Fluorine Containing Groups Modification Of Hole Transport Materials For Efficient And Stable Perovskite Solar Cells

Since 2009,organic-inorganic hybrid perovskite materials with bipolar transmission characteristic have been used in solar cell devices.After optimizing devices fabrication and materials,the efficiency of perovskite solar cells（PSCs）has been increased from 3.8%to 25.5%,making PSC becoming an attractable photovoltaic technology with great commercial application.In general,the structure of PSCs device is composed of five parts:working electrode（ITO or FTO glass）,electron transport layer（ETL）,perovskite light absorption layer,hole transport layer（HTL）and metal electrode（Au,Al,Ag,etc.）.Among them,HTL plays an important role,which can not only rapidly extract the photogenerated holes in the perovskite layer,but also avoid the degradation of perovskite caused by water and oxygen in the air.The ideal hole transport material（HTM）should have high hole mobility,energy level matching with perovskite material,excellent thermal stability and solubility,and good hydrophobicity.The fluorine-containing group has good hydrophobicity and strong electron-withdwawing property,so the introduction of fluorine into HTM has become a research hotspot for high and efficient PSCs.In this thesis,a series of novel small molecular HTMs containing fluorine groups were designed and synthesized.The effect of modified mode of fluorine groups in HTMs on the properties of HTMs and PSCs was studied.The following experimental results were obtained:（1）Six small molecular HTMs with different fluorine-containing groups（single fluorine atom,3,4,5-trifluorobenzene and trifluoromethyl）as the peripheral terminating groups were designed and synthesized by using two structures of spirodifluorene（SBF）and spiro[fluoren-9.9’-oxanthracene]（SFX）as the core,respectively,namely SBF-PF,SBF-P3F,SBF-PCF₃,SFX-PF,SFX-P3F and SFX-PCF₃.It was found that the introduction of fluorine-containing group can reduce the Highest Occupied Molecular Orbital（HOMO）levels of the materials and improve the hole mobility and hydrophobic ability.The compounds SBF-PCF₃ and SFX-PCF₃modified with trifluoromethyl showed high hole mobility and good hydrophobicity.The hole mobility of SBF-PCF₃ and SFX-PCF₃ reached 7.69×10^-5 cm² V^-1 s^-1 and 6.30×10^-5cm² V^-1 s^-1,and the water contact angle were 91°and 91.5°,respectively.The dopant-free PSCs based on SFX-PF,SBF-PF and SBF-PCF₃ as HTM exhibited PCE of 1.28%,3.60%and 9.84%,respectively.（2）Two novel small molecular HTMs,SBF-POCCF₃ and SFX-POCCF₃,were synthesized by linking trifluoromethyl into HTMs based on SBF and SFX through alkoxy.The results showed that the compounds with non-conjugated trifluoroethoxy group had excellent hydrophobicity and higher hole mobility(SBF-POCCF₃:2.31×10^-4 cm² V^-1 s^-1;SFX-POCCF₃:1.22×10^-4 cm² V^-1 s^-1).The dopant-free PSCs based on SBF-POCCF₃,SFX-POCCF₃ and Spiro-OMe TAD as HTM exhibit PCE of 15.21%,11.96%and 13.28%,respectively.And the dopant PSCs based on SBF-POCCF₃,SFX-POCCF₃ and Spiro-OMe TAD as HTM exhibited PCE of 21.11%and 20.16%,respectively.The device efficiency based on SBF-POCCF₃ is higher than that of Spiro-OMe TAD（20.44%）.The unencapsulated SBF-POCCF₃-based PSC can maintain 93.03%of the original efficiency after being placed in an air humidity of 60%for250 h,showing excellent long-term stability.