Effect of Fluorine on the Photovoltaic Property of Benzotriazole-Based Copolymers Containing Thieno3, 2-bthiophene π Bridge

Two wide bandgap copolymers, PTTBDT-BTA and PTTBDT-FBTA, have been prepared by the Stille coupling polymerization method in the presence of tris(dibenzylideneacetone) dipalladium (Pd₂(dba)₃) and tri(o-toyl) phosphine (P(o-tol)₃), based on the two dimensional 4, 8-bis(5-(2-butyloctyl)thieno3, 2-bthiophen-2-yl)benzo1, 2-b:4, 5-b'dithiophene (BDT) as donor (D) moiety, thieno3, 2-bthiophene (TT) as conjugated π bridge and benzod 1, 2, 3triazole (BTA)/5, 6-difluorobenzod 1, 2, 3-triazole (FBTA) as acceptor (A) unit, respectively. Both copolymers were characterized by a set of methods including ¹H-NMR, ¹³C-NMR, elemental analysis, gel permeation chromatography (GPC), thermogravimetric (TG) analysis, UV-Vis absorption and cyclic voltammetry (CV), etc. And the effects of fluorine on thermal stability, film-forming property, absorption spectra, aggregation ability in chlorobenzene (CB) solution, energy level and photovoltaic performance were investigated. Compared with PTTBDT-BTA, PTTBDT-FBTA showed an increase for decomposition temperature (T_d, 5% mass-loss) of 20℃, decreased solubility, slightly narrowed absorption range, significantly enhanced aggregation in CB solution and a decrease for the highest occupied molecular orbital energy level (E_HOMO) of 0.10 eV. The photovoltaic devices were fabricated with a structure of ITO/PEDOT:PSS/active layer/PFN/Al, using PTTBDT-BTA or PTTBDT-FBTA as donor and 6, 6phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) as acceptor, and their photovoltaic properties were investigated under the AM 1.5, 100 mW/cm². It was found that the optimal device based on fluorinated PTTBDT-FBTA exhibited a 49.3% increase in power conversion efficiency (PCE), which was originated from a 16.9% enhancement of open circuit voltage (U_OC), a 13.2% improvement of short circuit current density (J_SC) and a 11.8% rise of fill factor (FF) compared with PTTBDT-BTA-based devices. These results demonstrated that fluorination was an effective strategy for tuning the material structure and improving the photovoltaic property.