title: Controlling the Morphology of BDTT-DPP-Based Small Molecules via End-Group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells.
abstract: 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 (JSC) 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.
authors.affiliation: Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University , Busan 609-735, Republic of Korea.
authors.forename: Ji-Hoon
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University , Busan 609-735, Republic of Korea.
authors.forename: Jong Baek
authors.initials: JB
authors.lastname: Park
authors.affiliation: Department of Applied Organic Materials Engineering, Optoelectronic Hybrids Research Center, Inha University , Incheon 402-751, Republic of Korea.
authors.forename: Hoichang
authors.initials: H
authors.lastname: Yang
authors.affiliation: Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) , Daejeon 305-600, Republic of Korea.
authors.forename: In Hwan
authors.initials: IH
authors.lastname: Jung
authors.affiliation: Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT) , Daejeon 305-600, Republic of Korea.
authors.forename: Sung Cheol
authors.initials: SC
authors.lastname: Yoon
authors.affiliation: Department of Chemistry, Kyonggi University , San 94-6, Iui-dong, Yeongtong-gu, Suwon 443-760, Republic of Korea.
authors.forename: Dongwook
authors.initials: D
authors.lastname: Kim
authors.affiliation: Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University , Busan 609-735, Republic of Korea.
authors.forename: Do-Hoon
authors.initials: DH
authors.lastname: Hwang
keywords: benzodithiophene; diketopyrrolopyrrole; inverted tandem solar cells; organic photovoltaic device; small molecules