SCALED METHOD FOR MANUFACTURING LEAD IODIDE (PBI2) THIN FILMS WITHOUT THE USE OF ORGANIC SOLVENTS

Abstract

In this work, a new method for synthesizing lead iodide (PbI₂) thin films was developed using the chemical bath deposition (CBD) method, which consisted of two stages. In the first stage, tin oxide films were synthesized to act as a seed layer, allowing for better adhesion of the film to be deposited. The second stage consisted of synthesizing the lead iodide films. Finally, heat treatments (100, 150°C, 200°C, 250°C, 270°C) were performed to promote polytype transitions in the material and to study the effect of these structures on the optical, chemical, and structural properties of the material. The direct influence of the heat treatment on the induction of polytypes was determined, and these polytypes clearly influence the structure and chemical composition of the resulting films.The results showed that the formulation sequence is crucial for generating a heterogeneous mechanism, which leads to the formation of lead iodide films. The samples obtained exhibited a characteristic yellow color of lead iodide films. Structural and compositional characterization using X-ray diffraction and transmission electron microscopy established that the films, both with and without heat treatment, exhibit a 2H and 4H polytypical structure. Furthermore, an increase in the contribution of the 4H structure and crystallinity was observed with increasing heat treatment temperature, a result consistent with the hypothesis and objectives of the project.Using Raman microscopy, we were able to identify the vibrational modes of the films with and without heat treatment, which correspond to the vibrational modes of 2H and 4H polytypical structures. Finally, their optical properties were measured using UV-Vis optical microscopy, establishing that temperature does not have a significant effect on the onset of fundamental absorption of the material. Applying the Kubelka-Munk method, we were able to determine the band gap for the lead iodide films, Eg 2.4 eV.

Description

SCALABLE METHOD FOR THE LOW-COST MANUFACTURE OF LEAD IODIDE (Pbl2) THIN FILMS AT ROOM TEMPERATURE AND WITHOUT THE USE OF ORGANIC SOLVENTS TECHNICAL FIELD The present invention falls within the technical field of Chemistry, as it is a method for the homogeneous growth of highly uniform, adherent and reproducible Pbk thin films by chemical bath deposition. BACKGROUND OF THE INVENTION In recent decades, the family of solids in which atoms are arranged in layers with strong internal chemical bonds and weak interlayer bonds has attracted considerable interest in fundamental research due to its potential technological applications. This is driven by the ability to model novel properties resulting from the intercalation of atoms, ions, and molecules between the layers of the host material. The macroscopic physical properties of these materials are strongly anisotropic. Graphite is probably the most familiar of these layered materials, but there are other examples such as sulfides, silicates, and halides. Among the latter is lead iodide (PbI), a fascinating and much-studied solid because it readily accommodates a guest atom of varying charge or size in the interlayer space. It has emerged as an important basis for quasi-two-dimensional anisotropic semiconductors and exists primarily in two crystal structures (hexagonal and rhombohedral). It is characterized by a spatial repetition of three planes: I-Pb-I. In its elemental form, a PbI2 crystal exhibits a high electron density within the layers, which gives rise to strong interactions, mainly covalent, but between the layers that are stacked along a growth direction the interactions are weak, mainly of the Van Der Waals type, which generates multiple stacking configurations, called polytypic structures, which are random crystallization forms on a growth axis. Lead iodide (Pbb) in its crystalline and polycrystalline forms is considered an attractive candidate material for studying the formation of clusters and for studying thin films of atoms and molecules that can be placed between layers (intercalation). With its high mass density (6.2 g / cm³) and composition of elements with high atomic numbers (ZPb = 82 and Zl = 53), it exhibits high photon stopping power (due to its high atomic absorption coefficient). It has potential applications as room-temperature photocells, X-ray detectors, and low-energy gamma-ray detectors. It is used as a suspension for colloidal nanoparticles and for constructing quantum wells. Due to its wide band gap (Eg), it also has applications as a semiconductor.Finally, the