Syntheses and characterization of new low-band gap polymers containing 4H-cyclopenta[def]phenanthrene unit and 4,7-di(thien-2-yl)-2H-benzimidazole-2-spirocyclohexane for photovoltaic device

Two new low-band gap polymers, poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PCPP-DTCHBI) and poly[(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PBEHPCPP-DTCHBI), were synthesized and characterized for the photovoltaics. These polymers showed typical characteristics of low-band gap polymers through the internal charge transfer (ICT) between 4H-cyclopenta[def]phenanthrene as the electron-rich unit and di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane as the electron-deficient unit. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are −5.52 and −3.82 eV for PCPP-DTCHBI, and −5.36 and −3.76 eV for PBEHPCPP-DTCHBI, respectively. Optical band gaps of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.70 and 1.60 eV, respectively. As compared to the case of poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2,1,3-benzothiadiazole))] (PCPP-DTBT), PCPP-DTCHBI shows deeper HOMO energy levels by 0.12 eV and lower band gap by 0.3 eV. The FET mobilities of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.19 × 10−4 and 5.11 × 10−5 cm2/V s, respectively, and the power conversion efficiencies of the solar cell devices of the PCPP-DTCHBI and PBEHPCPP-DTCHBI blended with [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) are 1.01% and 0.53%, respectively. The newly designed DTCHBI unit can be used as the electron-deficient moiety inducing efficient ICT for low band gap generation while keeping deep HOMO energy level of the polymer.