Design,Synthesis,and Photovoltaic Propertiesof Conjugated Polymers Based On Difluorobenzoxadiazole

It is clear that the traditional fossil energy is nonrenewable and has negative consequences on environment,such as greenhouse effect.Exploring sustainable energy has attracted amount of attention.Solar energy has the capability to meet the word's demand.Solar cells which can convert the sunlight into electricity are one of the most promising technologies to harvest this energy.Especially,polymer solar cells hold promise for fabricating light-weight,large-area,and flexible devices via low-cost and high-throughput manufacturing such as roll to-roll processing and inkjet printing,which spurs extensive research efforts.Nowadays,benefiting from efforts in material innovation and device engineering,significant progress in device performance has been achieved in the past two decades,resulting in power conversion efficiencies in excess of 15%.With respect to difluorobenzothiadiazole that has been widely explored for applications in polymer solar cells,difluorobenzoxadiazole can significantly reduce the HOMO and LUMO of the polymers while the optical bandgap remains unchanged.This difference can improve the V_oc while decrease the energy loss.Besides,difluorobenzoxadiazole has a larger dipole moment and planar backbone which will lead to a stronger intermolecular interaction.So the polymer based on difluorobenzoxadiazole would have a stronger aggregation tendency in solution and a higher crystallinity in blend films.In the second chapter,we synthesized two alternating donor–acceptor conjugated polymers BDT-ff BX-OD and BDT-ffBX-DT based on 5,6-difluoro-2,1,3-benzoxadiazole?ffBX?and benzo[1,2-b:4,5-b?]dithiophene?BDT?through rational molecular design.Both polymers possess outstanding comprehensive properties.Especially,BDT-ffBX-DT is a champion among recent photovoltaic polymers that are promising for true realworld applications.When blended with a low-cost acceptor(C₆₀-fullerene derivative),BDT-ffBX-DT produces a PCE of 9.4%at active layer thickness over 250 nm.Moreover,BDT-ffBX-DT devices can be fabricated from nonhalogenated solvents at low processing temperatureIn the third chapter,based on the success of BDT-ffBX-DT,we extendedthe absorbance range via replacing the donor unit BDT bythienothiopene?TT?toimprovethe short-current density and power conversion efficiency.In the forth chapter,we designed and synthesized four D-A1-D-A2 random terpolymers.5,6-difluoro-4,7-dithieno[2,1,3]thiadiazole?DTffBT?and5,6-difluoro-4,7-dithieno[2,1,3]oxadiazole?DTffBX?servedasA1,while5,6-difluorobenzo[2,1,3]thiadiazole?ffBT?and 5,6-difluorobenzo[2,1,3]oxadiazole?ffBX?served as A2.Bycopolymerizingwith the donor unit of thienyl-substituted benzo[1,2-b:4,5-b?]dithiophene?BDT?,we obtained four D-A1-D-A2 random terpolymers,they are 2TBT-BT,2TBX-BT,2TBT-BX,and 2TBX-BX.Using these four polymers,we investigated the relationships between chemistry structure and devices performance of the random terpolymers,and the charge generation,charge transportand recombination,and morphology were studied as well.