Replacing alkyl side chain of non-fullerene acceptor with siloxane-terminated side chain enables lower surface energy towards optimizing bulk-heterojunction morphology and high photovoltaic performance

Towards a good control of the morphology of bulk-heterojunction (BHJ) active layers for polymer solar cells (PSCs), selecting an appropriate side chain for a polymer donor and a nonfullerene acceptor (NFA) is very crucial. In this work, two novel NFAs i-IE-4F and i-IESi-4F comprising alkyl and siloxane-terminated side chains on the central indacenodithiophene (IDT) core, respectively, were synthesized. Attaching the siloxane-terminated side chain in i-IESi-4F affords surface energy (γ) of 33.32 mN/m, much lower than that of 39.83 mN/m for i-IE-4F, supplying a big chance to tune miscibility with a polymer donor. Two fluorobenzotriazole-based polymer donors J52 and PBZ-2Si bearing alkyl and siloxane-terminated side chains, respectively, show γ values of 36.08 and 33.10 mN/m, respectively. The estimated Flory-Huggins interaction parameters (χD, A) indicate that the i-IESi-4F is more miscible than i-IE-4F in pairing with J52 or PBZ-2Si. The resulting i-IESi-4F-based blend films exhibits low film roughness and accompanies obviously improved BHJ uniformity. In PSCs, the J52:i-IESi-4F and PBZ-2Si:i-IESi-4F active layers display power conversion efficiencies (PCEs) of 12.67% and 14.54%, respectively, all remarkably higher than PCEs ≼ 7.34% of the i-IE-4F-based active layers. Interestingly, the PBZ-2Si:i-IESi-4F active layer, a donor:acceptor blend system comprising siloxane-terminated side chains (DSi:ASi matching) with the highest BHJ miscibility due to the combinatory effect of the side chains, shows the highest efficiency, as supported by efficient exciton dissociation, the lowest bimolecular recombination, and the optimal charge transports. Our results demonstrate that attaching siloxane-terminated side chains in NFAs, as a side chain engineering, has big potential in lowering its surface energy towards fine control of BHJ morphology and leading to a better donor:acceptor blend system.