Flexible–Rigid Synergetic Strategy for Saddle-Shaped Perylene Diimide Acceptors in As-Cast Polymer Solar Cells

Pioneering work referring to perylene diimide (PDI) acceptors has been conducted for a long period, and great efforts have been devoted to hinder the excessive self-aggregation. The main strategy adopted is constructing a three-dimensional molecular structure either with a twisted core or by forming a congestion center. In addition, it is especially important to maintain an essential intermolecular stacking affinity to endow the acceptor with a high charge-transport ability. With these considerations in mind, herein, we adopt a saddle-shaped cyclooctatetrathiophene (COTh) during the PDI acceptor design and integrate the flexibility of COTh into the final PDI acceptor molecules. The geometries for these acceptors have been verified by quantum calculation, and the main excitations have been well analyzed, which demonstrates that the linking manners between PDI and COTh would directly impact their entire configurations, conjugations, and electronic wave-function distributions. The three-dimensional skeleton guarantees the isotropic crystalline characters and effectively suppresses the severe aggregation. When the saddle feature and the large π-extended characteristic intimately combine in COTh-FPDI, balanced mobility and favorable morphology can be achieved. Finally, a power conversion efficiency (PCE) of 8.37% can be accomplished by the as-cast polymer solar cells of the COTh-FPDI:PBDB-T blend. All of the results demonstrate that this flexible saddle core could provide a new block for the novel PDI-based acceptor design in photovoltaic applications.