Fullerenes appear as small silver spheres spread consistently throughout a network of small molecules, or polymers, in this schematic illustration of the morphology of a BHJ film with solvent additives. Image: Oak Ridge National Laboratory.
Fullerenes appear as small silver spheres spread consistently throughout a network of small molecules, or polymers, in this schematic illustration of the morphology of a BHJ film with solvent additives. Image: Oak Ridge National Laboratory.

Advances in ultrathin films have made solar panels and semiconductor devices more efficient and less costly. Now, in a paper in Scientific Reports, researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL) say they've found a way to manufacture the films more easily, too.

Typically, the films, which are used by organic bulk heterojunction (BHJ) solar cells to convert solar energy into electricity, are created in solution by mixing together conjugated polymers and fullerenes, soccer ball-like carbon molecules also known as buckyballs. Next, the mixture is spin cast on a rotating substrate to ensure uniformity, then sent to post-processing to be annealed. Annealing the material – heating then cooling it – reduces the material's hardness while increasing its toughness, making it easier to work with.

This pliability makes BHJs more appealing than their more costly crystalline silicon counterparts, but the annealing process is time consuming. ORNL researchers have now discovered that a simple solvent may make thermal annealing a thing of the past.

In a collaboration between ORNL's Spallation Neutron Source (SNS) and the Center for Nanophase Materials Sciences (CNMS), both DOE Office of Science User Facilities, postdoctoral researcher Nuradhika Herath led a team of neutron and materials scientists in a study of the morphology, or structure, of BHJ films.

"Optimizing a film's morphology is the key to improving device performance," Herath said. "What we want to find out is the relationship between the blend structures and photovoltaic performance." Finding ways to tune the film's morphology is as important as working out why certain film morphologies are more favorable than others, she added.

Researchers compared thermal annealing with a method that adds a small amount of solvent that aids in dissolving the fullerenes within the blend and helps to make the film's structure more uniform. The idea is to get the most uniform mixture of light-absorbing molecules (e.g. polymers or other molecules) and fullerenes throughout the film. If the mixture is not uniform, clusters form that cause passing electrons to be absorbed, weakening the film's ability to transport electrical current and so decreasing device performance.

Because the films are typically about 100nm thick and their chemical composition is highly complex, special instruments are needed to measure the material's morphology. For this, the researchers turned to neutron scattering.

After mixing and spin casting two different samples at CNMS – one annealed, the other with the solvent additive – the team put both films under the eye of SNS's Magnetism Reflectometer (MR). The MR provided them with an accurate depiction of the structural profiles, which revealed exactly how the polymers and fullerenes were arranging themselves throughout both films. The difference between them was evident.

Whereas the annealed sample's morphology clearly showed significant separation between the polymers and fullerenes, the sample containing the solvent additive was remarkably consistent throughout and performed better.

"The reason is that when we use a solvent instead of annealing, the sample dries very slowly, so there is enough time for the system to become fully optimized," explained MR lead instrument scientist Valeria Lauter. "We see that additional annealing is not necessary because, in a sense, the system is already as perfect as it can be."

Neutron reflectometry is a powerful analytical method because it effectively makes many materials transparent, Lauter explained. Instead of searching for the key that opens the metaphorical black box that prevents researchers from seeing a material's atomic structure, she says, neutrons simply go straight through it, providing researchers with both qualitative and quantitative information about their problem.

Not only will the information obtained from neutron reflectometry increase the efficiency of the solar cells' performance, but it will also help streamline the process of manufacturing them. Using solvent additives to optimize the morphology of BHJ films could help to save time, money and resources by doing away with the annealing step.

"In addition, optimization of photovoltaic properties provides information to manufacture solar cells with fully controlled morphology and device performance," Herath said. "These findings will aid in developing 'ideal' photovoltaics, which gets us one step closer to producing commercialized devices."

This story is adapted from material from ORNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.