Synthesis, characterization of bay-substituted perylene diimide based D-A-D type small molecules and their applications as a non-fullerene electron acceptor in polymer solar cells

We report a series of bay substituted perylene diimide based donor-acceptor-donor (D-A-D) type small molecule acceptor derivatives such as S-I, S-II, S-III and S-IV for small molecule based organic solar cell (SM-OSC) applications. The electron rich thiophene derivatives such as thiophene, 2-hexylthiophene, 2,2′-bithiophene, and 5-hexyl-2,2′-bithiophene were used as a donor (D), and perylene diimide was used as an acceptor (A). The synthesized small molecules were confirmed by FT-IR, NMR, and HR-MS. The small molecules showed wide and strong absorption in the UV-vis region up to 750 nm, which reduced the optical band gap to <2 eV. The calculated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were comparable with those of the PC₆₁BM. Scanning electron microscope (SEM) studies confirmed the aggregation of the small molecules, S-I to S-IV. Small molecules showed thermal stability up to 300 °C. In bulk heterojunction organic solar cells (BHJ-OSCs), the S-I based device showed a maximum power conversion efficiency (PCE) of 0.12% with P3HT polymer donor. The PCE was declined with respect to the number of thiophene units and the flexible alkyl chain in the bay position.