Synthesis And Performance Of Nonhalogenated‐solvent-processable Terpolymer Acceptor Materials

With the continuous development and utilization of high-efficiency photoactive active layers and the optimization of device processes,the power conversion efficiencies（PCE）of polymer solar cells have been improved steadily.Among them,the development of non-fullerene electron acceptor materials（NFAs）has greatly promoted the progress of polymer solar cells.Compared with small molecule NFA acceptors,polymer NFA acceptors have better film-forming properties,and their all-polymer blend films have better morphology stability,so the photovoltaic stability of all-polymer solar cells is better.However,the development of polymeric NFAs has been relatively slow compared with small-molecule NFAs.In addition,most high-performance polymer NFAs are still processed with halogenated organic solvents.Therefore,designing and developing high-performance and green-solvent-processable polymer NFAs is one of the research focuses of all-polymer solar cells（A-PSCs）in order to protect the environmental.Accordingly,two types of terpolymer acceptor materials,which are suitable for processing with non-halogenated organic solvents,were designed and synthesized,and the effects of the electron-withdrawing（A）and electron-donating（D）units as well as their contents on its performance were studied in detail.The main research contents are as follows:1.Three D-A₁-D-A₂ ternary NFAs materials,named as PT-MDI25,PDTPy-MDI25 and PFBDT-MDI25,were designed and synthesized with the octyl-substituted IDTIC derivative as the first electron-withdrawing unit（A₁）,the pyromellitic acid diimide（MDI）derivative as the second electron-withdrawing unit（A₂）,and pyrrole（DTPy）and benzodithiophene as D units.The structure of the D units affects the molecular geometry,energy level,absorption spectrum and aggregation state of the terpolymers.The three terpolymers showed strong intermolecular aggregation properties in chloroform,but weaker intermolecular aggregation properties in xylene solution.Therefore,the three terpolymers were suitable for preparing A-PSCs with o-xylene as the solvent.The HOMO energy level of PDTPy-MDI25 is close to that of PM6,which could lead to a inefficient hole transfer from PDTPy-MDI25 to PM6.Therefore,the exciton separation efficiency of PDTPy-MDI25:PM6 blend film is low,so its A-PSCs device showed the lowest J_sc(5.40 m A cm^-2),FF（43.66%）and PCE（2.05%）values.In addition,the results show that FBDT is beneficial to obtain better-performing polymer NFAs when it is used as the D unit,and the PCE of the PFBDT-MDI25:PM6-based device reached a PCE of 6.96%with a V_oc of 0.958 V,a J_sc of 14.39m A cm^-2,and a FF of 50.48%.2.Four D-A₁-D-A₂ terpolymer acceptor materials PFBDT-NDI10,PFBDT-NDI15,PFBDT-NDI20 and PFBDT-NDI25 were designed and synthesized by using octyl-substituted IDTIC derivatives as A1 units,FBDT as D units,and 10%,15%,20%,and25%mole fractions of NDI（naphthalimide）derivatives as A₂units,respectively.The results indicate that the HOMO energy level of the terpolymer gradually decreased with the increase of NDI content.Moreover,the fluorescence quenching,the exciton dissociation and charge transfer of the polymer:PM6 blend film present the trend of increasing first and then decreasing with the increase of NDI content,which resulte in similar photovoltaic performance trend of the devices.The A-PSCs were prepared by using PFBDT-NDI10,PFBDT-NDI15,PFBDT-NDI20,and PFBDT-NDI25 as the acceptor materials,PM6 as the donor material,and o-xylene as the solvent.The PCE of the PFBDT-NDI10-based device was 4.95%,the PCE of the PFBDT-NDI15-based device was 7.33%,while the PCE of the PFBDT-NDI20 and PFBDT-NDI25-based devices were 5.95%and 2.87%,respectively.This result indicates that the ternary copolymerization strategy of introducing NDI units can effectively weaken the aggregation of the polymer NFAs itself,so that o-xylene can be used as a solvent to prepare A-PSCs devices.Moreover,the NDI content also affects the energy levels and conformations of the terpolymers as well as the exciton dissociation and charge transfer of the blended films,which determine the photovoltaic performance of A-PSCs.On the one hand,the aggregation of the polymer NFAs cannot be effectively weakened,which is adverse to preparing high-performance A-PSCs if the NDI content is too small.On the other hand,too high content of NDI also seriously restrains intermolecular aggregation and charge transport,which is also adverse to high photovoltaic performance.