Study On The Effect Of Temperature On Copolymers Properties By First-principle Molecular Dynamics

Diketopyrrolopyrrole(DPP)-based copolymers have become one of the more popular materials for high-efficiency organic solar cells due to their good electron and hole transport capabilities,narrow band gap,and good light absorption width.There are many factors that affect the photoelectric performance of copolymer,such as:molecular structure,magnetic field,electric field,etc.,where temperature is also one of the factors that affect the efficiency of solar cells.In this thesis,the first-principles density functional theory is used to study the effect of the number of thiophenes in the copolymer PDPP-n T(n = 2-6)at 0 K on its electronic structure and optical properties.The method of Ab initio molecular dynamics(AIMD)studied the photoelectric properties of PDPP-n T at different temperatures(0 K,60 K,120 K,180 K,240 K,300 K,360 K).Then introduce ?-bridge pyridine(Py,P2),thiazole(Tz,P3)and furan(THF,P4)into the copolymer PDPP-2T(P1),calculate the electronic structure and optical absorption spectrum at 0 K,and the effects of these three ? bridges on the photoelectric properties of copolymer PDPP-2T were analyzed.Finally,AIMD method was used to simulate the copolymer P4 after the introduction of THF at a temperature of 300 K,and the photoelectric properties were studied.The main research contents of this article are as follows:First,this paper studied the photoelectric properties of the copolymer PDPP-n T at 0 K,60 K,120 K,180 K,240 K,300 K,and 360 K.The specific results are as follows:(1)At 0 K,as the the number of thiophenes increases,the band gap value of the copolymer PDPP-n T gradually increased,and the position of LUMO orbit also moved away from the Fermi level,but the position of the HOMO orbit almost did not change.Through the analysis of the optical absorption spectrum of the copolymer,it is found that with the increase of n,the position of the optical absorption peak of the copolymer is blue-shifted.(2)At 60 K,120 K,180 K,240 K,300 K,and 360 K,the band gap of the thermal equilibrium structure of the copolymer PDPP-n T almost increases.The band gap value of the copolymer PDPP-3T at 300 K and the copolymer PDPP-5T at 360 K is smaller than the band gap at 0 K.(3)After the temperature changes,the position of the HOMO orbit of the copolymer PDPP-n T does not change,but the position of LUMO orbit changes significantly.(4)With the change oftemperature,the position of the long-wave absorption peak of the copolymer has a blue shift with respect to the position at 0 K.Secondly,this thesis studied the photoelectric properties of the three ? bridges(Py,Tz,THF)introduced into the copolymer PDPP-2T at 0 K and the photoelectric properties of the copolymer P4 at 300 K.The specific results are as follows:(1)After the introduction of three ? bridges of Py,Tz,and THF,the band gap increases,but the curvature increases and the effective electron mass becomes smaller,which may improve the mobility of the material.At the short wave of the light absorption spectrum,the light absorption peaks are red-shifted.At the long wave,the long wave peaks of P2 and P4 are blue-shifted,while the long wave peaks of P3 do not change much,and the long-wave absorption intensity is reduced.The HOMO orbit is hardly affected by the ? bridge,the LUMO orbit is greatly affected by the ? bridge,and its position is far from the Fermi level.Therefore,the introduction of ? bridges in PDPP-2T to improve charge transfer is a problem that needs comprehensive consideration.(2)Through the analysis of Bader charge,it was found that after the introduction of the three ? bridges,the intramolecular charge transfer amount of P2 and P3 decreased,and the intramolecular charge transfer amount of P4 increased slightly,and the ? bridge can be either an electron-acquiring unit or an electron loss unit in the copolymer.(3)At 300 K,after introducing ?-bridge THF,the thermal stability of the copolymer at room temperature is still very good,and its band gap value is reduced relative to P1 at room temperature.The HOMO position has not moved,and the position of LUMO has obviously moved.(4)At 300 K,the position of the light absorption peak of copolymer P4 is red-shifted with respect to P1.In short,the innovation of this paper is to calculate the electronic structure and absorption spectrum of the copolymer PDPP-n T at different temperatures through the AIMD method,and find that the temperature change mainly affects the LUMO energy level of the copolymer PDPP-n T.It is found that the introduction of ? bridge in the copolymer is a problem that needs comprehensive consideration.