Solar cells based on block copolymers, self-assembling organic materials that arrange themselves in distinct layers, could pave the way for a new class of solar energy devices. The photovoltaic devices created in collaboration at Rice and Pennsylvania State universities could easily outperform other cells with polymer compounds as active elements, reaching about 3 percent efficiency — surprisingly better than other labs have achieved using such compounds, the researchers say. Commercial, silicon-based solar cells turn about 20 percent of sunlight into electricity and experimental units top 25 percent. An undercurrent of research into polymer-based cells could greatly reduce the cost of solar energy, said Rafael Verduzco, a chemical engineer at Rice. “You need two components in a solar cell: one to carry (negative) electrons, the other to carry positive charges,” Verduzco said. The imbalance between the two prompted by the input of energy — sunlight — creates useful current. Since the mid-1980s, scientists have experimented with stacking or mixing polymer components with limited success, he said. Later polymer/fullerene mixtures topped 10 percent efficiency, but the fullerenes — in this case, enhanced C-60 Buckyballs — are difficult to work with. Rice lab discovered a block copolymer — P3HT-b-PFTBT, which was created in the presence of a glass/indium tin oxide (ITO) top layer at 165 ºC — that separates into bands that are about 16 nm wide and whose natural tendency is to form bands perpendicular to the glass. Researchers at Rice and Pennsylvania State universities have created solar cells based on block copolymers, self-assembling organic materials that arrange themselves into distinct layers. Courtesy of Verduzco Laboratory/Rice University. With a layer of aluminum on the other side of the device constructed by the Penn State team, the polymer bands stretched from the top to bottom electrodes and provided a clear path for electrons to flow. “On paper, block copolymers are excellent candidates for organic solar cells, but no one has been able to get very good photovoltaic performance using block copolymers,” Verduzco said. “We didn’t give up on the idea of block copolymers because there’s really only been a handful of these types of solar cells previously tested. We thought getting good performance using block copolymers was possible if we designed the right materials and fabricated the solar cells under the right conditions.” Mysteries remain, he said. “It’s not clear why the copolymer organizes itself perpendicular to the electrodes. Our hypothesis is that both polymers want to be in contact with the ITO-coated glass. We think that forces this orientation, though we haven’t proven it yet.” The investigators now hope to experiment with other block copolymers and learn how to control their structures to increase the solar cell’s ability to capture photons and turn them into electricity. Once higher performance from the cells is achieved, the scientists will look at long-term use. “We’ll focus on performance first, because if we can’t get it high enough, there’s no reason to address some of the other challenges like stability,” Verduzco said. Encapsulating a solar cell to keep air and water from degrading it is easy, he said, but protecting it from UV degradation over time is hard. “You have to expose it to sunlight,” he said. “That you can’t avoid.” Researchers from Argonne and Lawrence Berkeley national laboratories also contributed to the study, which appeared in Nano Letters (doi: 10.1021/nl401420s). For more information, visit: www.rice.edu or www.psu.edu
Photosensitizers