New Hole Transport Materials Based On Bezothiadiazole And Its Derivatives And Their Application In Perovskite Solar Cells

In recent years,the metal halide perovskite materials have become one of the most attractive photovoltaic materials because of its excellent photovoltaic performance and low cost.So far,the power conversion efficiency?PCE?of the perovskite solar cells?PSCs?has made a breakthrough,which has been increased from 3.8%to 24.2%in the past 10 years.In PSCs,the hole transport layer contacts with the perovskite layer directly and extracts the hole from the perovskite layer,to avoid the charge quenching caused by the direct contact between perovskite materials and electrodes.Therefore,in order to improve the device performance,it is very important to develop appropriate hole transport materials?HTMs?for PSCs,in addition to optimization of the perovskite materials and device structures.In the design of HTMs,biphenyl,tetrastyrene,spirofluorene,aromatic heterocycles and etc are commonly used as core units,while electron-withdrawing groups are seldom used.However,in 3D type twisted molecular structures,introducing electron-withdrawing core units can adjust energy levels and increase the dipole-dipole interaction between molecules.Therefore,in this thesis,we have designed and synthesized a series of organic small molecule HTMs based on benzothiadiazole and its derivatives.The effects of the introduction of these electron-withdrawing units on the photophysical properties,photochemistry properties and thermal stability of molecules have been studied.We have applied them to perovskite solar cells and explored the effects of molecular structure on device performance.The main contents of this thesis include the following four aspects:1.JY5,a novel organic HTM based on benzothiadiazole?BT?,was designed and synthesized.It was applied to a planar PSC.The effect of introducing electron-withdrawing units into the core of the molecule on the hole transport performance and PCE was studied.The results show that the efficiency of the device with X51,a biphenyl-based HTM,is 13.20%.The PCE of JY5-based PSC increases to 16.87%by introducing BT unit into biphenyl.The improvement of efficiency is mainly due to the increase of short circuit current density(J_sc).The BT unit can improve the planarity of the central structure of JY5,which is beneficial to charge transfer.Therefore,PSC with JY5 as HTM has better performance than X51.It turns out that introducing electron-withdrawing unit BT into HTMs with twisted structure can promote hole transport,improve J_sc,and ultimately improve device performance.2.JY6 and JY7,organic HTMs based on fluorobenzothiadiazole?FBT?,were designed and synthesized,and applied to PSCs.The effects of fluorine atoms on hole transport performance and device efficiency were systematically studied.The core structure of JY6ismonofluorobenzothiadiazole,whilethecorestructureofJY7is difluorobenzothiadiazole.JY5 using BT as the core is used as a reference.The results show that in the HTMs studied,JY6 has higher hole mobility and more effective hole extraction capacity,and its surface morphology is more uniform,so its corresponding device efficiency is up to 18.54%.It is noteworthy that the device based on JY6 obtains a high fill factor?FF?of 81%,which is one of the highest FF values in PSCs.The device efficiency of JY5 is 16.87%,while the efficiency of JY7 is only 15.71%.This is mainly due to of the increased fluorine atoms increasing the crystallinity of JY7 molecule,thus resulting in rough film morphology,which affect the hole mobility and charge transfer ability of JY7-based device.It turns out that proper introduction of fluorine atoms into the BT unit can improve the hole transport performance of HTM and ultimately the performance of PSCs.3.JY8,a HTM based on pyridylthiadiazole?PT?,was designed and synthesized.It was applied in PSC.The effect of enhanced electron-withdrawing ability of core units on hole transport performance and device efficiency was studied.Compared with the previously reported JY5 with BT as core structure,the PT unit with strong electron-withdrawing ability enhances the dipole-dipole interaction between the molecules.In addition,the dihedral angle between the PT group and one of adjacent benzene ring is very small,which provides a good planarity for the central part of JY8,and extends the?conjugation and electron delocalization of the central part of JY8.The results show that JY8 has higher hole mobility than JY5,and JY8 HTM has higher hole extraction and transport capability,and its surface morphology is more uniform.Therefore,JY8-based device efficiency reaches 19.14%,which is higher than the efficiency of JY5-based devices under the same conditions.It turns out that it is feasible to replace BT unit with PT unit to improve the PSCs performance in this system.4.The organic small molecule J1 and J2 with 1,2-dimethoxyphenyl as the end group were designed and synthesized.The core structures of J1 and J2 are tetrastyrene and spirofluorene,respectively.Compared with the commonly used methoxy end groups,there are two methoxy groups on the 1,2-dimethoxyphenyl,which enhances the electron-donating ability of the terminal groups and can also adjust the energy level.In addition,the introduction of benzene ring can prolong the?conjugation of the whole molecule,which is beneficial to the charge delocalization and charge transporting.The results show that the two compounds have good thermal stability,suitable energy levels and good hole transport ability.The PCE of the planar PSCs based on the J2 is 15.07%,which is close to device performance based on Spiro-OMeTAD measured at the same test condition.