Effects of fluorination of benzothiadiazole units on the properties of alternating cyclopentadithiophene/benzothiadiazole donor/acceptor polymers

Abstract

A set of narrow bandgap conjugated polymers was prepared, using cyclopentadithiophene (CDT) donor units coupled with benzothiadiazole (BT) acceptor units substituted with either no fluorine atoms (A1), one fluorine atom (A2) or two fluorine atoms (A3), using the Stille cross coupling reaction. The addition of two electron-withdrawing fluorine atoms to the BT units was observed to deepen the HOMO energy level of the resulting copolymer, while only slightly affecting the LUMO level, as evidenced by cyclic voltammetry examination. The alternating copolymers (CDT-A1, CDT-A2 and CDT-A3) possess small optical bandgaps of 1.37, 1.43 and 1.51 eV (which should efficiently harvest a broad part of the solar spectrum), and a moderate HOMO level of −5.00, −5.05 and −5.12 eV, respectively. CDT-A3 displayed the highest optical/electrochemical-bandgap and the deepest HOMO level, a consequence of the addition of the fluorine atoms on the BT moieties. Inclusion of two fluorine atoms resulted in sharper X-ray diffraction peaks in the CDT-A3 copolymer with respect to its analogues CDT-A1 and CDT-A2 copolymer indicating a greater crystallinity. These findings clearly demonstrate that fluorination of BT units is an effective approach for adjusting the energy levels and optical properties of BT-based materials for use in organic solar cells devices as well as for other applications.