Packing Arrangements And Photovoltaic Properties Of Non-Fullerene Isomeric Acceptors

It is urgent to develop renewable and green energy to meet the growing needs of mankind.Organic solar cells（OSCs）have gradually emerged in new energy materials due to their low price,light-weight,various colors,and the realization of integrated construction.In the past two decades,great efforts have been made and the power conversion efficiency（PCE）of OSCs has increased rapidly from less than 1%to more than 18%,showing great potential in commercial applications.Among them,the design of new non-fullerene acceptor（NFA）materials has become one of the most effective strategies to the rapid development of OSCs.Apart from fast development of new NFA materials,better understanding of internal mechanism of these materials is also quite important,especially the influence of their packing models and aggregation behaviors on charge transport,which is helpful in designation and development of acceptors with excellent performance.In this thesis,a series of NFA materials were designed and synthesized,especially isomers with similar structures but hugely different performance.Single-crystal analysis etc.were employed to investigate the relationships between structure and performance and its impacts on photovoltaic performance.In the A-D-A system,the isomer separation of the end-group IC-Cl-m was achieved,and the chlorine-substituted isomers ITIC-2Cl-δand ITIC-2Cl-γwere synthesized.According to the single-crystal analysis,ITIC-2Cl-δshows a linear packing structure while ITIC-2Cl-γshows a 3D network packing mode,forming a rectangular network charge transmission channels,which would provide more electron jump nodes.Therefore,the devices based on ITIC-2Cl-γachieved an excellent PCE of more than 13%.This shows that even simple chlorine substitution isomerism is very effective for improving the PCE of OSCs.To further explore the effect of chlorination,two isomers ITIC-2Cl-βand a-ITIC-2Cl with different symmetries were synthesized,and their packing arrangement and photovoltaic performance were also compared.The results show that the asymmetric strategy can also induce to form a 3D network transmission structure,leading to higher mobilities and improved current in the device.By adjusting the substitution position of chlorine atom on end group to realize the transformation of molecular packing models,which further affects its own physicochemical properties and corresponding device performances.Further exploration of chlorination was implied in the A-D-A-D-A system.Dichlorine-substituted isomers BTIC-BO4Cl-βγand BTIC-BO4Cl-βδwere designed and synthesized.Single crystal analysis shows that BTIC-BO4Cl-βδexhibits the best molecular planarity,shortestπ···πstacking distance,and minimum binding energy among the three acceptors,leading to the tightest 3D network packing structure.The devices based on BTIC-BO4Cl-βδachieves higher carrier mobility,less bimolecular recombination,and lower non-radiative energy loss,leading to the highest PCE of17.04%.This shows that change of molecular packing arrangement,especially the size of 3D network skeleton,has a great impact on the charge transfer,and finally affects the device performances.In addition,the reasons for frame size of this system were also explored:different alkyl chains lead to various molecular packing modes.Ethylhexyl substituted molecules form a loose structure with large frame,while butyl octyl substituted molecules form a tight structure with small frame.This affects the size of the 3D network frame skeleton,which is finally reflected on the packing orderliness of acceptor materials.On the basis of chlorination,the effect of trifluoromethylation on molecular properties was further studied.Combined with isomerization strategy above,an ultra-narrow band-gap molecule BTIC-CF₃-γwas constructed.The single crystal structure of BTIC-CF₃-γis helpful to understand the packing arrangement of acceptors in this system:the cooperative H/J aggregations form a 3D network packing structure of elliptical frame.When the OSC devices are prepared,the devices based on BTIC-CF₃-γexhibit PCE of 15.59%,which is much higher than that of devices based on F and Cl substituted acceptors（13.61%and 13.16%）.In conclusion,trifluoromethyl substitution is an effective strategy for designing ultra-narrow band-gap NFAs.On this basis,combined with the chlorination strategy above,the asymmetric acceptor BTIC-2Cl-γCF₃ with higher crystallinity and good solubility was obtained.The PCE of OSCs devices treated with toluene is as high as 16.31%.When PC₇₁Th BM is added,the PCE of obtained ternary devices was improved to 17.12%.In addition,the semi-transparent devices based on BTIC-2Cl-γCF₃ achieve a PCE of 13.06%with a light transmittance of 24.45%,which shows application potential in the production of building integration.The asymmetric molecular design combined with chlorination and trifluoromethylation achieves the ideal aggregation state of the material,which has certain crystallinity and solubility in green solvents.In addition to the positional isomerism strategies,other isomerism strategies have also been explored.First,functional group isomerizations:two isomers BTIC-OCOMe and BTIC-COOMe were synthesized.After fabricated as OSC devices,the PCEs of acetoxy and ester substitution systems were 8.32%and 13.25%,respectively,which can be attributed to three main reasons:absorption,energy levels and exciton splitting,etc.This result is a supplement to the study of isomers caused by different position substitution.Second,core isomerism:two cis-trans isomers BDT_tIC-γCl and BDT_cIC-γCl were synthesized.The theoretical calculation shows that the energy level distribution of two acceptors is quite different,which is manifested in the difference of absorption spectrum.Therefore,BDT_cIC-γCl based devices achieve an efficiency of7.61%,compared with that of devices based on BDT_tIC-γCl（0.71%）has been greatly improved.GIWAXS shows that there are more regular and ordered molecular packing arrangement in BDT_cIC-γCl neat film and blend film,which is consistent with its higher mobilities,thus,higher current and fill factor are obtained in OSC device.This research supplement the previous research on the backbone isomerization,and help to understand central core design of NFAs materials.