crystalline compound Pbb is an intrinsic semiconductor with a wide optical band gap and high electrical resistivity, which is why it can be used as a precursor for third-generation perovskite solar cells. Lead iodide has a high melting point (Tm) (~402–410 °C), low vapor pressure, and high structural stability. Therefore, it can be synthesized as monocrystalline and polycrystalline thin films using various methods, including solution, vapor, gel, and melt synthesis. This leads to the generation of numerous polytypes of the material. Due to the synthesis method employed, these polytypes significantly affect the optical and electrical properties, and especially the phase stability, of PbK. This is primarily due to the structural lattice disorder and non-stoichiometry of the manufactured material samples, thus having a significant impact on applications where symmetry is critical, such as solar cells. This undesirable phenomenon is very likely to exist in amorphous and polycrystalline films of these materials, particularly if the film deposition is not controlled. This leads to an inability to determine the polytypes that may form. This phenomenon is amplified when the starting materials are chemically impure. / cnonn / zznz / e / Yi Recent research on the optoelectronic properties of lead iodide focuses on photovoltaic applications in metal halide perovskite solar cells. Lead iodide is one of the precursors in these cells, achieving energy conversion efficiencies exceeding 22%. However, a significant bottleneck for further development of this technology is overcoming performance losses caused by the instability of the cells. This instability likely stems primarily from the inappropriate selection of the synthesis process and a lack of research on the precursor materials. Consequently, it is impossible to determine the polytypes that can form and their influence on the final properties of the perovskite films, impacting their standardization and reproducibility. Currently, the synthesis of PbK thin films is carried out using physical methods that employ specialized equipment requiring high currents and high vacuum. Therefore, the cost of obtaining a film is high. Traditional chemical methods employ expensive and polluting organic solvents, which generate films of low stability, causing significant degradation of the lead iodide films and the resulting products. Another problem with these traditional methods is the need to use temperatures above 80°C to evaporate the solvents used, which negatively impacts the quality of the final films.Therefore, it is necessary to find manufacturing methods that produce more stable films while reducing costs, avoiding the use of specialized equipment and expensive, polluting organic solvents that affect film quality. To date, no articles or patent documents in the prior art describe the method that will be described below. OBJECT OF THE INVENTION The object of the present invention is a method for the homogeneous growth of highly uniform, adherent, and reproducible Pbb films by chemical bath deposition. This method solves several problems in the synthesis of lead iodide films, such as synthesis on substrates of the required shape, greater film stability due to the absence of solvent traces that would cause degradation, the simplicity of the instruments and equipment needed to obtain the films makes it a method that is easy to disseminate and industrially scalable, and the films exhibit high stability in aqueous solutions. DESCRIPTION OF THE INVENTION The characteristic details of the invention are clearly shown in the following description and in the accompanying figures, using reference numbers to indicate the parts shown. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a schematic of the method for obtaining the seed layer, where: a) 0.1 M tin chloride solution; b) Chemical bath deposition; c) Tin oxide seed layer; d) Tin oxide seed layer; e) 0.1 M silver nitrate solution; f) Second chemical bath deposition; g) SnCte / AgO (Tin oxide / Silver oxide). Figure 2 shows a schematic of the method for obtaining lead iodide Pbb thin films, where: a) 4 ml potassium iodide 0.2M, 3.5 ml ammonium citrate 0.025M, 58.5 ml distilled water, 4.0 ml lead nitrate 0.1 M; b) Chemical bath deposition at 30°C for 300 minutes; c) Obtaining lead iodide films. Figure 3 is a photograph of a thin film of lead iodide Pbb where: a) Commercial glass substrate; b) Lead iodide Pbh film. / cnonn / zznz / e / Yi Figure 4 is a photograph of the films without heat treatment (ST) and with heat treatment at 100, 150, 200, 250 and 270°C. Figure 5 is the TEM analysis