The Design, Synthesis And Applications Of Novel Conjugated Organic Materials Containing Alkyl Imide For Organic Solar Cells

With the global energy and environmental issues becoming increasingly severe, it is an urgent need for people to search for clean and renewable energy. Among the variety of new energy, solar energy has been paid much attention for its inexhaustible supply, no geographical restrictions, no pollution, no noise and so on. Compared with the conventional inorganic silicon-based solar cells, organic polymer solar cells(PSCs) have aroused widespread concern in both academia and industry due to its advantages such as low cost, flexible, roll-to-roll printing process, thin and large area substrates and so on. Currently, the power conversion efficiency of PSCs has exceeded 11%, but there are still many problems waiting to be resolved for the commercial applications, such as low efficiency compared with silicon-based solar cells, poor stabiliy, large-area devices with low efficiency and so on.In this thesis, based on the unique charm of alkyl imide group which allows the integration of the regulation of both electronic energy level and solubility and starting from the design concept of optoelectronic organic semiconductor materials, we have creatively incorporated alkyl imide into common electron-deficient unis such as benzothiadiazole, quinoxaline, dibenzophenazine, benzotriazole and so on, thus several novel acceptor units with improved performance were obtained. Based on the novel acceptor units, we also have synthesized a series of new organic conjugated polymers or small molecules which demonstrated relatively satisfactory results when used in organic solar cells.In the second chapter, three novel donor-acceptor type of conjugated polymers derived from 2,1,3-benzothiadiazole-5,6-dicarboxylic imide and thiophene derivative were designed and prepared by Stille polymerization. We have carefully studied the regulating effect of the stucture of donor units on the absorption spectra, energy levels and photovoltaic properties of the conjugated polymers, which has brought great reference value for the design and optimization of the polymer materials for PSCs application.In the third chapter, we have designed and synthesized two new acceptor units of cyclic imide substituted quinoxaline(TPQD) and dibenzo[a,c]phenazine(TBPDI) which were copolymerized with benzodithiophene(BDT) and indacenodithiophene(IDT), and four novel donor-acceptor type conjugated polymers were obtained. In comparison with the copolymers based on the electron-deficient unit of TPQD, the resultant copolymers based on the enlarged coplannar TBPDI unit exhibited more compact intermolecular packing, red-shifted absorbance and enhanced absoption coefficiency, and higher hole mobility. The best device performance with a power conversion efficiency of 5.58% was achieved by using the IDT-alt-TBPDI copolymer as the photoactive layer. These observations indicated that the developed TBPDI unit that has enlarged coplanarity can be a promising building block for the construction of highly efficient conjugated polymers for solar cell applications.In the fourth chapter, we have designed and synthesized a new acceptor units of 4,8-di(thien-2-yl)-pyrrolo[3,4-f]benzotriazole-5,7-dione(TZBI) for the first time. Compared with benzotriazole, TZBI exhibited stronger electron-withdrawing ability due to the incorporation of cyclic imide. Furthermore, the modifiability of the N atom on imide provides another straightforward route toward alkylation for excellent solubility of the resulting polymers. Through the copolymerization of TZBI and benzodithiophene, a wide-bandgap polymer PTZBIBDT was obtained and used as donor material in PSCs devices. A remarkable photovoltaic performance with a power conversion efficiency of 8.63% was achieved, which opens the door for the application of TZBI unit and lays the foundation for the development of TZBI-based organic materials.In the fifth chapter, we have carried out the further research on TZNI unit by copolymerizing it with dithienobenzodithiophene, and a horizontally developed copolymer PDTBDT-TZBI was obtained. Compared with the polymer PTZBIBDT in the last chapter, PDTBDT-TZBI demonstrated red-shifted absorbance and deeper highest occupied molecular orbital energy level, but the absoption coefficiency of PDTBDT-TZBI was weakened. As a result, the PSCs device based on PDTBDT-TZBI exhibited a decreased power conversion efficiency of 7.66% in terms of the decreased short-circuit current and fill factor. We have every reason to believe that the photovoltaic performance will be improved by optimizing the chemical structure of PDTBDT-TZBI.In the sixth chapter, we have designed and synthesized an n-type small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2. This molecule exhibits a moderately low-lying lowest unoccupied molecular orbital energy level of-3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performance with a power conversion efficiency of 0.77%. The relatively low efficiency can be attributed to the strong crystallinity of this molecule that cause unfavorable phase separation. Future work will concern how to control bulk-heterojunction film morphology and its relationship with photovoltaic performances.