Photovoltaic properties of a new quinoxaline-based copolymer with Thieno[3,2-b]thiophene side chain for organic photovoltaic cell applications

•Quinoxaline-based electron-accepting unit with a thienothiophene side chains was synthesized.•The synthesized accepting unit was copolymerized with benzodithiophene donor unit.•The synthesized accepting unit increased crystalliniity and hole mobility of the polymer.•The photovoltaic device fabricated using the polymer showed PCE of 3.42%.

A new quinoxaline-based electron-accepting building block with thieno[3,2-b]thiophene side chains was synthesized and copolymerized with the alkoxy-substituted benzodithiophene (BDT) donor building block to obtain poly[(4,8-bis[(2-butyloctyl)oxy]-benzo[1,2-b:4,5-b′]dithiophene)-alt-(2,3-bis(5-octylthieno[3,2-b]thiophene-2-yl)-5,8-di(thiophen-2-yl)quinoxaline)] (PBDT- TTDTQx) copolymer. To compare the effect of the addition of the thieno[3,2-b]thiophene side chain in the quinoxaline moiety, an analogous copolymer with a thienyl side chain in the quinoxaline unit was also synthesized to yield the poly[(4,8-bis[(2-butyloctyl)oxy]-benzo[1,2-b:4,5-b′]dithiophene)-alt-(2,3-bis(5-octylthiophene-2-yl)-5,8-di(thiophen-2-yl)quinoxaline)] (PBDT-TDTQx) copolymer. The optical, electrochemical, morphological, and photovoltaic properties of the two polymers were investigated. Bulk heterojunction photovoltaic devices were fabricated using the polymers as a p-type donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the n-type acceptor. The power conversion efficiencies of the devices fabricated using PBDT-TTDTQx and PBDT-TDTQx are 3.42 and 2.26%, respectively. The replacement of the alkylthienyl moiety with thieno[3,2-b]thiophene on quinoxaline could expand the UV–visible absorption range due to extended π-conjugation and also enhance the crystallinity of the polymer.

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