Synthesis And Photovoltaic Properties Of The Fluorinated Tetrapolymer As Donor With Broad Absorption Range

Nowadays, energy shortage has become a serious problem so that human being is paying more and more attention to the development and utilization of solar energy. Organic solar cells with the advantages of low weight, low cost,material abundance, easy fabrication, flexible properties, have great potential in application, but needs to be further improved for commercial applications.Design and synthesis of high-performance organic copolymer materials have recently become the key to improve the efficiency of organic photovoltaic(OPV) cells. In this dissertation, we have carried out the following works:(1) In Chapter 3, two low band-gap conjugated copolymers(P3-1 and P3-2)are designed and synthesized via Stille cross-coupling polymerization by choosing benzodithiophene(BDT) as electron-donating unit and three compounds, diketopyrrolo[3,4-c]pyrrole(DPP), benzothiadiazole(BT) and difluorobenzene(DFB) or tetrafluorobenzene(TFB) as electron-accepting units.DFB unit of tetrapolymer PBDT-DFB1-DPP1-BT1 and TFB unit of tetrapolymer PBDT-TFB1-DPP1-BT1 have both a broad absorption spectrum from 300 nm to the near infrared region and the narrow optical band gapsranging from 1.51 to 1.52 eV. Additionally, the fluorine copolymer exhibited a long-term thermal stability for 5% weight loss at 370 °C, 403 °C and relatively lower-lying highest occupied molecular orbital(HOMO) energy level.Preliminary investigation on the bulk heterojunction(BHJ) solar cells fabricated with these polymers as donors and PC60 BM as an acceptor(1:1wt) achieved a power conversion efficiency(PCE) of 1.56% and1.26% in AM 1.5G, power of100 mW/cm2 simulated sunlight.(2) In Chapter 4, four low band-gap conjugated copolymers(P4-1—P4-4)are designed and synthesized via Stille cross-coupling polymerization by choosing benzodithiophene(BDT) as electron-donating unit and three compounds, diketopyrrolo[3,4-c]pyrrole(DPP), hexylthiophene(HT) and benzene(B)/fluorobenzene(FB)/difluorobenzene(DFB)/tetrafluorobenzene(TFB) as electron-accepting units, for which the molecular structure, thermal stabilities, optical absorption and electrochemical properties were studied.PBDT-B1-DPP1-HT1, PBDT-FB1-DPP1-HT1, PBDT-DFB1-DPP1-HT1 and PBDT-TFB1-DPP1-HT1 have both a wide spectral range, from 300 to 900 nm and relatively deeper lying HOMOs(-5.61 to-5.66eV). Moreover, from the thermogravimetric analysis(TGA) measurements, the onset temperatures with5% weight loss of all polymers are higher than 330 °C, showing good thermal stability. Experimental results have shown the incorporation of fluorine atoms to the polymer backbone can fine-tune the polymer energy levels and molecular packing. The tetrapolymer investigated with broad absorption spectrum makeprogress to application and development of high-performance OPVs technology.(3) In Chapter 5, four alternating copolymers(P4-4—P5-3) are designed and dsynthesized via Stille cross-coupling polymerization by choosing benzodithiophene(BDT) as electron-donating unit and three compounds,diketopyrrolo[3,4-c]pyrrole(DPP), hexylthiophene(HT) and tetrafluorobenzene(TFB) as electron-accepting units, and the photophysical and electrical properties were investigated. PBDT-TFB1-DPP1-HT1, BDT-TFB2-DPP1-HT1,PBDT-TFB1-DPP2-HT1 and PBDT-TFB1-DPP1-HT2 have both a broad absorption spectrum from 300 nm to the near infrared region and the narrow optical band gaps ranging from 1.56 to 1.58 eV. The short-wavelength absorption of polymers can be adjusted by changing the proportion of electron-donating and electron-accepting units. Additionally, the fluorine copolymers exhibited a long-term thermal stability for 5% weight loss at 335 °C and relatively deeper-lying energy level, which can provide new tetrapolymer donor materials for solar cell applications.