Method for forming tin oxide quantum dots for use as electron transport layer

Abstract

A colloidal SnO2 solution and method of preparing the same is disclosed wherein the method comprises dissolving a tin salt and a urea-based ligand in a solvent and agitating at room temperature until tin oxide (SnO2) particles having a size of less than 10 nm are formed and suspended within the solvent. The solution contains a quantity of suspended SnO2 quantum dot particles having a size of less than 10nm no sulfur or sulfur containing compounds. The solution may be utilized to form an electron transport layer on a substrate which in turn may be used in the preparation of a solar cell.

Description

[0001] METHOD FOR FORMING TIN OXIDE QUANTUM DOTS FOR USE AS ELECTRON TRANSPORT LAYER

[0002] CROSS REFERNCE TO RELATED APPLIATIONS

[0003] The present application claims the benefit of priority to co-pending U.S. provisional application No. 63 / / 653,479 filed May 30, 2024, the contents of which are herein incorporated by reference.

[0004] BACKGROUND

[0005] 1. Technical Field

[0006] This disclosure relates generally to electron transporting layers and in particular to sulfur-free SnO2 quantum dot electron transport layers, to methods for their preparation and uses thereof.

[0007] 2. Description of Related Art

[0008] Organic-inorganic hybrid perovskite (OIHP) materials have garnered significant research interest due to their exceptional properties, including material abundance, (Zhao et aL, 2024) long carrier diffusion lengths, tunable bandgap, and high absorption coefficients (Koo et aL, 2024). OlHPs have shown great potential for various optoelectronic applications. Perovskite solar cells (PSCs) are among the emerging and promising photovoltaics technology that have achieved a power conversion efficiency (PCE) over 26.15% (Chen et aL).

[0009] The electron transport layer (ETL) is a vital component of PSCs, fulfilling the dual functions of facilitating electron transport and blocking holes (Tian et aL, 2024). SnO2has emerged as an advantageous ETL material due to its exceptional optoelectronic properties, such as its high electron mobility, suitable energy levels, excellent optical transparency, and robust stability (Song et aL, 2024). However, structural imperfections in SnO2introduce numerous pinholes, unavoidable defects that trap free electrons, impair carrier transport, and lead to charge accumulation at the SnO2 / perovskite interface, thereby reducing the efficiency of PSCs (Kokaba et al. 2024). As a result, SnO2quantum dots (QDs) have garnered significant attention as ETLs in PSCs (Xu et al. 2021) due to their exceptional optoelectronic properties (wide bandgap, greater than 3.8 eV, good electron mobility, a favorable conduction band edge, enabling more efficient charge transfer), high thermal stability, and compatibility with solution processing techniques.

[0010] The conventional approaches to synthesizing SnO2QDs frequently rely on thiourea as a stabilizer and facilitator during synthesis (Yang et al. 2018). While effective, thiourea may pose serious environmental and health concerns, including respiratory conditions like bronchitis and nose, throat, and lung irritation. Moreover, residual sulfur from thiourea on SnO2QD thin films can interact with the perovskite layer, forming volatile organic compounds that degrade device stability over time (Liu et al., 2019). Studies indicated that traditional SnO2QDs doped with thiourea induced surface positive-charge protonation, which caused recombination of transferred electrons and extended their migration path. This, in turn, decreased electron-transfer efficiency while enhancing surface photocatalytic activity (Ziang et aL, 2024). Addressing these issues typically requires high-temperature annealing (above 200°C) to eliminate thiourea residues, which increases energy costs and limits compatibility with temperature-sensitive substrates.

[0011] SUMMARY OF THE DISCLOSURE

[0012] According to a first embodiment of the present disclosure is a method of preparing a colloidal SnO2 solution comprising dissolving tin salt and a urea- based ligand in a solvent and agitating at room temperature until tin oxide (SnO2) particles having a diameter of between 5 and 30 nm are formed and suspended within the solvent.

[0013] Tin salt may comprise stannous chloride dihydrate. The solvent may comprise water. The urea-based ligand may be selected from the group consisting of urea, N-methylurea (NMU), Dimethylurea (DMU) and Tetramethylurea (TMU). The urea-based ligand may be urea. The stannous chloride dihydrate and a urea-based ligand may be mixed in a mass ratio of 3:1. The stannous chloride, dihydride and urea based ligand may be dissolved in the water at a concentration of between 0.05 and 0.20 mol / L. The solution may be exposed to atmospheric conditions during stirring. The solution may be agitated until yellow and transparent.

[0014] According to a further embodiment of the present disclosure is a method of forming an electron transport layer on a substrate comprising applying a thin layer of the colloidal SnC solution formed by dissolving stannous chloride dihydrate and a urea-based ligand in water and agitating at room temperature until a tin oxide (SnOz) particles having a diameter of between 5 and 30 nm are formed and suspended within the water to a substrate and annealing the thin layer.

[0015] The thin layer may be applied by solution deposition method. The solution deposition method may be selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

[0016] The substrate may be selected from a group consisting of a fluorine-doped tin oxide coated glass (FTO glass substrate), iridium tin oxide (ITO) glass or transparent conducting oxides (TCOs). The substrates may be etched and cleaned before application of the colloidal Sno2 solution.

