Effect of Replacing Alkyl Side Chains with Triethylene Glycols on Photovoltaic Properties of Easily Accessible Fluorene-Based Non-Fullerene Molecular Acceptors: Improve or Deteriorate?

Hydrophilic oligo(ethylene glycol) (OEG) side chains are more flexible than alkyl chains and can facilitate the π–π stacking of conjugated polymer backbones, which has been proven to be a key structural feature leading to better device performance in polymer:fullerene blend polymer solar cells (PSCs). So far, little has been known about the influence of OEG side chains on the performance of non-fullerene acceptors in polymer:non-fullerene acceptor blends. Based on an easily accessible conjugated backbone of dicyanomethylene indanone–thiophene–fluorene–thiophene–dicyanometnylene indanone (DICTF), two non-fullerene molecular acceptors were synthesized with decyl (DICTF-C10) and triethylene glycol monoether (DICTF-TEG) side chains to blend with the polymer donor of PTB7-Th for PSCs. Replacing the decyl side chains with the TEGs on DICTF does improve the π–π stacking between the conjugated backbones as expected. For DICTF-TEG, nevertheless, forming the BHJ blend with the PTB7-Th donor leads to unfavorable film morphology structure of the active blend and deteriorated device performance with much lower PCE of 3.09% compared to the blend of PTB7-Th:DICTF-C10 with the maximum PCE of 6.93% at optimized conditions. It is found that charge generation, transport, and recombination within the PSC devices of the two acceptors were greatly impacted by introducing TEG side chains instead of alkyls into the conjugated backbone of DICTF. This comparative study on understanding the side chain effect on device performance allows the establishment of guidance on molecular engineering of non-fullerene molecular acceptors to obtain high performance PSCs.