Synthesis and photovoltaic application of low-bandgap conjugated polymers by incorporating highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units

- Two polymers based on pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized and characterized.
- The LUMO and bandgap were lowered by introducing a more electron deficient unit.
- The polymers maintained the deep HOMO level and the devices exhibited a PCE of 3.66% with a high Voc of 0.91 V.

Two donor-acceptor polymer semiconductors based on highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized by Stille cross-coupling polymerization, and their thermal property, photophysical property, electrochemical property, microstructure, application as organic thin-film transistors, and photovoltaic property were investigated. Because of the strong electron-accepting characteristic of pyrrolo[3,4-d]pyridazine-5,7-dione, the new polymers (P1 and P2) exhibited much wider absorption, smaller bandgaps (1.70 vs 1.98 eV), and deeper LUMO levels (−3.60 vs −3.37 eV) than those of a phthalimide-based polymer PBDT-PhBT (Figure 1). The fabricated organic thin-film transistor devices exhibited hole-transport behavior, and the highest mobility of 1.14 × 10−3 cm2 V−1 s−1 was obtained. The bulk-heterojunction solar cells based on the two polymers as the electron donors and PC71BM as the electron acceptor showed a high open-circuit voltage and achieved a power conversion efficiency of 2.71% and 3.66% for polymers P1 and P2, respectively.

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