Characterization of Interfacial Structure in Polymer-Fullerene Bulk Heterojunctions via ${}^{13}\mathrm{C}$ $\{{}^2\mathrm{H}\}$ Rotational Echo Double Resonance NMR

We introduce a new application of solid state NMR measurements towards characterizing the donor-acceptor interfaces within bulk heterojunction (BHJ) films. Rotational echo double resonance (REDOR) is used to measure dipolar couplings between ${}^{13}\mathrm{C}$ nuclei on the acceptor phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester (PCBM) fullerene cage, which is $\approx 18\%$ isotopically enriched with ${}^{13}\mathrm{C}$, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are $>95\%$ isotopically enriched with ${}^2\mathrm{H}$. The ${}^{13}\mathrm{C}-{}^2\mathrm{H}$ dipolar couplings are used for constraining possible models of molecular packing in the amorphous mixed phase of a $\mathrm{P}3\mathrm{HT}/\mathrm{PCBM}$ BHJ. The films studied are highly mixed ($>80\%$) and have a maximum length scale of composition nonuniformity of $\approx 6\mathrm{nm}$ in the mixed phase, as demonstrated by ${}^1\mathrm{H}$ spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than $\approx 6\mathrm{nm}$, neat P3HT and PCBM clusters exist on $\approx 3\mathrm{nm}$ size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.

Figure 1

${}^{13}\mathrm{C}$ cross polarization magic angle spinning spectra both without (black) and with (gray) ${}^2\mathrm{H}$ REDOR dephasing pulses. The REDOR pulse sequence is given above. The dephasing time was 13.6 ms.

Figure 2

Experimental (dots) and simulated (lines) REDOR curves. The gray and black lines get darker with increasing ${(3\mathrm{HT})}_9$ content. The lines correspond to ${(3\mathrm{HT})}_9$ on the face(s) of the cuboid and no ${(3\mathrm{HT})}_9$ on the corners. Dotted lines correspond to cuboids with ${(3\mathrm{HT})}_9$ molecules on the cuboid and ${(3\mathrm{HT})}_9$ molecules on all four corners.

Figure 3

(a) Small angle neutron scattering curve of $\mathrm{P}3\mathrm{HT}/\mathrm{PCBM}$ (dots) and model fits. (b) The spatial correlation function for a value of $\zeta =3.3\mathrm{nm}$. (c) The probability distribution for finding ${(3\mathrm{HT})}_3$ on the face of the simple cubic lattice assuming a 0.15 probability and a binomial distribution.