Synthesis and Photovoltaic Properties of Cyclopentadithiophene-Based Low-Bandgap Copolymers That Contain Electron-Withdrawing Thiazole Derivatives

We have synthesized four types of cyclopentadithiophene (CDT)-based low-bandgap copolymers, poly-[{4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl}-alt-(2,2'-bithiazole-5,5'-diyl)](PehCDT-BT), poly[(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt(2,2'-bithiazole-5,5'-diyl)](PocCDT-BT), poly[{4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl}-alt-{2,5-di(thiophen-2-y1)thiazolo-[5,4-d]thiazole-5,5'-dyl}](PehCDT-TZ), and poly[(4,4-dioctyl-4H-cycloprnta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5'-diyl}[(PocCDT-TZ), for use in photovoltaic applications. The intramolecular charge-transfer interaction between the electron-sufficient CDT unit and electron-deficient bithiazole (BT) or thiazolothiazole (TZ) units in the polymeric backbone induced a low bandgap and broad absorption that covered 300 nm to 700-800 nm. The optical bandgap was measured to be around 1.9 eV for PehCDT-BT and PocCDT-BT, and mound 1 S eV for PehCDT-TZ and PocCDT-TZ. Gel permeation chromatography showed that number-average molecular weights ranged from 8000 to 14 000 g mol(-1). Field-effect mobility measurements showed hole mobility of 10(-6) -10(-4) cm(2) V(-1) s(-1) for the copolymers. The film morphology of the bulk heterojunction mixtures with [6,6]phenyl-C(61)-butyric acid methyl ester (PCBM) was also examined by atomic force microscopy before and after heat treatment. When the polymers were blended with PCBM, PehCDT-TZ exhibited the best performance with an open circuit voltage of 0.69 V, short-circuit current of 7.14 mA cm(-2), and power conversion efficiency of 2.23 % under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm(-2)).