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

摘要

Weintroduce 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 ¹³C nuclei on the acceptor phenyl-C61-butyric acid methyl ester (PCBM) fullerene cage, which is ≈18% isotopically enriched with ¹³C, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are >95% isotopically enriched with ²H. The ¹³C — ²H 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 ¹H 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.