[0017] According to a further embodiment of the present disclosure is a method of preparing a solar cell comprising forming an electron transport layer on a substrate comprising applying a thin layer of the colloidal SnOz solution formed by dissolving stannous chloride dihydrate and a urea-based ligand in water and agitating at room temperature until a tin oxide (SnOz) particles having a diameter of between 5 and 30 nm are formed and suspended within the water to a substrate and annealing the thin layer. The method further comprises forming a perovskite layer on the electron transport layer, forming a hole transport layer on the perovskite layer and forming a cathode on the hole transport layer. The perovskite layer may be formed by depositing a thin layer of a mixed-cation perovskite solution on the electron transport layer and annealing. The mixed- cation perovskite solution may be formed to a thin layer by a solution deposition method. The solution deposition method may be selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

[0018] The hole transport layer may be formed on the perovskite layer by depositing a thin layer of a spiro-OMeTAD solution thereto. The spiro-OMeTAD solution may be formed to a thin layer by a solution deposition method. The solution deposition method may be selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

[0019] According to a further embodiment of the present disclosure is a method of preparing a solar cell comprising forming an anode on a substrate, forming a hole transport layer on the anode and substrate, forming a perovskite layer on the hole transport layer, forming an electron transport layer on the perovskite layer using the method of any one of claims 8 through 12 and forming a cathode on the hole transport layer.

[0020] According to a further embodiment of the present disclosure is solution for use in forming an electron transport layer comprising an aqueous base; a quantity of suspended SnO2 particles having a size of less than 10nm; and the solution having no sulfur or sulfur containing compounds.

[0021] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings constitute part of the disclosure. Each drawing illustrates exemplary aspects wherein similar characters of reference denote corresponding parts in each view,

[0023] Figure 1 shows chemical structures of the ligands used to synthesize SnCh QDs according to methods of the present disclosure.

[0024] Figure 2 shows schematic illustration of a synthesis route for SnO2 QDs (a), Spin coating of SnO2 QD solution for ETL (b), dynamic light scattering (DLS) analysis for particle size and distribution (c), zeta potential analysis of SnO2 QDs (d) of the present disclosure.

[0025] Figure 3 is a synthetic pathway of urea and methylated urea-based SnO2 QDs

[0026] Figure 4 is a Raman spectra of SnO2 QDs solutions and ligands.

[0027] Figure 5 is a Raman spectra of urea-based ligands and corresponding SnO2

[0028] QDs solutions.

[0029] Figure 6 shows the calculated optical bandgaps of SnO2 QD (U).

[0030] Figure 7 shows top-view SEM image of SnO2 QD (U)film on FTO (a), transmission electron microscopy (TEM) images of SnO2 QDs (b), HRTEM image (C), and SAED pattern of SnO2 QD (d).

[0031] Figure 8 shows top-view SEM images at low magnification for SnO2 QDs derived from (a) thiourea, (b) urea, (c) NMU, (d) DMU and (d) DMU deposited on FTO.

[0032] Figure 9 shows a high-angle annular dark-field (HAADF) STEM image maps of SnO2(U)

[0033] Figure 10 shows transmission electron microscopy (TEM) images of SnO2 QDs (methylated urea (a), HRTEM image (b), and SAED pattern of SnO2 QD (c).

[0034] Figure 11 show XPS Survey spectra (a) of SnO2 QDs. The high-resolution XPS of Sn 3d and O1s are represented in (b), and (c), respectively.

[0035] Figure 12 illustrate laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) elemental distribution of S on SnO2 QDs synthesized form TU annealed at: 150 °C a(a), (b) and 200 °C (c), (d). Figure 13 show laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) elemental distribution of S on SnO2QDs synthesized from U: S_32 (a), and S_34 (b), (Annealing temperature: 150 °C)

[0036] Figure 14 illustrate l-V curves of devices with the structure of FTO / SnO2QDs / Ag for DC conductivity measurements, (a), the schematic device structure of electron-only device (b), dark J-V measurements of the electron-only devices displaying VTFL behavior of: SnO2 QDS film (TU) (c), SnO2 QDS film (U),(d) and SnO2QDs film (NMU).

[0037] Figure 15 are SEM images of CS0.05FA0.80MA0.15Pbl2.75Br0.25 on: (a) SnO2 QDs thin film (TU), (b) SnO2 QDs thin film (, and a) SnO2QDs thin film (methylated, NMU).

[0038] Figure 16 illustrate (a) a device structure of the PSCs using SnO2 QDs as ETLs, (b) J-V curves of champion planar SnO2 QDs -based PSCs under reverse and forward scan directions, (c) dark current curve of SnO2QDs -based PSCs, and (d) and device stability under dry box storage.

[0039] Figure 17 show statistical box plots of the PV parameters obtained from devices using various SnO2 QDs as ETL.

[0040] Figure 18 is a J-V curves of methylated urea-derived SnO2QDs-based PSCs

[0041] DETAILED DESCRIPTION

[0042] According to one embodiment of the present disclosure is included a method of preparing a colloidal SnO2solution using urea-based ligand in an aqueous colloid. In particular, method provides a room-temperature solution- processable strategy employing less expensive, environmentally friendly sulfur- free urea-based ligands as accelerator and stabilizer ligands in a water-based solvent to produce a colloidal solution of SnO2QDs in ambient conditions.

