Measuring the Extent of Phase Separation in Poly-3-Hexylthiophene/Phenyl-C₆₁-Butyric Acid Methyl Ester Photovoltaic Blends with ¹H Spin Diffusion NMR Spectroscopy

We demonstrate that 1 H spin diffusion NMR is a valuable method to estimate the domain size distribution in bulk heterojunction (BHJ) active layers composed of poly-3-hexylthiophene (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM). Bulk samples were annealed at several temperatures, and a distribution of domain sizes was observed that ranged over hundreds of nanometers. Unannealed samples exhibited a large amount of PCBM in small domains (1 to 5 nm) and smaller amounts in moderate (tens of nm) and large (>100 nm) domains. Annealing the samples at 100 °C had no effect on the morphology as evidenced from 1 H spin diffusion NMR, grazing-incidence diffraction, and calorimetry, but phase separation was observed after annealing at 150 °C. Even with this higher temperature processing, the 1 H NMR showed conclusively that phase separation remained incomplete; this finding was confirmed with photoluminescence quenching measurements.