Design,Synthesis And Photovoltaic Performances Of Novel Conjugated Polymers Containing Imide Benzotriazole Unit

With the annually increase in global energy demand,energy has gradually become the core of the economic development of countries all over the world.As an inexhaustible clean energy,solar energy is currently one of the largest energy sources that can be developed in the world.Organic solar cells that can use solar power to generate electricity have the specific characteristics of light weight,low price,flexibility,etc.,and have attracted widespread attention in the scientific and industrial circles.In recent years,with the unremitting efforts of domestic and foreign scientific researchers,novel organic and polymer semiconducting materials have continuously emerged,and the power conversion efficiency of organic photovoltaic devices based on organic / polymer semiconductor materials has exceeded 18%.This dissertation is devoted to the development of novel polymer photovoltaic materials based on an imide benzotriazole(Tz BI)unit,and explorating the relationships between the molecular structures of these electron-donating conjugated polymers,and ultimately,the potential applications of these novel polymers in polymer solar cells.In the second chapter,two donor-acceptor(D-A)type conjugated copolymers,namely PTz BI-BDF-T and PTz BI-BDF-Ph,were developed.These two electron-donating polymers are constructed based on the benzo[1,2-b:4,5-b?]difuran unit(BDF)unit as the electron-rich unit,and Tz BI unit as the electron-deficient unit.The effects of alkylthienyl side chains and alkylphenyl side chains on the optoelectronic and thermal properties of these polymers were systematically studied.To reveal the effects of the molecular structures on the photovoltaic performances of these copolymers,polymer solar cells were fabricated by blending these copolymers with electron-accepting materials to fabricate the bulk-heterojunciton lightharvesting layers.It was found that the copolymer PTz BI-BDF-T presented bathochromic shift in absorption profile in film state,and the copolymer PTz BI-BDF-Ph presented much deeper frontier molecular orbital energy levels,as confirmed by cyclic voltammetry measurement.Device based on PTz BI-BDF-T exhibited much higher short-circuit current density than that of PTz BI-BDF-P device,while the device based on PTz BI-BDF-Ph exhibited much higher opencircuit voltage than that of PTz BI-BDF-T device.The resulting polymer solar cells based on these polymers presented reasonable power conversion efficiencies of over 6%.In the third chapter,two novel conjugated polymers,namely PTz BI-S and PTz BI-Ph,were designed and synthesized by incorportating alkyl-side chains containing sulflur atom into the benzodithiophene(BDT)derivatives.Here the molecular design strategy involves the virtues of the relatively weak electron-rich properties of the alkylphenyl unit,and the existence of empty 3d-orbital in the sulfur atoms of the alkyl-sulfur chains.The effects of molecular structures of the side groups on the physical and chemical properties of these novel conjugated polymers are systematically studied.To evaluate the photovoltaic performances of these polymers,polymer solar cells were fabricated by blending these new copolymers with fullerene and non-fullerene acceptors as the light-harvesting layers.It is noted that the incorporated alkylside chains containing sulfur atom can obviously reduce the highest occupied molecular orbital energy level,and thus obviously enhance the open-circuit voltage of the resulting polymer solar cell to above 0.9 V.Of particular importance is that the device based on PTz BI-Ph and a fullerene acceptor presented an impressive power conversion efficiency of 8.84%,which is among the highest value of so far reported fullerene based polymer solar cells based on widebandgap conjugated polymers containing Tz BI unit.By incorporating alkoxylphenyl side chain and fluorine atom into the molecular chain of BDT-Tz BI,two novel wide-bandgap conjugated polymers,namely PTz BI-Ph O and PTz BIPh O-2F,were designed and synthesized.The effects of alkoxylphenyl side chain and fluorine atom substitution on the optical,electrochemical and thermal properties of polymers were studied.By blending these copolymers with a non-fullerene acceptor as the light-harvesting layer,the resulting copolymer containing fluorine substitution presented much higher photovoltaic performance,which showed a champion power conversion efficiency of 8.76%.Futher studies indicated that the enhancement in photovoltaic performance of the fluorinated copolymer can be attributed to the simultaneously improved charge carrier mobility and opencircuit voltage.The atomic force microscope(AFM),transmission electron microscope(TEM)and the relevant film morphology characterizations indicated that the incorporation of fluorine substitution into the side chain of the alkoxylphenyl group obviously improved the film morphology.It is found that the introduction of fluorine atom has many favorable factors for polymer solar cells.Thus,fluorine atoms were introduced into the pristine conjugated polymer PTz BI,and the modification of alkyl chains gave four wide-band gap conjugated polymers,namely PTz BI,PTz BI-2F,PTz BIEH-2F and PTz BIEHp-2F.The influence of fluorine atom on the properties of the polymer and photovoltaic devices was systematically studied.It is found that the introduction of fluorine atom has little effect on absorption spectrum and thermal properties,but it can deepen the molecular orbital energy level of polymer and improve the charge carrier transportation.The optimization of alkyl side chain can increase the solubility of polymer,facilitate device processing,improve charge mobility and improve photovoltaic performance.Among them,the devices based on PTz BIEHp-2F can achieve the power conversion efficiency of over 12%.In addition to the incorporation of fluorine atom,recent progress indicated that the incorporation of chlorine atom also has pronounced effects of the optoelectronic properties of organic semiconducting materials.In chapter 6,a new conjugated polymer PTz BIEHp-2Cl was devleoped by introducing chlorine atoms into the research system,which was compared with the fluorinated polymer PTz BIEHp-2F.It was found that the optical properties of the two polymers were slightly different,while PTz BIEHp-2Cl had a deeper molecular orbital energy level.Devices based on polymer donor of fluorinated or chlorinated polymer donors,PTz BIEHp-2F and PTz BIEHp-2Cl,were blended with the small molecule acceptors of fluorinated IT-4F and the chlorinated IT-4Cl,respectively.It was realized that the incorporation of chlorine substitution lead to decreased molecular orbital energy level of the electron-donating polymer,and reduce the band-gap of the the elelctron-acceptor and thus accounting for the redshifted absorption profile.The combination of these merits lead to enhanced open-circuit voltage and short circuit voltage.In particular,the devices based on PTz BIEHp-2Cl:IT-4Cl presented the champion power conversion efficiency of 12.86%,presenting the great potential of these novel conjugated polymers for the application as the electron-donating polymer for high-performance polymer solar cells.