Mimicking the highly efficient light-harvesting structures of blades of grass could boost power-conversion efficiency in solar panels. “For decades, scientists and engineers have placed great effort in trying to control the morphology of p-n junction interfaces in organic solar cells,” said Dr. Alejandro Briseno of the University of Massachusetts Amherst. “We … have at last developed the ideal architecture composed of organic single-crystal vertical nanopillars.” Nanopillars have shown to be particularly effective at converting light to energy more efficiently. They resemble blades of grass in their high-density array system and vertical orientations. Vertical nanopillars may help boost solar power-conversion efficiency for electronic devices. Courtesy of UMass Amherst. Using single crystalline organic nanopillars, the researchers have demonstrated how to avoid discontinuous pathways, which is challenging when using blended systems such as bulk heterojunction donor-acceptor, p-n junctions for harvesting energy in organic solar cells. This advance not only addresses the problem of discontinuous pathways that make for inefficient energy transfer, the researchers said, it also solves instability problems where the materials in mixed blends of polymers tend to lose their phase-separated behavior over time. The researchers noted that materials in blended systems tend to be amorphous to semi-crystalline at best and “this is a disadvantage, since charge transport is more efficient in highly crystalline systems.” The team controlled the molecular orientation and packing on electrode surfaces, and got the necessary compounds to stack like coins. This configuration has the largest charge transport anisotropy, a phenomenon where electrons flow faster along a particular crystallographic direction due to close molecule-molecule interactions. In this case, the anisotropy is along the nanopillar, perpendicular to the substrate. Charge separation/collection is most efficient when perpendicular. “The biggest challenge in producing this architecture was finding the appropriate substrate that would enable the molecules to stack vertically,” Briseno said. “We had exploited essentially every substrate possible until we finally succeeded with graphene.” This new technique can be applicable to numerous donor and acceptor compounds that are commercially available. “We envision that our nanopillar solar cells will appeal to low-end energy applications such as gadgets, toys, sensors and short-lifetime disposable devices,” Briseno said. The research was published in Nano Letters (doi: 10.1021/nl501933q). For more information, visit www.umass.edu.