Synthesis and properties of thiophene- and quinoxaline-based random copolymers for organic photovoltaics

Abstract

In this study, a series of polymers (denoted as PTQxFs) were prepared from random copolymers—poly[(5,8-(6,7-difluoro-2,3-dihexylquinoxaline))-alt-(2,5-thiophene)]-ran-[(2,5-(3-dodecylthiophene))-alt-(2,5-thiophene)]—via Pd(0)-catalyzed Stille polymerization using thiophene as the electron-rich unit and 6,7-difluoro-2,3-dihexylquinoxaline as an electron-deficient unit, which is newly synthesized by our group. This series of polymers were utilized for organic photovoltaics, and their absorption spectra, energy levels, and photovoltaic properties were investigated. The two fluorine atoms in the 6,7-difluoro-2,3-dihexylquinoxaline unit, exhibiting strong electron-withdrawing properties, enhanced the open-circuit voltage of solar cell devices due to the decrease of HOMO energy levels. With increasing number of 6,7-difluoro-2,3-dihexylquinoxaline units in the random copolymers, the HOMO energy levels decreased from −5.33 eV for PTQxF1 to −5.50 eV for PTQxF9, resulting in increased open-circuit voltage.