of lead iodide films without heat treatment a) ST and with treatment b) 100°C; c) 150°C; d) 200°C; e) 270°C. Figure 6 shows the UV-Vis spectra obtained for lead iodide films at different heat treatment temperatures. Figure 7 shows the XRD patterns for lead iodide films without heat treatment and with treatment at 150 °C and 250 °C to crystallographic chart JCPDS #00-007-0235 and crystallographic chart JCPDS # 01 -073-1751. Figure 8 is an SEM image showing the morphology of iodide films, which corresponds to an arrangement of interconnected flakes or threads, thus generating a morphology with high porosity, which is the most well-known characteristic in the precursor films implemented in the synthesis of perovskites. Figure 9 is an energy-dispersive X-ray spectroscopy (EDS) image that determined the distribution and composition of the elements that make up the film. It shows that the film is composed exclusively of lead and iodine atoms, which are distributed homogeneously throughout the film. These results demonstrate the high efficiency and reproducibility of the films obtained using this method. The present invention describes a method for the homogeneous growth of highly uniform, adherent, and reproducible Pbk films by chemical bath deposition. This method solves several problems in the synthesis of lead iodide films, such as synthesis on substrates of the required shape, greater film stability due to the absence of solvent traces that would cause degradation, the simplicity of the instruments and equipment needed to obtain the films makes it a method that is easy to disseminate and industrially scalable, and the films exhibit high stability in aqueous solutions. / cnonn / zznz / e / Yi This method involves immersing a solid support (glass slide) in a series of precursor solutions poured into a reactor, which does not require heating because the lead iodide films are obtained at room temperature (25-30°C). The only solvent used is distilled water, which guarantees a stable film free of contaminants in its structure. The chemical bath deposition technique is easily applicable for obtaining films on both small and large scales. Lead iodide semiconductor films are obtained by chemical bath deposition using a two-stage synthesis strategy. The first stage involves the formation of a continuous and homogeneous seed layer of tin oxide / silver oxide; the second stage is the synthesis of the Pbl2 film. The methodological details and materials required for the synthesis are described below. The following reagents are required for this work: lead nitrate (Productos Químico Monterrey, 99.8%), acetic acid (Fermont, 99.1%), tin chloride (Fermont, 99.1%), ammonium citrate (Fagalab, 99.5%), potassium iodide (Fermont, 99.0%), and silver nitrate (Fermont, 99.5%). All reagents are used without further purification. The substrates used are commercial glass, 26 mm x 76 mm, with a thickness of 0.8 mm (on these substrates, it is possible to deposit the material in an area of ​​26 mm x 50 mm). For the manufacture of thin films using the chemical bath deposition method, the first step is to clean the substrates to remove any remaining organic matter, eliminate residues from the production process of the substrate itself, and to create a surface with an anchoring point. The cleaning process can be carried out by washing the substrates with distilled water and Alkanox® soap, then immersing them in a mixture of nitric acid (HNO3) and distilled water in a 1:10 v / v (volume) ratio for at least 20 minutes. After this, they are rinsed in isopropyl alcohol and then sonicated in distilled water for at least 15 minutes. Finally, they are left in distilled water until use. / cnonn / zznz / e / Yi Other ways to clean substrates are: a) Probe them for 20 minutes with distilled water; b) Immerse them in a solution of distilled water and alcohol for 20 minutes; oc) Clean them with a swab and isopropanol. Seed coat synthesis Once the cleaning protocol was established and executed, the tin oxide seed layer was synthesized. Four commercial glass substrates were placed in a 100 mL beaker, and 2.2 g of tin chloride were added to 100 mL of distilled water to obtain a 0.1 M concentration. The deposition was carried out with a reaction time of at least 60 minutes and a constant temperature of 40–70°C, without stirring. The resulting films were rinsed with distilled water. Subsequently, the films underwent a second process to generate a silver oxide layer on the film surface. This was achieved by immersing the substrates obtained in the previous step in a 0.1 M silver nitrate solution for at least 60 minutes