Star-shaped polyfluorene: Design, synthesis, characterization and application towards solar cells

• Star shaped polyfluorene having arylimide core.
• Suppression of keto defects by blocking the interchain exciton diffusion.
• Bulk hetero junction solar cell with star shaped polyfluorene.

The present manuscript reports the design, synthesis and characterization of three star-shaped polymers consisting of three different arylimides such as perylene (PR)-, naphthalene (NT)- and benzene (BZ) tetracarboxylicdiimide as core and polyfluorene (PF) as arms. Chemical structure of star-shaped polymers was aimed at broadening as much as possible their absorption profile. Arylimide cored star polymers (PF-BZ, PF-NT and PF-PR) were prepared through palladium catalyzed Suzuki polycondensation to tune the band gap of the polymers. The prepared polymers were characterized by elemental analysis, NMR, GPC, UV–Vis, photoluminescence and cyclic voltammetry studies. Electrochemical and optical responses of three polymers revealed the lowering of band gap from linear PF to star-shaped polymers. TCSPC study confirmed the partial energy transfer from PF arms to arylimide cores. The unexpected keto defect in linear PF was also reduced by preparation of star polymer with large arylimide cores. TGA exhibited the enhancement of thermal stability of star polymer than linear PF. By using star polyfluorenes as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor, bulk heterojunction (BHJ) solar cells of the structure ITO/PEDOT:PSS/star polymer: PCBM/Al were fabricated and studied with a solar simulator under AM1.5G (100 mW/cm2) irradiation intensity. Those cells showed the open circuit voltage (Voc) ∼0.52–0.55 V, the short circuit current density (Jsc) ∼0.84–1.13 mA cm−2, the fill factor ∼0.39–0.44 and the power conversion efficiency (PCE) ∼0.18–0.26%.

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