Synthesis And Properties Of Symmetric And Asymmetric Small-molecule Acceptor Materials Containing N,S-aromatic Heterocycles

In recent years,high-performance non-fullerene acceptor(NFAs)materials have received special attention in organic solar cells(OSCs)and have made great progress with a power conversion efficiency(PCE)exceeding 18%.Currently,the molecule design strategy of the most effective NFAs material is to construct A-D-A type small molecule acceptors(SMAs).According to the symmetry of the molecular structure,SMAs can be divided into two categories:symmetric and asymmetric.In addition to the similar benefits of symmetric small molecules,asymmetric small molecules also have stronger intermolecular binding energy and larger dipole moments relative to symmetric small molecules,which are conducive to enhancing intermolecular interactions and improving device performance.Therefore,in this paper,we designed and synthesized a series of symmetric and asymmetric SMAs materials containing N,S-aromatic heterocycles,focusing on the influence of the asymmetric center electron donating unit(D),terminal electron withdrawing unit(A)and alkyl side chains on the molecular structure,thermal stability,photophysics,electrochemical properties and photovoltaic performance of small molecule acceptor materials.The main research contents are as follows:1.Two asymmetric SMAs materials TTCIC and TCIC based on thiophene(thiophene)and indole units with multiple asymmetric factors,including a central core and side chains were designed and synthesized.The effects of multiple asymmetry factors(central core and side chains)and the expansion of the conjugated system of the central D unit on the molecular structure,absorption spectrum,energy level,carrier transport property,film morphology and photovoltaic performance of the materials are systematically studied.The results showed that multiple asymmetry factors led to different molecular conformations,so that the two molecules had different aggregation behaviors.TTCIC is H-type aggregation,which resulted in a blueshift of the absorption spectrum.Compared with thiophene,thieno[3,2-b]thiophene(TT)has stronger electron-donating property,which led to a higher LUMO energy level of TTCIC.Using polymer PBDB-T as a donor material and TTCIC or TCIC as the acceptor material to prepare OSCs devices,the photovoltaic performance was optimized,and the final PCE was 6.84%and 7.25%for TTCIC and TCIC,respectively.2.We designed and synthesized two symmetric A-D-A SMAs(IDTT-BOA and IDTT-BOACl)with 2-acetonitrile-benzoxazole(BOA) and 2-(5-chlorobenzo[d]oxazol-2-yl)acetonitrile(BOACl)as the A unit respectively,and an asymmetric A₁-D-A₂ SMA(ICCl-IDTT-BOACl)with the different A unit of BOACl(A₁)and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile(ICCl,A₂).The effects of A units(BOA,BOACl,ICCl)with different electron-withdrawing ability and asymmetric A units(A₁ and A₂)on the photophysical,electrochemical properties and photovoltaic performance of these SMAs were systematically studied.The results showed that IDTT-BOA exhibited an upshifted LUMO energy level compared with IDTT-BOACl and ICCl-IDTT-BOACl,due to the weak electron withdrawing ability of BOA unit relative to BOACl and ICCl,which is beneficial to increase the open-circuit voltage(V_OC)of the OSCs.On the other hand,ICCl-IDTT-BOACl with the asymmetric A unit(BOACl and ICCl)showed a raised LUMO energy level and broaden absorption spectrum,which are beneficial for improving V_OC and short-circuit current(J_SC)meantime.Using polymer PBDB-T as the donor material and the three synthesized small molecules as acceptor materials to prepare OSCs devices.After optimization,the obtained V_OC based on IDTT-BOA and IDTT-BOACl devices exceeded 1.1 V.The PCE of ICC1-IDTT-BOACl-based device reached 7.83%.3.Two symmetric A-?-D-?-A SMAs(BDI-T-IC and BDI-OT-IC)based on benzidiimidazole(BDI)were designed and synthesized.The two small molecules have the same total carbon number of the alkyl chains,but the alkyl chains and their positions are different.The influences of the different lengths of alkyl chains and their positions on the molecular structure,absorption spectrum,molecular energy level and photovoltaic performance of the SMA materials were studied in detail.The results show that the two small molecules have good thermal stability,strong absorption loated at 450-650 nm,and a high LUMO energy level.BDI-OT-IC showed a higher-lying HOMO energy level relative to BDI-T-IC due to the introcution of alkoxy side chains.OSCs are constructed with polymer PBDB-T as the donor material and BDI-T-IC or BDI-OT-IC as the acceptor material.Preliminary results showed that OSCs based on these two SMAs produced a relatively high V_OC over 0.9V.