[0043] In particular, as disclosed herein, SnO2 QDs are fabricated using urea based ligand and may be utilized as electron transporting layer for perovskite solar cells. The methods and approaches applied for the synthesis of colloidal SnO2QD Solution and thin films is further described below. In accordance with some embodiments of the present invention, a tin salt , such as by way of non-limiting example stannous chloride dihydrate (SnCl2-2H2O) along with one or more urea-derivative ligands may be dissolved in a solvent within a vessel, container or the like. Although SnCl2-2H2O is used herein, the tin salt may also be selected from any suitable tin salt, including, by way of nonlimiting example, Tin(ll) chloride, Tin(ll) fluoride, Tin(ll) bromide, Tin(ll) iodide, Tin(ll) sulfate, Tin(ll) nitrate, Tin(ll) acetate, Tin(ll) oxalate, Tin(ll) carbonate, Tin(ll) hydroxide, Tin(ll) oxide, Tin(IV) chloride, Tin(IV) fluoride, Tin(IV) bromide, Tin(IV) iodide, Tin(IV) sulfate, Tin(IV) nitrate, Tin(IV) acetate, Tin(IV) oxalate, Tin(IV) carbonate, Tin(IV) hydroxide, Tin(IV) oxide.

[0044] Furthermore, although the presented example utilizes water as the solvent, it will be appreciated that the solvent may be selected from any suitable solvent including, by way of non-limiting example, ethanol, methanol, DMSO and DMF, benzene, toluene, chloroform, acetone and glycols. The water may be purified, such as by way of non-limiting example, deionized water. It will be appreciated that other forms of water, including distilled, filtered or otherwise may also be utilized depending on intended purpose of the SnC film formed therefrom. In particular, the Stannous chloride dihydrate may be combined with the urea- based ligand, such as urea, methylated urea-derivatives or the like in a ratio selected to complete the formation of SnO2 therefrom. It will be appreciated that a radio of 3:1 maybe be useful in processing using urea as the ligand while other ratios may also be utilized. It will also be appreciated that variations from this ratio may also be utilized where purpose to which the resulting solution is intended as greater tolerance for remnants of one or more of the precursor substances. The SnCl2-2H2O and ligand, such as by way of non-limiting example are mixed in water at any desired concentration so as to provide adequate suspension of the resulting SnC>2. By way of non-limiting example it has been found that concentrations of between 0.05 to 0.2 mol / L has been useful and in particular 0.15 mol / L has been useful in solutions for forming the electron transport layer of perovskite solar cells. The urea based ligand act as reaction accelerators and stabilizers to facilitate the hydrolysis, dehydration, and oxidation of SnCl2 2H2O in distilled water so as to synthesize the colloidal SnC QDs. The solution may be agitated, such as by way of non-limiting example, stirred vigorously under magnetic stirring at room temperature resulting in a milky suspension. In practice it has been found that 2-4 days of stirring has been adequate although it will be appreciated that other times may be useful as well. It will be appreciated that common methods of determining transparency, such as light scattering techniques may be utilized and furthermore appreciated that depending on the intended use of the solution, less than complete reaction may be provided by shorter durations of stirring. The method of agitation may be selected from any commonly known methods, including, by way of non-limiting example, stirring, mixing, agitation, ultrasonic baths and the like. Under such conditions, a yellow and transparent colloidal SnO2 QD solution is formed having particle sizes of between 5 and 30 nm (diameter) by way of non-limiting example. It will be appreciated that other particle sizes may also be utilized. The SnC>2 QD solution may also be filtered such as by way of nonlimiting example, using a 0.45 pm PTFE filter before use.

[0045] The SnO2 QD solution may be useful for a range of products, including, for use as an electron transport layer in the formation of perovskite solar cells, batteries, gas sensors, photodetectors, light-emitting diodes, and biomedical applications like Biosensors and drug delivery systems, water splitting (PEC cells) and photodegradation of pollutants by way of non-limiting example. In particular, as used on in a perovskite solar cell, a substrate, such as formed of fluorine-doped tin oxide (FTO) / glass substrate may have an electron transport layer formed using the SnO2 QD solution. It will also be appreciated that other substrates including, by way of non-limiting example iridium tin oxide (ITO) or transparent conducting oxides (TCOs) may also be utilized. The electron transport layer may be formed by applying the SnO2 QD solution to the substrate so as to form a think uniform layer thereon by known means. In the present disclosure, a solution deposition method may be utilized to form a thin layer of the electron transport layer on substrate. It will be appreciated that such methods include applying a quantity of a solution containing the Sno2 quantum dots in a suspended colloid to the substrate and thereafter distributing the solution into a think uniform layer. Non-limiting examples of such methods may include but are not limited to spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating. In particular, the electron transport layer may be formed to have a thickness of up to 50 nm with a thickness of approximately 30 nm for use on perovskite solar cells having been found to be useful although it will be appreciated that other thicknesses may also be useful.

[0046] After formation of the SnOz QD, such as for use as forming an electron transport layer, the a perovskite layer may be formed on the electron transport layer (in the case of forming a perovskite solar cell and a hole transport layer thereafter formed on the on the perovskite layer. Finaly, a cathode may be provided on the hole transport layer. It will be appreciated that the perovskite layer, hole transport layer and cathode may be formed by any known means including spin coating and the like.

[0047] The present disclosures provide a novel route to fabricate SnO2QDs for PSCs that not only mitigates environmental and stability challenges associated with thiourea but also advances the development of stable, scalable, sustainable, and high-performance photovoltaic devices with an improved hysteresis index. It will be appreciated that utilizing the present methods permits a lower processing temperature and therefore either an inverted perovskite solar cell or a convention perovskite solar cell may thus be formed. Such inverted perovskite solar cells (p-i-n) may be formed as a structure of a substrate (usually glass or flexible plastic); a bottom electrode (Anode) typically Indium Tin Oxide (ITO) or Fluorine-doped Tin Oxide (FTO); a hole transport layer (HTL), commonly formed of PEDOT:PSS, NiOx, or PTAA to fFacilitates hole extraction and blocks electrons; a Perovskite Absorber Layer, such as by way of non-limiting example MAPbl3, FAPbl3, or mixed halide perovskites which is responsible for light absorption and charge generation; an electron Transport Layer (ETL), commonly formed of PCBM, C60, ZnO, or SnO2.by way of nonlimiting example operable to extract electrons and blocks holes; and a top Electrode (Cathode) typically formed of silver (Ag), gold (Au), or aluminum (Al) by way of non-limiting example.

