Field-effect Properties Of Dithienothiophene Derivatives

Recently,organic semiconductors based on dithienothiophene?DTT?derivatives have attracted particular attention because of their potential applications in the field of organic optoelectronics.DTT unit exhibits much advantages including electron-rich,planarity,inoxidizability.In addition,high polarizability of sulphur atoms in dithienothiophene benefits the forming of multiple intermolecular interactions,such as S…H interactions,S…S interactions,S…C interactions etc,which provide convenient pathway for efficient carrier transportation and therefore improve device performance.There are six isomers of DTT units,while only two isomers have been widely studied.However,understanding of the relationship between the molecule structure and device performance is still a significant challenge.In this thesis,the optical and electrical properties of films based on DTT-core derivatives were characterized via many methods such as AFM,XRD,UV-vis,UPS.On this basis of these results,film transistors were fabricated and their field-effect properties were also studied.The relationship between molecular structures and properties were analysized on the point view of molecular feature?position of sulfur atom,capping groups,connection type?.This will guide the design of new organic semiconductor materials with high properties.1)OFET properties of ss-DTT derivatives have been characterized.Only compound W-3 shows good field-effect performance,the maximum mobility is 0.18 cm² V^-1s^-1,threshold voltage is about-15 V,on/off ratio is over 10⁵.Unfortunately,no signal was detected with the other compounds.This may be correlated with their nonlinear molecular configuration and short effective conjugation length.2)OFET properties of bb-DTT derivatives have been characterized.The two DTT units in W-5,W-6,W-7 are connected with single bond,C=C double bonds,1,4-divinyl benzene,respectively,the maximum mobilities are 0.13,0.22,0.40 cm² V^-1s^-1,respectively.With the increase of molecular length,the mobility increases slightly,the threshold voltage decreases obviously,on/off ratio is more than 10⁵.All three compounds display good environment stability due to their larger band gap.A distinct blue shift was observed in the uv-vis absorption spectra.Nevertheless,the connection type of DTTs does not affect the HOMO levels of these materials obviously.3)OFET properties of bt-DTT derivatives have been characterized.TM-7 shows the best field-effect performance in these derivatives of bt-DTT,the maximum mobility is 1.16 cm² V^-1s^-1,threshold voltage is about-10 V,on/off ratio is over 10⁷.Conclusions as following can be obtained from comparative analyses of relationship between structure and performance:a.S atomic position:the mobility of 2-?position derivatives are at least ten times higher than 5-?derivatives.Compared from 5-?position derivatives,absorption spectrum of 2-?position derivatives is bathochromic shift in different extent.While there is no obvious difference between their HOMO energy level.b.capping group:introducing capping groups?phenyl or styryl?results into the increase of the mobility obviously.The absorption spectrum is bathochromic shift in different degree.There is little effection to HOMO energy level after intruding capping groups.c.connection type:there is no homogeneous response in the connection style how to affect the mobility.It has to simultaneously consider the effect of S atomic position and substituent group.When the connection style ranges single bond from ethenyl,ranges ethenyl from 1,4-divinyl benzene,the absorption spectrum is similar and red shift in different degree.There is over 0.26 eV decline of HOMO energy level as the connection style changes to 1,4-divinyl benzene from single bond or ethenyl.