In this manuscript we show that 1H spin diffusion NMR is a valuable method for estimating the domain sizes in thin films of a polymer-fullerene blend for bulk heterojunction (BHJ) photovoltaics. Variations in common BHJ film processing parameters have dramatic effects on the domain sizes in the blend films, which in turn affect power conversion efficiency. Spin-coating rotational speed, thermal annealing schedule, and solvent annealing all strongly affect BHJ morphology. The 1H spin diffusion NMR data clearly show that films spun at fast speeds are primarily composed of small (< 10 nm) domains of each component and exhibit low power conversion efficiencies (eta = 0.4%). These films are improved substantially upon thermal annealing due to nanoscopic phase separation which leads to higher efficiency (eta = 2.2%). Films spun at slow speeds and then slowly dried display larger domains and exhibit even higher efficiencies (eta = 2.6%), but do not benefit from thermal annealing. The 1H spin diffusion NMR results show that the presence of domains in the tens of nm range is a common characteristic of samples with higher efficiencies. We thus demonstrate the utility of solid-state NMR for the measurement of BHJ morphology.
Citation: Advanced Functional Materials
Pub Type: Journals
ORGANIC PHOTOVOLTAICS, BULK HETEROJUNCTION, SOLID-STATE NMR, DOMAIN SIZE, MORPHOLOGY