Molecular Skeletons Modification Induces Distinctive Aggregation Behaviors and Boosts the Efficiency Over 19% in Organic Solar Cells

Understanding the molecular packing arrangement and aggregation behaviors of organic semiconductor materials is crucial in comprehending their unique properties, particularly in complex structures required for solution processing in organic photovoltaics. However, there has been limited focus on studying the diverse self-assembly behaviors induced by varying molecular skeletons. To address this issue, we designed and synthesized i-9R4Cl, i-7R4Cl, and 7R4Cl with nine- and seven-membered ring backbones, respectively. The single crystal structures revealed a standard H-type aggregate in i-9R4Cl, which is rare fully face-to-face packing in nonfullerene acceptors. Conversely, i-7R4Cl exhibited a typical J-type aggregate, while 7R4Cl demonstrated a synergistic H/J-type aggregate as conventional Y-series acceptors. Moreover, it reveals a unique three-dimensional (3D) network packing structure dominated by H-aggregation in i-9R4Cl, a linear packing structure in i-7R4Cl, and an elliptical 3D network packing structure in 7R4Cl. The grazing incidence wide-angle X-ray scattering tests confirmed that the packing arrangement in crystal structures was preserved in the film state. Despite i-9R4Cl's favorable properties in stacking, it achieved a lower power conversion efficiency (PCE) of 1.97% compared to the other two acceptors, which should be attributed to poor exciton separation and carrier recombination induced by the morphology of aggregation regulation. Surprisingly, the electron paramagnetic resonance indicates that i-9R4Cl possesses radical properties, and when introduced as the third component in the PBDB-TF: BTIC-C9-4Cl based devices, it led to an enhancement in PCE from 18.42% to 19.08%, making it one of the highest efficiencies based on the BTIC-C9-4Cl system. It underscores how even subtle changes in molecular structure can significantly impact material properties. Our work aims to control the aggregation states of molecules, transitioning from standard H-type to J-type and to synergistic H/J-type aggregates, subsequently investigating the corresponding relationship between aggregation to overcome the bottlenecks in efficiency.