Theoretical Studies On The Electronic Structure Changes Of C₈-BTBT Under Strain

Due to the ease of fabrication,low-cost,and other features,organic semiconductor has been widely applied as organic light-emitting diode,organic photovoltaic cell,and organic field-effect transistor.However,charge carrier mobility is still the key factor that hinders organic electronic devices many important applications.In this thesis,we perform non-equilibrium molecular dynamics of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene derivative,C8-BTBT under shear and tensile strain,to study the changes of molecular packing and band structure.On one hand,we try to understand the influence in charge transport by molecular packing,on the other hand,we expect to guide material design for organic semiconductor with higher charge carrier mobility.We begin with experiment crystal structure.Energy minimization and NPT relaxation are first performed to release the inner stress.NPT relaxation from 300 K to 1K at an interval of 20 K indicates the C8-BTBT lattice undergoes a transition from monoclinic to triclinic crystal system at the temperature below 160 K.After that,we perform non-equilibrium molecular dynamics under shear strain in the xy plane,and tensile strain along x direction and y direction respectively.In the shearing process,the crystal keeps the herringbone structure,however,the cell parameter changes significantly,especially the cell angle ?,which indicates that the molecular packing has been modulated effectively.We find two linear region in the stress-strain curve.Except for the one including the original structure,a new linear stress-stain region is found from = 0.1 to = 0.22.It has been identified a low-temperature triclinic phase of C8-BTBT by later NPT relaxation.In the tensile process,the distances between conjugated rings have been adjusted along x direction and y direction respectively.We then extract the deformed structures from shear and tensile simulations to perform band structure calculation.In the shearing process,the band increases first,and then decreases with the shear strain increasing.In contrast,the band width of C8-BTBT varies monotonically in the tensile process.Structure under 6% of shear strain exhibits the largest band width,with up to 15% enhancement as compared to the unstrained structure.The band width of C8-BTBT shows as large as 8% enhancement under 2% of compressive strain in the x direction,and 14% enhancement under 4% of tensile strain in the y direction.To understand the changes of band structures under shear and tensile strain,we calculated the transfer integral in the deformed structures.Three important three charge transport channels T1,T2,and T3 are investigated.The two edge to face channels T1 and T2 constitute main charge transport pathways in the y direction,while the face to face channel T3 is responsible for charge transport along the x direction.The variation of transfer integrals between the edge to face pairs T1 and T2 with strains agrees with that of the band width along the ?Y direction,and that for the T3 channel agrees with the band width variation along the ?X direction.In the tensile process,the band width depends on the distance between conjugated rings.Smaller distance indicates stronger electronic overlap and better charge transport properties.