Effect Of Number And Position Of Fluorine Atom On The Photoelectric Properties Of Dithiobenzodithiophenyl Based Polymers

Polymer solar cells（PSCs）have attracted great interest in recent years due to their advantages of light weight,flexibility,low cost.After decades of development,its power conversion efficiency（PCE）has exceeded 20%.Fluorine substitution is an effective strategy for designing polymer donors for high efficiency.This paper employ dithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene（DTBDT）as the donor unit,benzo[1,2-c][1,2,5]thiadiazole（BT）and its derivatives as the acceptor unit to construct D-A copolymers.The effects of fluorine substitution on the absorption spectrum,self-aggregation behavior,energy level and photovoltaic properties of the polymers were systematically studied by introducing different numbers of different positions fluorine atoms.The relationship between polymer donors structure and their photovoltaic properties is revealed.In the second chapter,three fluorinated D-A polymers PDTBDT-F-BT,PDTBDT-F-FBT and PDTBDT-F-2FBT,consisting of DTBDT modified by 2-（2-butyl-octyl）-3-fluorothiophene side chains and benzothiadiazole（BT）with different numbers of fluorine substitution（0,1 and2）,were synthesized.The three polymers all have good absorption in the wavelength range of300-700 nm and similar optical band gap.With the number of fluorine atoms increasing,the absorption spectrum of the polymer blue-shifted 20 nm,the HOMO energy levels were decreased from-5.42 e V to-5.54 e V.In addition,the introduction of fluorine atoms on the polymer acceptor structure unit enhanced the crystallization property of the polymer,which is conducive to the better charge transport performance of PSCs devices.The PDTBDT-F-FBT-based device showed the best PCE of 9.92%,with an open circuit voltage(V_OC)of 0.777 V,a short circuit current density(J_SC)of 23.22 m A·cm^-2 and a filling factor（FF）of 54.92%.In the third section of the paper,conjugated polymers PDTBDT-F-2FBT and PDTBDT-2F-2FBT were polymerized with 5,6-difluorobenzene benzo[1,2-c][1,2,5]thiadiazole and DTBDT which flanked monofluorinated or difluorinated thiophene side chain,respectively.The polymers all exhibited good absorption at 300-700 nm,compared with PDTBDT-F-2FBT,the absorption spectra of PDTBDT-2F-2FBT was slightly blue-shifted.the HOMO energy levels decreased and the V_OC of the device blended with Y6 increased from 0.761 V to 0.830 V.Moreover,the increased of the number of fluorine atoms enhanced the interaction between polymer chains,which is conducive to improving the crystallization property of polymer,and effectively inhibited the bimolecular recombination of photovoltaic devices and boosted the exciton dissociation rate.As a result,the PDTBDT-2F-2FBT based photovoltaic devices showed the best efficiency of 9.34%which profits from higher J_SC and V_OC.In the fourth chapters,two conjugated polymers PDTBDT-F-2FBT and PDTBDT-2F-BT are synthesized to study the influence of fluorine atom position on the polymers and its photovoltaic performance of the polymer donors.The two polymers all have good absorption in the wavelength range of 300-700 nm and similar optical band gap,and the aggregation of PDTBDT-2F-BT in film was poor.Compared with PDTBDT-2F-BT,PDTBDT-F-2FBT exhibited lower HOMO energy levels,higher hole mobility and better exciton dissociation.Therefore,PDTBDT-F-2FBT-based device achieve the best PCE of 9.17%,with a V_OC of 0.761V,a J_SC of 22.92 m A cm^-2 and an FF of 52.6%.