Single-walled carbon nanotube (SWCNT) semiconductors are well suited for solar photovoltaic systems because they can potentially convert sunlight to electricity or fuels without losing much energy, finds the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).
NREL says its research builds on the Nobel Prize-winning work of Rudolph Marcus, who developed a fundamental tenet of physical chemistry that explains the rate at which an electron can move from one chemical to another. The Marcus formulation, however, has rarely been used to study photoinduced electron transfer for emerging organic semiconductors such as SWCNT that can be used in organic PV devices.
In organic PV devices, after a photon is absorbed, charges (electrons and holes) generally need to be separated across an interface so that they can live long enough to be collected as electrical current, explains NREL. The electron transfer event that produces these separated charges comes with a potential energy loss as the molecules involved have to structurally reorganize their bonds. This loss is called reorganization energy, but NREL researchers have found little energy was lost when pairing SWCNT semiconductors with fullerene molecules.
“What we find in our study is this particular system – nanotubes with fullerenes – have an exceptionally low reorganization energy, and the nanotubes themselves probably have very, very low reorganization energy,” says Jeffrey Blackburn, a senior scientist at NREL and co-author of the paper titled, “Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions.” The paper appears in the new issue of the journal Nature Chemistry.
Organic PV devices involve an interface between a donor and an acceptor. In this case, NREL says the SWCNT served as the donor, as it donated an electron to the acceptor (here, the fullerene). The NREL researchers strategically partnered with colleagues at Colorado State University (CSU) to take advantage of expertise at each institution in producing donors and acceptors with well-defined and highly tunable energy levels: semiconducting SWCNT donors at NREL and fullerene acceptors at CSU.
This partnership enabled NREL’s scientists to determine that the electron transfer event didn’t come with a large energy loss associated with reorganization, meaning solar energy can be harvested more efficiently. For this reason, NREL says, SWCNT semiconductors could be favorable for PV applications.
Funds for the research came from the Energy Department’s Office of Science.