at a constant temperature of 40–70°C. Figure 1 illustrates this step of the method. Obtaining the lead iodide film Using the seed-layer glass substrates obtained in the previous step, lead iodide films are prepared. For this, 2 to 4 ml of 0.1 M to 0.2 M potassium iodide and 2 to 4 ml of 0.015 M to 0.05 M ammonium citrate were placed in a 100 ml beaker. Then, 57 ml of distilled water and 3 to 5 ml of 0.1 M lead nitrate were added. Once the precursor solution was prepared, four commercial glass substrates were immersed in the solution. These substrates should be positioned vertically against the walls of the beaker, as the reaction occurs most efficiently in this area. / cnonn / zznz / e / Yi Finally, the mixture was placed in a preheated thermal bath at a temperature of 25 to 50°C for a reaction time of at least 300 minutes. The bath solution was kept undisturbed. After deposition, the Pblz films were washed with distilled water to remove lead iodide particles not firmly attached to the film and were air-dried. As shown in Figures 8 and 9, the morphology of the resulting films has an arrangement of interconnected flakes or threads, thus generating a highly porous morphology, which is the most well-known characteristic of the precursor films used in perovskite synthesis. Furthermore, they have a thickness ranging from 300 nm to 7 micrometers. This invention involves coating a wide variety of materials, such as glass, plastic, and ceramics, with a thin layer of Pbl2. This method allows for the synthesis of materials with any substrate morphology and size, including irregular shapes and varying dimensions. Regarding its applications, lead iodide is a two-dimensional material, allowing it to be used in diverse fields such as electrical, magnetic, optical, and nonlinear optics. Currently, the most promising applications of lead iodide are as a nanogenerator or piezoelectric strain sensor. This material is also used as a detector of low-energy X-rays and gamma rays. It has applications as a room-temperature photodetector. It is used as a precursor in the synthesis of third-generation perovskite solar cells. Finally, by intercalating copper atoms into its two-dimensional structure, this material is used in the manufacture of hydrogen fuel cells. A key characteristic of the films obtained by this method is their high quality; that is, the films adhere well to the substrate and provide uniform coverage, as shown in Figure 3. The high quality of the films is maintained during heat treatment, where their homogeneity is readily apparent, even upon detailed inspection. No color change is observed with increasing treatment temperatures up to 200°C. At higher treatment temperatures, a darkening of the films is observed, as shown in Figure 4, which could indicate a change in physical and / or chemical properties, leading to the formation of polytypic structures.Furthermore, these results showed that the chemical bath process is a viable alternative for producing reproducible, high-quality films, which is of great importance for scaling up the process. Moreover, it is important to note that the quality of the films does not deteriorate even after exposure to ambient conditions, suggesting that this route is a favorable alternative for obtaining lead iodide thin films. Using transmission electron microscopy (TEM), a morphological analysis was performed on lead iodide films to determine the shape, size, and dispersion of the samples and to ascertain how the heat treatment temperature influences morphological and structural changes. As shown in Figure 5, increasing the treatment temperature leads to greater crystallinity in the material, resulting in a more defined structure with less dispersion and a homogeneous and symmetrical particle size. This result is consistent with reports in the literature, where this behavior is primarily attributed to the slippage of the lead iodide layers, caused by a gradual loss of weak interactions, thus enabling the structural and compositional homogenization of the material.Another result that supports the previous analysis is obtained from high-resolution micrographs, through which the electron diffraction pattern was determined. It can be observed how the films with heat treatment at temperatures of 200°C and 270°C present a diffraction pattern of greater crystallinity, evidence and corroborates the effect of temperature on the lead iodide films. The UV-Vis spectra obtained for the lead iodide films are shown in Figure 6. These were acquired using the diffuse reflectance model due to the 5.2 pm thickness obtained for both heat-treated and untreated samples, which