Simple A-D₂-A Nonfullerene Acceptors for Efficient Binary Bulk Heterojunction Organic Solar Cells

Two new nonfused ring nonfullerene electron acceptors, NFAs, (dicarbazolyl)bis(2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile) (MDCzM-4H) and -(2-(5,6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile) (MDCzM-4F), thus exhibiting an A-D₂-A motif, were synthesized and characterized. As thin films, they exhibit the lowest energy absorption signature near 540 nm, extending down to ∼700 nm. This band is due to an intramolecular charge transfer process from the DCz (nonfused dicarbazoyl; DCz) moiety to the malononitrile-based units (M) based on density functional theory calculations (DFT), which are also corroborated by time-dependent DFT (TDDFT) computations. Both NFAs fluoresce in the near-IR region exhibiting a band maximum peaking near 750 nm with biphasic lifetimes in the 75-410 ps time scale. Electrochemical measurements permitted the determination of their HOMO (∼-5.7 eV) and LUMO (∼-4.0 eV) energies. The absorption bands are complementary to those of the commercial copolymer PTB7-Th, which was used to prepare binary blends for photovoltaic cell performance assessments (ITO/PEDOT:PSS/active layer/PFN-Br/Ag). The power conversion efficiencies (PCE) are found to be 10.17% for MDCzM-4H/PTB7-Th (short-circuit current J_SC = 15.87 mA cm^-2; open-circuit voltage V_OC = 1.03 V; fill factor FF = 0.622) and 14.09% for MDCzM-4F/PTB7-Th (J_SC = 20.92 mA cm^-2; V_OC = 0.965 V; FF = 0.698). The use of nonfused ring NFAs achieving such high performances is significant and reveals a path toward simpler NFAs for use in organic photovoltaics.