Nuclear relaxation measurements in organic semiconducting polymers for application to organic spintronics

NMR measurements of spin-lattice relaxation of hydrogen nuclei in two prototype organic semiconducting solids, MEH-PPV and DOO-PPV, were carried out for temperatures between 4.2 K and room temperature, and for applied magnetic fields between 1.25 and 4.7 T. These $\pi$-conjugated polymers are of interest for use as the active semiconducting layer in spintronic devices. They typically exhibit weak spin-orbit coupling, and the interaction with inhomogeneous hyperfine fields generated by the nuclear spins plays a significant, if not dominant, role in the spin coherence and spin relaxation of electronic charge carriers. Our studies were conducted on unbiased bulk material with no photo-illumination. The characteristic ${}^1\mathrm{H}$ longitudinal relaxation times in these materials ranges from hundreds of milliseconds to $>1000$ s, and are predominantly nonmonoexponential. We present the data both in terms of a recovery time, $T_{1/2}$, corresponding to 50% recovery of thermal magnetization from saturation and in terms of a “$T_1$ spectrum” produced via a numerical Laplace transform of the time-domain data. The evidence best supports relaxation to paramagnetic centers (radicals) mediated by nuclear spin diffusion as the primary mechanism: the observed relaxation is predominantly nonmonoexponential, and a characteristic $T_1$ minimum as a function of temperature is apparent for both materials somewhere between 77 K and room temperature. The paramagnetic centers may be somewhat-delocalized charge-carrier pairs (i.e., polarons) along the polymer backbone, although the concentration in an unbiased sample (no carrier injection) should be very low. Alternatively, the centers may be localized defects, vacancies, or impurities. Our results may also be used to judge the feasibility of Overhauser-type dynamic nuclear polarization from polarized charge carriers or optically pumped exciton states.

Figure 1

Molecular structures for (a) MEH-PPV (260.18 Da per monomer) and (b) DOO-PPV (358 Da per monomer). Bond lengths vary between 0.1 and 0.15 nm, but the disordered packing behavior of the long polymer chains can cause spacing between adjacent chains to be much larger.

Figure 2

(a) Magnetization recovery of ${}^1\mathrm{H}$ vs time in DOO-PPV at 1.25 T and 77 K. This is a relatively rare instance in these measurements where the decay fits reasonably well to a single exponential. The boxed graph shows corresponding Laplace transform having a single peak corresponding to $T_1=2.28\pm 0.04\text{s}$ (uncertainty extracted from the fit to the time-domain data). (b) Magnetization recovery of ${}^1\mathrm{H}$ vs time in MEH-PPV at 2.5 T and 10 K. Here, the decay is strongly nonmonoexponential. The boxed graph shows corresponding Laplace transform which shows many peaks, some of which are not reflected in the multiexponential fit to the time-domain data.

Figure 3

The relaxation spectrum for ${}^1\mathrm{H}$ in DOO-PPV and MEH-PPV as determined by taking the Laplace transform of time-domain saturation-recovery data. If one assumes a multiexponential decay model, then the intensities correspond to the weights of the various $T_1$ components. Nonmonoexponential behavior, in general, tends to be most prevalent at lowest temperatures and highest fields. Significant dependence of the relaxation behavior on both temperature and magnetic field is observed; the largest $T_1$ component may pass through an apparent minimum between 77 K and room temperature.