Understanding the Role of Non-Fullerene Acceptors Crystallinity on the Charge Transport Properties and Performance of Organic Solar Cells

The active layer crystallinity has long been associated with favourable organic solar cells (OSCs) properties such as high mobility and Fill Factor. In particular, this applies to acceptor materials such as fullerene-derivatives and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing 19% of Power Conversion Efficiency. Despite these advantages are being commonly attributed to their 3-dimensional crystal packing motif in the single crystal, the bridge that links the acceptor crystal packing from single crystals to solar cells has not clearly been shown yet. In this work, we investigate the molecular organisation of seven NFAs (o-IDTBR, IDIC, ITIC, m-ITIC, 4TIC, 4TICO, m-4TICO), following the evolution of their packing motif in single-crystals, powder and thin films made with pure NFAs and donor:NFA blends. In general, we observed a good correlation between the NFA single crystal packing and their molecular arrangement in the bulk heterojunction. However, the NFA packing motif is not directly affecting the device parameters but it provide an impact on the material propensity to form highly crystalline domain in the blend. Although that NFA crystallinity is required to obtain high mobility, the domain purity is more important to limit the bimolecular recombination and to obtain high efficiency organic solar cells.