Synthesis Of Flouene-terminated Hole Transporting Materials And Application In Perovskite Solar Cells

Owing to the world is facing energy crisis and environmental pollution,the development of new energy sources have become scientific research goals.Perovskite materials have received great attention from researchers because of their excellent PV performance,easy processing and low cost.In recent years,organometal halide perovskite solar cells（PSCs）have become a research hotspot of new generation solar cells,and their photoelectric conversion efficiency（PCE）has rapidly increased from the initial 3.8%to 25.5%.At present,the common small molecule hole transporting material（HTM）is still spiro-OMe TAD.Due to the excellent performance and dopability,it is regarded as an irreplaceable HTM in PSCs.However,due to its high synthesis cost,low thermal stability and chemical stability,it is important to develop low-cost,thermally stable hole transporting materials.Diphenylamine（DPA）or its derivatives usually show a high triplet excited state energy（E_T）value and moderate hole transport properties.Due to its lower HOMO level,DPA group is close to the HOMO level of widely used hole transporting materials and thus achieve efficient hole injection.Fluorene compounds have always been favored by people because they have broad application prospects in many fields such as photovoltaic materials,solar cells and biomedicine.The structural characteristics of fluorene compounds are as follows:（1）the fluorene ring is a special biphenyl structure with high thermal stability and photochemical stability.（2）the 2-,7-and 9-carbons are easy to modify the structure to introduce various functional groups;（3）the price is low and the raw materials are easily available.Therefore,this thesis designed and synthesized three types of small molecule hole transporting materials with fluorene-substituted diphenylamine as the peripheral group,systematically studied the photophysics,thermal stability,electrochemistry and other properties of the materials,and applied them in perovskite solar cells.The experimental results are obtained as follows:1.Three compounds SFX-F,SFX-FM and SFX-FP,which were SFX-centered and N-（4-methoxyphenyl）-N’-（9,9’-dimethylfluoren-2-yl）amino（FPA）as the peripheral substituent group,along with different substituent positions in the SFX core were synthesized.All materials are simply prepared through a two-step reaction and have high yields,which have the potential for large-scale production.All the fluorene-terminated compounds SFX-F,SFX-FM and SFX-FP have matched HOMO energy level with the perovskite Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃,high hole mobility and hydrophobicity.Among them,the hole mobility of SFX-FM reaches1.26×10^-4 cm² V^-1 S^-1,which is conducive to the collection and transport of holes;T_g is154℃,which can inhibit the formation of grain boundaries and is conducive to uniform film formation.The meta-substituted SFX-FM showed a high PCE of 17.29%in the planar PSC,higher than the spiro-OMe TAD-based PSC（15.14%）.Under the same conditions,SFX-FP and SFX-F based PSCs with a PCE of 15.45%and 14.54%,respectively.2.In order to study the influence of fluorene-substituted peripheral groups on hole transport materials,compounds KZ-1,KZ-2 and KZ-3 were synthesized with low-cost carbazole as the core and fluorene-substituted diphenylamines with different numbers of methoxy groups as peripheral groups.The results show that the HOMO energy levels of KZ-1,KZ-2 and KZ-3 increase with the number of methoxy groups,and their oxidation potentials also increase,which may be due to the enhanced molecular conjugation.In addition,the ortho-substituted KZ-1 has a greater steric hindrance to reduce the degree of conjugation,while KZ-2 and KZ-3 containing two and three methoxy groups both enhance their p-πconjugation effect.The greater the number of methoxy groups,the better the conjugation effect.The device PCEs based on KZ-1,KZ-2,and KZ-3 are 16.67%,17.06%,and 16.58%,respectively.Stability test results show that KZ-2 based PSCs devices can maintain 97%initial efficiency when exposed to air for more than 59 days;KZ-1 and KZ-3 based PSCs devices can maintain the initial efficiency of 95%and 83%when exposed to air for more than 47days,respectively.These HTMs show good long-term stability.3.Chemical passivation is an effective method to repair surface defects of perovskite crystals,reduce charge recombination loss,and increase open circuit voltage.The S atom can more effectively passivate the defects in the perovskite by coordinating the Pb²⁺vacancies.Therefore,on the basis of FPA,methylthio groups areintroducedtoreplacemethoxygroups,and N-（4-methylthiophenyl）-N’-（9,9’-Dimethylfluoren-2-yl）amino（F_SPA）is synthesized to achieve surface interface passivation.Therefore,in Chapter 3,we synthesized two hole transporting materials based on the tristyrene core,the CJ-S with methylthiodiphenylamine peripheral group and the CJ-FPA with F_SPA peripheral group.The test results show that both CJ-S and CJ-FPA have suitable HOMO energy levels,which can be well matched with the perovskite layer.More importantly,the introduction of fluorenyl group into the substituent makes CJ-FPA have better thermal stability than CJ-S:T_g increases from 70℃to 102℃,and T_d increases from 235℃to 308℃.However,because the solubility of CJ-FPA is worse than that of CJ-S,resulting rougher film,which also has a certain impact on the efficiency of the device.The device efficiencies based on undoped CJ-S and CJ-FPA are 7.74%and 4.81%,respectively.After being placed at a humidity of 20%and room temperature for 29days,the efficiency of the undoped CJ-S-based device increased from 7.74%to12.04%,and the efficiency of the undoped CJ-FPA-based device increased from4.81%to 11.18%.