Inner alkyl chain modulation of small molecular acceptors enables molecular packing optimization and efficient organic solar cells

With the generation of Y6, organic solar cells have reached remarkable achievement of over 19% efficiency. Alkyl chain is of importance to modulate intermolecular stacking and possibly enhance optoelectronic properties of small molecule acceptors (SMAs). Three alkyl chains of 2-ethylhexyl, 2-butylocyl and 3-ethylheptyl were selected to obtain G6-EH, G6-BO and G6-EHep molecules, respectively. Compared to G6-EH and G6-BO, G6-EHep was found inducing unfavourable large domain size. Furthermore, we discover that 2-butyloctyl effectively inhibits monomolecular and bimolecular recombination, improves molecular packing, generates more balanced carrier mobility and enhances exciton dissociation. The SMA with 2-butyloctyl alkyl chains (G6-BO) shows the best electrical and morphological characteristics, achieving a higher power conversion efficiency (PCE) of 17.06%, with an open circuit voltage of 0.912 V, a short-circuit current of 24.22 mA cm⁻² and a fill factor of 77.25%. Finally, using the ternary strategy by incorporating the G6-BO acceptor into PM6:BTP-eC9, we achieved a higher PCE of 18.13% with enhanced electron transport. [Figure not available: see fulltext.].