Synthesis And Characterization Of Asymmetric Indenothiophene Based Donor-acceptor Copolymers For Efficient Polymer Solar Cells

In the design and synthesis,most of the donor polymers are based on symmetric D-A structure,while few are based on asymmetric structure.In this thesis,indenothiophene(IT)was selected as the donor building unit,and a series of novel asymmetric donor polymers were designed and synthesized,which were combined with fullerene derivative acceptor material PC71BM.The photovoltaic performance of indenothiophene based organic solar cells was systematically studied.(1)Side-chain fluorination of conjugated polymers has been proved to be a highly effective approach for optimizing optical and electrical properties of the relative polymers.However,current studies on the fluorination are all based on symmetric molecular structures.In this thesis,two new D-A type photovoltaic polymers,namely PITPh-DTffBT and PITPhf-DTffBT,based on asymmetric indenothiophene(IT)donor units with alkoxyphenyl or fluoroalkoxyphenyl substituents were designed and synthesized.Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure,ordering structure,photovoltaic properties,and charge generation and recombination dynamics were investigated.It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymer with a deeper HOMO level,higher and more balanced charge mobilites,increased charge dissociation efficiency and reduced bimolecular recombination.As a result,the bulk heterojunction solar cell based on the blend film of PITPhf-DTffBT and PC71BM demonstrated an efficiency of 7.03%,whereas the cell efficiency based on PITPh-DTffBT was only 5.68%.These results indicate that our design strategy by introducing a fluoroalkoxyphenyl unit as side chain to fabricate asymmetric IT-based polymer is efficient in improving the photovoltaic performance.We believe that the results provide new insights into the design of high-performance semiconducting photovoltaic polymers.(2)Two new D-A type photovoltaic polymers,namely PITPh-DfQx and PITPhf-DfQx,based on asymmetric indenothiophene(IT)donor units with alkoxyphenyl or fluoroalkoxyphenyl substitutes were designed and synthesized.Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure,ordering structure,photovoltaic properties,and charge generation and recombination dynamics were investigated.It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymers with a deeper HOMO level,higher and more balanced charge mobilites,increased charge dissociation efficiency and reduced bimolecular recombination.As a result,the bulk heterojunction solar cell based on the blend film of PITPhf-DfQx and PC71BM demonstrated an efficiency of 6.10%,whereas the cell efficiency based on PITPh-DfQx was only 3.00%.In addition,a triblock copolymer,poly(styrene-block-butadiene-block-styrene)(SBS),was employed for the first time as a polymer additive into the active layers based on PITPh-DfQx/PC71BM and PITPhf-DfQx/PC71BM devices to promote donor crystallization and tune the extent of phase separation between the donor and acceptor.The presence of SBS obviously improved the molecule packing and induced the crystallization of the two polymers,giving rise to a better phase separation due to enhanced aggregation effect of photovoltaic polymers.Therefore,with a small addition of SBS,the optimal PCE was further increased from 6.10%to 6.60%for PITPhf-DfQx based device and from 3.00%to 5.50%for PITPh-DfQx based device.The positive effects of SBS additive on the performance of photovoltaic polymer/fullerene BHJ solar cells provide a new strategy for developing high performance polymer solar cells.(3)Two new D-A type photovoltaic polymers,namely PIT-DTBT and PIT-DTffBT,based on asymmetric indenothiophene(IT)donor unit and nonfluorinated or difluorinated benzothiadiazole acceptor unit were firstly designed and synthesized,respectively.Then a triblock copolymer,poly(styrene-block-butadiene-block-styrene)(SBS),was employed as a polymer additive into the active layers of PIT-DTBT/PC71BM and PIT-DTffBT/PC71BM based devices,and the SBS effect on the performance of the two kinds of devices was investigated.The results demonstrated that,difluorinated PIT-DTffBT based devices exhibit better photovoltaic performance than nonfluorinated PIT-DTBT based ones.Meanwhile,polymer additive SBS has a positive effect only on PIT-DTffBT based devices,but is not very sensitive to PIT-DTBT based devices.The presence of SBS obviously improves the molecule packing and induces the crystallization of PIT-DTffBT,giving rise to a better phase separation.However,almost no improvement in molecule packing and crystallization was observed in nonfluorinated PIT-DTBT based devices.After addition of SBS,the optimum power conversion efficiency(PCE)can be increased from 4.61%to 6.57%for PIT-DTffBT based device,while only from 2.94%to 3.13%for PIT-DTBT based device.