The Design,Synthesis And Performance Characterization Of Small-Molecule Donor Materials Containing Thiazolo[5,4-d] Thiazole Unit

In recent years,organic solar cells(OSCs)have attracted wide attention due to their light weight,low cost,fabrication of flexible and semitransparent devices and indoor applications.High performance OSCs are always polymer soalr cells(PSCs)which based on blend of polymer donor and non-fullerene small molecule acceptor as active layer.However,polymer materials have the shortcomings of difficult purification,difficult control of molecular weight and molecular weight distribution and poor batch repeatability,which to some extent limit their commercial application.Compare with polymer materials,small molecule materials have the merits of defined molecule weight,easy purification and low batch-to-batch variations.Therefore,all-small molecule organic solar cells(SM-OSCs)have great commercialization potential.However,the development of SM-OSCs is seriously limited by the lag of the development of small molecule materials.In this paper,we regulate the absorption spectrum,molecular energy level,aggregation and blend film morphology of materials by modifying the chemical structure of small molecule donor materials,and thus improve the photovoltaic performance of devices.To develop efficient small molecule donor materials and investigate the relationship between molecular structure and properties.The main research contents and innovations are as follows:1.We synthesized three small molecule donors SL1,SL2 and SL3 using thiazolo[5,4-d]thiazole(TTz)as the bridging unit,thienyl benzodithiophene(BDTT)as electron-donating core with different end groups.The effects of different end groups on the properties of small molecule donors were investigated.The results show that the absorption spectrum and molecular energy level of the small molecule donor are basically unchanged,the crystallization and the phase separation in the blend film decreases when the length of the flexible chain of the rhodanine end group is extended.Compare with rhodanine,the introduction of 2-ethylhexyl 2-cyanoacetate end group makes the absorption spectrum blue-shifts,the molecular energy levels increase,the crystallization decreases,and the phase separation in the blend film become smaller.In addition,solvent vapor annealing(SVA)treatment effectively promoted the crystallization of donor and acceptor in the blend film,thus increased the phase separation and phase purity,made the morphology more suitable,and significantly improved the power conversion efficiency(PCE).Finally,the SL1:Y6 based photovoltaic devices achieved PCE of 13.9%and open-circuit voltage(Voc)of 0.88 V,which is one of the best SM-OSCs results reported so far.At the same time,the device obtained a non-radiative energy loss similar to that of inorganic solar cells(0.18 eV).In addition,the fabrication of the devices are not sensitive to to the time of SVA,which is conducive to its processing and fabrication.2.We synthesized three small molecule donors SL4,SL5 and SL6 by using TTz as the bridging unit,3-butylrhodanine as the end group and BDTT containing different alkyl substitutions as electron-donating cores.The effects of different alkyl substituents on the performance of small molecule donors were investigated.The results showed that the absorption spectra of the small molecule donors with different alkyl substituents were similar.In addition,with the extension of the alkyl chain,the molecular energy level of the small molecule donor decreases slightly.The small molecule donors which side chains are 2-ethylhexyl and linear alkyl chains have stronger crystallization in face-on orientation and better miscibility with the acceptor Y6.The SL6 with the shortest alkyl chain has the closest molecular stacking and the highest hole mobility.Finally,the photovoltaic devices based on SL4/SL5/SL6:Y6 show PCEs of 11.5%,12.7%and 12.4%,respectively.