Figure 2. Specular neutron reflectivity data for the different Soxhlet fractions of PCDTBT polymer studied and the unfractionated polymer, showing data with associated errors and fits to the data (black lines).

The data are offset by factors of 10 for clarity. The unfractionated data was measured using a magnetic reference layer (NiFe) with polarised neutrons and so we have measured spin up and spin down to give two reflectivity datasets. The scattering length density of pure PCDTBT was taken as 1.34 × 10⁻⁶Å⁻² (from previous NR thin film measurements of PCDTBT) and PC₇₁BM as 4.46 × 10⁻⁶ Å⁻² was calculated using the NIST online database.