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-diy1}-alt(2,2'-bithiazole-5,5'-diy1)] (PehCDTBT), poly[(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt(2, 2'-bithiazole-5,5'-diyl)] (PocCDTBT), poly[{4,4-bis(2-ethylhexyl)-4H- cyclopenta[2,1-b:3,4-b)']dithiophene-2,6diyl}-alt-{2,5-di(thiophen-2-y1) thiazolo[5,4-d]thiazole-5,5'-diy1}] (PehCDTTZ), and poly[(4,4-dioctyl-4H- cyclopenta[2,1-b:3,4-b']dithiophene-2,6diyl)-alt-{2,5-di(thiophen-2-y1) thiazolo[5,4-d]thiazole-5,5'-diy1}] (PocCDTTZ), for use in photovoltaic applications. The intramolecular charge-transfer interaction between the electronsufficient 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 700800 nm. The optical bandgap was measured to be around 1.9 eV for PehCDT-BT and PocCDT-BT, and around 1.8 eV for PehCDT-TZ and PocCDT-TZ. Gel permeation chromatography showed that number-average molecular weights ranged from 8000 to 14000 g mol-¹. Field-effect mobility measurements showed hole mobility of 10 ⁶-10-⁴ Cm²V-¹ s-¹ for the copolymers. The film morphology of the bulk heteroj unction mixtures with [6,6]phenyl-C₆₁-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 mWcm-²).