Study of the nanoscale morphology of polythiophene fibrils and a fullerene derivative.

Nanoscale blending of electron-donor and electron-acceptor materials in solution-processed bulk heterojunction organic photovoltaic devices is crucial for achieving high power conversion efficiency. We used a classic blend of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) as a model to observe the nanoscale morphology of the P3HT fibrils and PCBM nanoclusters in the mixture. Energy-filtered transmission electron microscopy (EFTEM) clearly revealed a nanoscopic phase separation. Randomly connected and/or nonconnected P3HT fibrous networks and PCBM domains, revealed by 2-dimensional micrographs, were observed by collecting electron energy loss spectra in the range of 19-30 eV. From EFTEM images, the average length and the diameter of P3HT fibrils were found to be approximately 70 ± 5 and 15 ± 2 nm, respectively. Combining the EFTEM, selected area electron diffraction, and X-ray diffraction results, the number and spacing of the ordered chains in P3HT fibrils were determined. There were 18 ± 3 repeating units of P3HT perpendicular to the fibril, ∼184 layers of π-π stacking along the fibril, and ∼9 layers of interchain stacking within the fibril. These conclusive observations provide insight into the number of molecules found in one instance of ordered-plane stacking. This information is useful for the calculation of charge transport in semicrystalline polymers. Using cross-section samples prepared with a focused ion beam technique, the vertical morphology of each phase was analyzed. By collecting 30 eV energy loss images, the phase separation in the P3HT/PCBM system was distinguishable. A higher P3HT concentration was observed at the top of the cell, near Al contact, which could possibly cause loss of carriers and recombination due to a mismatch in the P3HT and Al energy bands.