Introduction of Photoconversion Rare Earth Phosphors Fabrication

: Heterojunction photovoltaic cells exhibit two types of parasitic absorption, namely band edge absorption of ITO thin film and light absorption by doped amorphous silicon layer. These two types of parasitic absorption result in a decrease in EQE or IQE of heterojunction photovoltaic cells in the short wavelength region. Rare earth elements are widely used in optical functional materials due to their unique electronic configuration, which is attributed to the 4f electronic properties. The photoconversion phosphors composed of different rare earth elements are sorted with up/down conversion rare earth luminescent materials. The results of spectral allocation can make up for the shortcomings of the low absorption efficiency of the short wavelength of heterojunction photovoltaic cell. The preparation processes of rare earth phosphors generally include high-temperature solid-state method, precipitation method, sol-gel method, hydrothermal method, combustion method, etc. In this paper, the main preparation methods of rare earth phosphors are presented, and some typical cases are listed.


Introduction
Crystalline silicon cells have evolved into their third generation [1] .Currently, amorphous-siliconbased heterojunction with intrinsic thin-film (HIT/HJT), composed of two distinct semiconductor materials, has a robust competitive advantage in the N-type cell market.The factors affecting the photoelectric conversion efficiency of crystalline silicon cells encompass both optical and electrical losses.Specifically, the optical loss is partially attributed to the low absorption efficiency of HJT cell for short-wavelength sunlight, a characteristic significantly inferior to other cell types such as PERC and TOPCon.The aforementioned reasons can be attributed to the band-edge absorption of ITO thin films and the light absorption of doped amorphous silicon layers [2,3] .Given its unique electronic configuration, photoconversion rare earth phosphors can absorb short wavelengths (ultraviolet wavelengths) and emit long wavelengths (visible and near-infrared wavelengths), thus matching the optimal band gap width of crystalline silicon.Therefore, auxiliary materials like films and glass incorporating photoconversion rare earth phosphors have emerged as the preferred choice for HJT photovoltaic modules.This paper reviews recently reported methods for preparing photoconversion phosphors, including the high-temperature solid-state method, precipitation method, sol-gel method, hydrothermal method, and combustion method.Notably, the high-temperature solid-state method is the most employed one.

High-temperature solid-state reaction method
The high-temperature solid-state method is suitable for preparing various inorganic materials, such as metal oxides, silicates, and sulfides.It offers the advantage of a relatively straightforward preparation process, yielding materials excellent crystallization and thermal stability.However, this method also has some limitations, notably its requirement for high temperatures and prolonged reaction periods, signifying its characteristic of a slow reaction rate.
The preparation process starts with preparing different pure oxides and weighing them according to the chemical ratio (mole ratio).Then, these oxides are sequentially placed in an agate mortar for mixing and grinding.After the particles are uniformly fine, they are transferred to a corundum crucible.The crucible is then put into a high-temperature furnace, typically a muffle furnace, where the temperature is set to 1000℃ or above, and a nitrogen or argon atmosphere is introduced for sintering.Finally, these oxides are taken out from the furnace for crushing and re-grinding.The introduction of inert gases is essential to prevent some oxides from oxidizing into other valence states in the presence of oxygen, ensuring the acquisition of the target valence state.
The process flow of the high-temperature solid-state reaction method is shown in Figure 1.The widely studied matrix material for photoconversion materials is yttrium aluminum garnet (YAG), to which rare earth substances, such as Ce 3+ , Yb 3+ , Tb 3+ , or their combinations, are added.Through the high-temperature solid-state method, down-conversion rare earth phosphors of yellow powder series can be prepared, which can improve the photoelectric conversion efficiency of photovoltaic modules to varying degrees.Other researched rare earth materials include red powder series, such as CaAlSiN3: Eu 2+ .

Chemical co-precipitation method
The chemical coprecipitation method involves adding a precipitant into solutions containing two or more cations, generating precursor precipitates through stirring.These obtained precipitates are then filtered, washed, dried, and calcined to yield the required phosphor powder.Despite its lengthy production process, complex influencing factors, and inconsistent repeatability, this method can produce high-performance products.
The process flow of the co-precipitation method is shown in Figure 2. In a typical case [5] , raw materials AlCl3• 6H2O, SrCO3, NH4HCO3, and H3BO3 of the analytical reagent grade, along with an aqueous solution of EuCl3 with a concentration of 66.8 g/L, were prepared first.Based on the mole number of AlCl3• 6H2O, SrCO3 was weighed to achieve the composition (Sr0.96Eu0.04)3Al2O6,with the addition of 3% (mass fraction) H3BO3 as a fluxing agent and 0.04 mol EuCl3 as an activator.SrCO3 was dissolved in an HCl solution (0.06 mol/L), and a homogeneous solution was formed by adding AlCl3• 6H2O, EuCl3, and a suitable amount of distilled water.Subsequently, NH4HCO3 was added dropwise into the solution under magnetic stirring to ensure homogeneity, during which the system PH was maintained between 7 and 7.5.Afterward, the resulting solution was stirred for 1 hour, aged for another hour, and then filtered and rinsed with distilled water.After being dried at 80℃ for 12 hours, the obtained material underwent uniform grinding.It was then calcined at 300℃ for 1 hour, followed by successive calcination at 800℃, 900℃, 1000℃, 1100℃, 1150℃, and 1200℃ to determine the optimal calcination temperature.Finally, the material was maintained at the optimal temperature for 1 hour, 2 hours, 4 hours, and 5 hours to identify the optimal holding time.

