Synthesis, Optical Properties And Perovskite Photovoltaic Applications Of Rare-earth Doped Fluoride Nanocrystals

Recently,lanthanide-doped fluoride upconverted nanomaterials have drawn widespread attention as a sort of important luminous materials,due to their unique physical and chemical properties.These types of nanomaterials are with great potential utilization in volumetric displays,photocatalysis,bioimaging,laser devices,non-contact optical temperature sensors and photovoltaic cells.Exploring new rare-earth doped fluoride nanomaterials,developing new fabrication technologies,studying growth mechanisms and tailoring materials with controllable sizes,morphologies and crystal growth processes has important research value and practical significance for optimizing the optical properties and achieving applications in photo-electronic devices.The protocols to improve the luminescent efficiency and optical temperature sensing properties,via introducing tervalent lanthanide ions or doping elements different from those of the host atoms into fluoride hosts,have attracted considerable attention.Moreover,researchers have a great interest in organic/inorganic hybrid perovskite solar cells?PSCs?due to their high power conversion efficiencies?PCEs?,appropriate bandgaps,high carrier mobility,and low-cost solution-processing techniques.However,the spectral response of PSCs or other typed thin film solar cells is limited to just a tiny portion of the solar spectrum.Thus,the photovoltaic?PV?cell wastes a large fraction of the energy that is abundant in sunlight.In especial,in the case of near infrared?NIR?or IR light,the sub-bandgap sunlight photons have no sufficient energy to create electron-hole pairs,either transmitting through the solar cell or being absorbed to produce heat,and hence having no contribution to produce electricity for the PV cell.One promising approach to decrease the sub-bandgap loss is the application of the rare earth element-doped nanophosphors that absorb NIR or IR light and upconvert it to higher visible energy photons.In this way,the spectral response of the PSC device could be broadened to the subbandgap photons.The main research contents of this dissertation are as follows:?1?KMnF₃:Yb,Er upconverting nanocrystals?UCNCs?were prepared by a simple hydrothermal approach with Ethylenediaminetetraaceticaciddisodiumsalt?EDTA?as a complexing agent.The influence of the complexing agent content on the morphologies and the structures of the formed UCNCs was investigated.It is found that the size of the UCNCs decreases with the increase of the EDTA concentration.KMnF₃:Yb,Er nanocrystals with well-controllable morphologies and narrow size distributions were obtained by controlling the content of the complexing agent.The UC emission spectra of the synthesized KMnF₃:Yb,Er UCNCs were measured at various temperatures under the irradiation of a 980 nm diode laser.The dependence of the emission intensity ratios of Er³⁺ions of adjacent energy levels on the temperature was investigated.In accordance with the sensitivity definition,the maximum sensitivity(S_max)values of KMnF₃:Yb,Er UCNCs were obtained.The mechanism of optical temperature sensing is discussed.The S_maxax value of KMnF₃:Yb,Er UCNCs formed at an EDTA molar ratio of 1 is0.94%K^-1,indicating that these UCNCs can be used as optical temperature sensors for temperature measurement.We present a new structured and near-infrared perovskite solar cell?PSC?via incorporating KMnF₃:Yb,Er nanocrystals into the perovskite active layers.KMnF₃:Yb,Er UCNCs synthesized at an EDTA molar ratio of 1 was introduced into the perovskite precursor solution.The size of the UCNCs is 49 nm.It is found that the added KMnF₃ based UCNCs were served as nucleation sites to induce the growth of perovskite crystals during the spin-coating and annealing processes.The resulted CH₃NH₃PbI₃ perovskite absorber films embedded with UCNCs display a uniform pinhole-free morphology with enlarged crystal grains.The CH₃NH₃PbI₃solar cells fabricated with KMnF₃ based UCNCs achieved a PCE of 19.11%underAM1.5G conditions,an increase over 24%compared with conventional PSCs?15.41%?.?2?A new strategy is reported to enhance the upconversion?UC?luminescence emission of LiYF₄:Yb,Er nanocrystals?NCs?using magnesium as a dopant.We carried out systematic experimental studies on the crystal structure,grain size,and UC emitting property of the tetragonal LiYF₄:Yb,Er with varied concentrations of Mg²⁺.The UC luminescence properties were examined under 980 nm laser illumination with various excitation power densities.At a proper doping concentration,co-doping of Mg²⁺ions into LiYF₄:Yb,Er is found to result in efficient reinforcement in both the green and red upconverted emissions.Remarkably,the maximum green and red luminescence intensities were reinforced by sevenfold and fivefold,respectively,when 7 mol%Mg²⁺was co-doped into tetragonal LiYF₄.The possible origin and mechanism for boosting UC emission were explained according to the alteration of the cell volume and the local crystal field surrounding the Er3+ions by co-doping of Mg²⁺.Moreover,the emission-optimized LiYF₄ UCNCs were further investigated to understand thermalsensing behaviors employing the fluorescence intensity ratio?FIR?approach from the two neighboring thermal coupled states(²H_11/2/⁴S_3/2).The optimization of Mg²⁺co-doping in LiYF₄:Yb,Er allowed the resultant UCNCs to be an excellent luminescent thermometer over a wide range of temperature.Applying the optimized UCNCs as an optical thermometer,a maximum thermal sensitivity of 0.0543 K^-1 was achieved at room temperature and a low-power excited upconversion(1 W cm^-2).The achieved S_max value is more advanced than most of the Er-based nanophosphors reported heretofore.This paper provides a perspective scheme to design and grow high-quality upconversion nanomaterials for achieving the preconditions of the pragmatic application in temperature sensing,optically heating,and color display devices.?3?Mono-disperse,single tetragonal-phase Li?Gd,Y?F₄:Yb,Er upconversion nanocrystals?UCNCs?were prepared by a thermal decomposition method.The grown Li?Gd,Y?F₄:Yb,Er UCNCs displayed tetragonal bipyramidal morphologies and intense UC green luminescence.The octahedral UCNCs were blended into the Spiro-OMeTADbased HTL with different weight ratios.At an optimal weight ratio of UCNCs,the PSCs based on the upconverting transport hole layer achieved an average power conversion efficiency of 18.34%under AM1.5G illumination,an increase over25%compared with conventional HTL based PSCs?14.69%?.We systematically explore the optical and electrical enhancement effects of Li?Gd,Y?F₄:Yb,Er UCNCs via UV–visible absorption,diffused reflectance,external quantum efficiency?EQE?,steady-state and time-resolved photoluminescence,photocurrent property and the space-charge-limited-current?SCLC?measurements.It was found that with the addition of the octahedral UCNCs in a proper proportion the composite HTL exhibits high charge carrier mobility and charge carrier transfer/collection capability.Besides,the embedded UCNCs boost the light harvesting of the PSCs over a large range of wavelengths from 400 nm to 800 nm.Furthermore,with a 980 nm near-infrared?NIR?laser irradiation,the embedded UCNCs acted as efficient upconverting centers to up-convert the NIR light for promisingly expanding the effective absorption spectrum for PSCs.