Synthesis And Photovoltaic Performance Of A-DA′D-A Type Five-membered-fused Rings Acceptors

In recent years,A-DA’D-A type fused-ring small molecule acceptors（SMAs）and polymerized small molecule acceptors（PSMA）are developing rapidly,especially the optimum power conversion efficiency（PCE）of single-junction organic solar cells（OSCs）based on A-DA’D-A type SMAs have exceed 19%.Although the PCE of PSMAs is slightly inferior to that of SMAs,its high thermal stability also attracts much attention.However,in addition to photovoltaic parameters such as PCE,cost and stability must be considered.At present,A-DA’D-A type SMAs mostly adopt the seven-membered DA’D fused-ring structure based on thiophene[3,2b]thiophene（TT）unit,but the main drawbacks are the difficulty of synthesis and the cost.Therefore,it is particularly important to design efficient A-DA’D-A type SMAs that can be easily synthesized.In this paper,some simply-structured A-DA’D-A SMAs based on five-membered DA’D fused rings are synthesized by replacing TT units with thiophene units.And the effects of these subtle controls on the photoelectrical properties about these SMAs are investigated.In addition,it is also important to improve the stability of materials.So,PSMA is synthesized by the copolymerization of A-DA′D-A type five-membered-fused rings SMA and thiophene unit,and the photoelectric properties and thermal stability of SMA and PSMA are compared.（1）We synthesize two low-cost A-DA’D-A five-membered-fused-ring SMAs BZIC2F（Y25）and BZIC2F-C6（Y26）using 5,6-difluoro-3-（dicyanimymethyl）indone（DFIC）as end groups and thiophene and 3-hexyl thiophene as D units,which are easier to synthesize.In this paper,we compare the photoelectric properties of Y25 and Y26,and study the effect of alkyl chain modification inβsite of thiophene on charge transport,J_SC and FF.After the introduction of hexyl in thiopheneβsite,the relative steric resistance becomes larger,torsion barrier of the single bond connected with end group DFIC and DA’D core increases,resulting in reduced disorder,stable configuration,dominant molecular stacking and desirable morphology,so the charge transport and collection enhance,higher J_SC(21.63 m A?cm^-2),FF（74.33%）and PCE（13.34%）are achieved.（2）We synthesize A-DA’D-A type five-membered-fused rings SMA BTIC2F-C6 based on BZIC2F-C6 by replacing benzotriazole（BTz）with benzothiadiazole（BT）.By comparing the photoelectric properties of the two acceptors,the effect of the subtle regulation of the two A′units BT and BTz on the device performance is investigated.BTIC2F-C6 has stronger crystallinity than BZIC2F-C6 due to the larger atomic radius of sulfur atoms in BT and the lack of a site for adding alkyl chain modification on the nitrogen atom compared with BTz,which results in stronger self-aggregation of BTIC2F-C6 and worse morphology when blended with PM6,which is not conducive to charge transport.So the device based on PM6:BTIC2F-C6 shows lower J_SC(19.15 m A?cm^-2),FF（67.60%）and PCE（11.03%）than PM6:BZIC2F-C6（Y26）.（3）Based on BZIC2F-C6,the five-membered-fused rings SMA BZIC-HD is prepared by increasing the length of the alkyl chain in theβsite of thiophene and the site on the nitrogen atom of pyrrole ring,using 3-（dicyanlimethyl）as the end group.The alternating copolymer acceptor P-BZIC-HD is prepared by coupling BZIC-Br-HD and 2,5-bistrimethyltin,and the photoelectric properties of BZIC-HD and P-BZIC-HD are compared.The results show that PBDB-T:P-BZIC-HD blend films have stronger self-aggregation tendency,which leads to higher surface roughness and phase separation size,and the devices based on PBDB-T:P-BZIC-HD have more charge recombination,thus showing low J_SC and FF.At the same time,the devices based on the two acceptors are subjected to thermal annealing（TA）under the same conditions to observe their tolerance to TA.After treatment,PBDB-T:P-BZIC-HD blend films show serious monomolecular recombination,accompanied by serious voltage loss,J_SC and FF also attenuated correspondingly,and the PCE retention rate is only 27%.However,the phase separation size of PBDB-T:BZIC-HD blend films shows an opposite trend after thermal annealing,which leads to the increase of bimolecular recombination and decrease of device parameters.So,the PCE retention rate of PBDB-T:BZIC-HD devices is only 47%.