Sol-gel method
The basic principle of the sol-gel method is to dissolve metal alkoxide or metal inorganic salts in water or organic solvents, leading to hydrolysis and alcoholysis reactions in the solution to produce particles that agglomerate to form sols.The sol is then transformed into a gel through drying, and the required target powder is obtained through subsequent heat treatment.This method has the advantages of high reactant purity, excellent chemical homogeneity, fine particle size, low sintering temperature, and energy saving, while its drawbacks include a lengthy reaction time, low yields, high costs, and the need for further performance improvement.
The process flow of the sol-gel method is shown in Figure 3.A typical instance [6] employed the sol-gel method to prepare Gd2O3: Bi 3+ , Gd2O3: Yb 3+ , and Gd2O3: Bi 3+ , Yb 3+ down-conversion rare earth phosphors.A suitable amount of citric acid (twice the amount of metal cations) was dissolved for later use.According to the specified ratios, Gd2O3(4N), Yb2O3(4N), and Bi2O3(4N) were weighed properly and dissolved in hot concentrated nitric acid.The solution was then added to the citric acid solution for complete dissolution, followed by slow stirring at a constant temperature of 80℃ in a water bath to form a sol, eventually transforming into a gel.The gel was then dried at 120℃ for 4 hours in a blast drying oven to obtain xerogel.After being ground, the xerogel was subjected to muffle furnace calcination at 600℃ and further crystallization in a tube furnace at a specified temperature in an air atmosphere, thus yielding the samples.The optimal crystallization temperature for Gd2O3: Bi 3+ was determined to be 1000℃, with a crystallization time of 3 hours, during which the excitation and emission fluorescence spectra reached their maximum values.

Hydrothermal synthesis method
Hydrothermal synthesis refers to the process of preparing raw materials into a solution according to a certain ratio at a temperature of 100~1000℃ and a pressure of 1 MPa~1 GPa and then pouring it into a sealed reactor (usually a reaction kettle) for reaction.As a chemical component, water also participates in the reaction, while the reactor serves as the core equipment in a hydrothermal synthesis plant.However, this method has several disadvantages, including low luminescence efficiency and unstable fluorescence performance of the resulting phosphors, the need for process improvement, high equipment requirements, challenging process control, and substantial difficulties in industrialization.
The process flow of the hydrothermal synthesis method is shown in Figure 4.In a typical case [7] , CeF3: 0.2 mol% Tm 3+ , 2.8 mol% Tb 3+ , 4 mol% Eu 3+ synthetic phosphors were prepared by the hydrothermal method.During the synthesis, stoichiometric Ce(NO3)3• 6H2O and RE(NO3)3• 6H2O were dissolved in 10 ml of deionized water.Subsequently, NH4F dissolved in 5 ml of deionized water was slowly dripped into the rare earth ion solution with continuous stirring.The mixed solution was stirred with a magnetic stirrer for 1 hour and then put into a sealed Teflon high-pressure tank.After being heated at 120℃ for 12 hours, it was cooled to room temperature, centrifuged at 10000 r/min for 5 minutes, and washed with deionized water and ethanol in turn.Finally, the obtained product is dried at 60℃ for 2 hours.

Combustion synthesis method
The combustion synthesis method achieves its target products by burning precursor materials.This method employs soluble metal salts (mainly nitrates) and organic fuels (such as urea, citric acid, and amino acetic acid) as reactants.While the former acts as the oxidant, the latter serves as the reducing agent.The precursor is ignited in a certain way and then experiences an oxidationreduction reaction.Sustained by the released heat, the combustion process concludes within minutes.The resulting combustion product is the required target material.This method offers notable advantages in terms of safety, time efficiency, and energy saving, marking it as an innovative method.
The process flow of the combustion synthesis method is shown in Figure 5.In a typical case [8] , YAG doped with 0.3 mol% Ce 3+ was synthesized through microwave solution combustion (MSC) at various microwave periods.The precursor materials included Y(NO3)3 (99.8%),Al(NO3)3 (98.5%),Ce(NO3)3 (99.99%),CO(NH2)2, and C2H5NO2.With the atomic ratio of Y:Al set as 3:5, the metal nitrate and fuel were weighed and dissolved in deionized distilled water.The mixture was heated and stirred at 70°C for 30 minutes.Afterward, the solution was heated in a microwave oven for 5, 10, and 15 minutes, during which ignition and flame were observed, and the solution evaporated instantaneously.This combustion process arises from the dehydration and decomposition of nitrates and fuels, emitting a substantial amount of gas.After combustion was completed, the powder was extracted, finely ground with a mortar and pestle, then annealed at 1050 °C for 5 hours, and finally cooled to room temperature.

Conclusions
With the rapid industrialization of HJT cell modules, photoconversion rare earth phosphors will play a pivotal role in the coming years.Consequently, the preparation methods and performance optimization of these phosphors have become research hot-spots, which include developing new preparation methods, optimizing current techniques, understanding the influence mechanism of phosphor particle morphology/size on fluorescence, and determining how to match fluorescence characteristics with diverse cell.

Figure 1 :
Figure 1: Schematic diagram of the high-temperature solid-state reaction method

Figure 2 :
Figure 2: Schematic diagram of the co-precipitation method

Figure 3 :
Figure 3: Schematic diagram of the sol-gel method

Figure 4 :
Figure 4: Schematic diagram of the hydrothermal synthesis method

Figure 5 :
Figure 5: Schematic diagram of the combustion synthesis method