Delineation of Thermodynamic and Kinetic Factors that Control Stability in Non-fullerene Organic Solar Cells

Although non-fullerene small molecular acceptors (NF-SMAs) are dominating current research in organic solar cells (OSCs), measurements of thermodynamics drivers and kinetic factors determining their morphological stability are lacking. Here, we delineate and measure such factors in crystallizable NF-SMA blends and discuss four model systems with respect to their meta-stability and degree of vitrification. We determine for the first time the amorphous-amorphous phase diagram in an NF-SMA system and show that its deep quench depth can result in severe burn-in degradation. We estimate the relative phase behavior of four other materials systems. Additionally, we derive room-temperature diffusion coefficients and conclude that the morphology needs to be stabilized by vitrification corresponding to diffusion constants below 10−22 cm2/s. Our results show that to achieve stability via rational molecular design, the thermodynamics, glass transition temperature, diffusion properties, and related structure-function relations need to be more extensively studied and understood. In recent years, the performance of organic solar cells (OSCs) has greatly improved with the development of novel non-fullerene small molecular acceptors (NF-SMA). The rapid increase in power conversion efficiency, now surpassing 15%, highlights an immediate and increasing need to understand the longevity and lifetime of NF-OSCs. However, the field relies mainly on a laborious trial-and-error approach to select polymer:NF-SMA pairs with desirable device stability. Here, we provide a structure-property relation that explains the morphological stability and burn-in degradation due to excessive demixing or crystallization. The framework presented in our study shows that a specific balance of interactions between polymer and NF-SMA can offer a short-term solution against excessive demixing. Long-term morphological stability that also suppresses crystallization can only be achieved by freezing in the initial quenched morphology through the use of polymers and/or NF-SMAs with low flexibility. This research provides a structure-property relation that sheds light on morphological stability of NF-OSCs by using the thermodynamic and the kinetic perspectives. We show that NF-OSCs can suffer from excessive amorphous-amorphous phase separation in the blends and crystallization of NF-SMA. The former instability channel can be eliminated in systems with an optimal miscibility, whereas the excessive phase separation in low miscibility systems and NF-SMA crystallization need to be suppressed through the utilization of polymers or NF-SMAs with low flexibility.