Theoretical Study Of Degradation And Photolumincesce Red-shift Of Hydrogen-terminated Silicon Induced By Water Molecule

Silicon clusters are under intense investigation because of their inexpensive,nontoxic,biocompatible and huge silicon electronics market.In particular,as a result of the startling discovery of efficient emission from porous silicon by Canham in 1990,the color tunable emission from free-standing Si clusters have kindled the development and study of photoluminescence properties of Si for the potential applications such as light-emitting diodes?LEDs?,lasers,solar cells and fluorescent tags.In these applications,the surrounding environment is inevitably present in vapor or liquid.Water is a particularly common solvent in the synthesis process,biosensor,and the electrode of light-emitting and solar cells of Si QDs.Many recent experimental studies have shown that water induced field effect,which caused the electrical conductivity of hydrogen-terminated silicon substrates changed.Furthermore,the photoluminescence?PL?spectra of silicon surface affected by water vapor can be used as humidity sensor,and Si clusters being used as the electrode in application to light-emitting or solar cells are susceptible to degradation by water vapor.In the early theories,the excited state properties of Si crystals have been limited to isolated nanoclusters without considering most experiments performed under the humid or aqueous condition.On the one hand,the water environment effects on the emission spectrum of Si clusters have not yet been considered.On the other hand,the mechanism of external processes of silicon lightinduced degradation is still elusive.Therefore,for Si clusters a critical fundamental question of significant importance is whether and how the electronic structure and hence the PL properties and light-induced degradation is altered at nanoscale,in particular,with the presence of water.We analyze the impact of water cluster molecules on the optical properties of Si clusters based on time-dependent formulation of density functional theory.In contrast to the negligible impact on the absorption spectra with water cluster molecules adsorption on Si clusters,the fluorescence spectra with the peak position at ³66 nm shows a red-shift about 30 nm compared to the vacuum environment.More interestingly,the emission spectra of?H₂O?₃ and?H₂O?₄ adsorbed on the Si surface display a dualband with maxima centered at ³66 nm and ⁵10 nm.The physical mechanism is that the behaviors of?H₂O?₃ and?H₂O?₄ adsorption on Si clusters may provide weak elastic response of the environment near the surface,which leads to form self-trapped exciton.Here,we find that Si clusters in low-humid environment under light excitation induce a dual-band photoemission and a weak Si-Si dimer,which explain the experimental observations:?1?the difference of fluorescence of Si clusters between aged in water and in air;?2?a source of light-induced degradation of hydrogenated silicon.Our study of the detailed water molecule clusters effect on the fluorescence of Si clusters not only provides new explanations for the fluorescence mechanism of Si clusters at a particular size,but also shed light on the Si-water interaction that is important for the development of optical devises based on Si coated surfaces and fluorescent tags.Based on time-dependent formulation of density functional theory,we show that in water vapor environment the Si-Si bond will be more stretched under irradiation than that in vacuum,which will remarkably accelerate the degradation process.The degradation is the key factor to determine the efficiency and stability of the solar cells.In particular,light induced degradation?LID?of solar cell,known as Staebler-Wronski effect?SWE?,reduces the conversion efficiency of photovoltaic energy from sunlight to electrici-ty in hydrogenated silicon devices,which has negative impact on the financing and operation of solar cell.Since LID was firstly recognized in the 1970's,the rapid growth in solar cell deployments and the dramatic reduction the material cost have brought continuous interest in this phenomenon.A wide variety of models,such as hydro-gen complexes model,hydrogen collision model,Si dangling-bond defect model,and surface microvoids model,have been proposed to explain the LID.In these models,the mechanism of LID was mainly focused on the intrinsic degradation process,but the mechanism of external processes of degradation is still elusive.Water is a particularly common extrinsic environment on the silicon light-emitting diodes and silicon solar cells.Numerous experimental studies have shown that silicon used as the electrode in light-emitting and solar cells are susceptible to degradation by water vapor under illumination.For example,some studies at the National Renewable Energy Lab?NREL?found that controlling the humidity environment in the photovoltaic?PV?module can dramatically decrease the degradation processes within the PV module.However,the microscopic mechanism of LID in hydrogenated silicon devices at water vapor environment is still unclear.The electric dipole moment of Si₃₂H₃₈ in ground state is 0.1 Debye,while large enhancement of electric dipole moment(4.4 Debye for Si₃₂H₃₈)from exciton in Si QDs is induced by photo excitation.Large enhancement of electric dipole moment of Si cluster in excited state is strongly coupled to the dipole moment of water clusters,which results in the more elongation of Si-Si bond.Interestingly,this Si-Si bond is slightly more stretched which will significantly facilitate O₂ attack and further dissociation to create new Si-O-Si arrangements to initiate the degradation.This external degradation process is different from the intrinsic degradation mechanisms of light-induced degradation in hydrogenated silicon devices,which are caused by formation of metastable hydrogen complexes.The ab initio molecular dynamics?AIMD?results show the detail degradation process that within 150 fs,the whole O₂ molecule attaches to this Si atom,accompanying with the elongation of O-O bond.But for the vacuum environment,O₂ is repelled from the surface of Si cluster.This is further confirmed by the transition state calculations of degradation pathway.The light irradiation will lead a weak Si-Si bond to reduce the reaction energy barrier of first step?O₂ attack?to 13.0 kcal/mol in vacuum,which could be further reduced by the water vapor environment to only 4.0 kcal/mol.Further studies indicate that the size and disorder of the silicon clusters will highly affect the degradation process.Our results not only shed new insight for understanding the molecular mechanism of light-induced degradation under water vapor,but also may provide the theoretical foundation for controlling the light-induced degradation of silicon solar cell in real application.