First-principles Study Of Rare Earth Doped ?-NaYF₄ Upconversion Luminescent Materials

With the depletion of fossil energy,the energy crisis has become a serious problem which restricts the further development of human beings.Solar cells can convert solar energy into electricity efficiently,which is one of the most promising photovoltaic devices to completely solve the energy crisis.However,solar cells can only absorb photons with energy in the visible light range,photons of infrared light cannot be utilized,which account for?50%of the solar energy.The up-conversion photoluminescent materials,which are composed of matrix materials and rare earth elements,can be employed in solar cells to extend the photo-absorption to the infrared region,thereby improving the photovoltaic conversion efficiency of solar cells significantly.As the most effective matrix material at present,the hexagonal phase ?-NaYF4 has been widely used in experiments,and the rare-earth-doped ?-NaYF4 system has also become an research hotpot for scientists.In this paper,based on the density functional theory,first-principles simulations are used to systematically study the ground state structure,electronic structure,light absorption and light emission properties of ?-NaYF4 and its doping system.The main work includes the following three aspects:1.The ground state geometric and electronic properties of undoped ?-NaYF4 shows that ?-NaYF4 is a stable and indirect band gap semiconductor material,with ultralow photo-reflection coefficient and light absorption coefficient in the entire spectral range,which ensuring low light loss and high light transmittance,thus making sure enough infrared photons can be absorbed by the rare earth ions and enough visible light photons can be absorbed by the solar cells.Therefore,?-NaYF4 is a promising matrix material for up-conversion photoluminescence with superiority over other materials.2.The electronic and optical properties of several rare earth single-doped ?-NaYF4 systems with different doping elements were also studied.Comparing with the intact ?-NaYF4,new defect states are found between the conduction band and valence band for all the doped systems,which is beneficial to the absorption of low-energy photons.The optical absorption spectra of doped systems further prove that the light absorption of doped ?-NaYF4 in the near-infrared light portion increases significantly compared with that of the undoped system.This effect is particularly prominent in Er3+doped ?-NaYF4 with more than 3 times larger photo-absorption in the near-infrared range.At the same time,the light reflection remains almost the same before and after doping,proving that rare earth doping is an effective way to improve the up-conversion luminescence efficiency of ?-NaYF4.Furthermore,the up-conversion luminescence properties of ?-NaYF4:Er3+ with different Er3+concentrations were studied.Within a certain concentration range,the infrared light absorption of?-NaYF4 increases with the doping concentration.When the doping concentration is 33.3%,the infrared light absorption of ?-NaYF4:Er3+ is enhanced by more than 70 times.In addition,the up-conversion luminescence properties of the Er-Ho double-doped ?-NaYF4 system are also investigated.The simulation results reveal that the near-infrared photo-absorption of ?-NaYF4:Er3+/Ho3+ is increased by 12.5 times compared with the single-doped system,which indicating that double-doping is another valid way to improve the up-conversion luminescence efficiency of ?-NaYF4.3.Interstitial doping with metal elements can reduce the symmetry of the crystal field around the rare earth ions,which greatly improves the probability of electron radiation transition,thereby increasing the up-conversion luminescence efficiency of the up-conversion luminescent material significantly.In order to further improve the up-conversion luminescence efficiency of ?-NaYF4,the up-conversion luminescence properties of 2%and 4%Li-doped ?-NaYF4:Er3+ system were studied.It was found that,compared with the undoped case,the infrared light absorption of Li-doped ?-NaYF4:Er3+increased by 18.3 times for 2%Li concentration and 76.4 for 4%Li concentration.Consequently,lithium doping provides a novel way to enhance the photo-absorption of ?-NaYF4:Er3+ in the infrared light range,hence optimizing the up-conversion luminescence effectively.The influences of different dopants,doping concentration,double-doping and Li-doping on the up-conversion luminescence efficiency of rare-earth doped ?-NaYF4 are studied systematically and meticulously in this work by first principle simulations based on the density functional theory.Our results provide novel ideas on improving the up-conversion luminescence efficiency of rare-earth doped ?-NaYF4,which has particle significance on promoting the application of up-conversion luminescence materials in solar cells and innovative meanings on optimizing the photovoltaic performance of solar cells,thus facilitating large-scale implementation of solar cells and solving the pressing energy crisis and environmental pollution.