The Synthesis of Conjugated Polymer Based Benzofuran and the Study of Its Photovoltaic Performance

Author:Gao Yue Yue

Supervisor:yang yu lin zhang yong


Degree Year:2018





Organic polymer solar cells(PSCs)possess many advantages,such as light weight,low cost,good flexibility,and the potential of fabricating large-area devices via roll to roll form with spin-coating,screen printing or ink-jet printing processing.PSCs are considered as the third generation solar cells,which are expected to be large-scale application in the future.At present,the majority of donor materials in PSCs are benzothiophene-based polymers.However,thiophene,which is the raw material,has limited reserves,consumes large energy and generates pollution when prepared.The above disadvantages limit the large-scale preparation of benzothiophene-based polymers.Furan,which is in the same group with thiophene,possesses extensive source,and the preparation process is mature without pollution.Most importantly,benzofuran-based polymers,which possess the same structure as polymers based benzothiophene,have shown comparable and even higher photovoltaic performance than polymers based benzothiophene.Nevertheless,no matter from the number of polymers or the highest PCE of PSCs,benzofuran-based polymers lag behind polymers based benzothiophene.For nourishing the system of polymers based benzofuran,this paper modified the backbone and side chain of polymers to prepare a series of D-A type polymers based benzofuran,and systematically studied the thermal properties,molecular configuration,optical properties,electrochemical properties,molecular stacking behaviour and photovoltaic properties of these benzofuran-based polymers.The main work could be divided into the following parts:We adopted thieno[2,3-f]benzofuran(TBF)and benzo[1,2-b:4,5-b′]dithiophene(BDT)as donor units to copolymerize with thieno[3,4-c]pyrrole-4,6-dione(TPD)acceptor unit,and prepared PTBFTPD and PBDTTPD.We systematically investigated the effect of introducing furan into the polymer backbone on thermal properties,conjugated degree,absorption spectra,electrochemical level,molecular stacking behaviour and photovoltaic properties of polymer.In comparision with PBDTTPD,PTBFTPD had larger conjugated degree,lower highest occupied molecular orbital(HOMO)energy level,smaller layer spacing.The PCE of PTBFTPD-based fullerene PSCs was 4.33%.Under the same condition,the PCE of PBDTTPD-based fullerene PSCs was only 3.29%.We adopted difluorobenzotriazole(fBz)acceptor unit with thiophene asπbridge,which possessed weaker steric hindrance,and used it to copolymerize with thieno[2,3-f]benzofuran(TBF)to prepare PTBFBz.We systematically investigated the thermostability,molecular configuration,optical properties,HOMO energy level,XRD properties and photovoltaic properties of PTBFBz.PTBFBz possessed large conjugated plane,low HOMO energy level,small lamellar spacing and layer spacing.The PCE of PSCs based PTBFBz:PC71BM was 4.52%.Since PTBFBz is more closely matched with small molecule ITIC in absorption spectra and energy level,the PCE of PSCs based PTBFBz:ITIC could reach up to 8.33%.In order to enhance the conjugated degree of polymer based benzofuran,we replaced thieno[2,3-f]benzofuran(TBF)unit of PTBFBz with benzo[1,2-b:4,5-b′]difuran(BDF)unit,and meanwhile introduced alkylfuryl side chain into BDF.Finally,PBDFT-Bz and PBDFF-Bz were prepared.We systematically studied the effect of changes of backbone and side chain of polymers on photophysical properties,molecular torsion angle,frontier orbital level,and photovoltaic properties of PBDFT-Bz and PBDFF-Bz.Compared with PTBFBz,the molecular torsion angle of PBDFT-Bz and PBDFF-Bz decreased and the absorption red-shifted.PBDFF-Bz possessed wide absorption,lower HOMO energy level,smaller lamellar spacing and layer spacing than those of PBDFT-Bz.The PCE of PSCs based PBDFF-Bz/PBDFT-Bz:PC71BM were 5.07%and 4.04%,respectively.The PCE of PSCs based PBDFF-Bz/PBDFT-Bz:ITIC were 9.46%and 9.26%,respectively.The PCE of PSCs based PBDFF-Bz/PBDFT-Bz:m-ITIC increased to 10.28%and 9.84%,respectively.The PCE(10.28%)of PSCs based PBDF-Bz:m-ITIC renovated the highest record in PSCs based benzofuran-based polymer.In the view of the condition that PBDFF-Bz showed better PCE with low Jsc,we introduced sulfur atom into the side chain of BDF,then used it to copolymerize with difluorobenzotriazole(f-Bz)and benzotriazole(Bz)with thiophene asπbridge,and got PBDFS-fBz and PBDFS-Bz.We systematically analyzed the effect of the changes of side chain and backbone of polymers on conjugated degree,thermostability,absorbance ability and electrochemical level.We carried out a preliminary study on the photovoltaic properties of PSCs based PBDFS-fBz/PBDFS-Bz:ITIC.In comparison with PBDFF-Bz,the conjugated degree of PBDFS-fBz and PBDFS-Bz decreased,but the molecule of PBDFS-fBz and PBDFS-Bz still keepd better conjugated plane.The absorption of PBDFS-fBz and PBDFS-Bz blue-shifted in comparison with that of PBDFF-Bz,whereas,the molar absorption coefficient of PBDFS-fBz and PBDFS-Bz increased significantly.Compared with PBDFS-Bz,PBDFS-fBz presented lower HOMO energy level,smaller lamellar spacing and layer spacing.Through the preliminary exploration,the PCE of PSCs based PBDFS-fBz/PBDFS-Bz:ITIC were9.00%and 8.07%,respectively.