Donor/acceptor interfacial structure from ¹³C {²H} REDOR NMR

Robust relationships between structure and function are generally lacking in organic photovoltaic (OPV) thin film active layers. To predict performance there exists a need for tools that can measure structure on length scales fine enough to be relatable to inter-molecular energy transfer. Electron microscopy lacks sufficient spatial resolution due to a lack of electron density contrast, and scattering curves can be ambiguous because there typically is not a unique fitting model. In this talk, I will give highlights of recent ¹³C {²H} REDOR measurements to characterize the donor/acceptor interfaces in bulk heterojunction thin films. Heteronuclear couplings are measured between ¹³C nuclei on the acceptor C₆₀ cage and thiophene hydrogens on the donor main chain, which has been isotopically enriched with ²H. I will discuss models of the interface that are used to fit the REDOR dephasing curve, and the constraints that these models have on local composition and packing. We will also show that the REDOR measurements can help solve the mystery of which model to use in fitting small angle neutron scattering curves.