Synthesis of π-Extended Dithienobenzodithiophene-Containing Medium Bandgap Copolymers and Their Photovoltaic Application

Abstract

<div><p>Two medium bandgap alternating conjugated copolymers, namely, poly{5,10-di(2-hexyldecyloxy)dithieno[2,3-<i>d</i>:2′,3′-<i>d</i>′]benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,7-diyl-<i>alt</i>-thiophene-2,5-di-yl} (<b>P1</b>) and poly{5,10-di(2-hexyldecyloxy)dithieno[2,3-<i>d</i>:2′,3′-<i>d</i>′]benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,7-di-yl-<i>alt</i>-thieno[3,2-<i>b</i>]thiophene-2,5-diyl} (<b>P2</b>), were prepared by the palladium-catalyzed Stille polycondensation and characterized by gel permeation chromatography (GPC), UV-Vis absorption and photoluminescence (PL) spectra, thermal gravimetric analysis (TGA), cyclic voltammetry (CV) <i>etc</i>. The resultant copolymers show moderate solubility in common organic solvents and enough stabilities for photovoltaic application. And both copolymers absorb the solar light from 300–600 nm, with the optical band gaps () calculated from the onset of absorption in the solid film of ca. 2.1 eV. The highest occupied molecular orbital (HOMO) levels of two copolymers determined by CV were at about −5.35 eV. Photovoltaic propertites of the polymers were investigated by using the polymers as donor and [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) as acceptor with a weight ratio of polymer:PC₇₁BM of 1:2. The power conversion efficiencies (PCEs) of polymer solar cells based on PDTBTT-TT reached 2.50%, with an open-circuit voltage (<i>V</i>_oc) of 0.70 V, a short-circuit current density (<i>J</i>_sc) of 6.89 mA cm⁻², and a fill factor (<i>FF</i>) of 52%, under the illumination of AM1.5, 100 mW cm⁻². These results indicate that dithieno[2,3-<i>d</i>:2′,3′-<i>d</i>′]benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene (DTBDT) is a promising building block for the high-performance organic electronic materials.</p></div