Effects of inserting keto-functionalized side-chains instead of imide-functionalized side-chain on the pyrrole backbone of 2,5-bis(2-thienyl) pyrrole-based polymers for organic solar cells

A 2,5-bis(2-thienyl)pyrrole (TPT)-based new monomer unit incorporating the electron-withdrawing 1-decanone (ketone) at the 3- and 4-positions of the pyrrole unit of TPT was prepared. The resulting keto-functionalized TPT unit (approximate to TPTK) was polymerized with distannyl derivatives of 4,8-bis(5-(2-ediylhexyl)thiophen-2-yl)benzo[1,2b:4,5-b']dithiophene (BDTT), 4,7-bis(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), and 3,6-bis(thiophen-2yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DKPP), to obtain three new polymers (P(BDTT-TPTK), P (DTBT-TPTK) and P(DKPP-TPTK)), respectively. The photophysical, electrochemical, and energy conversion efficiency of the TPTK-based polymers were thoroughly studied and compared with those of structurally similar polymers (P(BDTT-TPTI), P(DTBT-TPTI), and P(DKPP-TPTI)) incorporating an imide-functionalized TPT unit (approximate to TPTI). Overall, the TPTK-based polymers showed a higher band-gap (E-g approximate to 2.42 eV, 1.92 eV, and 1.42 eV), deeper highest occupied molecular orbital (HOMO approximate to -5.58 eV, -5.48 eV, and -5.31 eV, respectively) energy levels and lower carrier mobilities than those of the corresponding TPTI-based polymers. These combined effects led to relatively poor solar to electrical energy conversion efficiency for TPTK-based polymers compared to those of TPTI-based polymers.