Semiconductor Nanocrystalline Enhanced NaYF₄:Yb,Er Upconversion Luminesence And Its Application

In recent years,due to the exhausting of traditional fossil energy?coal,oil and natural gas?and serious environmental pollution,the development and utilization of the rich and pollution-free solar energy is imperative.However,it is found that most applications can only use the energy of visible light,but near-infrared light,which accounts for nearly half of the total energy of the solar spectrum,cannot be directly utilized.Therefore,it is an important way to improve the utilization rate of solar energy by expanding the response range of applications to the solar spectrum.Rare-earth doped up-conversion luminescent materials can effectively absorb near-infrared light and convert it into visible light,which has been widely used in the fields of solar cells,biomedical imaging and fluorescent labeling.However,the traditional rare earth doped up-conversion luminescent materials?NaYF₄: Yb,Er/Tm?have a small absorption cross section and low up-conversion luminescence efficiency,which greatly limits their further application.At present,using the plasma characteristics of noble metals and semiconductor nanocrystals to conduct local field regulation is one of the most effective methods to enhance the rare earth conversion luminescence.Noble metals cause the collective vibration of free electrons in metals through their unique nanostructural properties,namely local surface plasmon resonance?LSPR?.The LSPR properties of semiconductor nanocrystals are caused by excessive free carrier collective oscillation caused by defects or impurities,so their LSPR properties can be easily regulated by controlling the doping ratio or stoichiometric ratio.As an antenna,plasma can improve the sensitivity of lanthanide fluoride,reduce the threshold,and expand the absorption spectrum of lanthanide fluoride.Moreover,local surface plasmon resonance?LSPR?can greatly enhances the electric field intensity near the plasma,and increases the excitation rate of luminescent molecules,so as to achieve the purpose of enhancing luminescence.The LSPR absorption of most noble metals is located in the visible region,while the LSPR absorption of semiconductor nanocrystals is located in the near infrared region,so abundant near infrared energy can be fully utilized.At present,the optical mechanism between semiconductor nanocrystals and rare earth upconversion luminescent particles is rarely studied.Based on the above analysis,the following two parts are mainly done:Part I: firstly,Cu_1.8S nanoparticles were selected as the plasma,insulator NaYF₄ as the intermediate layer,and NaYF₄: Yb,Er as the luminators to form Cu_1.8S@NaYF₄@NaYF₄: Yb,Er core shell nanoparticles.Compared with NaYF₄@NaYF₄: Yb,Er,the green and red light of Cu_1.8S@NaYF₄@NaYF₄: Yb,Er were enhanced by 6.9 and 7.5 times respectively.Then,the intermediate layer was doped with Yb ions of appropriate concentration to form Cu_1.8S@NaYF₄: Yb@NaYF₄: Yb,Er core-shell nanoparticles,Cu_1.8S@NaYF₄: 30%Yb@NaYF₄: Yb,Er the green and red enhancement factors?FF?of nanoparticles reached 28 and 101 respectively,and was enhanced by 40 times in total.After analysis and discussion,up-conversion luminescence enhancement is mainly attributed to two aspects :?1?excitation field enhancement caused by local surface plasmon resonance of Cu_1.8S semiconductor nanocrystals in the core;?2?doping Yb ions with appropriate concentration in the intermediate layer NaYF₄ can increase the absorption of near-infrared energy,and then transfer to Er ions in the luminescent layer to enhance their luminescence.Finally,the relationship between the excitation power and the fluorescence red-green ratio and fluorescence intensity of core-shell nanoparticles was explored.It was found that Cu_1.8S@NaYF₄: 30%Yb@NaYF₄: Yb,Er core-shell nanoparticles had a high fluorescence red-green ratio under different power,and the number of pumped photons was less in the up-conversion luminescence process.The results can be attributed to the saturation effect and local thermal effect caused by the LSPR property of Cu_1.8S nanoparticles.Part ?: With NaCsWO₃ nanorods as plasma.NaCsWO₃@NaYF₄: Yb@NaYF₄: Yb,Er core-shell nanoparticles are synthesized by the solvothermal method.The fluorescence effect is 600 times stronger.Next,NaCsWO₃@NaYF₄@NaYF₄: Yb,Er nanoparticles were doped into Spiro and used as hole transport materials for perovskite solar cells?PSCs?,and the efficiency was achieved by 18.28%.Then,on the basis of Spiro doping by UCNPs,UCNPs was used to modify perovskite layer,and the results showed that the efficiency of PSCs was increased to 18.89%?compared with the reference PSCs,efficiency increased by 17.99%?.Through analysis,we attribute the improvement of PSCs performance to the following three aspects:?1?UCNPs broadens the response range of perovskite to solar spectrum,increases light absorption and improves photocurrent.?2?the introduction of UCNPs into the PSCs enhances the scattering and reflection of light,prolongs the optical path length,leads to the visible light reabsorption,and generates more photocurrent.?3?UCNPs can modify the perovskite surface,which reduced the surface defects of perovskite,and effectively filled the holes at the grain of perovskite.In summary,the application of efficient UCNPs to the PSCs can effectively improve the cells performance.