Two New A-D-A Type Small Molecule Acceptors Based On C-2v-Symmetric Dithienocyclopentaspiro[Fluorene-9,9 '-Xanthene] Core For Polymer Solar Cells

A C-2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9'-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C-2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550-750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (V-oc) of 0.89 V, short circuit current density (J(sc)) of 15.27 mA cm(-2) and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with V-oc of 0.83 V, J(sc) of 15.48 mA cm(-2) and FF of 66.16%. The higher V-oc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C-2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.