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Article

Molecular Structure and Dynamics at the Interfaces within Bulk Heterojunction Materials for Solar Cells

Cuiying Yang,^† Jerry G. Hu,^‡ and Alan J. Heeger*^†

Contribution from the Center of Polymers and Organic Solids, University of California, Santa Barbara, California 93106-5090 and Materials Research Laboratory, University California, Santa Barbara, California 93106-5121

J. Am. Chem. Soc., 2006, 128 (36), pp 12007–12013

DOI: 10.1021/ja063707f

Publication Date (Web): August 17, 2006

†

Center of Polymers and Organic Solids.

‡

Materials Research Laboratory.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Abstract

The molecular structures within the interfaces of the bulk heterojunction material comprising regioregular-poly(3-hexylthiophene-2,5-diyl), rrP3HT, and C₆₀ or its soluble derivative, [6,6]-phenyl-C₆₁butyric acid methyl ester, PCBM, have been studied by one- and two-dimensional nuclear magnetic resonance (NMR). The local structure within the interface was inferred from chemical shift (CS) data obtained from composite films (CFs) fabricated at room temperature (PCBMCF-RT and C₆₀CF-RT) and from CFs that had been subsequently annealed at 150 °C for 30 min (PCBMCF-A150 and C₆₀CF-150A). In PCBMCF-RT, the alkyl side chains of rrP3HT are close to the C₆₀ ball; C₆₀ is essentially ‘wrapped' by the alkyl side chains. In PCBMCF-A150, the alkyl side chains self-assemble such that rrP3HT and PCBM are separated. The observation of well-defined splittings in the CS spectrum of the ¹³C of C₆₀ in C₆₀CF-A150 indicates a distortion from spherical symmetry. Measurements of the spin−lattice relaxation rate, 1/T₁, of C₆₀ imply local magnetic field fluctuations that arise from the dynamics of the C₆₀ distortion.