The Design,Synthesis And Photovoltaic Performance Of Novel Conjugated Polymers Based On (E)-1,2-Di(thiophen-2-yl)ethene

In the recent decades,organic solar cells have attracted tremendous attention due to the advantages of light weight,low cost,flexibility,and the potential for the fabrication of large-area flexible devices.To date,power conversion efficiency(PCE)of the single layer bulk-heterojunction polymer solar cells and tandem solar cells have exceeded 12%.In this process,the design and synthesis of new materials used in the field of organic solar cells to achieve high performance is still the direction of our efforts.Therefore,the main focus of this paper is to explore the relationship between the structure of the material and its photovoltaic properties.Thiophene vinyl thiophene(TVT)unit,its excellent coplanarity and carrier transport characteristics make it widely used in the field of organic field effect transistor(OFET),but it was less investigated in the application of organic solar cells.In this paper,a series of weak electron-donating units based on the thiophene vinyl thiophene were synthesized by means of chemical synthesis,and then copolymerized with different electron withdrawing units(naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole(NT),5,6-difluoro-2,1,3-benzothiadiazole(DFBT)and benzo[1,2-c:4,5-c']dithiophene-4,8-dione(BDD)).The photovoltaic performance of conjugated polymers based on thiophene vinyl thiophene derivatives were studied in detail.In the second chapter,we synthesized three kinds of weak electron donating units based on TVT unit by the introduction of cyano group(CNTVT)and ethoxy group(EOTVT)into the vinyl group,and then copolymerized with the electron withdrawing unit NT to obtain three narrow bandgap D-A polymer donor materials.Electrochemical analysis of the polymers indicated that the introduction of cyano groups effectively reduced the maximum occupied molecular orbital energy level(HOMO)of the polymers and the introduction of ethoxy groups could raise the HOMO energy level.The UV-vis absorption spectra showed that the cyano group promoted the aggregation of the polymer strongly,which facilitated the aggregation of polymer chains,but had a negative impact on the solution processing.The introduction of ethoxy could effectively improve the solubility of the polymers and were not able to produce a strong aggregation behavior.In the basic device characterization,the efficiency of the polymer obtained by the copolymerization of CNTVT and NT was the highest,reaching 6.17%.But its strong aggregation behavior made it difficult to optimize.The conjugated polymers obtained by copolymerization of TVT and NT were optimized and their devices achieved an energy conversion efficiency of 6.89% when the thickness of the active layer was 250 nm.The efficiency of the device based on EOTVT and NT was only 2.78%.Although the efficiency of these three was not high,the modification of vinyl groups of thiophene vinyl thiophene systematically studied the effect on the energy level and bandgap,the aggregation behavior and photovoltaic properties,which was undoubtedly a reference.In the third chaptor,two novel difluorobenzothiadiazole(DFBT)-based polymeric donors,DFBT812 and DFBT1014,possessing 2-octyldodecyl and 2-decyltetradecyl side chains,respectively,were designed,synthesized,and applied for polymer solar cells.Alkyl chains of 2-octyldodecyl and 2-decyltetradecyl were incorporated into the polymer to tune the solubility and crystallization of the polymers and morphology of the blend films.Morphological study showed that the blend film of DFBT812 with PC61 BM presented much better phase separation domain,which is beneficial for charge transport and collection of charge carriers in the blend film.Photovoltaic devices based on DFBT812 exhibited a power conversion efficiency(PCE)of 6.25%,outperforming those based on DFBT1014.Moreover,DFBT812-based photovoltaic devices exhibited a PCE over 6% even when the thickness of the active layer was 220 nm.In the fourth chapter,we used three weak electron donating units(TVT,CNTVT,EOTVT)based on thiophene vinyl thiophene to copolymerize with BDD units to obtain three wide band gap D-A polymer donor materials(PTVT-BDD,PCNTVT-BDD,PEOTVT-BDD).In these three polymers,the film of PEOTVT-BDD was easy to air oxidation because of its high HOMO energy level.Compared with the polymer PTVT-BDD,the aggregation behavior of the PCNTVT-BDD was stronger and the HOMO level was lower,and the corresponding device had higher open circuit voltage and finally achieved a PCE of 3.6%.