Design Of Organic Acceptor Molecules Modified With Se Atoms And Alkoxy Groups And Theoretical Study Of Photoelectric Properties

As a new type of solar cell,organic solar cells（OSCs）has attracted wide attention due to its advantages of flexibility,thinness and low cost.The structure of the acceptor molecule in the active layer of OSCs heterojunction plays an important role in charge separation and transport process,and its performance directly affects the efficiency and stability of the cell.In recent years,the molecular modification of OSCs acceptor in active layer has become a hot research direction.At the same time,computational chemistry and experimental chemistry are complementary to each other,which is conducive to multi-scale and comprehensive study of OSCs acceptor materials.In this paper,two kinds of Y6 derivative molecules were designed based on the Se atom substitution in thiophene and the modification of the inner alkyl chain,and a series of non-thick ring acceptor molecules were designed using alkoxy substitution.On the basis of density functional theory（DFT）and time-dependent density functional theory（TD-DFT）,the properties of molecular planarity,frontier molecular orbitals（FMOs）,electron-hole transport,optical absorption properties,and recombination energy are systematically studied by quantum chemical calculation methods.The influence of molecular structure changes on molecular planarity and electronic structure was explored,which provided effective theoretical support for experimental development and screening of new active layer molecules.The main research content of this paper is as follows:Part 1:Systematic theoretical study of A’and D unit Selenium substitution in non-fullerene acceptor Y6.Five new Y6 derivative molecules were designed by replacing weakly electronegative sulfur atoms with highly electronegative selenium atoms,the ground state electronic structure,UV-absorption spectrum and charge transfer mechanism of all molecules were calculated and analyzed based on quantum chemistry theory.Firstly,Gauss View and Multiwfn software were used to analyze the molecular bond length,dihedral Angle and AIM topological analysis to study the effect of selenium atom substitution on the molecular plane.It was found that the non-covalent bond between selenium atom and the terminal acceptor oxygen atom formed a"conformational lock"when selenium atom was located in the outer thiophene,which was conducive to the establishment of a better conformation of coplanar molecules.Secondly,the properties of FMOs,electrostatic potential（ESP）and state density（DOS）of Y6derivatives were analyzed.The results show that the selenium-substituted molecule has a stronger electron-deficient ability and lower energy gap than Y6.Finally,the intramolecular electron excitation properties and transport mechanism were investigated by means of intramolecular charge transfer（ICT）,electron-hole transport characteristics and Marcus theory recombination energy.The binding and recombination energies of Se substituted derivatives are lower than those of Y6,indicating that the internal excitons are more likely to dissociate into electrons and holes,further promoting charge transfer.This work further elucidates the effect of selenium atom substitution on electron-hole transport and charge transfer within Y6 molecules,which has theoretical significance for designing more efficient acceptor materials in the active layer in the future.Part 2:Effects of modification of inner alkyl chain(-C₇H₁₄)on electronic structure and photoelectric properties of Y6 molecule.Five Y6 derivatives were designed to modify the Y6medial chain by screening side chain groups.Based on DFT and TD-DFT methods,geometric optimization,single point energy and excited states of all molecules were calculated by suitable functional.The effects of different alkoxy substitutions on the planarity of Y6 derivatives were investigated by dihedral Angle,bond length and AIM topology analysis.The results show that the bonding caused by partial group substitution can weaken the non-covalent bond between the end group and the core atom,which is not conducive to the stability of molecular conformation.Although the amino modified molecule（Y6-C-N₂H₄）caused the dihedral Angle between the terminal group and the central nucleus to change by about 1（?）,it had no significant effect on the molecular planarity.By means of FMOs,ESP and DOS,the ground-state electronic structure of Y6 derivative molecules was analyzed.It was found that Y6-C-N₂H₄ had the smallest energy gap,the most obvious absorption spectrum red-shift and the uniform distribution of frontier orbital energy levels.The properties of electron-hole transfer,ultraviolet visible absorption spectrum,Transition Density Matrix（TDM）,Charge Transfer Matrix（CTM）and recombination energy were analyzed by Multiwfn software.The results show that all molecules exhibit a mixture of charge transfer excitation（CT）and local excitation（LE）,and the charge is transferred from 2,1,3-benzothiadiazole（BT）to end-group（IC-2F）.This work revealed the synergistic mechanism of the effects of medial chain modification on the geometry,electronic structure and photoelectric properties of Y6 molecules.Part 3:The effect of alkoxy modification on intramolecular charge transfer in non fused ring acceptors was investigated.Six kinds of A-π-D-π-A non-thick ring acceptor molecules were designed with benzo[1,2-b:4,5-b’]dithiophene（BDT）and thiophene as core cores.All the molecules were geometrically optimized and the single point energy and excited states were calculated.The effect of alkoxy substitution on molecular planarity was studied by means of bond length dihedral Angle and AIM topology analysis.The results show that oxygen atoms in alkoxy group and sulfur atoms in thiophene are easy to form non-covalent bonds between sulfur and oxygen,which makes all molecular fragments tend to be planar and further reduces the steric hindrance of charge transport.In addition,the analysis of UV-vis absorption spectra shows that W1 has the maximumλ_max（798.87 nm）and good optical absorption performance in the infrared region.Compared with R1,W1 has an obvious red-shift,which is conducive to promoting intramolecular charge transfer.The properties of electron-hole transport,charge and hole recombination energy are analyzed by Multiwfn software.The results show that the distance between the hole and the electron center of mass of the molecule（W3）replaced by a single methoxy group is the largest,which is conducive to the separation of electron and hole.At the same time,all the internal charges of the molecule are transferred from the central nucleus to the terminal group of IC-2F,the molecule（W2）with BDT core and 3,4-ethyldioxyethiophene（EDOT）bridge with the donor molecule PBDB-T showed excellent V_oc and FF.The results show that the introduction of alkoxy can effectively improve the molecular planarity and promote charge transfer,which has a certain reference for the design of non-fused ring molecules.