Synthesis and optical properties of a chlorinated small-molecule acceptor for organic photovoltaic cells

Chlorinated end group is a common design strategy to achieve high-performance non-fullerene acceptors (NFAs). Here, we designed and synthesized a monochlorinated small-molecule acceptor Y-BO-Cl-p with well-defined structure and studied its optical properties. The strong electronegativity of the chlorine (Cl) atom and the large polarity of the C-Cl bond will lead to an enhanced intramolecular charge transfer (ICT) effect from the electron-donating core to the chlorinated end group. Therefore, Y-BO-Cl-p has a narrow bandgap of 1.39 eV, and both its chloroform solution and solid thin film exhibit broad absorption spectra. In chloroform solution, the absorption range of Y-BO-Cl-p is mainly in 450-800 nm, and its maximum absorption peak is at 730 nm. In contrast, the absorption range of the film is mainly in 500-900 nm, and its maximum absorption peak is at 803nm. The large red shift of the absorption spectrum from solution to film is attributed to the tighter molecular packing in the Y-BO-Cl-p film. In addition, the larger Cl-substituted dipole moment also modulates the intermolecular interactions, which is beneficial to the formation of more ordered intermolecular π-π stacking. In conclusion, the chlorination modification of end group is an efficient approach to realize high-performance small molecule acceptors.