Spin relaxation dynamics of radical-pair processes at low magnetic fields

TL;DR

This study measures spin-relaxation times of charge-carrier spins in a conjugated polymer at low magnetic fields, revealing how hyperfine interactions influence radical-pair processes and spin dynamics.

Contribution

It provides the first detailed experimental analysis of spin relaxation and decoherence in organic semiconductors under low magnetic fields using pulsed magnetic resonance techniques.

Findings

Spin-lattice relaxation time varies with magnetic field in low-field regime.

Spin decoherence time remains constant across low magnetic fields.

Experimental confirmation of hyperfine field influence on radical-pair spin dynamics.

Abstract

We report measurements of room-temperature spin-relaxation times $T_1$ and $T_2$ of charge-carrier spins in a $\pi$-conjugated polymer thin film under bipolar injection and low ($1\mbox{ mT}\lesssim B_0\lesssim 10\mbox{ mT}$) static magnetic fields, using electrically detected magnetic resonant Hahn-echo and inversion-recovery pulse sequences. The experiments confirm the correlation between the magnetic-field sensitive observables of radical-pair processes, which include both the spin-dependent recombination currents in organic semiconductors and the associated spin-relaxation times when random local hyperfine fields and external magnetic fields compete in magnitude. Whereas a striking field dependence of spin-lattice relaxation exists in the low-field regime, the apparent spin decoherence time remains field independent as the distinction between the two is lifted at low fields.