Characterization of Interfacial Structure in Polymer-Fullerene Bulk Heterojunctions via C 13 { H 2 } 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 C13 nuclei on the acceptor phenyl-C61-butyric acid methyl ester (PCBM) fullerene cage, which is ≈18% isotopically enriched with C13, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are >95% isotopically enriched with H2. The C13-H2 dipolar couplings are used for constraining possible models of molecular packing in the amorphous mixed phase of a P3HT/PCBM BHJ. The films studied are highly mixed (>80%) and have a maximum length scale of composition nonuniformity of ≈6 nm in the mixed phase, as demonstrated by H1 spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than ≈6 nm, neat P3HT and PCBM clusters exist on ≈3 nm size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.