[0048] On the other hand, the hysteresis phenomenon forSnO2-based PSCs has been successfully addressed through the incorporation of ETL bilayer structures, or the use of carbon-based interfacial layers like PCBM, or Ceo-SAM, the addition of such extra layers complicates the PSCs, raises their cost and lengthen the processing time. In accordance with the present disclosure, the perovskite device was found to be with a small hysteresis without any further treatment or additional layer. This approach enables the formation of high-quality SnO2thin films at lower processing temperatures, reducing energy requirements and broadening substrate compatibility. Furthermore, the residual urea on the SnO2thin film acts as a surface passivation agent, facilitating improved perovskite crystallization and contributing to enhanced device stability and efficiency. Moreover, the effects of methylated urea on SnCh QDs ink and thin films as ETLs for perovskite photovoltaics have been studied.

[0049] Examples

[0050] Example 1 :

[0051] Preparation of Colloidal SnO2 QD Solution

[0052] SnCl2-2H2O and ligands with a mass ratio of 3:1 were dissolved in 30 mL deionized water under vigorous magnetic stirring at room temperature, resulting in a milky suspension. To prepare SnC QD dispersions, 1015 mg SnCl2-2H2O and 338.33 mg of ligands were dissolved in 30 mL DI water in an open beaker. During the reaction, the solution was exposed to air. After stirring at room temperature, a yellow and transparent colloidal SnO2 QD solution was formed.

[0053] Perovskite Solar Cells Fabrication:

[0054] The FTO / glass substrate was initially etched using 4 mol / L HCI, followed by sequential cleaning with a detergent solution in deionized water, pure deionized water, acetone, and isopropanol, with each step involving 15 minutes of sonication. All substrates were further cleaned by UV-ozone for 15 min before deposition of SnCh QDs. The SnO2 QD dispersions were deposited on the cleaned FTO substrate by spinning the as-prepared colloidal SnC>2 QD solution at 500 rpm for 5 seconds and 3000 rpm for 30 seconds. The as-deposited SnOz QD thin films were then annealed on a hot plate at different temperatures (70, 100, 150, or 200 °C) for 1 hour in air. Before depositing the perovskite layer, the SnO2 QDs coated-FTO substrate was treated with UV-ozone for 15 minutes. The SnO2 QD thin films were then transferred to the nitrogen-filled glovebox.

[0055] The mixed-cation perovskite solution was prepared by mixing 1.05 M FAI, 0.2 M MABr, 1.24 M Pbl2, 0.07 M PbBr2, and 0.07 M Csl in DMF: DMSO (3:1 , v / v) to form Cs0.05FA0.80MA0.15Pbl2.75Br0.25. The perovskite solution was deposited via spin coating at 1000 rpm for 10 s (200 rpm ramp) and 6000 rpm for 20 s (2000 rpm ramp). 250 L of chlorobenzene used for quenching dropped 3 seconds before the end of the second spin-coating step and annealed at 150 °C for 10 min. Then spiro-OMeTAD solution in 1 mL CB containing 73.2 mg spiro-OMeTAD, 28.8 pL t-BP, 9.1pL Li-TFSI and 13.5 mg Co-TFSI, was spin- coated onto the perovskite layer at 4000 rpm for 30 s, and annealed at 80 °C for 30 min. Finally, a thickness of 100 nm Ag was deposited by thermal evaporation to complete the device.

[0056] Characterization and Instrumentation

[0057] The thin film morphology and elemental composition were analyzed using a Tescan Mira 3 XMU scanning electron microscope with a field emission gun. Elemental distribution maps were obtained via energy-dispersive X-ray analysis at 10 keV and processed with the Aztec software suite. Thin film thickness was measured using a Bruker Dektak XT Stylus Profiler with a stylus radius of 2.5 pm and a stylus force of 5-10 mg. Dynamic light scattering (DLS) and Zetapotential ( ) measurements were conducted at 25 °C using a Zetasizer Nano ZSP from Malvern Panalytical Technologies, UK, operated with Malvern Zetasizer software version 7.13. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Analytical Axis Ultra DLD spectrometer with a monochromatic aluminum source (Al Ka, 1486.6 eV) at 150 W (10 mA emission current and 15 kV HT). For some analyses, the aluminum source operated at 75 W (5 mA emission current and 15 kV HT). The analysis covered a 700 x 300 pm2area of the samples. High-resolution scans were conducted with a 50 meV step size, a dwell time of 1000 ms per step, and a pass energy of 20 eV. The Kratos charge neutralizer was used for insulating samples, and energy scale linearity was calibrated using Al and Mg X-ray sources on Argon sputter- cleaned gold and copper substrates. The efficiency of perovskite solar cells was measured using a Keithley™ 2401 source meter with a 4-point probe under a simulated AM1.5G spectrum, calibrated with a reference silicon photodiode from a Science Tech AAA Solar Simulator System. The J-V curves were recorded in both reverse (1.2 to -0.2 V) and forward (-0.2 to 1.2 V) scan directions at a rate of 0.1 V / s. The active area was defined by the Ag electrode, measuring 0.16 cm2

