Decay in spin diffusion length with temperature in organic semiconductors: An insight of possible mechanisms

This article presents a comparison of spin transport mechanism in two π-conjugated organic polymers namely, regiorandom and regioregular poly (3-hexyl thiophenes) with same elemental composition but different regioregularity of the constituent atoms leading to different crystallinity and charge carrier mobility. Spin-valve devices made with both polymers show substantial low temperature giant magnetoresistance (GMR) response. However, the GMR signal decreases drastically at higher temperatures where charge carrier mobility is higher. Our results suggest that in both the polymers spin diffusion length at low temperature is almost similar, but, temperature dependence of spin diffusion length is greater in the disordered polymer compared to the more structured one. Comprehensive analysis of our experimental data suggest that at low temperature, in the VRH hopping regime (5–50 K), spin relaxation due to hyperfine interaction and Elliot-Yafet momentum scattering is the dominant spin relaxation mechanism while in the thermally activated regime Dyakonov–Perel mechanism contribution becomes significant. However, mobility dependence of spin scattering rate in both systems differ from traditional Dyakonov–Perel model signifying that there are coexisting contributions from several spin scattering effects present in the system. Proper understanding and careful modification of spin–orbit coupling in organic semiconductors can be very useful for organic based spin devices.