Design And Synthesis Of Novel Organic Small Molecule Hole Transport Materials And Their Application In Perovskite Solar Cells

Perovskite solar cells（PSCs）have become the leader of the third generation of solar cells due to their advantages of high photoelectric conversion efficiency（PCE）,low cost,and easy manufacturing.After 14 years of development,its PCE has increased from 3.8%to 25.8%.However,in commercial applications,PSCs still face the problems of theoretical efficiency breakthrough and stability improvement.Among the strategies to solve these problems,the strategy of developing high-performance hole transport materials（HTMs）plays a crucial role.At present,small organic molecules HTMs have attracted much attention because of their clear molecular structure and easy to regulate structural relationships.Among them,the most representative organic small molecule HTM is 2,2’,7,7’-tetrakis[N,N-bis（4-methoxyphenyl）amino]-9,9’-spirodifluorene（Spiro-OMe TAD）.Due to its advantages such as energy level matching with perovskites and good film-forming,PSCs based on Spiro-OMe TAD have always occupied a place in high-performance perovskite solar cells.However,the three-dimensional orthogonal configuration of spiral Spiro-OMe TAD tends to inhibit the accumulation of molecules and reduce the crystallinity,resulting in poor intrinsic hole mobility.Therefore,a large number of studies have begun to explore the core and peripheral groups of organic small molecule HTMs,in order to obtain HTMs with better performance through reasonable molecular design.Although there are many studies on the molecular configuration of HTMs,the relationship between molecular structure and performance still needs to be improved.Based on this,in this work,we study the structure of HTMs,so as to systematically explore the effects of the molecular structure of HTMs on their molecular optical properties,intrinsic hole mobility and the photovoltaic performance of corresponding perovskite solar cells.The main research contents are as follows:（1）With tetraphenylethylene as the core structure and N-ethylcarbazole as the peripheral group,two small organic molecules HTMs（TPE-2Cz and TPE-3Cz）were designed and synthesized.The difference between the two molecules is that N-ethylcarbazole is substituted at the C₂ or C₃ sites,respectively.Subsequently,the effects of N-ethylcarbazole on HTMs performance and device efficiency at different substitution sites were systematically discussed by comparing TPE-2Cz and TPE-3Cz.From UV-vis absorption spectroscopy and density functional theory（DFT）calculations,N-ethylcarbazole has greater steric hindrance at the C₃ site than at the C₂ site,which makes the TPE-3Cz molecule have a more twisted structure.Thermodynamic performance tests showed that TPE-3Cz had better thermal stability and film-forming.Through scanning electron microscopy（SEM）and atomic force microscopy（AFM）tests,it is found that TPE-3Cz film has a more uniform and smooth surface topography.The space-charge limiting current（SCLC）test shows that the TPE-3Cz film has higher hole mobility than the TPE-2Cz film.Steady state photoluminescence（PL）and time-resolved photoluminescence（TRPL）spectra showed that TPE-3Cz films exhibited better hole extraction ability than TPE-2Cz films.Transient photovoltage（TPV）and transient photocurrent（TPC）tests show that TPE-3Cz based devices have more efficient charge transport than TPE-2Cz based devices.Therefore,the TPE-3Cz based device shows better photovoltaic performance,and its PCE is as high as 20.94%.In addition,the PCE of the TPE-3Cz based device can still maintain 88.7%of the original efficiency after 1000 h of operation at room temperature and 30%humidity,which is higher than that of the TPE-2Cz based device under the same conditions,showing a more stable photovoltaic conversion efficiency.This work shows that when tetraphenylethylene is used as the core structure,N-ethylcarbazole is more conducive to improving the performance of HTMs at the C₃ site than at the C₂ site.（2）With thieno3,2-bthiophene and diethyl terephthalate as the core structure,two linear molecules DTTTP-DPA and DTTTP-TPA were designed and synthesized.The introduction of thieno3,2-bthiophene can extend theπconjugation length of molecules,enhance the accumulation ofπ-πbetween molecules,and give corresponding molecules better hole transport ability.The ester group in diethyl terephthalate can not only improve the solubility of molecules,but also provide lone pairs of electrons to interact with Pb²⁺in perovskite to passivate uncoordinated Pb²⁺defects,thereby improving the photoelectric conversion efficiency of PSCs.DTTTP-TPA has a longer conjugate length than DTTTP-DPA.Next,we applied the two compounds as dopant free HTMs to PSCs to explore how the conjugation length of linear molecules affects the comprehensive properties of hole transport materials and the corresponding device efficiency.DFT calculations and XRD test results show that DTTTP-TPA has a longer conjugation length,which is conducive to the ordered stacking of molecules.SCLC tests showed that DTTTP-TPA obtained higher intrinsic hole mobility.It can be seen from steady-state PL and TRPL spectra that DTTTP-TPA films exhibit better hole extraction ability than DTTTP-DPA films.TPV and TPC tests show that DTTTP-TPA based devices have higher carrier extraction efficiency than DTTTP-DPA fabrication devices.Therefore,the DTTTP-TPA prepared device shows better photovoltaic performance,and its PCE is as high as 21.62%.In addition,the PCE of DTTTP-TPA based devices is still 93.2%of the original efficiency after 1000 h of operation,showing excellent stability.This work shows that the ingenious construction of long conjugated structures in linear molecules is indeed an effective strategy to obtain high performance HTMs.