Unraveling the influence of non-fullerene acceptor molecular packing on photovoltaic performance of organic solar cells.

Ye, Linglong; Weng, Kangkang; Xu, Jinqiu; Du, Xiaoyan; Chandrabose, Sreelakshmi; Chen, Kai; Zhou, Jiadong; Han, Guangchao; Tan, Songting; Xie, Zengqi; Yi, Yuanping; Li, Ning; Liu, Feng; Hodgkiss, Justin M; Brabec, Christoph J; Sun, Yanming

In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π-π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π-π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π-π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.