Isomeric Effect of Wide Bandgap Polymer Donors with High Crystallinity to Achieve Efficient Polymer Solar Cells.

Two highly crystalline polymer donors (PBTz4T2C-a, PBTz4T2C-b) with isomers (4T2C-a, 4T2C-b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C-b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C-a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C-b:IT2F blend film. In contrast, PBTz4T2C-a with 4T2C-a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high-performance polymer donors.

Novel Copolymers Based Tetrafluorobenzene and Difluorobenzothiadiazole for Organic Solar Cells with Prominent Open Circuit Voltage and Stability.

Two novel copolymers based on benzothiadiazole (BT) or difluorobenzothiadizole (ffBT) with 2,2'-(perfluoro-1,4-phenylene)dithiophene (2TPF4), namely PBT-2TPF4 and PffBT-2TPF4, are synthesized for applications in polymer solar cells (PSCs). A noticeably high open-circuit voltage (Voc ) of 1.017 and 0.87 V are achieved for PffBT-2TPF4 and PBT-2TPF4-based devices, respectively. Although only a moderate efficiency (5.7%) of PBT-2TPF4-based devices is obtained, it is first demonstrated that 2TPF4 is a promising acceptor block for construction of the donor copolymers which possess high Voc , prominent crystallinity, and long-term stability, simultaneously. Besides, two thienyl flanking the tetrafluorophenylene can decrease torsion angle between conjugated units, resulting in a high coplanar structure of copolymers to enhance their charge carrier mobility. The findings may open a promising and practical way to accelerate the commercialization of PSCs by developing a series of new donor copolymers for efficient and long-term stable thickness bulk heterojunction PSCs.