Tuning Surface Energy of Conjugated Polymers via Fluorine Substitution of Side Alkyl Chains: Influence on Phase Separation of Thin Films and Performance of Polymer Solar Cells

High Resolution Image Download MS PowerPoint Slide Different contents of fluorine in side alkyl chains were incorporated into three conjugated polymers (namely, PBDTTT-f13, PBDTTT-f9, and PBDTTT-f5) whose backbones consist of benzodithiophene donors and thienothiophene acceptors. These three fluorinated polymers, in comparison with the well-known analogue PTB7-Th, show comparable energy levels and optical band gaps. However, the fluorination of side alkyl chains significantly changed the surface energy of bulk materials, which leads to distinctly different self-assembly behaviors and phase separations as being mixed with PC 71 BM. The increased mismatch in surface energies between the polymer and PC 71 BM causes larger scale phase domains, which makes a sound explanation for the difference in their photovoltaic properties.