Development Of Small Molecular Hole Transporting Materials Based On Spiro-core And Its Application In Perovskite Solar Cells

Perovskite solar cell(PSC)is a type of solar cell that uses metal halide perovskite materials as the light-absorbing layer.As a new type of solar cell,its power conversion efficiency(PCE)has rapidly increased from the initial 3.8%to 25.8%,and due to the simple preparation process and relatively low cost of PSC,it has attracted extensive attention from researchers in recent years.The most common highefficiency PSC consists of a hole transporting layer(HTL)an electron transporting layer,a perovskite light-absorbing layer sandwiched between them,and transparent conductive glass and electrodes.Among them,HTL plays an essential role in extracting and transporting photogenerated holes,inhibiting electron backflow,and protecting the perovskite layer.So far,a large number of different kinds of hole transporting materials(HTMs)have been developed and applied in PSCs,and spiroOMeTAD maintains the highest efficiency of PSCs due to the advantages of spiro molecules with charge transport isotropy and inhibition of aggregation.However,the poor molecular stability of spiro-OMeTAD and the complex synth,esis of spirobifluorene core in the molecule limit its application.In this paper,the efficiency and stability of the device were improved mainly by replacing the terminal anisole of spiro-OMeTAD with the carbazole group,further developed a molecule based on a low-cost spiro[fluorene-9,9’-anthracene]core,and investigated the relationship between structure and device performance.The main achievements are as follows:(1)Taking the core of spiro-OMeTAD,that is,9,9’-spirobifluorene(SBF)as the core,a new type of spiro-type molecule was designed and synthesized,in which the methoxyphenyl groups on spiro-OMeTAD were fully replaced by N-ethylcarbazole,named spiro-carbazole.Compared with spiro-OMeTAD,the newly developed molecule exhibits a lower HOMO energy level,higher hole mobility,and an extremely high glass transition temperature(Tg=196℃).The spiro-carbazole-based PSCs exhibited a PCE of 22.01%,surpassing that of spiro-OMeTAD(21.12%).More importantly,compared with spiro-OMeTAD,the spiro-carbazole-based PSC device has better thermal stability,humidity stability,and long-term stability.(2)Four small-molecule HTMs were prepared by introducing oxygen atoms onto the core of spiro-OMeTAD,that is,spiro[fluorene-9,9’-anthracene](SFX,spiro[fluorene-9,9’-xanthene])(SFX-1,SFX-2,SFX-3,and SFX-4).The study found that the site of terminal substitution has a significant impact on the properties of the molecule.Among them,SFX-3,whose terminal group is located at the 3,6substitution site on the SFX fluorene side,has high conductivity,and its HOMO energy level matches well with the perovskite layer.The SFX-3 with 3,6-substitution site exhibited better film-forming performance and better hole extraction ability than molecules with other substitution sites.Applying SFX-3 to PSCs achieved a higher PCE(22.42%),although it was slightly lower than spiro-OMeTAD(22.96%)under the same conditions,it had better stability and lower cost.