Synthesis and optical properties of photovoltaic materials based on the ambipolar dithienonaphthothiadiazole unit

Abstract

Dithieno[3′2′:5,6;2′′,3′′:7,8]naphtho[2,3-c][1,2,5]thiadiazole (DTNT) was designed to control the band energies of the polymers for photovoltaic materials. Electrochemical analysis showed that DTNT acts as both an electron donor and an electron acceptor, revealing the ambipolar nature of the DTNT unit. The direct arylation polymerization of DTNT with 2,2′-bithiophene (BTh) and 3,6-bis(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) afforded four polymers that differed in either the unit of copolymerization or the chosen side chains. In the PDTNT-BTh series, a shoulder absorption band was observed at a longer wavelength than the intense absorption band. The PDTNT-DPP series exhibited a narrow band gap of less than 1.4 eV and a low HOMO energy of −5.43 eV. An organic photovoltaic cell that contained a PDTNT-BTh polymer with 2-ethylhexyl groups and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as an active layer afforded the best performance among the studied compounds, with a J_SC of 6.98 mA cm⁻³, a V_OC of 0.758 V, a FF of 0.52, and a PCE of 2.76%.