[0058] Result and discussion

[0059] To understand the physical and chemical properties of the synthesized SnO2 QDs, a series of investigations were conducted onto the as-synthesized SnC QDs, utilizing urea and methylated urea as facilitators. Thiourea-derived SnC QDs were also utilized as a reference material. As illustrated in Figure 1 , the chemical structure of the ligands used in this work is shown. Those Lewis bases are known to be complex with several (Lewis acidic) metal chlorides, and the Lewis basic oxygen / sulfur coordinate with the Lewis acidic metal (Clark et al., 2019). Urea is a chemically interesting compound with two amino groups (- NH2) joined by a carbonyl functional group. In the current reaction system, the HCI generated by hydrolysis and dehydration of SnCL can be consumed by reaction with ligands such as urea (Lu et al., 2016). As observed from Figure 1 , the solution is a milky white suspension at the beginning of the preparation process. With further stirring, the suspension gradually turned yellow as illustrated in Figure 3, and finally, a clear yellow solution was obtained that demonstrated the formation of SnC QDs. The ligands facilitate the formation of SnO2 by consuming the HCI produced during hydrolysis. Additionally, it interacts with metal ions through its oxygen and possibly nitrogen atoms, surrounding the resulting SnO2 QDs with a positive charge from its protonated amino groups. This positive charge prevents the aggregation of SnO2 QDs. Strong complexes are important to prevent metal chloride volatilization during annealing before forming SnOz QDs thin film (Clark et aL, 2019). The ligands act as an important accelerating and stabilizing agent, which played an important role on the formation of the colloidal SnO2 QDs solution. In the absence of these ligands, the milky suspension requires several days to transform into a transparent yellow SnOz solution. However, this aqueous solution is unstable, tending to precipitate and rapidly change from yellow to white, or it may be challenging to achieve the desired final product. Similarly, the methylated urea derivatives (R-U) consistently exhibited a transparent yellow color. We observed that methylation of urea accelerates the formation of SnO2 QDs. The average size distributions and zeta potentials of SnO2 QD in aqueous solutions were analyzed using dynamic light scattering (DLS) measurement as shown inFigures 2c and 2d. For the SnO2 QDs solution, we observed only one Gaussian distribution for all ligands which indicated that SnO2 QDs are uniformly distributed in the solution with a small deviation in particle size. Another peak is visible in the case of the SnO2 QD synthesized from thiourea. This additional peak is attributed to other impurities from the precursors. DLS measurements of the resulting SnO2 QDs solutions revealed a peak at 9.9, 10.99, 11.28, 11.26, 12.30, and 13.50 nm for the SnO2 QD derived from thiourea, urea, N-methylurea (NMU), 1 ,3-dimethylurea (DMU, and 1,1 ,3,3-tetramethylurea (TMU), respectively. It is important to note that the DLS method typically overestimates the size of particles or clusters because of the effects of multiple scattering and an electrical double layer surrounding the particles Ren et aL, 2021 ). The actual size of these QDs is less than 10 nm. On the other hand, the measured zeta potentials of the urea, thiourea (TU), urea (U), NMU, DMU, and TMU-based SnO2 QDs solutions were 20.3, 15,7, 12.69, 14.5, and 16.1mV, respectively. These particles exhibit moderate stability in solution with a low tendency to aggregate over time, as their zeta potential exceeds +10 mV. A higher zeta potential value for SnO2 QDs reflects strong colloidal stability. The zeta potential indicates the mutual attraction or repulsion between particles in a solution, and its value can indicate the stability of a colloidal dispersion. Consequently, these SnO2 QDs remained dispersed and maintained their stability for an extended period, lasting several months. This is because a high zeta potential signifies strong electrostatic repulsion between the particles, which prevents them from aggregating or clumping together (Chen et al., 2024).

[0060] Raman spectroscopy was conducted on SnO2QDs solutions to examine the coordination mechanisms between the precursor and the ligands (TU, U, NMU, DMU, TMU) in water. As shown in Figure 4, the primary Raman modes for TU include the N-C-N bend at 476 cm-1, the C=S stretch at 727 cm"1, and the C-N stretch at 1093 cm-1, consistent with earlier findings.31The shift of the C=S stretching vibration from 733 cm"1(in thiourea) to 727 cm"1(in SnO2QD ink) suggests that the tin-ligand bond forms through the sulfur atom. For urea-based SnO2QDs, the C=O stretching vibration shifts from 1540 cm"1in pure urea to 1562 cm-1in SnO2QD ink, indicating an interaction between the urea’s carbonyl oxygen and the Sn atoms on the QDs surface. The amino (-NH2) groups in urea may also interact with Sn atoms. Additionally, the significant blue shift in the N-C-N symmetric stretching confirms that urea chemically interacts with the quantum dots rather than existing as free molecules in the solution. These findings demonstrate that the complexes in the solutions are formed via Sn-S interactions for TU-based systems and Sn-0 interactions for urea-based SnO2QD solutions. For the methylated urea, including but not limited to N- methylurea (NMU), Dimethylurea (DMU) and Tetramethylurea (TMU), the shifting and splitting of various Raman peaks shows the existence of molecular interaction between the ligands and SnO2QDs and a change in the molecular environment in the SnO2QDs solution in Figure 5. Determining the enthalpies of the complex formation of each ligand with the SnO2QDs precursor material is essential to completely understand the nature of the interaction and quantify the interaction strength. The optical bandgap of the SnO2 QDs was calculated from UV-vis absorption spectrum as shown in Figure 6, and the SnO2 QD ETL exhibits a wider optical bandgap of 4.02 eV.

