Can electron superhighway drives organic solar panel innovations?
The discovery, reported in the journal Nature Communications, will help progress the drive for alternatives to traditional silicon-based electronics.
A new generation of organic semiconductors may allow these kinds of flexible electronics to be manufactured at low cost, claimed University of Vermont physicist and materials scientist Madalina Furis.
Many types of flexible electronic devices will rely on thin films of organic materials that catch sunlight and convert the light into electric current using excited states in the material called ‘excitons’. Roughly speaking, an exciton is a displaced electron bound together with the hole it left behind. Increasing the distance these excitons can diffuse — before they reach a juncture where they are broken apart to produce electrical current — is essential to improving the efficiency of organic semiconductors.
Using a new imaging technique, the UVM team was able to observe nanoscale defects and boundaries in the crystal grains in the thin films of phthalocyanine — roadblocks in the electron highway. “We have discovered that we have hills that electrons have to go over and potholes that they need to avoid,” explained Furis.
To find these defects, the UVM team built a scanning laser microscope. The instrument combines a specialized form of linearly polarized light and photoluminescence to optically probe the molecular structure of the phthalocyanine crystals.
“Marrying these two techniques together is new; it has never been reported anywhere,” claimed Lane Manning ’08 a doctoral student in Furis’ lab and co-author on the new study.
The technique allows the scientists a deeper understanding of how the arrangement of molecules and the boundaries in the crystals influence the movement of excitons. It is these boundaries that form a “barrier for exciton diffusion” describes the team.
The researchers say “this energy barrier can be entirely eliminated”. The trick involves carefully controlling how the thin films are deposited. Using a novel’pen-writing’ technique with a hollow capillary, the team worked in the lab of UVM physics and materials science professor Randy Headrick to form films with jumbo-sized crystal grains and “small angle boundaries”.
Although the study focused on one organic material, phthalocyanine, the research provides a way to explore many other types of organic materials, too — with particular promise for improved solar cells. A recent U.S. Department of Energy report identified one of the fundamental bottlenecks to improved solar power technologies as “determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity.”
The UVM study — led by two of Furis’ students, Zhenwen Pan G’12, and Naveen Rawat G’15 — opens a window to view how increasing ‘long-range order’ in the organic semiconductor films is a key mechanism that allows excitons to migrate farther. “The molecules are stacked like dishes in a dish rack,” explained Furis, “these stacked molecules — this dish rack — is the electron superhighway.”
Though excitons are neutrally charged — and cannot be pushed by voltage like the electrons flowing in a light bulb — they can, in a sense, bounce from one of these tightly stacked molecules to the next. This allows organic thin films to carry energy along this molecular highway with relative ease, though no net electrical charge is transported.
“One of today’s big challenges is how to make better photovoltaics and solar technologies,” said Furis, who directs UVM’s program in materials science, “and to do that we need a deeper understanding of exciton diffusion. That’s what this research is about.”
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