Crystallinity dictates the selection of fullerene or non-fullerene acceptors in a small molecule organic solar cell

The side chains of a molecule affect the molecular packing and crystallinity which are crucial factors determining the photovoltaic properties in organic solar cells (OSCs). In this work, two small molecule donors, BDT(X1) and BDT(X2), with identical backbone and slightly different side chains on the end groups have been designed and synthesized. Compared with BDT(X1) with linear alkyl chain, BDT(X2) featuring a branched alkyl chain in the terminal groups has larger steric hindrance and exhibits weaker crystallinity and less compact molecule stacking. Application of the two small molecule donors in OSCs shows that BDT(X1) is more suitable for IDIC, a nonfullerene acceptor, while BDT(X2) demonstrates higher OSC performance when blended with fullerene acceptor PC71 BM. Morphology and grazing-incidence wide-angle X-ray scattering (GIWAXS) studies reveal that crystallinity plays a key role in determining the molecular packing and phase separation with fullerene/non-fullerene acceptors which in turn influence the charge transport and photovoltaic properties. Our results provide some guidance on selection of acceptors for a given donor and demonstrate the importance of fine tuning of the crystallinity in small molecule OSCs.

Automated summary

The side chains of a molecule significantly impact the photovoltaic properties of organic solar cells, with differences in performance observed when different small molecule donors are blended with specific acceptors. Crystallinity plays a key role in determining molecular stacking, phase separation, charge transport, and overall photovoltaic properties in small molecule OSCs.