The Design,Synthesis And Photovoltaic Performances Of Nonfullerene Acceptors Via Side Chain Engineering

As a promising photovoltaic technology for next generation,organic photovoltaics(OPVs)have attracted widespread attention owing to their multiple advantages,such as light weight,mechanical flexibility,semitransparency,and solution processability.In recent years,owing to the great breakthrough of non-fullerene acceptors(NFAs),the power conversion efficiency(PCE)of OPVs have been improved rapidly,and over 18%of PCE has been reported.As for the design of high performance NFAs,the research priorities mainly focus on four aspects:central core design,side chain engineering,?-spacer modification,and end-group decoration.There in,side chain engineering as a method which can quickly change optoelectronic properties of acceptor material to tune morphology of active layer,undoubtedly will play key role in the next breakthrough in PCE of OPVs.Thus,in the thesis,a variety of the NFAs were designed and synthesized based on three side chain modification strategies(novel side chain design,side chain combination and side chain length effect),and systematically investigated the impacts of side chain engineering for OPVs performances.In Chapter 2,two novel functionalized side chains for non-fullerene acceptors were developed.The siloxane-terminated alkyl side chain and siloxane-terminated alkoxyl side chain were successfully introduced into?-spacer unit in NFAs,and then two acceptors with narrow bandgaps,namely IECSi-4F and IEOSi-4F,were synthesized.Notably,both IECSi-4F and IEOSi-4F exhibited high electron mobilities over 10^-3 cm² V^-1 s^-1.With Si25 as polymer donor,the IEOSi-4F based devices displayed lower PCE of 5.51%,due to the larger phase separation in active layer,leading to serious charge recombination and unbalanced carrier transport.Relatively,when blend with PM6,the IECSi-4F which has weaker aggregation could form suitable microscopic morphology to suppress charge recombination,thus the IECSi-4F based devices showed larger J_sc over 25.42 m A cm^-2 and obtained higher PCE of 10.37%.In Chapter 3,the combinatory effect of side chains of a polymer donor and a NFA acceptor on the performance of OPVs was investigated.A new main-chain twisting medium-bandgap acceptor namely i-IEOSi-4F was synthesized,which was the first report of a NFA that comprised siloxane-terminated side chain.If compare with IEOSi-4F,the i-IEOSi-4F with twisted backbone could broaden the bandgap,up-shift the energy level,and provide a large extinction coefficient of 2.36×10⁵ mol^-1 L cm^-1 in solution.Two polymers J52 and PBZ-2Si with the same backbone units but different side chains(a branched alkyl side chain vs.a siloxane-terminated alkyl side chain)were employed as the donor to construct OPVs and showed PCE of 10.46%and 12.66%,respectively.The higher PCE achieved by PBZ-2Si based device should be related to the combinatory effect of the same kind side chains.Specifically,the PBZ-2Si:i-IEOSi-4F blend film exhibited more compact?-?stacking distances and stronger face-on orientation,fairly smooth surfaces,and well interpenetrated network structure for balanced carrier transport,finally leading to larger J_sc of 19.67 m A cm^-2and higher FF of71.45%.In Chapter 4,the green synthesis and side-chain engineering for nonfused acceptor were investigated.Two new benzodithiophenedione(BDD)-based nonfused acceptors,BDDEH-4F and BDDBO-4F,were successfully designed and synthesized via ligand-free direct heteroarylation approach for the first time.With PM6 as the donor,the BDDEH-4F and BDDBO-4F based devices exhibited PCE of 12.59%and 9.80%,respectively.The BDDEH-4F with shorter alkyl side chain afforded better device performance due to suitable phase separation resulted favorable morphology with fibrillar network,which could show more efficient exciton dissociations,weaker bimolecular recombination,and more balance carrier mobilities.Moreover,the BDDEH-4F-based devices also displayed good thermal stability and storage stability.