  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.
