Solid-state NMR Studies of Non-Fullerene Acceptor-Based Organic Photovoltaic Active Layers*

The efficiency of organic photovoltaics has undergone remarkable increases in power conversion efficiencies (PCEs) recently achieving values in excess of 17% due, primarily, to the advent of non-fullerene acceptors (NFAs). To further increase PCEs, understanding and manipulating the morphology over broad range of length scales is necessary. The similarity in the chemical composition of the donors and NFAs limit current methods based on electron densities or refractive indices in discerning details of the morphology. To fill this gap we have used solid-state NMR (ssNMR) where donor and acceptor nuclei have distinct relaxation times that can be used to characterize the crystallinity, crystal size and state of mixing. Here, we present results on several donors and NFA combination where characteristics of the morphology were obtained. Donor and NFA blend film and their pristine films were studied by ssNMR. For PTB7:ITIC, the crystalline forms of ITIC show sensitive dependence on processing conditions. Furthermore, T₁ and T_1ρ relaxation times were used to elucidate the phase separation behavior. For PM6:Y6, spin diffusion experiments based on the contrast between the T_1ρ relaxation times of PM6 and Y6 are able to detect phase separation at a length scale of <15 nm.