Cyclopentadithiophene Based Donor And Acceptor Materials Via Direct C–H Arylation For Organic Solar Cells

At present,solution processed bulk heterojunction（BHJ）organic solar cells（OSCs）have attracted extensive attention from researchers for their advantages of lightweight,thin,soft,low-cost and translucency.Thanks to the development of active layer materials,especially small molecule acceptor materials,OSCs has achieved breakthroughs in power conversion efficiency（PCE）,showing great commercial potential.In the past development of photovoltaic materials,the construction of sp²C–C bond mainly adopts the classical coupling reactions such as Stille and Suzuki.When these reactions are used,they often require the pre-functional group of the substrate,resulting in the rise of synthetic cost,and will produce toxic and harmful wastes（organotin,etc.）,which is unfriendly to the environment and greatly restricts the future large-scale production.In this paper,the synthesis method of direct C–H arylation is used to construct sp²C–C bond,which has the characteristics of atom economy and green,and can avoid the problems existing in traditional Stille,Suzuki and other coupling methods.The main research results are as follows:（1）Three novel star-shaped unfused ring electron acceptors（SSUFREAs）,i.e.,H1-3,with and without fluorine-substituent in phenyl core or peripheral group are designed and synthesized as third components via direct C–H arylation to incorporate into PM6:Y6 BHJ films.The structure-property-performance dependence study reveals that the isotropic charge transfer,complementary star-shape structure and light absorption,and energy level cascades with PM6and Y6 allow all Hs:PM6:Y6 ternary BHJs to have higher PCE compared to the PM6:Y6 binary BHJ.Among them,the ternary BHJ involving fluorine-free H1 possesses the highest PCE（16.57%）owing to the high-lying frontier molecular orbital and the enlarged torsion angle,which enhances open circuit voltage,inhibits the excessive crystallization of Y6,and facilitates exciton dissociation as well as collection.Our findings indicate that SSUFREAs have great potential to serve as third components to optimize morphology and improve the open-circuit voltage(V_OC)of BHJ OSCs.（2）Four oligomer intermediates with progressively increasing conjugated length BTDTn（n=1-4）were synthesized by direct C–H arylation.The main chain structure of BTDTn was alternately composed of difluorobenzothiadiazole（2FBT）and CPDT.Subsequently,three unfused ring electron acceptors（BTDT）n-IC（n=1-3）and a donor（BTDT）4-RD were prepared by formylation and Knoevenagel condensation.The relationship between conjugation degree and structure and performance is studied systematically.When these oligomer acceptors and donor materials PBDB-T are prepared into devices,（BTDT）1-IC has better morphology and higher carrier mobility,which promotes charge transport and exciton ionization,and improves the J_SC and FF of the devices.Therefore,PBDB-T:（BTDT）1-IC OSCs exhibited the highest PCE of 9.14%.However,the phase separation of the acceptor with the longest conjugate length（BTDT）3-IC was too small after mixing with the donor PBDB-T,and the energy level mismatch between the donor and the acceptor resulted in the inability of excitons to dissociate effectively between the interface of the donor and the acceptor,resulting in a low J_SC and the lowest PCE of 3.24%.The efficiency of 4.51%was obtained after blending oligomer donor（BTDT）4-RD with Y6.The results show that direct C–H arylation is a successful method to synthesize complex conjugated compounds,and its controllability is higher than that of Stille,Suzuki,etc.,which will have reference value for the synthesis of photovoltaic materials in the future.In summary,this paper focuses on the direct C–H arylation to synthesize OSCs donor and acceptor materials,and conducts a detailed research on the photoelectric properties,photovoltaic devices,film morphology and other aspects of the synthesized materials,so as to provide ideas for the design and synthesis of photovoltaic materials in the future.