Toward Thermal Stable and High Photovoltaic Efficiency Ternary Conjugated Copolymers: Influence of Backbone Fluorination and Regioselectivity

Narrow band gap conjugated polymers with a D-A-D'-A repeat unit architecture, namely, PF-0, PF-1a, PF-1b, and PF-2, were designed and synthesized. By precisely controlling the orientation of the asymmetric fluorobenzo[c][1,2,5]thiadiazole (FBT) fragments as well as incorporating different electron acceptors benzothiadiazole (BT) and difluorobenzo[c][1,2,5]thiadiazole (DFBT), regioregularity and graded fluorination have been achieved over the polymer backbone. There are evident differences between the properties of PF-1a and PF-1b due to different regioselectivity within the polymer backbone. In addition, the fluorinated analogues can exhibit increased light absorbance, higher electron density in the solid state, a lower-lying valence band, and more ordered solid film structure. The monofluorinated polymer PF-1b with the optimal regioselectivity and bis-fluorinated polymer PF-2 demonstrated improved charge transport as well as thermally resistant film structure (up to 300 degrees C) in organic field-effect transistors. Moreover, the fluorinated polymers exhibit dramatically increased efficiency from 5.58% to 8.42% in solar cells with lower amount of processing additive, indicating the important role of fluorination and regioselectivity in determining polymer properties. Thus, our systematical study on fluorination may provide an effective approach to precisely control the polymer regioselectivity and improve device performance as well as long-term durability under various environmental stresses.