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

• Published in: Physical review letters 2018-07, Vol.121 (2), p.026101
• Main Authors: Nieuwendaal, R C, DeLongchamp, D M, Richter, L J, Snyder, C R, Jones, R L, Engmann, S, Herzing, A, Heeney, M, Fei, Z, Sieval, A B, Hummelen, J C
• Format: Article
• Language: English
• Subjects: Butyric acid; Carbon 13; Coupling (molecular); Couplings; Fullerenes; Heterojunctions; NMR; Nonuniformity; Nuclear magnetic resonance; Phase separation; Structural analysis
• ISSN: 0031-9007; 1079-7114

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 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.