Researchers at Oak Ridge National Laboratory, Department of Energy have discovered a method that makes solar panels more cost-effective and efficient. They used a simple solvent. Researchers explained that ultra thin films for solar panels and other semiconductor devices are now possible to make more quickly. The films, which are used in BHJs (organic Bulk Heterjunction) to convert sunlight into electric power, are typically made by mixing conjugated polymers with fullerenes. These carbon molecules are shaped like buckyballs. Scientists discussed their results in Scientific Reports (citation above). This schematic illustrates the morphology and properties of a BHJ Film with solvent additives. Fullerenes are small, silver spheres that appear in a network made up of many polymers. Image: ornl.gov. Next, the mixture is spun cast onto a rotating substrate. It is then sent for post-processing. The heat is applied to allow it to cool down slowly to reduce internal stress and to harden it (annealed). The material is made harder by annealing, which makes it less difficult, but also more durable. It is easier to work in. A simple solvent can transform the process. BHJs will be more flexible than crystallized silicon. The annealing process can be time-consuming. Nuradhika Herath (postdoctoral researcher) and her colleagues believe that thermal annealing may be obsolete if a simple solvent is used. Herath stated that optimizing a film’s structure is key to better device performance. The scientists stated that they are interested in the relationships between blend structures and solar performance. They also want to know how to adjust film morphologies to make them more appealing than others. Herath’s team also compared thermal-annealing to a process that only a tiny amount of solvent was used. This solvent helps dissolve the fullerenes and makes the structure of the film more consistent. It is important to have a uniform mix of light absorbing molecules and polymers in the film. A mixture that is less homogeneous will result in more clusters, which can cause electrons to be absorbed. This reduces the film’s ability to carry electrical current and decreases its performance. The films typically measure 100 nanometers thick, while human hair measures 75,000 micrometers. Because of this complexity and because the thickness profile of the materials is complex, scientists require special tools to determine the material’s morphology. They used neutron scattering to do this. The researchers mixed and spun cast two samples, one with solvent additive and one annealed. They then put the films under the microscope of a Magnetism Reflectometer. They were able to see the exact structural patterns of both films using the Magnetism Reflectometer. They were clearly different. The film performed better when containing solvent. While the sample that contained the solvent additive performed much better and was more consistent, the annealed specimen’s morphology showed significant separation of fullerenes from polymers. Valeria Lauter (MR Lead Instrument Scientist) stated that the solvent is used instead of annealing because the sample dries slowly so the system can become optimized. Lauter said that extra annealing was not required because the system, in a way, is already perfect. Neutron reflectometry makes any material transparent and is a very powerful tool. Lauter said, “Instead searching for the key to unlock the metaphorical blackbox that blocks researchers from seeing a materials atomic structure,” neutrons just go through the material, providing researchers with both quantitative and qualitative information. This information will increase the efficiency of solar cells as well as streamline the manufacturing process. BHJ Film morphology optimization can be achieved by adding solvent. This will save money, time and resources. Herath stated that optimization of photovoltaic characteristics provides valuable information for the manufacture and performance control of solar cells. These findings will aid in developing ‘ideal’ photovoltaics, which gets us one step closer to producing commercialized devices.” Citation: “Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells,” Nuradhika Herath, Sanjib Das, Jong K. Keum, Jiahua Zhu, Rajeev Kumar, Ilia N. Ivanov, Bobby G. Sumpter, James F. Browning, Kai Xiao, Gong Gu, Pooran Joshi, Sean Smith & Valeria Lauter. Scientific Reports 5, Article Number: (*_ ). DOI: 10. 1038/srep13407.
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