Studies Of Photovoltaic Properties Of Conjugated Polymers Constructed By Random Copolymerization And Isomerization Strategies

As a promising solar photovoltaic device,organic solar cells possessing the advantages of light weight,lower cost,room temperature solution processing and the potential of fabricating flexible devices.In recent years,breakthroughs have been made in the power conversion efficiency（PCE）of organic solar cells,and the device efficiency of single-junction organic solar cells has exceeded 19%.In order to further improve the PCE of organic solar cells,the design and synthesis of new donor and acceptor materials become a crucial aspect.However,the molecular design of donor and acceptor materials generally faces some challenges including the limited development of new structural units and highly development cost.To solve these problems,the random copolymerization strategy and isomerization effect were used to modify or design the polymer donor and acceptor materials in this thesis.At the same time,the relationship between the changes of molecular structure and the differences of device performance was studied in detail.The specific research work and results are as follows:1.A series of random polymer donor materials based on polymer PM6 were designed and synthesized through random copolymerization strategy,by introducing benzodithiophene（BDT）and benzodithiophene dione（BDD）units as the third unit,respectively,After the introduction of BDT unit,a decreased HOMO energy level and the gradually blue-shifted absorption spectrum were observed in random polymers,while when BDD unit was introduced,the change trend is almost the opposite.Furthermore,the density functional theory（DFT）calculations showed that the coplanarity of random polymer molecules was optimized after the introduction of BDT units.Benefiting from the lower HOMO level,enhanced face-on orientation and significantly improved carrier mobility in the blend film,the PM6-5%BDT:Y6-based devices achieved a PCE approaching 16.8%,which is much higher than that of PM6:Y6 and PM6-5%BDD:Y6 based devices（15.65%and 13.87%）.In addition,the device stability test under continuous illumination for more than 600 h showed that the devices based on PM6-5%BDT still maintained more than 85%of the original device efficiency,showing a much more excellent device stability than that of PM6.2.The simple structure of bisfluoro-substituted thiophene（2FT）and dicyano-substituted thiophene（2CNT）were selected as the third unit to be introduced into the molecular structure of polymer PM6.The introduction of 2CNT unit effectively reduced the HOMO energy level of the random polymers,while the HOMO energy level increased after the 2FT unit was introduced.In addition,by comparing the absorption spectra of polymer films,it can be found that the crystallinity of random polymers based on two different third units has changed significantly.Although the open-circuit voltage(V_oc)of PM6-10%2FT:IT4F-based devices decreased due to the higher HOMO level,but the device short-circuit current density(J_sc)and fill factor（FF）were greatly improved due to the higher carrier mobility and lower charge recombination,and resulting a high PCE of 12.65%,which is similar to the performance of the device based on PM6:IT4F.However,after the introduction of 2CNT unit,although the V_oc gradually increased in the devices,the device performance still decreased due to the decreased J_sc and FF.This work provides a molecular design reference for the use of simple structures as the third unit to modify polymers in random copolymerization strategies.3.By adjusting the position of ester functional groups,an isomer 4T2C（4T2C-a and 4T2C-b）with different steric hindrance effects and non-covalent bonding effects were constructed.At the same time,the isomers were systematically simulated by topological analysis and the surface electrostatic potential（ESP）distribution,and the relationship between molecular conformation and electronic properties was studied.Benefited to the appropriate crystallinity,lower HOMO energy level,higher film absorption coefficient and charge mobility,the devices based on polymer PBTz4T2C-a with 4T2C-a unit achieved a PCE of more than 11%.This work not only studies the internal relationship between isomerization effect and device performance,but also provided a feasible research method for studying the changes of molecular geometry and electrical properties caused by isomerization effect.4.The polymer acceptor materials PA1 and PA2 with isomeric effect were constructed by changing the substitution position of fluorine atoms,and the polymer acceptor PA3 which containing multiple fluorine atoms was also designed and synthesized.The similar main chain configuration but significantly different average surface electrostatic potential（ESP）distribution of polymer molecules with isomerization effect were discovered from DFT calculations.All-polymer organic solar cells based on these three different polymer acceptor materials were fabricated.Among them,the PA1-based device showed best device performance with a PCE over 13%,while PA2-based devices,which has an isomeric effect with PA1,showed a dramatic drop in device performance with a PCE less than 3%due to the serious problems of exciton recombination and low charge separation efficiency.At the same time,the effects of isomerization and multi-fluorination on the stability of the device were studied.Considering the current situation that fluorine atoms are widely used in small molecule acceptors,this work provides a certain reference for the molecular design and the optimization of device stability of fluoropolymer acceptors.