Abstract
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 nuclei on the acceptor phenyl--butyric acid methyl ester (PCBM) fullerene cage, which is isotopically enriched with , and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are isotopically enriched with . The dipolar couplings are used for constraining possible models of molecular packing in the amorphous mixed phase of a BHJ. The films studied are highly mixed () and have a maximum length scale of composition nonuniformity of in the mixed phase, as demonstrated by spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than , neat P3HT and PCBM clusters exist on size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.
- Received 9 January 2018
DOI:https://doi.org/10.1103/PhysRevLett.121.026101
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