Synthesis And Photovoltaic Properties Of D-A Copolymers Based On Fluorinated Benzothiadiazole Or Benzodipyrrolidones

Abstract:In this work, the major work is focusing on the design and synthesis of new D-A conjugated polymer donor materials for the application of polymer solar cells. The main results are as follows:(1)Two new D-A alternating copolymers, PBDFFBT and PBDTFBT, featuring a fluorinated benzothiazole (BT) electron-acceptor unit and the electron-donor segments of benzo[1,2-b:4,5-b’]difuran (BDF) or benzo[1,2-b:4,5-b’]dithiophene (BDT), were prepared by a standard Stille cross-coupling reaction. The resulting copolymers were characterized by TGA, GPC, UV-vis absorption spectroscopy and cyclic voltammetry measurements. PBDFFBT and PBDTFBT show good film forming properties. Bulk heterojunction polymer photovoltaic devices based on an active layer of the electron donor copolymers (PBDFFBT and PBDTFBT), blended with an electron acceptor [6,6]-phenyl-C7] butyric acid methyl ester (PC71BM) in three weight ratios of1:1,1:2and1:3were investigated. The devices with the configuration of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al demonstrated promising power conversion efficiencies of3.3%for PBDFFBT:PC71BM (1:3, w/w) and4.0%for PBDTFBT:PC7,BM (1:2, w/w) under the illumination of AM1.5,100mW/cm2.(2) A new donor-acceptor conjugated polymer (PBDTPO-FBT), which consists of a fluorinated benzothiadiazole (FBT) electron-acceptor unit and an electron-donor segment of alkoxylphenyl substituted benzo[1,2-b:4,5-b’]dithiophene (BDTPO), was synthesized according to the Stille cross-coupling reaction. The resulting copolymer was characterized by TGA, GPC, UV-vis absorption spectroscopy and cyclic voltammetry measurements. PBDTPO-FBT possesses good thermal stability with5%weight loss temperature of410℃and shows broad absorption at300-800nm with an optical bandgap of1.53eV. Cyclic voltammetry exhibits HOMO and LUMO energy levels of-5.43eV and-3.72eV, respectively. The hole mobility of PBDTPO-FBT:PC71BM (1:1, w/w) reaches up to3.8×10-3cm/(V’s) by the space-charge-limited current (SCLC) method. By using1,8-diiodooctane (DIO) as the solvent additive, the polymer solar cell with the configuration of ITO/PEDOT:PSS/PBDTPOFBT:PC71BM(1:1, w/w)/Ca/Al demonstrates a power conversion efficiency of2.70%with Voc=0.70V, Jsc=7.23mA/cm2and FF=47%, under the illumination of AM1.5G (100mW/cm2).(3) A new D-A conjugate polymer containing benzodipyrrolidones unit and benzodithiophene unit was synthesized by Stille cross-coupling polymerization. The thermal, optical and electrochemical properties were well investigated. PBDPDPBDT is soluble in common solvents, and shows broad absorption at300-700nm with an optical bandgap of1.60eV. The preliminary results showed the device based on the classical structure of ITO/PEDOT:PSS/polymer:PC61BM(1:3,w/w)/Ca/Al displayed the best photovoltaic performance with a PCE of0.16%under illumination of AM1.5G (100mW/cm2).