Design And Synthesis Of Wide-Bandgap Dopant-Free Hole Transport Materials And Applications In Perovskite Solar Cells

In the past 20 years,with the continuous consumption of fossil energy such as oil and natural gas,energy shortage has become a global problem.To alleviate the increasing demand for energy,it is urgent to seek alternative energy sources.Solar power is independent of Earth’s surface and climate and is a very reliable form of clean energy compared to wind,hydropower and others.Over the past decade,perovskite solar cells(PSC)have made incredible progress in their photovoltaic conversion efficiency(PCE)from 3.8%to 26.2%.In addition to high efficiency,how to improve the long-term stability of PSC in large-scale commercial applications has become a key problem that must be solved.Over the past 10 years,with the development and application of new high-efficiency hole transport materials(HTM),the performance of PSC has improved rapidly.It is clear that HTMs,as an important part of PSC,has a huge impact on the overall performance of devices.The efficient HTMs represented by Spiro-OMe TAD generally have low intrinsic mobility and usually require chemical doping to improve the hole mobility/conductivity.Doping on the one hand increases the overall cost of the device and on the other hand affects the long-term stability of the device.The development of low-cost,dopant-free HTMs has become one of the research hotspots of PSC related materials.The study of dopant-free HTM mainly includes two categories:small molecule and polymer.The design of small-molecule dopant-free HTM mainly considers the direct effect of the structure itself on the mobility.The study of dopant-free polymer HTMs is often arbitrary.Compared with small-molecule HTMs,polymer HTMs have many advantages,such as diverse structure,heat resistance,hydrophobicity and film forming ability,showing excellent device efficiency and stability in different types of device structures.In addition,in terms of the device structure of PSCs,the traditional PSC using doped HTMs has higher efficiency,but requires transparent electrodes to deposit mesoporous films at high temperature,which is not conducive to the implementation of large area coating on flexible substrates.inverted Planar PSCs are much simpler in structure and can be machined at room temperature or low temperature.They are better suited for large area coating.However,their efficiency is very low compared to inverted planar devices.Based on the above considerations,this paper intends to carry out research on dopant-free small molecule and polymer hole transport materials,and apply them to inverted PSC devices,explore and compare the differences in design strategies of small molecule and polymer HTMs,especially to analyze the influence of molecular structure on device efficiency and device stability.In the first part of this paper,the dimethoxyaniline group is introduced into the 2-site,1,3-site and 1,4-site bridging biphenyl to obtain ortho-,meso-and para isomerized isomers:TP-o-TPA,TP-m-TPA and TP-p-TPA.Through the selection of the bridging mode,the space extension and plane conjugation characteristics of the three molecules are regulated,and then the energy level,electronic structure and transport characteristics are regulated.The undoped inverted perovskite solar cells devices prepared based on these three HTMs show excellent PCE.Among them,the PSC device based on TP-m-TPA has the highest efficiency(18.4%),while the cell efficiency based on TP-o-TPA and TP-p-TPA is only 16.1%and 15.6%,respectively.As a comparison,the device efficiency of PSC with the same device structure was only 13.8%based on commercial hole transport material PEDOT:PSS.On the one hand,this study provides a molecular design idea to construct non-doped hole transport based on the center symmetric core,and also proves that the introduction of dimethoxyaniline into HTM by interphenyl can obtain better photoelectric conversion efficiency.Three TP-TPA isomers achieve high efficiency PSC under non-doping conditions,which inspires us to convert doped HTM into dopant-free HTM.To that end,the second part of this paper studies the determination of a doped small molecule HT2(9,9-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-N~2,N~2,N~7,N~7-tetrakis(4-methoxyp henyl)-9H-fluorene-2,7-diamine)was designed as a molecular template,and 9,9-bis(triaromatic amine)fluorene was used as a repeating unit to design and synthesize a low-cost and high-efficiency non-doping type hole transport polymer,PFTPA.PFTPA has good modulability,photothermal stability and visible light transmittance.The HOMO energy level of PFTPA is 5.33 e V,which is higher than that of common hole transport materials,and it has very good water and oxygen stability.Using PFTPA as undoped hole transport material,inverted planar PSC(perovskite layer as MAPbI3)was prepared in air,and the high light conversion efficiency of 16.8%was obtained,which was much higher than that of the same device based on PEDOT:PSS(13.8%).Meanwhile,the non-doping trans planar PSC devices based on PFTPA have very high air storage stability.In this chapter,we explore the possibility of doped small molecule hole transport materials after polymer conversion to dopant-free HTM,providing a new idea for the design of efficient dopant-free polymer hole transport materials.