Synthesis of random copolymer using Zig-Zag Naphthodithiophene for bulk Heterojunction polymer solar cell applications

Abstract

Synthesized a random copolymer here, namely P(zNDT-TPDBT), consisting of an electron-rich unit ‘zig-zag’ 4,9-bis-(2-ethylhexyloxy)naphtho[1,2-b:5,6-b’]dithiophene (zNDT) and different ratio of two-electron acceptor units of thieno[3,4-c]pyrrole-4,6-dione (TPD) and benzodiathiazole (BT) via Stille coupling polymerization. The photophysical, electrochemical, and photovoltaic properties of P(zNDT-TPDBT) were investigated. The differences in the photophysical property of P(zNDT-TPDBT) indicate (i) red-shifted and broadening absorption spectrum located at 509 nm in thin-film as compared to solution (λmax = 492 nm) (ii) the fluorescence spectrum shows dual emission bands at 545 nm and 608 nm in the longer wavelength in solution, however, relatively complete quenching fluorescence property was observed in blended film with PC71BM. The copolymer exhibited an optical bandgap of 2.03 eV, with a highest occupied molecular orbital (HOMO) level of −5.87 eV. The optimized structure for the copolymer was also determined through DFT calculation. The bulk-heterojunction (BHJ) polymer solar cell (PSC) with a device structure of ITO/PEDOT:PSS/P(zNDT-TPDBT):PC71BM/Al exhibited a promising efficiency of 2.4% with the short circuit current density (Jsc) of 5.7 mA cm−2, open-circuit voltage (Voc) of 0.91 V, and fill factor (FF) of 47%, without processing addictive. The clear correlations between film morphology and device efficiency were observed through atomic force microscopy technique.