Isotope effect in spin response of π-conjugated polymer films and devices

Abstract

Recent advances in organic spin response include long polaron spin-coherence times measured by optically detected magnetic resonance (ODMR), substantive room-temperature magnetoelectroluminescence and magnetoconductance obtained in organic light-emitting diodes (OLEDs) and spin-polarized carrier injection from ferromagnetic electrodes in organic spin valves (OSVs). Although the hyperfine interaction (HFI) has been foreseen to have an important role in organic spin response, no clear experimental evidence has been reported so far. Using the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and OSV devices based on pi-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI does indeed have a crucial role in all three spin responses. OLED films based on the D-polymers show substantially narrower magneto-electroluminescence and ODMR responses, and as a result of the longer spin diffusion obtained, OSV devices based on D-polymers show a substantially larger magnetoresistance.

Observation of a large spin-dependent transport length in organic spin valves at room temperature

“…To the best of our knowledge, this thickness is the largest SDT length reported at room temperature in OSs with vertical spin valve structures to date. We suggest that when compared with other reported OSs, such as rubrene and Alq 3 , the weaker spin-orbit coupling and hyperfine interaction of the C 60 molecule 14,28,29 significantly reduces spin flip in the organic film. Thus, a large SDT length was obtained for the C 60 layer.…”

“…In particular, the hyperfine interaction in C 60 is considerably weak because of the absence of polarized hydrogen nuclei and the low natural abundance of 13 C nuclear spins in the C 60 molecule 11,12,14,15 . The weak hyperfine interaction has been predicted to increase the SDT length 28 , which has been demonstrated at low temperatures using a deuterated polymer as a spacer in organic spin valve devices 29 . In addition, the C 60 molecule is nearly isotropic because of its high symmetry and rotational freedom.…”

“…The spherical structure of C 60 makes the sustenance of the upper metallic layer easy. The 'ill-defined' layer may also be related to the fabrication methods 7,8,10,29 . The Raman spectrum for the C 60 layer was measured for an organic spin valve device Fig.…”

“…Thus, one possible reason for the presence of fitting error is that other interactions, such as the exchange interaction or/and spin-orbit coupling, also contribute to the spin transport in OSs, as was suggested in ref. 29 and studied in ref. 38.…”

Traps and trions as origin of magnetoresistance in organic semiconductors

“…It has been demonstrated that hyperfine fields are involved in the spin mixing. 9 However, there is still on-going debate as to whether spin-orbit coupling might play a role in certain materials. 10 Regardless of the exact underlying nature, an external magnetic field suppresses this mixing and can consequently adjust the spin ratio of the subsequent particles.…”