[0061] The optical bandgap of the SnO2quantum dots (QDs) derived from urea was determined using the UV-vis absorption spectrum as shown in Figure 6. The SnO2QD ETL demonstrates a broader optical bandgap of 4.02 eV. Extensive morphological characterization of the SnO2QD thin films was conducted using SEM, HRTEM, and SAED, as depicted in Figure 7 (a-d). The SEM image reveals that the SnO2QD films are so thin that the morphologies of the underlying FTO substrate remain visible even after deposition. The SnO2QD films synthesized using thiourea and urea exhibit smooth and homogenous surfaces, whereas those based on methylated-urea display a rough surface. The ligands in methylated urea-based ones leave residues after annealing, as shown in Figure 8. This issue may hinder the smooth crystallization and formation of a perovskite layer.

[0062] The low magnification TEM) image of SnO2 QDs on the TEM copper grid is displayed in Figure 8. The TEM image (Figure 7b) demonstrates a uniform distribution of SnO2QDs on the TEM copper grid, while the HRTEM image (Figure 7c) provides additional details, showing an outer d-spacing of 0.34 nm. This corresponds to the (110) crystalline plane of the rutile phase of SnO2, with particle sizes about 1 nm. HRTEM further confirms the high crystallinity of the SnO2QDs, which is advantageous for producing defect-reduced SnO2films. The SAED pattern (Figure 7d) verifies the formation of rutile SnO2QDs, with diffraction rings indexed to the (110), (101), (211 ), and (310) planes, confirming the polycrystalline nature of the QDs. These findings align with the tetragonal rutile structure.3233The selected area HAADF-STEM image (Figure 9 shows that the SnO2QD films synthesized using urea consist of Sn and O elements. A similar TEM image was found for the SnO2QD synthesized from methylated urea (NMU as demonstrated in Figure 10, while the selected area HAADF- STEM image for the methylated urea (Figure 10) displays additional elements like Cl, C and Fe impurities that may originate from the precursor or crosscontamination during sample analysis. The primary distinction between QDs synthesized using urea and those derived from methylated urea likely lies in their film morphology. X-ray Photoelectron Spectroscopy (XPS) analysis was performed to examine the chemical composition of SnO2QD films deposited on silicon substrates. The comprehensive XPS survey spectrum shown in Figure 11 confirms the presence of Sn and O, and Sn 4d, 4s, 3s, 3ds / 2, 3ps / 2, and 3pi / 2, 3ds / 2, and 01s peaks were identified. Figure 11 b-c shows the high- resolution XPS spectra of Sn and O in the SnOz QD samples. The peaks at the binding energy of 484.04 and 492.47 eV in Figure 11b attributed to Sn 3ds / 2 and 3ds / 2, respectively for the Sn4+ion, and thus suggest the formation of SnO2. The peak-to-peak separation was estimated to be 8.43 eV. Figure 11 c illustrates the O 1s core-level signals for SnO2QDs synthesized using urea, which can be resolved into two distinct peaks. The peak at a binding energy of 527.81 eV corresponds to the Oz“ state in SnO2, while the peak at 529.09 eV is attributed to the adsorption of surface oxygen groups. These results confirm that SnC>2 is the main component of the surface of the as-prepare sample free from any contaminations.

[0063] The photoelectron binding energy of the Sn 3d and O 1s core-level signals for the SnO2QD film shows a red chemical shift (toward lower binding energy) compared to previously reported SnO2QD films. This shift could be attributed to several factors such as quantum confinement effects, interactions with surface ligands, and possibly the presence of oxygen vacancies. The as- synthesized QDs have a much smaller size compared to the previously reported SnO2 QDs and quantum confinement effects in the QDs modify the electronic structure, and then result in in shifts in core-level binding energies. Besides, as the SnO2QDs are capped with ligands can cause the binding energy to shift lower. Moreover, the presence of oxygen vacancies in the SnO2QDs could shift toward lower binding energy.

[0064] Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) was employed to quantify the sulfur content on the SnO2 QDs thin film after annealing, as shown in Figure 12. The analysis revealed a substantial amount of sulfur present on the thin film after annealing for one hour at 150°C. Even at an annealing temperature of 200°C, residual sulfur was still detected. This residual sulfur from the ETL (SnO2 QDs) negatively impacts the performance of perovskite solar cells, as sulfur can interact with the perovskite layer, forming volatile organic compounds that degrade device stability over time. Mitigating this issue often requires high-temperature annealing (above 200°C) to remove thiourea residues. However, reports indicate that treating thiourea-based SnOz quantum dots (QDs) at high temperatures reduces their n-type conductivity, making it harder for electrons to move and be extracted compared to low- temperature annealing25. Besides, such temperatures increase energy costs and are incompatible with temperature-sensitive substrates. Therefore, urea serves as a more sustainable and effective alternative for synthesizing SnC QDs, offering benefits in both environmental impact and device performance. The LA-ICP-MS for urea-based SnO2 QDs thin film is displayed in SnO2 QDs in Figure 13 with no sulfur is detected and as predicted.

