Spin Dynamics in Organic Semiconductors

Organic semiconductors exhibit exceptionally long spin lifetimes, and recent observations of the inverse spin hall effect as well as micrometer spin diffusion lengths in conjugated polymers have spiked interest in employing such carbon-based materials in spintronics applications. The charge transport and photophysics of organic semiconductors have been intensely studied for optoelectronic applications, revealing subtle relationships between molecular geometry, morphology and physical properties. Similar structure-property relationships remain mostly unknown for spin dynamics, where the charge carrier spins couple to their environment through hyperfine (HFI) and spin-orbit interactions (SOC). HFIs provide a pathway for spin relaxation while SOC plays a dual role in such materials: it couples the spin to its angular momentum and therefore enables both spin-to-charge conversion and spin relaxation. Understanding the molecular SOC, and finding a means to tune its strength, therefore is fundamentally important for materials design and selection. However, quantifying SOC strengths indirectly through spin relaxation effects has proven difficult due to competing relaxation mechanisms. We initially present a systematic study of the g-tensor shift in molecular semiconductors and establish it as a probe for the SOC strength in a series of high mobility molecular semiconductors. The results demonstrate a rich variability of molecular g-shifts with the effective SOC, depending on subtle aspects of molecular composition and structure. We then correlate the above g-shifts to spin-lattice relaxation times over four orders of magnitude, from 200 µs to 0.15 µs, for isolated molecules in solution and relate our findings to the spin relaxation mechanisms that are likely to be relevant in solid state systems. Isolated molecules provide an ideal model system to investigate a spin’s interactions with its environment but device applications commonly employ thin films. The second half of this thesis investigates polaron spin lifetimes in field effect transistors with high-mobility conjugated polymers as active layers. We use field-induced electron spin resonance measurements to demonstrate that spin relaxation is governed by the charges' hopping motion at low temperatures while Elliott-Yafet-like relaxation due to short-range, rapid spin density dynamics likely dominates high temperature spin lifetimes. Such a microscopic relaxation mechanism is highly sensitive to the local conformation of polymer backbones and we show its dependence on the degree of crystallinity in a polymer film