Effect of Fluorine Atom on Photovoltaic Properties of Triphenylamine-Substituted Quinoxaline-Based D-A Type Polymers

A series of two-dimensional triphenylamine (TPA)-substituted quinoxaline-based D-A-type polymers, in which the electron-donating indacenodithiophene unit was linked to the TPA-substituted electron-accepting quinoxaline derivatives, were synthesized. Fluorine, which exhibits a strong electron-withdrawing nature, was systematically incorporated into the structure of the polymers and its effect on the properties of the polymers was investigated. Three target polymers, namely, PIQx-TOF, PIQx-TIF, and PIQx-T2F were synthesized. The Arabic numerals in the names of the polymers indicate the number of fluorine atoms at the 6,7-positions of the triphenylamine-substituted quinoxaline units in the polymers. The structural, optical, and electrochemical characteristics of the polymers were investigated. The PCE of the polymer solar cells with the inverted-type configuration based on the mono-fluorinated polymer, PIQx-TIF, was the highest (5.30%) followed by those of the devices based on PIQx-TOF (4.49%) and the di-fluorinated polymer, PIQx-T2F (4.65%). This enhanced photovoltaic performance of the device with the polymer with a single fluorine atom can be attributed to the increase in the hole mobility, charge generation and dissociation, molecular ordering, and suppression of unfavorable recombination kinetics. Therefore, the incorporation of a single fluorine atom is the optimal condition for the synthesis of TPA-substituted quinoxaline-based polymers for photovoltaic applications.