The application of 1H spin diffusion nuclear magnetic resonance (NMR) is expanded to polymer-fullerene blends for bulk heterojunction (BHJ) organic photovoltaics (OPV) by developing a new experimental methodology for measuring the thin films used in poly-3-hexylthiophene–phenyl C61-butyric acid methyl ester (P3HT-PCBM) OPV devices and by creating an analysis framework for estimating domain size distributions. It is shown that variations in common P3HT-PCBM BHJ processing parameters such as spin-coating speed and thermal annealing can significantly affect domain size distributions, which in turn affect power conversion efficiency. 1H spin diffusion NMR analysis reveals that films spin-cast at fast speeds in dichlorobenzene are primarily composed of small (<10 nm) domains of each component; these devices exhibit low power conversion efficiencies (η = 0.4%). Fast-cast films improve substantially by thermal annealing, which causes nanometer-scale coarsening leading to higher efficiency (η = 2.2%). Films spin-cast at slow speeds and then slowly dried exhibit larger domains and even higher efficiencies (η = 2.6%), but do not benefit from thermal annealing. The 1H spin diffusion NMR results show that a significant population of domains tens of nanometers in size is a common characteristic of samples with higher efficiencies.