Measuring Domain Sizes and Compositional Heterogeneities in P3HT‐PCBM Bulk Heterojunction Thin Films with 1H Spin Diffusion NMR Spectroscopy

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.