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

We report measurements of room-temperature spin-relaxation times T_1 and T_2 of charge-carrier spins in a π-conjugated polymer thin film under bipolar injection and low (1≲ B_0≲ 10) 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.