Getting Around the Current-Voltage Trade Off in Organic Solar Cells
Department of Chemistry, University of Southern California
I will discuss a two approaches to get around the current-voltage trade-off typically observed for organic photovoltaic devices. The first approach is to use singlet fission to double the number of excitons generated from a single photon. We have prepared a number of chromophore dimers that show high efficiency (> 150% yield of triplets from one singlet) and very rapid rates of singlet fission. I will discuss the chemistry and physics of these interesting dimeric materials.
A key limitation of OPVs are their low open circuit voltages (VOC), which are typically well below the energy of the light absorbed. We achieve VOC values closer to the theoretical limits, using materials that involve symmetry breaking charge transfer (SBCT). These materials are symmetric molecules that spontaneously form an intramolecular charge transfer complex, with nearly complete one electron transfer from one part of the molecule to another. This minimizes the energy offset needed at the donor/acceptor interface to drive charge transfer and charge separation. Incorporating the SBCT process into an OPV pushes the VOC close to the theoretical limits. We have characterized the SBCT process in a family of different materials, with high potential in OPVs. The kinetics of formation of the SBCT process have been characterized by ultrafast spectroscopic techniques. We have explored these materials as both donors and acceptors in OPVs and found that they give good performance and high Voc.