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

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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.