Low-band gap copolymers of ethynylfluorene and 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione synthesized under microwave irradiation for polymer photovoltaic cells

Copolymers with alternative electron-donating and electron-withdrawing units can exhibit a low-band gap and can be used as the donor materials in polymer photovoltaic cells (PVs). This paper reports the synthesis of copolymers of electron-donating ethynylfluorene and electron-withdrawing 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyyrole-1,4-dione (DPP) via the Pd-catalyzed Sonogashira coupling and their application in polymer PVs. Applying microwave irradiation during the Sonogashira coupling can shorten the polymerization time from several days to a couple of minutes and improve the molecular weight. These polymers have band gaps of about 1.85 eV. Their energy levels indicate that they can be used as the donor materials for polymer PVs. Polymer PVs were fabricated using these polymers as donors and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor. Applying the microwave irradiation during the Sonogashira coupling can significantly improve the photovoltaic performance of the polymer PVs. Power conversion efficiency (PCE) of 2.25% was achieved on polymer PVs under AM 1.5G illumination (100 mW cm(-2)). (C) 2011 Elsevier B.V. All rights reserved.