[0065] To gain a deeper understanding of the electrical conductivity of SnO2 QDs, we constructed devices with the configuration FTO / SnO2 QDs / Ag. We evaluated their direct conductivity (a) under dark conditions (Figure 12a). We calculated the DC conductivity (a) of SnO2 QD films from the l-V (current-voltage) curve using the Eq. a = where I, d, V, and A denote the current, film thickness, voltage, and film area, respectively. The conductivity of SnO2 QDs thin films derived from TU, U, NMU, DMU, and TMU were estimated to be 3.53x1 O'6, 3.79x1 O'63.26x1 O'63.28x1 O'63.41x1 O'6, and 3.53x106S cm1, respectively. The methylated urea-based SnO2 QDs yield a relatively lower conductivity value. The superior conductivity of SnO2 QDs synthesized with urea and thiourea compared to those made with methylated urea ligands can substantially improve solar cells' fill factor (FF) by reducing series resistance.

[0066] Space-charge-limited current (SCLC) measurements were performed using electron-only devices with the structure FTO / SnO2 QDs / perovskite / PCBM / Ag, incorporating various SnO2 QDs as electron transport layers (ETLs). These measurements aimed to determine the charge carrier mobility and trap state density at the ETL / perovskite interface. Figure 14b-d presents the J-V characteristics of the electron-only devices, with the inset illustrating their structural configuration. The electron mobility in the SCLC region is calculated using the Mott-Gurney equation: / V — ^££o^TFL / lNt ~ ' eL2’ where Nt represents the trap-state density, E is the relative dielectric constant of perovskite (28.8), E0refers to vacuum permittivity (8.854187817xl0"14F / cm), VTFL is trap-filled limit voltage, e is the elementary charge of Electron (1.602176634 x 10"19C).) L denotes the thickness of the perovskite film. A smaller trap-filled limit voltage (VTFL) indicates a lower density of defect states, which suggests fewer complex centers and reduced carrier recombination. The trap-state densities were determined to be 9.55 x1015crm3, 7.39x1015cm-3, and 1.31x1016cm-3for SnCh QDs synthesized using thiourea (TU), urea (U), and methylated urea (NMU), respectively. The reduction in Nt in the SnOz QDs / perovskite interface effectively suppresses electron-hole recombination at the ETL / perovskite interface, enhances electron transport, and reduces the hysteresis in the corresponding PSCs.

[0067] The top SEM images of CS0.05FA0.80MA0.15Pbl275Br025 layer are shown in Figure 15, Supporting Information, and a compact perovskite film was formed. The average grain size of the perovskite deposited onto the urea and thioureabased SnO2 QDs thin films is larger than that of the methylated, NMU) one. The formation of larger grains perovskite reduces non-radiative recombination losses and enhances charge transport efficiency.

[0068] Performance of Perovskite Solar Cells

[0069] To evaluate the performance of SnO2 QDs as ETLs for PSCs, we fabricated planar devices with the configuration of FTO / SnO2 QDs / CS005FA080MA015Pbl275Br025 / spiro-OMeTAD / Ag. We systematically investigated the impact of the annealing temperature of SnO2 QDs thin films on the performance of PSCs. The performance statistics of PSCs, measured as a function of SnO2 QD ETL annealing temperature, and the corresponding data are summarized in Table 1 . Table S3: Photovoltaic parameters of PSCs based on the SnO2 QDs ETLs at various annealing temperatures

[0070] PSCs achieved their highest performance when SnOz QD ETLs were annealed at 150 °C, with an average efficiency of 17.50%. Since the solvent in the colloidal SnO2 QD solution is water, an annealing temperature of 100 °C is sufficient to quickly evaporate it. Devices with SnOz QD ETLs annealed at 150 °C and 200 °C exhibit nearly comparable photovoltaic performance. This suggests that a small residual amount of urea is crucial in passivating the SnOz QD ETLs and enhancing perovskite crystallization. In contrast, thiourea-based SnOz QD ETLs necessitate an annealing temperature of at least 200 °C, making them unsuitable for low-temperature processing and PSCs built on flexible substrates34. These devices were tested under ambient conditions without encapsulation. The device structure and the current density-voltage (J-V) characteristics of the champion PSCs using SnOz QDs under 1 sun illumination (AM 1.5 G) are depicted in Figure 16 a and 16b. Devices incorporating thiourea-based SnOz QDs achieved a Voc, Jsc, FF, and PCE of 1058.8 mV, 22.63 mA cm-2, 73.66%, and 17.65%, respectively, while those using urea-based SnOz QDs delivered a Voc, Jsc, FF, and PCE values of 1058.53 mV, 22.491 mA cm-2, 76.08%, and 18.11 %, respectively. The performance data for the champion devices utilizing thiourea and urea-based SnOz QDs is detailed in Table 2 below: Table 2: Solar cell performance parameters of champion devices with SnO2

[0071] QD-based ETLs

[0072] Additionally, the statistical analysis of the photovoltaic performance of SnO2 QD-based PSCs is illustrated in Figure 17. The superior photovoltaic performance of urea-based SnO2 QDs suggests that residual urea on the QD surface may act as a passivating layer and a bridge between the ETL and the perovskite layer. Additionally, urea-based SnOz QDs exhibited improved hysteresis index (0.02) compared to their thiourea counterparts (0.12), contributing to enhanced perovskite crystallization. However, the SnO2 QDs synthesized using methylated urea derivatives (NMU, DMU, and TMU) exhibit lower photovoltaic performance due to the low morphological and electronic properties, and the result is illustrated in Figure 18 and detailed in Table 3 below.

[0073] Table 3: Photovoltaic parameters of PSCs based on the SnC>2 QDs ETLs (methylated urea)

[0074] We have compared our findings with the existing reports and a summary of the performance metrics and synthesis approaches for various SnOz QDs is provided in Table 1.

