| 24 June 2012
Lin X. Chen at the US Department of Energy’s (DOE) Argonne National Laboratory is taking a fresh approach to improving organic solar cells by investigating the exciton, a “quasi-particle” few other researchers have focused on in the quest to help this promising yet seemingly lab-bound technology break out into the real world.
“My collaborators and I have learned that organic photovoltaic devices and detailed structures of the materials at the electron and atomic levels are really more closely correlated than people realized before,” says the Argonne senior chemist, who also is a professor of chemistry at Northwestern University in Evanston, Illinois (US). “Organic photovoltaic materials have been viewed as organic semiconductors in the past. Our studies revealed that they also have molecular-like properties that can dominate the device performance.”
Her discovery’s potential impact on future solar technologies lays in the design of new conjugated polymer materials, she says. ”We now know new rules for optimizing the device performance from building blocks of the materials, which will make the light-to-charge-carriers conversion easier and more efficient.”
“Like other researchers, we examined the multiple components in the materials, namely electron donors and acceptors corresponding to polymers and fullerene derivatives,” Prof. Chen explains. “The former are long chain, spaghetti-like molecules and the latter are tiny balls. The two mix together inside the active layer of the organic solar cell, similar to spaghetti and meatballs on a microscopic scale.” The excitons that are generated when photons strike this film need to split into holes and electrons to generate electricity (rather than heat). The closer the hole and the electron regions are inside an exciton, the more likely they are to recombine without generating electricity. However, if they are polarized, they are more likely to escape from this potential trap and become effective charge carriers.
Previous studies have shown that these excitons would only split at the boundaries of electron donor and acceptor—the spaghetti and meatballs—leaving the holes to the polymers and electrons to fullerene. The holes and electrons would travel inside the film and eventually be collected respectively by the cathode and anode to provide electricity. “We discovered that excitons could split inside a polymer chain without the fullerene if the polymer chain is constructed from alternating electron donor-like and electron acceptor-like blocks in a sequence,” Prof. Chen says. “When such polymers absorb photons, excitons are formed across adjacent blocks with displaced hole and electron positions, even if they are not yet split. Because the attractive Coulombic force between a hole and electron is inversely proportional to their separation distance, the excitons in alternating polymers are easier to split, and hence can enhance the device efficiency.”
What’s more, Prof. Chen and her team discovered they can “chemically tune the extend of electron donor/acceptor likeness in alternating blocks by attaching electron withdrawing/donating groups to the polymer backbone and, hence, determine how much separation of the hole and electron inside the excitons.” They also found that if the exciton is not polar enough, the exciton will be less likely to be split and more likely to be trapped without generating charge carriers for electricity generation. “We discovered that the device efficiency could be linearly correlated with the dipole moment change in each repeating unit with the alternating blocks,” Prof Chen says. “This discovery leads to a new design rule that could improve the material properties for high-efficiency organic solar cells. However, one needs to be award that the exciton splitting is not the only factor that determines the device performance. The whole thing is rather complicated. One efficiency bottleneck is removed, other ones may appear.”
Finding the reasons behind the observed phenomena—especially those unexpected, such as exciton splitting in a single polymer chain without the presence of electron acceptor—remains the biggest challenge in Prof. Chen’s research work. “We still do not completely understand everything that we need to know, but our discovery advances our understanding in making high performing organic solar cell materials,” she says.
Written by Sandra Henderson, Research Editor, Solar Novus Today
