Homo- and Heteromolecular interactions and Their Relevance for Morphological and Mechanical Stability of Non-Fullerene Organic Solar Cells

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy in high energy-per-weight or semi-transparent applications. Recently, the OSC field has been revolutionized through synthesis and processing advances, primarily through the development of numerous novel non-fullerene small molecular acceptors (NFA) with efficiencies now reaching &gt;19% when paired with suitable donor polymers. The device stability and mechanical durability of these non-fullerene OSCs is critical and developing devices with high performance, long-term morphological stability, and mechanical robustness remains challenging. Yet, morphological and mechanical stability is a prerequisite for OSC commercialization. Here, we discuss our current understanding of the phase behavior of OSC donor:acceptor mixtures and the relation of phase behavior and the underlying hetero- and homo-molecule interactions to performance, processing needs (e.g., kinetic quenches), and morphological and mechanical stability. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to x-ray scattering to determine critical morphology parameters and molecule packing and dynamic mechanical analysis (DMA) to assess specifically the hetero-interactions. The results presented and its ongoing evolution are intended to uncover fundamental molecular structure-function relationships that would allow predictive guidance on how desired properties can be targeted by specific chemical design. Comparative studies show that the molecular hetero-interactions between the donor and NFA are not always the geometric mean of the homo-interactions. The presentation will focus on these heterointeractions and the results underscores the limited success often encountered when Hanson Solubility Parameters and surface energies are used to estimate molecular interactions [1]. Additional insights into the molecular interactions are also provided and the relevance discussed in rubber-toughening of OSCs with a SEBS additive [2].<br/><br/>References.<br/>1. “Molecular interactions that drive morphological and mechanical stabilities in organic solar cells” S Siddika, Z Peng, N Balar, X Dong, X Zhong, W You, H Ade and BT O’Connor, Joule <b>7</b>, 1593 (2023)<br/>2. “Rubber-Toughened Organic Solar Cells: Miscibility–Morphology–Performance Relations” AA Shafe, HM Schrickx, K Ding, H Ade, and BT O’Connor, ACS Energy Letters <b>8</b>, 3720-3726 (2023)