[0075] The dark-state J-V curve of the complete device was analyzed as displayed in Figure 16c, revealing that SnO2 QDs derived from urea and thiourea exhibit lower dark current density than those based on methylated urea. This indicates a substantial reduction in defects, leading to minimized carrier recombination.21The stability assessment of thiourea-based versus urea-based SnO2 QDs as ETLs for PSCs reveals that urea-based SnO2 QDs exhibit enhanced stability (Figure 16d). This can be attributed to the sulfur residues in thiourea-derived SnO2 QD films, which are prone to interacting with the perovskite layer, forming volatile organic compounds. These byproducts accelerate degradation processes, ultimately compromising the long-term performance and stability of the device. The stability assessment of thiourea-based versus urea-based SnO2 QDs as ETLs for PSCs was tested and the result reveals that urea-based SnO2 QDs retained -90% while thiourea-based PSCs retained its -83% PCE after storing for about 90 days at a Dry box (Figure 16d, Supporting Information). Conclusion

[0076] In summary, we successfully synthesized ultra-small SnCh QDs using a low- temperature solution processed strategy with a sulfur-free facilitator and applied them as ETLs in PSCs. Our study demonstrated that traditional thiourea-based SnO2QD thin films retain a significant amount of residual sulfur, which leads to perovskite degradation. The as-synthesized urea-based SnO2QDs ETLs delivered improved photovoltaic performance and device stability compared to thiourea-based SnO2QD ETLs. Urea-based SnO2QDs as ETLs for PSCs maintained approximately 90% of its efficiency while thiourea-based PSCs retained its -83% PCE, stored in a Dry box. Furthermore, PSCs fabricated with urea-based SnO2 QDs showed a marked reduction in hysteresis (HI) compared to those using thiourea-based QDs. This enhancement demonstrates that urea is an effective and cost-efficient alternative facilitator for SnO2 QD synthesis, eliminating the need for additional interlayers. These findings provide critical insights for streamlining PSC fabrication processes, paving the way for stable, efficient, large-scale, and flexible PSCs and tandem PSCs fabricated at low temperatures.

[0077] While specific embodiments have been described and illustrated, such embodiments should be considered illustrative only and not as limiting the disclosure as construed in accordance with the accompanying claims.

[0078] All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. Where a term in the present application is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term

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Claims

What is claimed is:

1. A method of preparing a colloidal SnO2 solution comprising: dissolving a tin salt and a urea-based ligand in a solvent; and agitating at room temperature until tin oxide (SnCk) particles having a diameter of between 5 and 30 nm are formed and suspended within the sovent.

2. The method of claim 1 wherein the tin salt comprises stannous chloride di hydrate.

3. The method of claim 1 wherein the solvent comprises water.

4. The method of claim 1 wherein the urea-based ligand is selected from the group consisting of urea, N-methylurea (NMU), Dimethylurea (DMU) and Tetramethylurea (TMU).

5. The method of claim 4 wherein urea-based ligand is urea.

6. The method of claim 1 wherein the stannous chloride dihydrate and a urea-based ligand are mixed in a mass ratio of 3:1.

7. The method of claim 1 wherein the stannous chloride, dihydride and urea based ligand are dissolved in the water at a concentration of between 0.05 and 0.20 mol / L.

8. The method of claim 1 wherein the solution is exposed to atmospheric conditions during agitation.

9. The method of claim 1 wherein the solution is stirred until yellow and transparent.

10. A method of forming an electron transport layer on a substrate comprising: applying a thin layer of the colloidal SnO2 solution formed by the method of any one of claims 1 through 7 to a substrate; and annealing the thin layer.

11. The method of claim 10 wherein the thin layer is applied by solution deposition method.

12. The method of claim 11 wherein the solution deposition method is selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

13. The method of claim 10 wherein the substrate is selected from a group consisting of a fluorine-doped tin oxide coated glass (FTO glass substrate), iridium tin oxide (ITO) glass or transparent conducting oxides (TCOs).

14. The method of claim 13 wherein the substrates is etched and cleaned before application of the colloidal SnO2 solution.

15. A method of preparing a solar cell comprising: forming an electron transport layer on a substrate using the method of any one of claims 8 through 12; forming a perovskite layer on the electron transport layer; forming a hole transport layer on the perovskite layer; and forming a cathode on the hole transport layer.

16. The method of claim 15 wherein the perovskite layer is formed by depositing a thin layer of a mixed-cation perovskite solution on the electron transport layer and annealing.

17. The method of claim 16 wherein the mixed-cation perovskite solution is formed to a thin layer by a solution deposition method.

18. The method of claim 17 wherein the solution deposition method is selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

19. The method of claim 15 wherein the hole transport layer is formed on the perovskite layer by depositing a thin layer of a spiro-OMeTAD solution thereto.

20. The method of claim 19 wherein the spiro-OMeTAD solution is formed to a thin layer by a solution deposition method.

21. The method of claim 20 wherein the solution deposition method is selected from the group consisting of spin coating, spray coating, slot-die coating, blade coating, ink-jet coating, curtain coating and gravure coating.

22. A method of preparing a solar cell comprising: forming an anode on a substrate; forming a hole transport layer on the anode and substrate; forming a perovskite layer on the hole transport layer; forming an electron transport layer on the perovskite layer using the method of any one of claims 8 through 12; and forming a cathode on the hole transport layer.

23. A solution for use in forming an electron transport layer comprising: an aqueous base; and a quantity of suspended SnO2 particles having a diameter of between 5 and 30 nm diameter of between 5 and 30 nm; wherein the solution having no sulfur or sulfur containing compounds.

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