Non-adiabatic Molecular Dynamics Study Of Excited Carrier's Lifetime Affected By Defects In Semiconductors

Energy issue is the core in the sustainable development of human society.As a rich renewable energy source,solar energy is of great significane in its transformation and utilization.The conversion of solar energy includes several common methods such as photovoltaic,photocatalysis,and photosynthetic.Either way,the excitation and relaxation of electron-hole pairs in semiconductor material are the basic process in solar energy conversion.During the solar energy conversion,electron and hole can be non-radiatively recombined through interaction with phonons.In this process,the energy of electron and hole is converted into heat energy,and in-depth study of this process is the key to improve solar energy conversion efficiency.In this paper,we focus on the problem of electron-hole's non-radiative recombination in semiconductors,using ab initio non-adiabatic molecular dynamics(Hefei-NAMD)method based on time-dependent density functional theory(TDDFT)and surface hopping.We systematically study the time scale of electron-hole's non-radiative recombination in TiO2 and black phosphorus material and reveal the coherent dynamics of quasi-particles such as electron/hole,phonon,polaron and so on.In Chapter 1,we introduce two commonly used methods for solving multi-electron stationary Schrodinger eqution based on Born-Oppenheimer approximation:Hartree-Fock method based on wavefunction and density functional theory(DFT)based on electron density,which can both reduce the multi-electron problem to single-electron problem for electronic structure calculation.However,when deals with photo-excited carrier relaxation dynamics,Born-Oppenheimer failed.Then we introduce non-adiabatic molecular dynamics and usually adopt mixed quantum-classical dynamics including Ehrenfest method and surface hopping method.Finally,we introduce the combination of fewest surface hopping and TDDFT implemented in Kohn-Sham framework proposed by Professor Oleg V.Prezhdo.Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap.Taking three-dimension TiO2 with wide band gap as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics(NAMD),we find that the localization of impurity-phonon modes(IPMs)is the key parameter to determine the e-h recombination time scale.Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e-h recombination within several picoseconds.However,the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs.For different doping systems,the e-h recombination time is shown to depend exponentially on the IPM localization.This part will be introduced in Chapter 2.In addition to impurities,polarons formed by excess charge inside TiO2 also affect electron-hole recombination.Taking rutile TiO2 as a protype system and using NAMD,we find that the dynamics of polarons,phonons and photon-generated holes are strongly coherent.The hopping and delocalization of polarons significantly accelerate the energy relaxation of photon-generated holes through polaron-phonon coupling and thus speed up their quenching process.Our study of coherent dynamics of polaron,phonon and photon-generated carrier brings up the concept that polaron delocalization induces fast quenching through e-h recombination which has important applications for material design for nano-electronics,superconducting and solar energy conversion.This part will be introduced in Chapter 3.Phosphorene formed by a single layer of black phosphorus(BP)is a new promising two-dimensional(2D)semiconductor material,and performance of a material is closely connected with defects appearing unavoidably in fabrication.By performing NAMD simulation on three defects with distinct electronic structures,we demonstrate that instead of the expected acceleration,the e-h recombination is suppressed by the defects in the phosphorene.Such suppression of charge carrier recombination occurs for both shallow and deep gap states induced by the defects.Our analysis shows that because BP is a mono-elemental material,the distinct impurity phonon,which often induces fast e-h recombination,is not formed.Further,due to the flexibility of 2D phosphorene,'the defects scatter the phonons present in pristine phosphorene,generating multiple modes with lower frequencies.Through electron-phonon coupling with these low-frequency phonons,the nonadiabatic coupling and coherence time between the electronic states are reduced,which suppresses the e-h recombination.This part will be introduced in Chapter 4.The anomalous behavior that defects suppress e-h recombination in phosphorene has been further verified in the study of its oxidative defects.Therefore,we propose the oxide layer can serve as a self-protective layer to enhance the performance and application of BP.This part will be introduced in Chapter 5.In the last chapter,we summarize current works and look forward to future propescts.