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 C-13 nuclei on the acceptor phenyl-C-61-butyric acid methyl ester (PCBM) fullerene cage, which is approximate to 18% isotopically enriched with C-13, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are >95% isotopically enriched with H-2. The C-13 - H-2 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 approximate to 6 nm in the mixed phase, as demonstrated by H-1 spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than approximate to 6 nm, neat P3HT and PCBM clusters exist on approximate to 3 nm size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.