Application Of Functionalized Organic Small Molecule Hole Transport Materials In Perovskite Solar Cells

With the continuous consumption of non renewable energy on Earth and the huge demand for energy from social development,there is an urgent need to find more renewable energy as alternative energy sources.As an inexhaustible and inexhaustible energy source,how to efficiently utilize solar energy as a clean and renewable energy source has become a major challenge at present.Since Miyasaka et al.developed perovskite solar cells（PSCs）,PSCs have attracted attention due to their excellent photovoltaic performance,low production costs,and enormous potential in renewable energy.At present,the photoelectric conversion efficiency（PCE）of PSCs has exceeded25%,which is very close to the efficiency of traditional silicon based solar cells.However,the stability issue of PSCs has always been the main obstacle to their commercialization.Especially in the preparation of PSC devices,perovskite thin films prepared by solution method inevitably generate a large number of defects,such as uncoordinated Pb²⁺,I^-,and lead clusters.These defects result in a large amount of nonradiative recombination in the device,which seriously affects the performance and stability of the device.In recent years,researchers have designed and synthesized new functionalized hole transport materials（HTMs）with defect passivation capabilities,which not only enable HTMs to extract and transport holes,but also passivate defects at perovskite interfaces,suppress nonradiative recombination,and improve device performance and stability.At present,commonly used passivation groups include C=O,-CN,and-NH₂.Among them,the C=O functional group,as a Lewis base,can provide lone pair electrons to passivate uncoordinated Pb²⁺defects in perovskite.However,through literature research,it was found that a single C=O does not have a strong ability to passivate defects.In order to enhance the passivation ability of C=O,the passivation ability of the material can be improved by introducing bicarbonyl groups and increasing the number of passivation groups;In addition,the passivation ability of the material can also be enhanced by changing the passivation group from C=O to C=S.The work of this paper is as follows:1.Two functionalized HTMs,DB and PQ,were synthesized using benzoyl and phenanthraquinone containing dicarbonyl groups as the core and carbazole diphenylamine as the end group,and applied to PSC devices with positive mesopores.Among them,the core structures of benzoyl and phenanthraquinone both have two C=O passivation groups,but their structures are different.Therefore,as HTMs,they exhibit different hole extraction and defect passivation abilities.Firstly,through Fourier transform infrared spectroscopy,XPS,and defect density of states testing,it was found that DB has stronger defect passivation ability compared to PQ.Based on this,in order to further explore the impact of the difference in molecular structure between the two on the perovskite interaction,density functional theory（DFT）calculations were carried out.It was found that the benzoyl structure,as the core of the DB molecule,was conducive to the adjustment of the molecular structure itself,making it fully spread at the perovskite interface,thus enhancing the interaction between the DB molecule and perovskite,improving the defect passivation ability and charge transfer ability of the molecule.In contrast,the phenanthraquinone in the PQ core is not conducive to molecular spreading due to its structural rigidity,resulting in less effective contact with the perovskite surface and weaker intermolecular interactions.In addition,compared to PQ,DB also exhibits higher hole mobility and better hole extraction ability.Taking these factors into account,DB based PSCs devices achieved a high photoelectric conversion efficiency of 22.21%.Moreover,when tested in air with a relative humidity of 30%,DB based unpackaged devices retained 89%of their initial efficiency after>1100 hours.This chapter shows that the molecular structure of HTMs has a great impact on the performance and stability of perovskite solar cells,so this factor can also be considered in future HTMs design.2.Four functionalized HTMs were designed with fluorenone and its derivative structures as the core and carbazole diphenylamine as the terminal group.First,two simple linear HTMs,2,7-WO and 3,6-WO,were synthesized through a simple coupling reaction.After the simple treatment of 2,7-WO and 3,6-WO with Lawson reagent,C=O of fluorenone was changed to C=S,and 2,7-WS and 3,6-WS were synthesized.This work mainly explores the passivation ability of C=S and C=O on perovskite defects,as well as the influence of carbazole diphenylamine at two different sites,2,7 and 3,6,on device performance.Firstly,through infrared spectroscopy testing,XPS testing,and defect density of states testing,it was demonstrated that HTMs containing C=S have stronger defect passivation ability than C=O.This can be attributed to the larger range of action of the 3p lone pair electrons of S compared to the 2p lone pair electrons of O.Therefore,C=S may have stronger electron donating ability,allowing it to more fully and effectively passivate uncoordinated Pb²⁺defects in perovskite and suppress non radiative recombination.Subsequently,through SCLC,PL,TRPL,and TPC testing,it was demonstrated that compared to the other three HTMs,2,7-WS exhibited higher hole mobility and stronger hole extraction ability,which may be attributed to the advantage of2,7 site carbazole diphenylamine in forming linear conjugated structures with fluorenone.Therefore,PSCs devices based on 2,7-WS exhibit higher PCE（23.25%）and good long-term stability.The results of this chapter demonstrate that groups with different passivation abilities and HTMs at different sites have a certain impact on device performance.Therefore,these factors can be considered when designing new HTMs in the future.