Direct C-H Arylation-Derived Unfused-Ring Electron Acceptors For Optoelectronic Property And Photovoltaic Device

In recent years,fused-ring electron acceptors have been widely studied,which leads to new peaks in the power conversion efficiency（PCE）of organic solar cells.However,fused-ring electron acceptors suffer from cumbersome synthesis steps and difficulties in purification,resulting in relatively low yields and high costs.Unfused-ring electron acceptors can avoid low-yield cyclization reactions and maintain planar conformation through non-covalent interactions,which allowed device performance comparable to that of fused-ring electron acceptors.Thus,the design and synthesis of high performance of unfused-ring electron acceptors are essential for the development of the organic photovoltaic field.In addition,the sp² C-C bond inπ-conjugated organic photovoltaic materials is still mainly synthesized by classical Suzuki or Stille cross-coupling reactions,which usually results in the complex synthesis steps and expensive expenses with the harsh reaction conditions of strong alkali and low temperature to pre-functionalize the raw materials needed.Moreover,the threat of the environment is caused by the subsequent toxic by-products.Therefore,C-H bonds instead of C-M bonds combined with C-X bonds to form new C-C bonds are an ideal synthetic way for the synthesis of organic photovoltaic materials,which conforms to the atomic economy principle of green chemistry.To address the above challenges in the field of organic solar cells,the long-term research of synthesis of organic photovoltaic materials field uses atom-economic direct C-H arylation in our group.Eighteen novel unfused-ring electron acceptors with simple structures were efficiently synthesized via direct C-H arylation in this article.Moreover,optoelectronic properties and corresponding organic solar cells for the synthesized acceptor have been investigated systematically.The main research contents are as follows:（1）The A-D-A type unfused-ring electron acceptors with 4,4-bis（2-ethylhexyl）-dithiophenocyclopentadiene and 1,4-difluorobenzene as core with progressively increasing conjugation,four 5,6-difluoro-3-（dicyanomethyl）-3-indenone as terminal groups CPFB-IC-n（n=1-4）and three 5.6-dichloro-3-（dicyanomethyl）-3-indenone as terminal groups DFPC-2Cl-n（n=1-3）were synthesized via direct C-H arylation.The effect of conjugation length on optoelectronic properties and device performance was systematically investigated.Among them,CPFB-IC-4 has the longest conjugation length among the unfused-ring electron acceptors reported so far.The HOMO energy level of CPFB-IC-2 is closed to zero(ΔE_HOMO=0.06 e V),still obtaining the highest PCE of 7.55%due to the most balanced frontier molecular orbitals and complementary light absorption.In addition,the introduction of chlorine atoms in the end groups can effectively deepen the highest occupied orbital energy level and the hole can be transported efficiently from the acceptor to the donor.The present work shows that the optical properties and frontier orbital energy levels of unfused-ring electron acceptors can be fine-tuned by progressively elongating the conjugation length,thus which enables efficient screening of high-performance non-fullerene acceptors.Meanwhile,direct C-H arylation is an efficient synthetic method for the synthesis of oligomeric unfused-ring electron acceptors with stepwise extension of conjugated length.（2）Three A-DA’D-A type unfused-ring electron acceptors,i.e,IDC8CP-IC,IDC6CP-IC and IDC6IDT-IC,based on isoindigo with different alkyl chains as the central core,4,4-bis（2-ethylhexyl）-dithiophenocyclopentadiene and 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b’]dithiophene as the"D"unit,were synthesized via direct C-H arylation.The optoelectronic properties of the three molecules were systematically characterized.Among them,IDC8CP-IC and IDC6CP-IC can reach the near-infrared absorption range,and the corresponding devices were prepared by matching them with PBDB-T donor respectively to investigate the effect of alkyl chain length on molecular aggregation and stacking properties.IDC6CP-IC as acceptor to prepare bulk heterojunction organic solar cells possess the highest exciton dissociation efficiency and the most balanced charge transport behavior with a PCE of3.10%.The work shows that controlling the alkyl chain length of isoindigo can regulate molecular aggregation and crystallinity,resulting in the highest device performance.In addition,IDC8CP-IC and IDC6CP-IC reach near-infrared absorption,which is a reference value for the development of translucent organic solar cells.（3）Two"Z"and"Y"type unfused-ring electron acceptors were synthesized via direct C-H arylation and polymerized with different linkage units of thiophene,thiophene,trithiophene and pyrroroledione（DPP）to obtain eight polymers,i.e,P1-P8,respectively.The structure and conformational relationships of eight polymers were systematically characterized and investigated.Small molecule acceptors with narrow band gap and near-infrared absorption are combined with the advantage of good stability of polymers to construct a strategy for the polymerization of small molecule acceptors（PSMAs）.Compared with conventional fused-ring small molecule acceptors,this work shows that the stacking and aggregation between polymers and their optoelectronic properties can be fine-tuned by optimizing unfused-ring electron acceptor configurations and linking units,and then screening high-performance polymer acceptors for the preparation of high-performance all-polymer organic solar cells.