posted on 2014-08-13, 00:00authored byNopporn Rujisamphan, Roy E. Murray, Fei Deng, Chaoying Ni, S. Ismat Shah
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.