Double acceptor donor-acceptor alternating conjugated polymers containing cyclopentadithiophene, benzothiadiazole and thienopyrroledione: toward subtractive color organic photovoltaics

Double acceptor copolymers comprising benzothiadiazole (BT), thieno[3,4-c]pyrrole-4,6-dione (TPD) and two cyclopentadithiophene (CPDT) units, and a series of copolymers comprising CPDT-A-CPDT (A: acceptor=BT or TPD) with different numbers of thiophene units have been synthesized by combinations of Suzuki and Stille coupling, direct arylation and oxidative polymerization. Ultraviolet-visible absorption spectra and cyclic voltammograms were measured to compare the optical and electrochemical properties of the polymers. The double acceptor copolymer exhibited hybridized features resulting from the presence of both BT, as well as TPD. On the other hand, an increase in the number of electron donor units resulted in an increase in the band gap for both the TPD and BT series. Organic photovoltaics (OPV) devices were fabricated using the polymers and a fullerene derivative. These donor-acceptor alternating copolymers showed higher OPV performance (power conversion efficiency=3-4%) than other polymers. The active layer comprising the double acceptor polymer and a fullerene derivative showed subtractive color resulting from wide-range visible light absorption, which is advantageous for developing transparent building-integrated OPVs. The OPV performance of BT-based copolymers synthesized via direct arylation was comparable or lower compared with those prepared by conventional cross-coupling polymerizations.