Controlling the Morphology of BDTT-DPP-Based Small Molecules via End-Group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells

A series of narrow-band gap, -π-conjugated small molecules based on diketopyrrolopyrrole (DPP) electron acceptor units coupled with alkylthienyl-substituted-benzodithiophene (BDTT) electron donors were designed and synthesized for use as donor materials in solution-processed organic photovoltaic cells. In particular, by end-group functionalization of the small molecules with fluorine derivatives, the nanoscale morphologies of the photoactive layers of the photovoltaic cells were successfully controlled. The influences of different fluorine-based end-groups on the optoelectronic and morphological properties, carrier mobilities, and the photovoltaic performances of these materials were investigated. A high power conversion efficiency (PCE) of 6.00% under simulated solar light (AM 1.5G) illumination has been achieved for organic photovoltaic cells based on a small-molecule bulk heterojunction system consisting of a trifluoromethylbenzene (CF₃) end-group-containing oligomer (BDTT-(DPP)₂-CF₃) as the donor and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. As a result, the introduction of CF₃ end-groups has been found to enhance both the short circuit current density (J_SC) and fill factor (FF). A tandem photovoltaic device comprising an inverted BDTT-(DPP)₂-CF₃:PC₇₁BM cell and a poly(3-hexylthiophene) (P3HT):indene-C₆₀-bisadduct (IC₆₀BA)-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.30%. These results provide valuable guidelines for the rational design of conjugated small molecules for applications in high-performance organic photovoltaic cells. Furthermore, to the best of our knowledge, this is the first report on the design of fluorine-functionalized BDTT-DPP-based small molecules, which have been shown to be a viable candidate for use in inverted tandem cells.