Synthesis and Characterization of Cyclopentadithiophene-Based Low Bandgap Copolymers Containing Electron-Deficient Benzoselenadiazole Derivatives for Photovoltaic Devices

Abstract

We have synthesized two 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-(benzo[c] [1,2,5]selenadiazole-4,7-diyl)] (PCBSe) and poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(4,7- dithiophen-2-yl-benzo[c][1,2,5]selenadiazole-5,5'-diyl)] (PCT2BSe), for use in photovoltaic applications. Through the internal charge transfer interaction between the electron-donating CDT unit and the electron-accepting benzoselenadiazole, we realized exceedingly low bandgap polymers with bandgaps of 1.37-1.46 eV. The UV-vis absorption maxima of PCT2BSe were subjected to larger hypsochromic shifts than those of PCBSe, because of the distorted electron donor-acceptor (D-A) structures of the PCT2BSe backbone. These results were supported by the calculations of the D-A complex using the ab initio Hartree-Fock method with a split-valence 6-31G* basis set. However, PCT2BSe exhibited a better molar absorption coefficient in the visible region, which can lead to more efficient absorption of sunlight. As a result, PCT2BSe blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) exhibited a better photovoltaic performance than PCBSe because of the larger spectral overlap integral with respect to the solar spectrum. Furthermore, when the polymers were blended with PC(71)BM, PCT2BSe showed the best performance, with an open circuit voltage of 0.55 V, a short-circuit current of 6.63 mA/cm(2) and a power conversion efficiency of 1.34% under air mass 1.5 global illumination conditions. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:1423-1432, 2010