Structure Design And Photovoltaic Performance Of Fused-Ring Electron Acceptors

Organic solar cells(OSCs)have been the focus of academic and industrial attention for nearly 30 years because of their simple processing and low cost.With the continuous upgrading of organic photovoltaic materials,the representative fused-ring electron acceptors(FREAs)has promoted the rapid development of the field of OSCs and improved the efficiency(PCE)of OSCs.The active layer(the combination of donor and acceptor materials)plays the most central role in the structure of OSCs.Figuring out the relationship between the chemical structure of the active layer materials and photovoltaic properties,and developing the corresponding molecular design strategies,which play an important role in obtaining high-performance photovoltaic materials.In this dissertation,five strategies for designing FREA structures are proposed.By tailoring the molecular chemical structure,a series of novel FREAs were synthesized.The influence of molecular structure on their photovoltaic properties was studied,which realized the regulation of the basic properties of materials and the improvement of the performance of OSCs devices.Five strategies to improve the photovoltaic performance of FREAs are as follows:(1)Changing the molecular symmetry:Two isomers with different symmetries(F7TC-1 and F7TC-2)based on benzodithiophene units were designed and synthesized,and the effect of molecular symmetry on the properties of the materials was studied.All side chains were located on the same side of fused-ring core in axisymmetric F7TC-1,while those were evenly located on both sides of fused-ring core in centrosymmetric F7TC-2.From the research,it was found that the changing molecular symmetry showed notable effects on molecular packing and the morphology of the film,which can effectively improve the performance of OSCs devices.When J71 was used as the donor material,the optimal device of F7TC-1based yielded a PCE of 1.08%,while the PCE of the optimal device based on F7TC2 was improved to 11.14%due to lower voltage loss,more balanced charge transfer and lighter carrier recombination.(2)Introducing benzodipyrrole:First,a nine-fused-ring compound 9FIC was designed and synthesized,and then 9PIC was synthesized by replacing benzodicyclopentadiene in the center of 9FIC main chain with benzodipyrrole.It was found that this strategy had a great effect on the energy level and the absorption properties of the materials,especially for the absorption edge of the thin film state that even extending to more than 1000 nm.Using PTB7-Th as the donor material,the PCE of 9FIC-based devices showed only 1.21%,while 9PIC-based devices showed higher short circuit current density(Jsc)and open circuit voltage(VOC)due to the higher energy level and wider light absorption range of the acceptor material,and the PCE was increased to 4.21%.(3)Changing the ring closure sites:Using the characteristic of multiple reaction sites on the benzene ring,two novel asymmetric five-fused-ring isomer receptor materials(1PIC and P1IC)based on benzopyrrole were synthesized by changing the reaction sites on benzene ring in the ring closure reaction.Two groups of OSCs devices were prepared using PM6 as donor material and 1 PIC or P1IC as acceptor material,and the photovoltaic properties of two isomers based on different ring closure sites of benzopyrrole were studied.The PCE of 1 PIC-based devices exhibited 3.74%,while the PCE of P1IC-based devices was increased to 10.2%due to less carrier recombination,more efficient exciton dissociation,and more balanced charge transport.(4)Expanding pyrrole to pyrrolo[3,2-b]pyrrole:The pyrrole in the main chain of 1PIC was expanded to pyrrolo[3,2-b]pyrrole to obtain 2PIC,which was found that this strategy showed great influence on the absorption and energy level of materials.Under the same processing condition,because of the wider light absorption range,higher mobility and smoother film morphology of the active layer,the PCE of PM6:2PIC-based devices exhibited 12.6%,which was much higher than that of PM6:1 PIC-based devices.(5)The combination strategy of introducing pyrrolo[3,2-b]pyrrole and changing the ring closure site:By combining the above two strategies of introducing pyrrolo[3,2-b]pyrrole and changing the ring closure site,a pair of eightfused-ring isomers PPIC1 and PPIC2 based on the pyrrolo[3,2-b]pyrrole unit were synthesized.PPIC1 was asymmetric and PPIC2 was symmetric.Both acceptor materials had wide absorption range and narrow band gap,especially the absorption edge of PPIC2 was up to 1056 nm.When PTB7-Th was used as the donor material,the optimal device based on PPIC2 only yielded a PCE of 1.23%,the PPIC1 counterpart exhibited a larger PCE(8.11%)due to its higher exciton dissociation efficiency,less carrier recombination and lower energy loss.