neglected sometimes, for the macroscopic materials behaviors, especially under high magnetic field conditions (gt 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 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 showed 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.