Copolymers of Cyclopentadithiophene and Electron-Deficient Aromatic Units Designed for Photovoltaic Applications

• Published in: Advanced functional materials 2009-10, Vol.19 (20), p.3262-3270
• Main Authors: Bijleveld, Johan C., Shahid, Munazza, Gilot, Jan, Wienk, Martijn M., Janssen, René A. J.
• Format: Article
• Language: English
• Subjects: conjugated polymers; electronic processes; solar cells; thin films
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200900412

Alternating copolymers based on cyclopentadithiophene (CPDT) and five different electron‐deficient aromatic units with reduced optical band gaps are synthesized via Suzuki coupling. All polymers show a significant photovoltaic response when mixed with a fullerene acceptor. The frontier orbital levels of the new polymers are designed to minimize energy losses by increasing the open‐circuit voltage with respect to the optical band gap, while maintaining a high coverage of the absorption with the solar spectrum. The best cells are obtained for a copolymer of CPDT and benzooxadiazole (BO) with a band gap of 1.47 eV. This cell gives a short‐circuit current of 5.4 mA cm−2, an open‐circuit voltage of 0.78 V, and a fill factor of 0.6, resulting in a power conversion efficiency of about 2.5%. Optimization of the energy levels of alternating CPDT‐X copolymers is achieved by varying the electron‐deficient aromatic segment and results in materials that feature minimal energy losses in bulk‐heterojunction solar cells. For PCPDT‐BO:PCBM, the offset between the band gap and open‐circuit voltage cell is only ∼0.7 eV. This low offset contributes to its high power conversion efficiency (2.5%).