Abstract

In this work, a kind of thiophene-fused quinoxaline (QXT) unit has been designed and synthesized as an electron-deficient unit to build two D-A conjugated copolymers (PBDTT-QXT-TF1 and PBDTT-QXT-TF2) with a thiophene ring as “π-bridge” and thienyl substituted benzo[1,2-b:4,5-b’]dithiophene (BDTT) unit as an electron-rich unit, in which two adjacent side-chain thiophene rings with different number of fluorine atoms have been introduced at 2,3-position of QXT unit. It is found that the fusion of a thiophene ring onto QX unit can result in a red-shift of absorption and narrow the optical bandgap through stabilizing the quinoid structure of the conjugated backbone, and the introduction of carboxyl group at the fused thiophene can reduce the HOMO energy level that is favorable for the enhancement of VOC of the organic solar cells (OSCs). However, the introduction of fused thiophene ring results in partially twist of conjugated backbone, thus attenuating the intramolecular charge transfer (ICT) absorption intensity and finally lead to the decrement of the JSC for the OSCs. Therefore, the corresponding OSC devices based on the PBDTT-QXT-TF1 or PBDTT-QXT-TF2 with IDIC acceptor show much high VOC but low JSC. In addition, the introduction of side-chain thiophene rings with different number fluorine atoms also regulates the energy levels and the structural crystallinity, thus further influencing the photovoltaic performances. By comparison, the PBDTT-QXT-TF1:IDIC blended film exhibits higher and more balanced hole/electron charge mobilities than the PBDTT-QXT-TF2:IDIC blended film, and PBDTT-QXT-TF1:IDIC based device shows higher power conversion efficiency (PCE) of 6.67% (VOC = 0.951 V, JSC = 11.56 mA cm−2 and FF = 60.65%) than PBDTT-QXT-TF2:IDIC based device with a PCE of 5.12% (VOC = 0.975 V, JSC = 9.78 mA cm−2 and FF = 53.61%).

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