hindered the implementation of the specular reflectance method. The spectra obtained are characterized by an absorption range in the visible region of the electromagnetic spectrum. Regardless of the heat treatment employed, all spectra show an onset of absorption around 530 nm, as expected given the use of the same lead iodide composition. The reflectance spectra show a trend for the heat-treated films up to 150°C, which is an inflection point; above this temperature, the reflectance of the samples decreases.This behavior can be attributed to a greater orientation of the crystals due to a higher heat treatment temperature, thus causing a decrease in the material's reflectance. Another possibility raised in the literature is the relationship between impurities and temperature with the higher-order polytypical transformations of lead iodide. This behavior can be explained by the sandwich-like stacking of PbK. The electron density within each layer is very high, generating predominantly covalent bonds, but the electron density between adjacent sandwich layers is low, generating weak van der Waals-type bonds. The crystallinity of the obtained systems was confirmed by implementing X-ray diffraction. Figure 7 shows the diffraction patterns for the untreated and heat-treated films, which coincide with characteristic reflections of hexagonal lead iodide polytype 2H, with some peaks corresponding to the hexagonal polytype 4H formation, according to charts JCPDS 00-007-0235 and JCPDS 01-073-1751, respectively. This result agrees with the theoretical predictions; since this is a low-temperature synthesis process, the most stable polytypes obtainable are 2H and 4H. As mentioned previously, the lack of transformation or induction to higher polytypes with increased temperature could be due to insufficient heat treatment time, which did not promote complete transformation kinetics in the samples.This theory is reinforced by the presence of the stable 4H polytype in the heat-treated films, since this structure is the closest to the high-temperature stable 12R crystal structure. The unit cell of the 12R polytype actually consists of three 4H unit cells rotated 60° in succession (the Zhdanov sequence of 4H and 12R is ABCB43.... and ABCB / CABA / BCAC / ). / cnonn / zznz / e / Yi respectively), in other words, the growth kinetics of the synthesis technique leads to irregular growth and dislocation production, which in turn give rise to nucleation centers for further growth of the 4H polytype and not of higher ordering polytype structures.

Claims

1. A scalable, low-cost method for manufacturing lead iodide (Pbb) thin films at room temperature without the use of organic solvents, characterized in that it comprises: a) Cleaning the substrate to be used to remove residues from the production process of the same substrate and to generate a surface with an anchoring point; b) Forming a seed layer by placing the substrate in a container and adding 2.2 g of tin chloride to 100 ml of distilled water to obtain a concentration of 0.1 molar, allowing it to react for at least 60 minutes at a constant temperature of 40 to 70°C, without stirring; c) Forming a silver seed layer by rinsing the substrates with the films obtained in step a) with distilled water and immersing them in a 0.1 molar silver nitrate solution for at least 60 minutes at a constant temperature of 40 to 70°C; d) Form the lead iodide film by pouring 2 to 4 ml of 0.1 M to 0 potassium iodide into a container.2 M, and 2 to 4 ml of 0.015 M to 0.05 M ammonium citrate, then add 57 ml of distilled water and 3 to 5 ml of 0.1 M lead nitrate; e) Immerse the substrate obtained in step c) and leave it vertically in the solution prepared in step d); f) Place in a preheated water bath at a temperature of 25 to 50°C for a reaction time of at least 300 minutes, without stirring; g) Remove the substrate with the Pbb film formed and wash it with distilled water to remove any lead iodide particles not firmly attached to the film and air dry. / cnonn / zznz / e / Yi.

2. The method according to claim 1, characterized in that the substrate is cleaned with distilled water and soap, then immersed in a mixture of nitric acid (HNO3) and distilled water in a 1:10 v / v (volume) ratio for at least 20 minutes, after which it is rinsed in isopropyl alcohol and subsequently sonified in distilled water for at least 15 minutes, and then left in distilled water until use.

3. The films obtained from the method of claim 1, characterized in that they have a morphology of interconnected flakes which form a high porosity, and a thickness of 300 nm to 7 micrometers.