Recent advances in ultrathin films have made solar cells more efficient, and now scientists have discovered a way to also make them easier to manufacture.

Solar energy-converting films typically used by organic bulk heterojunction solar cells (BHJs) are created by mixing together conjugated polymers and "buckyballs" that are then spin cast on a rotating substrate to ensure uniformity, and heated and cooled to reduce hardness and increase toughness, Oak Ridge National Laboratory reported. The pliability of this material makes it more desirable than expensive crystalline silicon, but the process of heating and cooling (annealing) is extremely time consuming. New research suggests a simple solvent could solve the problem by replacing annealing.

"Optimizing a film's morphology is the key to improving device performance," said postdoctoral researcher Nuradhika Herath. "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 answering why certain film morphologies are more favorable than others."

The research team compared thermal annealing with a method that uses a solvent to aid in dissolving the buckyballs and keeping the structure uniform. If the mixture is not uniform, clusters form that absorb passing electrons and weakens the film's ability to transport electric currents, decreasing performance. The observations revealed annealing created a significant separation between the polymers and buckyballs, while the solvent additive led to more uniformity and better performance.

"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," said 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."

The findings were published in a recent edition of the journal Scientific Reports.