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

• Published in: Chemistry : a European journal 2010-03, Vol.16 (12), p.3743-3752
• Main Authors: Jung, In Hwan, Yu, Jinyoung, Jeong, Eunjae, Kim, Jinseck, Kwon, Sooncheol, Kong, Hoyoul, Lee, Kwanghee, Woo, Han Young, Shim, Hong-Ku
• Format: Article
• Language: English
• Subjects: Backbone; charge transfer; Chemistry; Copolymers; Derivatives; Heat treatment; Illumination; Open circuit voltage; Photovoltaic cells; photovoltaic effects; pi interactions; polymerization; Solar cells; thin layers
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200903064

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‐yl)thiazolo[5,4‐d]thiazole‐5,5′‐diyl}] (PehCDT‐TZ), and poly[(4,4‐dioctyl‐4H‐cyclopenta[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 around 1.8 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 cm2 V−1 s−1 for the copolymers. The film morphology of the bulk heterojunction 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 mW cm−2). Mind the bandgap: Four types of cyclopentadithiophene (CDT)‐based low‐bandgap copolymers with two thiazole derivatives based on thiazolothiazole (TZ) and bithiazole (BT) structures have been synthesized (see picture for examples). The internal charge‐transfer interaction between the electron‐sufficient CDT unit and electron‐deficient TZ or BT units in the polymeric backbone induces a low bandgap.