Spin transport in organic semiconductors

Abstract

The unique physical properties and low temperature solution processability of organic semiconductors have enabled many applications such as light emitting diodes, flexible logic and solar cells, they are unexploited in their potential for use in solid state devices for spintronics and spin-based information processing. Organic semiconductors composed of mainly light elements appeal to the field of spintronics due to their long spin lifetime originating from their weak spin-orbit coupling. The significant progress in improving carrier mobility of organic semiconductors in the past decade may lead to organic spin transport materials with both long spin diffusion length and spin lifetime which is important for spintronics applications. This dissertation explores the spin transport in organic semiconductors using a variety of experimental techniques from all electrical spin injection and detection to ferromagnetic resonance spin pumping and ISHE spin detection. Non-local spin valves and novel all electrical spin transport device architectures based on high mobility conjugated polymers were studied systematically. The intrinsic roadblocks for electrical spin injection-based measurements were identified as the current spreading effect (electrical cross-talk between the injector and detector electrodes) and the hopping conduction in organic semiconductors which makes all electrical nonlocal spin injection and detection measurements extremely challenging if not impossible for organic semiconductors. In addition, spin current transmission in the out of plane direction of organic semiconductors was studied by tri-layer spin pumping technique where the spin transport properties of organic semiconductors are correlated with their molecular structure and charge transport properties. Spin pumping, a charge-free spin injection method together with ISHE spin detection successfully overcome the impedance mismatch problem and the intrinsic roadblocks imposed by electrical spin injection-based techniques and enabled lateral spin current transport in organic semiconductors to be detected electrically. The lateral spin diffusion length of up to a micrometre was observed in doped conjugated polymers in agreement with theoretical calculations based on exchange mediated spin diffusion model and parameters obtained from first principle. Moreover, this non-local spin transport device structure provides a platform for studying spin transport in a wide range of organic semiconductors where the spin current propagates along the high mobility direction and could potentially be used as building blocks for high performance flexible spintronics devices.