Design, Synthesis, and Characterization of Small Molecules with Fluorinated Donor Moiety for Organic Solar Cells

In this work, a novel family of DTDCPB series with various numberous and positions of fluorine substitution have been synthesized as donor materials for small-molecule organic solar cells. The compound DTDCPB has an unsymmetrical donor−acceptor−acceptor (D-A-A) framework, which gave a remarkable PCE of 8.2%, is reported. Fluorine is the most electronegative element and the smallest electron-withdrawing group. Introduction of fluorine to our materials, the fluorinated DTDCPB derivatives exhibited decreased LUMO and HOMO energy levels, but with large band gap. The low-lying HOMO feature is beneficial to achieve reasonable highest Voc of 1.02 V for vapor-deposited small molecule OSCs. The substituted positions of fluorine on the donor unit (the fluorine ortho- or meta- to the) would affect the absorption wavelength and the extinction coefficient in their optical properties. We found there is a trade-off between the photovoltage and the photocurrent due to the modification of fluorine. By fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid bulk heterojunction devices, based on Fm–C70 gave a PCE of 6.12% along with Voc of 0.93 V, Jsc of 10.5 mA/cm2. Furthermore, we changed different backbone structure to adust molecular energy level for better light harvesting. The compound DTDCTB has a wonderful Jsc of 14.8 mA/cm2, because of the electron-rich and fortified quinoidal characters of thiophene. The new material DTDCTB-DF, with fluorinated thiophene, can improve the device performance as compared to DTDCTB. With an optimized blend ratio of DTDCTB-DF-C70, a high PCE of 5.3% was obtained, with Voc of 0.96 V, Jsc of 11.5 mA/cm2.