The morphology of the active layer in an organic photovoltaic bulk-heterojunction device is controlled by the extent and nature of phase separation during processing. Herein, we have examined the effects of fullerene crystallinity in the most widely studied system: poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. By utilizing a combination of focused ion-beam milling and energy-filtered transmission electron microscopy, we have directly monitored the local changes in phase distribution as a function of annealing time in model tri-layer structures consisting of a single layer of P3HT sandwiched between a pair of PCBM layers. While such structures have been characterized in the past using bulk techniques such as neutron reflectivity, dynamic secondary ion mass spectroscopy, and X-ray photoelectron spectroscopy, these techniques are insensitive to local variations in phase distribution and can only provide a representation of the overall structure averaged over a fairly large analytical volume. We directly show the local nanoscale mixing of amorphous PCBM within a surrounding P3HT matrix, and quantitatively describe its extent. In addition, we observe that the presence of a crystalline PCBM phase on one side of the tri-layer has a significant effect on the resulting morphology, such that amorphous PCBM will migrate through the intermediate P3HT layer to combine with a region of crystalline PCBM on the other side. The solubility of the crystalline component within P3HT is far lower than its amorphous counterpart, resulting in nearly complete uptake of all PCBM to the crystalline layer after a sufficient annealing time. However, when no crystalline PCBM is present in the initial tri-layer structure, large-scale PCBM aggregates form after long annealing times. The aggregates grow rapidly out of the original plane of the film by uptake of PCBM from the surrounding area, resulting in a large PCBM- depleted region in their immediate vicinity.
Citation: ACS Nano
Pub Type: Journals
TEM, EF-TEM, organic photovoltaics, microanalysis, P3HT, PCBM