Dynamics Of Singlet Photoexcitation In Conjugated Polymer

Organic conjugated polymer,as a new type of semiconductor materials,has aroused extensive interest in academic and industrial research.Compared with traditional inorganic semiconductor materials,organic conjugated polymers have the advantages of easy processing,low cost,light weight and so on,showing promising prospects in large-area display,information technology and new energy development.In recent years,it has been successfully applied in organic light-emitting diodes(OLEDs),organic solar cells(OSCs),organic field-effect transistors(OFETs)and other optoelectronic devices.Trans-polyacetylene has the simplest structure.However,the physical properties are the most special among the various of conjugated polymers.The molecular shows a conjugated structure with alternate single and double C-C bonds,which exists a twofold energy degeneracy of ground state energy.Therefore,the structure provides the possibility of the generation of elementary excitations—solitons.The solitons take the form of domain walls separating different districts of opposite single and double bonds alternation patterns.It also has unique spin-charge relationships.The dynamic of the soliton affects the properties of materials and device performance directly.Furthermore,soliton is a topological defect which induces topological properties of trans-polyacetylene.Ever since the seminal works by Su,Schrieffer and Heeger,trans-polyacetylene and its solitons dynamics has been extensively studied.The product after photoexcitation in the trans-polyacetylene has attracted substantive attentions over the past few years.In a previous work,we have mainly studied the formation and evolution of photogenerated soliton pairs in trans-polyacetylene at zero temperature.The results supported some experimental phenomenon.However,the characteristics of the materials in zero temperature are different from those in actual environment.In fact,thermal effects will induce the fluctuation of lattice atoms.The distribution of ? electrons is also changed since the electron-phonon interactions.The temperature effect has an important influence on the low-energy charge carriers.Based on these factors,the photoexcitation dynamics in trans-polyacetylene under non-zero temperature are investigated in our first part of work.The Su-Schrieffer-Heeger+Pariser-Parr-Pople(SSH+PPP)model is used to describe the trans-polyacetylene.Thermal effects are considered by adopting Langevin equations and are introduced through the lattices vibration.We adopted the multiconfigurational time-dependent Hartree-Fock(MCTDHF)method to handle the electron correlations.The evolutions of the electronic states and the atoms have been simulated using the nonadiabatic molecular dynamics method.The products after photoexcitation and their yields have been examined in detail.Compared with the zero-temperature condition,the time of oscillation before the formation of stable defects gets earlier,i.e.,the conversion time from charged to neutral soliton pair is shorter.And the yield of neutral soliton pair increases by considering the temperature effect.We also found that the center inversion symmetry of the system is broken,i.e.,the movement of the two domain walls is not symmetrical about the chain center.We have done simulations with different temperature values of T=5,50,100,150,200,250,300 K.The results have been taken as the ensemble average over multiple data.We found that lattice thermal oscillations enhance if the temperature increases.However,the generation time and the yield of neutral soliton pairs do not change significantly.The generation time of the neutral soliton pair is shortest at T=50K,at the same time the yield is also the highest.From the simulation results at high temperature,for example,300 K,it is found that the neutral soliton pair can still exist stably.So we can conclude that the soliton is a kind of stable excitation.In particular,we also have done the dynamics of the photoexcitation in trans-polyacetylene under non-zero temperature in the frame work of SSH model.The results show that only charged solitons are generated and neutral soliton pairs do not appear.Under the influence of the thermal fluctuations,the formed charged solitons performs a random walk along the polymer chain.The SSH model,based on the tight-binding approximation,only the nearest-neighbor hopping term appears in the model Hamiltonian.It works well when describing the systems which have a narrow band.However,the trans-polyacetylene chain has a planar geometry structure.The wave functions between non-nearest atoms could overlap.So the next-nearest-neighbor hopping interactions cannot be neglected.In addition,the next-nearest-neighbor hopping directly affects the charge conjugation symmetry of the SSH Hamiltonian and has an impact on the properties of materials.In the second part of this work,using the SSH+PPP Hamiltonian,we have studied the effects of electronic next-nearest-neighbor hopping on the ground state and dynamic properties.First,we have found that the next-nearest-neighbor hopping interactions significantly affect the properties of eigen energy levels.The eigen energy levels is no longer symmetric about E=0eV.The widths of the conduction and valence band are different.This is due to the fact that the electron-hole conjugation symmetry of SSH Hamiltonian is broken when the next-nearest-neighbor hopping is considered.Simulations have been done with different values of coupling strength P(0.0?0.25).Numerical results show that the valence band extends to a larger range while the conduction band acts to the contrary with increasing coupling strength P.The energy of LUMO(the highest occupied molecular orbital)and the energy of HOMO(the lowest unoccupied molecular orbital)increases.However,the gap and the bandwidth are hardly changed.The time of the transformation from charged into neutral soliton pair gets a little earlier if the next-nearest-neighbor hopping interactions are considered.With the increase of P,the generation time and yield of neutral soliton pairs have almost no changes.