Bulk heterojunction organic solar cells that are highly organized at the nanoscale are no more efficient at creating free electrons than cells with poorly organized structures. That is the finding of a research team led by North Carolina State University, which gauged cell efficiency using ultraviolet-visible absorption spectroscopy and grazing incidence x-ray diffraction. In previous organic solar cell research, there was ambiguity about whether differences in efficiency were due to exciton dissociation or charge collection because there was no clear method for distinguishing between the two. To address these questions, the North Carolina researchers developed a method that takes advantage of the polarization of absorbed light to compare ordered and disordered portions of the same cell. “In practical terms, this technique will help distinguish efficiency losses of newly developed materials, helping define which material and nanostructure features are needed to advance organic solar cell technology,” said professor Dr. Brendan O’Connor. The free-electron generation efficiency of ordered and disordered organic solar cells was compared by varying the polarization of absorbed light. Courtesy of North Carolina State University. Organic solar cells convert light into electric current in four steps, the researchers said. First, electrons are excited in the cell’s active layer when it absorbs sunlight. Each excited electron leaves behind a hole in the active layer. The electron and hole are collectively called an exciton. Next, the exciton hops around until it encounters an interface with another organic material in the active layer, a process called diffusion. Third, the exciton breaks apart, or dissociates, freeing the electron and hole. Finally, the free electron makes its way through the active layer to a point where it can be harvested. “We found that there was no relationship between dissociation efficiency and structural organization,” O’Connor said. “It was really a surprise, and it tells us that we don’t need highly ordered nanostructures for efficient free-electron generation.” The research was published in Advanced Functional Materials (doi: 10.1002/adfm.201403377). For more information, visit www.ncsu.edu.