Synthesis And Performance Studies Of Dopant-free Hole-transport Materials Based On Conjugated Acceptors And Spiro-core

Perovskite solar cells(PSCs)are rapidly emerging solar cells using perovskite-type organic-inorganic metal halide semiconductor materials as the light-absorbing layer,which have attracted the attention of researchers due to their low fabrication cost,good light absorption performance and amazing power conversion efficiency(PCE).As a key component in PSCs,hole-transport materials(HTMs)play an important role in extracting and transporting holes generated by photoexcitation,inhibiting electron-hole recombination and protecting the perovskite layer,which can effectively improve the efficiency and stability of the PSCs.Organic small-molecule HTMs have been widely used in PSCs due to their advantages of easy structural tunability,simple synthesis,good solubility,and generally excellent device performance.Currently,many small-molecule HTMs,including the star molecule 2,2?,7,7?-tetra[N,N-bis(4-methoxyphenyl)amino]-9,9?-spirobifluorene(spiro-OMe TAD),have achieved excellent PCE,but most of these small-molecule HTMs need the addition of dopants to improve their hole mobility when used.However,the use of hydrophilic dopants not only brings about a cumbersome doping process,but also accelerates the erosion of perovskite layers by moisture and oxygen in the environment,which seriously affects the stability of PSCs.Therefore,the design and synthesis of dopant-free small-molecule HTMs has important research significance.From the perspective of enhancing the intermolecular interaction force and hole mobility of HTMs,this thesis started with the core structure of the molecule and the peripheral donor units respectively,and designed and synthesized a series of conjugated acceptor-based D-A-D type dopant-free HTMs and spirofluorene-core dopant-free hole-transport materials with different donor units.Firstly,the basic properties of the HTMs were tested,such as the photophysical properties,electrochemical properties,thermal stability,film forming properties and hole mobility,and then the HTMs were applied to the PSCs to determine the photoelectric properties and stability of the devices.Through a series of characterization experiments,the relationship between the molecular structure of the HTMs and the device performance was systematically investigated.The specific research is summarized as follows:In the second chapter,four-ring-fused dithieno[3?,2?:3,4;2?,3?:5,6]benzo[1,2-c][1,2,5]-thiadiazole(DTBX,X = O,S and Se)acceptor was innovatively introduced as the core structures and 4,4?-dimethoxytriphenylamine was introduced as the terminal donor unit,and three D-A-D-type dopant-free HTMs(DTBF,DTBT and DTBS)in which the core acceptor structure of DTBF was dithiophenobenzofurazan,DTBT was dithiophenobenzothiadiazole,and DTBS was dithiophenobenzoselenadiazole were designed and synthesized.Through a series of experiments,the photophysical properties,electrochemical properties,and photovoltaic properties of the HTMs were systematically investigated,and the effects of chalcogenide atoms alteration in the core acceptor unit on the properties of the D-A-D-type HTMs were also studied.The results show that the HOMO energy level,thermal stability,film formation and hole mobility of selenium-containing DTBS are better than those of DTBF and DTBT.Therefore,the PSCs based on DTBS have the highest photovoltaic performance among the three novel HTMs,and the optimal PCE is 15.09%.In addition,the DTBS-based devices show good device stability.This study not only demonstrates the great potential of DTBS in the construction of low-cost and efficient PSC applications,but also provides reasonable design principles for the further development of efficient HTMs.In the third chapter,isoindigo conjugated acceptor unit with strong electron-withdrawing ability,and dibenzonaphthalenedione derivative structure were further introduced.Three D-A-D-type dopant-free small-molecule HTMs(IID-TPA,OO-DBND-TPA and NO-DBNDTPA)were designed,synthesized and applied to PSCs,and the effects of isomerism and different branched structures on the performance of the HTMs were systematically investigated.The results show that OO-DBND-TPA with two alkoxy chains and a dibenzonaphthalenedione acceptor core exhibit the lowest HOMO energy level,the best film-forming property and the highest hole mobility among the three new HTMs,and the highest PCE of 13.76% is obtained.The IID-TPA with an isoindigo core exhibits a maximum PCE of 9.80%;the dibenzonaphthalenedione-based NO-DBND-TPA with an N-substituted alkyl chain and an alkoxy chain achieves a maximum PCE of 12.05% This study shows that the isomeric molecules have great differences in molecular properties due to their different configurations,and the OO-DBND-TPA molecules with better planarity have better photoelectric properties.The two symmetric alkoxy side chain groups also contribute to the improvement of the performance of the HTMS compared with one N alkyl chain and one alkoxy side chain.In the fourth chapter,three spiro[fluorene-9,9?-xanthene](SFX)-based dopant-free HTMs with different donor units(SFX-2DPA,SFX-2CZ and SFX-4CZ)were designed and synthesized.The optical,thermal,electrochemical and photophysical properties as well as the photovoltaic characteristics of the three HTMs were systematically investigated through a series of characterization experiments.The results show that SFX-2CZ has higher hole mobility and more efficient hole extraction than SFX-2DPA and SFX-4CZ.The PCE of SFX-2CZ-based PSCs is 15.83%,which is higher than those of SFX-2DPA(14.25%)and SFX-4CZ(12.19%)based devices under the same conditions,and comparable to that of doped spiro-OMe TADbased devices(17.35%).This work not only provides reasonable design principles for efficient dopant-free HTMs,but also explores the feasibility of obtaining dopant-free HTMs by modifying the donor units.In the fifth chapter,fluorine atoms were further introduced into the MDCZ donor of SFX-2CZ,and the dopant-free HTMs SFX-2m FCZ containing fluorinated donor was designed and synthesized.We also introduced fluorine atoms into the diphenylamine donor and synthesized two other dopant-free HTMs with fluorinated donors,SFX-2m F and SFX-4m F.The photophysical,electrochemical,and thermal stability tests were conducted to investigate the effects of introducing fluorine atoms to the donor on the photophysical,electrochemical,and thermal stability of the HTMs.The experimental results show that the introduction of fluorine atoms on the donor units can effectively widen the optical band gap of the molecule,lower the HOMO energy level of the molecule,enhance the thermal stability of the HTMs,which will help to improve the optoelectronic properties of the material.This study provides a new idea for the design of dopant-free HTMs.In this thesis,a variety of dopant-free HTMs were designed and synthesized from the optimization of the molecular core structure and the terminal donor group.The relationship between molecular structure and performance was investigated through characterization experiments and device performance tests,and good photoelectric conversion efficiency and device stability have been achieved.The research results of this thesis will be helpful for the design of new dopant-free HTMs and the development of PSCs.