While organic solar cells are inexpensive, and a versatile alternative to inorganic solar cells, they also feature low efficiencies and limited lifetimes that currently preclude them from commercial use.
However, researchers from the Argonne-Northwestern Solar Energy Research Center (ANSER), a collaboration between Northwestern University and the U.S. Department of Energy’s (DOE) Argonne National Laboratory, have examined how the molecular structures of organic solar cells form, to possibly improve their performance and bring them closer to practical adoption.
The researchers used Argonne’s Advanced Photon Source (APS), a DOE Office of Science User Facility, to analyse how the crystal structures of organic solar cells develop, as they are produced under various conditions.
The focus of the study was the photoactive layer of the cell, built from thin films that absorb energy from sunlight, and then convert the energy into electric current. The team then produced the films with spin coating—a widely-used process for film fabrication in the laboratory.
The scientists dropped the material, dissolved in a solvent, on a spinning surface to cause it to spread into a thin, uniform sheet. They mounted the spin-coater at an X-ray beamline, at the APS, and watched the film’s crystal structure evolve in real time.
The researchers took advantage of a specific in-situ method called grazing-incidence wide-angle X-ray scattering (GIWAXS) to collect the X-ray diffraction data.
“It was the stability and reproducibility of this specific spin-coating setup that allowed this study to happen,” Northwestern graduate student Eric Manley, the first author of the study, said in a statement.
The researchers discovered how certain additives could significantly affect both the time it takes for the film’s structure to stop changing, and the intermediate structures the film adopts during evolution. The team found that even after the solvent dissolves, the structures can continue to change for anywhere from seconds to hours, depending on what additives are present.
The films produced more slowly with additives generally performed better than the more rapidly-formed films.
“Producers of solar cells will often go to the next step in production quickly after spin coating, which has the potential to lock the morphology while the structure is still forming,” Manley said. “This can significantly affect the cell’s performance, positively and negatively.
“We discovered that we needed to report the time between fabrication steps, to control conditions to reproduce optimised results.”
The next step will be to study structures that are more complex and examine how different choices can optimise performance.
“We hope this will pave the way to making these cells more viable for everyday applications,” Joseph Strzalka, a physicist and member of the Time-Resolved Research group within Argonne’s X-Ray Sciences division, said in a statement.
