Terminal-groups Engineering Of Asymmetric Fused-ring Electron-acceptor And Study Of Organic Solar Cells

Due to the rapid development of human society,people’s demand for energy is also increasing year by year.fossil fuels have long been unable to meet human needs,and the global climate changing caused by them is forcing people to search for new types of energy.Solar energy is free,widely distributed and clean.what’s more,it is the most extensive and effective renewable resource.After the development of three generations of photovoltaic cells,organic solar cells（OSCs）are considered to be one of the most promising candidates in photovoltaic technology due to their clean,non-toxic,low cost,light weight,good flexibility,solution processability,and suitability for large-scale production.At present,the key research problems in the field of organic photovoltaic materials are their low efficiency and poor stability,which is also the reason why they cannot be put into commercial production.OSCs are divided into polymer solar cells and small-molecule solar cells.In the past few decades,the acceptor materials of OSCs are usually bulk heterojunctions that use p-type polymers as electron-acceptors and n-type fullerenes as electron-acceptors.However,fullerene-based acceptors have significant shortcomings,for example,narrow absorption range,poor modification,and high synthesis cost.Therefore,non-fullerene fused-ring electron acceptors（FREAs）emerged,and such acceptors usually have acceptor-donor-acceptor（A-D-A）structure.The fused-ring ladder electron-rich core（D）is a rigid conjugated structure,which is conducive to charge transfer and molecular accumulation,and the large steric hindrance group attached to the D core can inhibit excessive molecular aggregation,so that it can demonstrate a good solid-state morphology.The terminal group（A）is an electron-deficient indanone group,which can enhance theπ-πstacking of neighboring molecules and promote near-infrared absorption of molecules.At present,a large number of groups have focused on symmetric FREAs.We will focus on asymmetric FREAs because of its increased dipole moment and dielectric constant,reduced exciton binding energy.This is benefit to the dissociation of excitons and the transfer of electric charges.Modification of the terminal group is an important methodology in improving the OSCs performances of FREAs.The commonly used terminal group is1,1-dicyanomethylene-3-indanone（IC）,and halogenation is the most commonly used method.The halogenation of IC can promote the near-infrared absorption,reduce the LUMO/HOMO energy level as well as adjusting the bandgap,and even improve the morphology of the blend film.The replacement of benzene with thiophene in the IC unit is also a effective method to improve the photoelectric properties of the acceptors.Recent studies have proved that the introduction of trifluoromethyl group into the electron-deficient terminal group can obtain an ultra-low band gap acceptor,and is beneficial to the formation of a 3D interpenetrating network and improving the charge transport ability of the blend film.Based on the above considerations,we mainly carried out the following works:（1）Four acceptor molecules,namely TPIC-2Cl-γ,TPIC-2Br-γ,TPIC-3Cl and TPIC-2Cl-Br,were designed and synthesized.Among them,TPIC-3Cl and TPIC-2Cl-Br were two double asymmetric FREAs.Compared with another high-efficient acceptor of TPIC-4Cl which has been reported by our group,we have studied the effects of the number of halogen atom modifications on their photophysics,electrochemistry,active-layer morphology,charge transport and photovoltaic performance in detail.With the gradual increase of halogen atoms（2,3 to 4）,these asymmetric acceptors show a gradual red-shifted absorption spectrum;as the electronegativity gradually increases,LUMO and HOMO gradually shift downward.When blended with the polymer donor PM6,the PCE of devices based on tri-halogen atom-substituted acceptors（TPIC-3Cl and TPIC-2Cl-Br）are always the highest,followed by tetra-halogen substitution,and two-halogen substitution the lowest.Among them,OSC based on PM6:TPIC-2Cl-Br achieved the highest performance(PCE=15.08%,V_OC=0.91 V,J_SC=21.59 m A/cm²,FF=0.77).（2）We designed and synthesized two thiophene-based terminal groups CPTCN-Cl and CPTCN-CF₃,and used them to synthesize the target products TPC-Cl and TPC-CF₃.Compared with TPC-Cl,the absorption absorption of TPC-CF₃has a slight red shift,and the LUMO/HOMO energy levels are all downshifted.Both acceptors have good thermal stability and can be used in the production of organic solar devices.However,due to time reasons,other photophysical property tests have not been tested,and battery devices based on these two acceptors are also being prepared.