Effect of Trifluoromethyl Group on Fluorinated and Cyanated Quinoxaline-based Conjugated Polymers in Photovoltaic Application

Abstract

We have successfully synthesized four new conjugated polymers (PF-0, PF-CF3, PCN-0, and PCN-CF3). Stille reaction was done to combine the 2-(2-ethylhexyl)thiophene-modified benzodithiophene (BDT) as the main backbone of electron donor and 2,3-diphenylquinoxaline (DPQ) as the main backbone of the electron acceptor. Various electron-withdrawing groups were incorporated into the DPQ. Cyano group or Fluorine atom were substituted to the main backbone of DPQ. Meanwhile, strong electron-withdrawing trifluoromethyl (CF3) groups were introduced to the meta position of the phenyl groups in the DPQ of the reference polymers (PF-0 and PCN-0) to afford PF-CF3 and PCN-CF3, respectively. The contribution of CF3 moiety in PF-CF3 has been proven to be effective in enhancing the photovoltaic performance by increasing all photovoltaic parameters to a high value of open circuit voltage (Voc) of 0.83 V, short-circuit current density (Jsc) of 13.25 mA cm--2, and fill factor of 63.6%, which results in power conversion efficiency (PCE) of 6.99%. On the other hand, although the introduction of CF3 groups can also dramatically increase the Voc of the device up to 0.91 V on cyanated polymer-based PSC, unfortunately, CF3 moiety affects negatively on the overall photovoltaic performance of PCN-CF3 due to the bad morphology of its blend film. Therefore, in the case of cyanated polymers, incorporating CF3 moiety is not a preferred way to enhance the photovoltaic performance. Interestingly, its identical cyanated polymer, PCN-0, exhibits the best overall device performance among the four polymers-based devices with a high Voc of 0.84 V, Jsc of 14.37 mA cm-2, and FF of 62.7% which lead to the maximum PCE of 7.57%. It proves that cyano group alone can be considered as promising and powerful substituents to achieve superior photovoltaic device performance.