Effect of Charge Localization on the Effective Hyperfine Interaction in Organic Semiconducting Polymers

Hyperfine interaction (HFI), originating from the coupling between spins of charge carriers and nuclei, has been demonstrated to strongly influence the spin dynamics of localized charges in organic semiconductors. Nevertheless, the role of charge localization on the HFI strength in organic thin films has not yet been experimentally investigated. In this study, the statistical relation hypothesis that the effective HFI of holes in regioregular poly(3-hexylthiophene) (P3HT) is proportional to $1/N^{0.5}$ has been examined, where $N$ is the number of the random nuclear spins within the envelope of the hole wave function. First, by studying magnetoconductance in hole-only devices made by isotope-labeled P3HT we verify that HFI is indeed the dominant spin interaction in P3HT. Second, assuming that holes delocalize fully over the P3HT polycrystalline domain, the strength of HFI is experimentally demonstrated to be proportional to $1/N^{0.52}$ in excellent agreement with the statistical relation. Third, the HFI of electrons in P3HT is about 3 times stronger than that of holes due to the stronger localization of the electrons. Finally, the effective HFI in organic light emitting diodes is found to be a superposition of effective electron and hole HFI. Such a statistical relation may be generally applied to other semiconducting polymers. This Letter may provide great benefits for organic optoelectronics, chemical reaction kinetics, and magnetoreception in biology.

Figure 1

(a) Raman spectra of isotope-labeled P3HT. The insets show the chemical structures of hydrogenated P3HT and full-deuterated P3HT. (b) Photoluminescence of the isotopes at a low temperature of 10 K. (c) IV characteristics and (d) MC (with 0.5 G resolution) of the isotope-based hole-only devices at 10 K.

Figure 2

(a) A cartoon of the P3HT as-cast thin film where the crystalline domains are mixed with the amorphous states, and the inset shows the crystallographic directions. [010] is the $\pi –\pi$ stacking direction. (b) The spatial distribution of electron wave function in the lamellar (100) plane. The small orange arrows represent the hydrogen nuclear spins that generate an effective hyperfine field $B_{\mathrm{HFI}}$ (large orange arrow) at the electron spin (blue arrow). (c) XRD spectra of annealed and as-cast films made by commercial P3HT. (d) MC (with 0.5 G resolution) of the hole-only devices corresponding to annealed and as-cast P3HT films at 10 K. (e) MC HWHM of the commercial P3HT hole-only devices versus the number of nuclear spins at 10 K (in log-log scale). The line shows a fit with the $1/N^p$ function. The current density in (d) and (e) is $\sim 50\mu \mathrm{A}/{\mathrm{mm}}^2$.

Figure 3

(a) Schematic representation of the organic spin valve (OSV) with four probe measurement. MR response at 20 K of OSVs (b) with film annealing at $160°\mathrm{C}$ and (c) without film annealing at different P3HT thicknesses. (d) Thickness-dependent MR magnitude (dots) at 20 K. The solid lines show fits using the modified Jullière model.

Figure 4

MC responses of an electron-only device (a) at different temperatures, and (b) their MC HWHM. Normalized MC responses of hole-only, electron-only, and bipolar devices made by (c) P3HT, (d) MEH-PPV, and (e) PFO. All MCs were measured with the current density of $10\mu \mathrm{A}/{\mathrm{mm}}^2$ and field resolution of 5 G.