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. Credit: ORNL.
(October 19, 2015) Advances
in ultrathin films have made solar panels and semiconductor devices more
efficient and less costly, and researchers at the Department of Energy’s Oak
Ridge National Laboratory say they’ve found a way to manufacture the films more
easily, too.
Typically the films—used by organic bulk heterojunction
solar cells, or BHJs, to convert solar energy into electricity—are created in a
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, which makes it easier to work with.
Pliability makes BHJs more appealing than their more costly
crystalline silicon counterparts, but the annealing process is time consuming.
Now ORNL researchers say 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 answering 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 and cause passing electrons
to get absorbed, weakening the film’s ability to transport electrical current,
which in turn decreases device performance.