Molecular structure and dynamics at the interfaces within bulk heterojunction materials for solar cells

The molecular structures within the interfaces of the bulk heterojunction material comprising regioregular-poly(3-hexylthiophene-2,5-diyl), rrP3HT, and C-60 or its soluble derivative, [6,6]-phenyl-C-61-butyric 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 degrees C for 30 min (PCBMCF-A150 and C60CF-150A). In PCBMCF-RT, the alkyl side chains of rrP3HT are close to the C-60 ball; C-60 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 C-13 of C-60 in C60CF-A150 indicates a distortion from spherical symmetry. Measurements of the spin-lattice relaxation rate, 1/T-1, of C-60 imply local magnetic field fluctuations that arise from the dynamics of the C-60 distortion.