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-1 H -inden-1-ylidene)malononitrile) ( MDCzM-4H ) and -(2-(5,6-fluoro-3-oxo-2,3-dihydro-1 H -inden-1-ylidene)malononitrile) ( MDCzM-4F ), thus exhibiting an A-D 2 -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.