The Effect of Diiodooctane on the Charge Carrier Generation in Organic Solar Cells Based on the Copolymer PBDTTT-C

Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network

Large-scale alignment of ABC block copolymer microdomains via solvent vapor treatment

We have studied the microdomain morphology of thin ABC triblock copolymer films supported by a solid substrate. The films were exposed to various solvent vapors, and the effect of the solvent removal speed on the resulting morphologies is investigated. Slow solvent extraction rates lead to a parallel alignment of lamellar microphases within the plane of the film. On fast drying, a perpendicular orientation of the lamellae is found. In the case of block copolymer samples with a highly anisotropic macroscopic shape, the microdomains can be aligned over large lateral areas. The results are discussed in view of the mechanical strain fields present during the drying process

Volume imaging of ultrathin SBS triblock copolymer films

We use scanning force microscopy and stepwise erosion in a radio-frequency (rf) plasma to obtain a three-dimensional volume image of an ultrathin poly(styrene-b-butadiene-b-styrene) triblock copolymer film. The free surface of the film exhibits domains of two distinctly different structures, i.e., hexagonal arrays of isolated dots and parallel stripes. A common forehand interpretation relates these surface morphologies to a cylindrical microdomain morphology with the cylinders oriented perpendicularly and parallel to the plane of the film, respectively. In contrast, our experiments show that the “dots” merge into parallel cylinders at some distance underneath the film surface. These results demonstrate that care has to be taken when the thin film morphology of block copolymers is inferred from their surface structure

Thin polymer films on chemically patterned, corrugated substrates

We study the effect of a chemical pattern on the wetting and dewetting behaviour of thin polystyrene (PS) films on regularly corrugated silicon substrates. Our results reveal that the film preparation, annealing method, and confinement play a critical role in the final film structure. On evaporating gold on both sides of the facets (such that it covered the crests of the facets, and not the troughs), we observed dewetting, which proceeded to the gold, demonstrating an enthalpic effect contrary to the outcome previously observed when gold was only evaporated on one side of the facet. We also coated the substrate with octadecyltrichlorosilane (OTS); this led to a gold and OTS striped structure. PS films several nanometres thick dewet such substrates, with a preferential direction for dewetting in the direction of the stripes forming droplets of a considerably larger size than the stripes