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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