Tuning of spin-orbit coupling in metal-free conjugated polymers by structural conformation

Abstract

Manipulating spin-orbit coupling (SOC) is a key achievement for spin-orbitronic applications since SOC determines spin-diffusion lengths and spin-to-charge conversion efficiencies. While in most organic semiconductors SOC is inherently very weak due to being composed of primarily light elements, the SOC in conjugated polymer systems is also intimately tied to the polymer's structural conformation and thus may be manipulated. Here we report a modification of SOC in conjugated polymers by altering torsion angle between conjugated units. Spin-pumping experiments are performed on three poly(3-alkylthiophene) polymer films with decreasing conjugation lengths and concomitantly increasing torsion angle. The more twisted polymer exhibits a shorter spin-diffusion length and a giant spin-mixing conductance (up to ${10}^{21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$ ), which is attributed to an increased SOC by structural conformation. This work offers a route for enhancing SOC and spin-injection efficiency in organic materials for spintronic applications.