A Low-Cost Synthetic Route of FAPbI3 Quantum Dots in Air at Atmospheric Pressure: The Role of Zinc Iodide Additives

Perovskite quantum dots (PQDs) have shown great promise in optoelectronic device applications. Typically, a traditional hot-injection method with heating and high vacuum pressure is used to synthesize these colloidal nanoparticles. In this article, we report a low-cost synthetic method for FAPbI3 PQDs in air at atmospheric pressure with the assistance of ZnI2. Compared with the FAPbI3 PQDs synthesized under vacuum/N2 condition, the air-synthesized Zn:FAPbI3 PQDs exhibit the same crystalline structure with a similar preferential crystallographic orientation but demonstrate higher colloidal stability and higher production yield. Furthermore, we examine the influence of ZnI2 during the synthesis process on morphologies and optoelectronic properties. The results show that the mean size of the obtained FAPbI3 PQDs is decreased by increasing the amount of added ZnI2. More importantly, introducing an optimal amount of ZnI2 into the Pb source precursor enables increasing the carrier lifetime of FAPbI3 PQDs, showing the potential beneficial effect on device performance.


Introduction
Organic-inorganic hybrid perovskite with a general formula of ABX 3 has attracted much attention over the past decade, in which A is a monovalent cation (such as methylammonium (MA + ), formamidinium (FA + ), or cesium (Cs + )), B is an inorganic metal cation (usually lead (Pb 2+ ) or tin (Sn 2+ )), and X is a halide anion (chlorine (Cl − ), bromine (Br − ), or iodine (I − )) [1]. Due to its superior semiconducting characteristics, such as high carrier mobility, long carrier diffusion length, and high defect tolerance, metal halide perovskite has shown great potential as the active material in optoelectronic applications [2]. Among them, formamidinium lead triiodide (FAPbI 3 ) has emerged as one of the most promising materials for highly efficient and stable perovskite solar cells, having achieved the record power conversion efficiency (PCE) of perovskite solar cells exceeding 25% [3][4][5][6][7][8][9][10]. Beyond photovoltaics, FAPbI 3 has also shown a large potential in lighting and displays, owing to the high brightness and good color purity [11]. More recently, the quantum dot (QD) form of FAPbI 3 has emerged as a promising material for next optoelectronic devices [12][13][14]. It additionally possesses many unique features from quantum dots such as wide tunable absorption range, multiple exciton generation, and high compatibility with a large-scale fabrication process [15].
Colloidal FAPbI 3 perovskite quantum dots (PQDs) are generally synthesized by a room-temperature ligand-assisted reprecipitation process (LARP) or a hot-injection method [16,17]. In LARP, since the surface capping ligands are commonly hydrophobic, polar solvents such as DMF and DMSO must be used to flocculate the PQDs, which are subsequently isolated via centrifugation. However, the high toxicity of such polar coordination solvents makes LARP inapplicable in industrial applications, according to ecological requirements. Furthermore, the synthesized PQDs are vulnerable to polar solvent that cannot be used especially in photovoltaic devices. Therefore, the hot-injection method, without using polar solvent, has so far been considered to be the only synthetic route for PQD-based photovoltaics, also in terms of production yield and surface ligand manipulation particularly with short-chain ligands. During the hot-injection synthetic route, a vacuum degassing process (usually <20 Pa) is first applied to the FA-oleate solution and the Pb-source precursor for removing moisture and volatile contaminants, which is typically unavoidable [17,18]. Then, the degassed FA-oleate solution is rapidly injected into the hot Pb-source precursor with a nonpolar solvent under inert atmosphere. Lastly, the mixture is cooled in an ice bath [17,19]. As can be seen above, the degassing process and maintaining the inert atmosphere are not industrially feasible due to their high costs and large time consumption. Thus, seeking a facile route for low-cost, large-scale, and controlled synthesis at atmospheric pressure in air is essential when exploring FAPbI 3 PQD-related practical applications.
Introducing active ligands or additives during synthesis has been proven to be effective for attaining PQDs with reduced defect density and improved structural stability. Recently, Liu et al. used trioctylphosphine as a ligand to achieve a stable and highly reactive PbI 2 precursor, and further obtained CsPbI 3 PQDs with a superior photoluminescence quantum yield of up to 100% [20]. Moreover, doping with additives is a generally efficient approach that not only enhances the charge transport properties but also controls the size and uniformity. In particular, creating an iodine-rich chemical environment during the PQD nucleation/growth process is favorable for minimizing the defect density. In addition, the existence of metal halides in the Pb-precursor could avoid the requirement of excess PbI 2 for achieving high-quality PQDs [21]. Herein, we develop an easy synthetic method for colloidal FAPbI 3 PQDs under atmospheric pressure in air with the assistance of ZnI 2 additives. The air-synthesized Zn:FAPbI 3 PQDs exhibit high structural and colloidal stability. By increasing the amount of ZnI 2 additives, the crystal size of the FAPbI 3 PQDs is decreased, while no significant changes in the crystal shape and size dispersion are observed. Surprisingly, with the addition of an optimal amount of ZnI 2 ,~30 mol%, the yielded FAPbI 3 PQDs possess a longer carrier lifetime than that obtained by the vacuum/N 2 -assisted method. This synthetic method described here provides an efficient, simple, and robust approach to the low-cost production of high-performance FAPbI 3 PQDs.
X-ray diffraction (XRD) patterns were obtained with Rigaku Ultima IV (Tokyo, Japan, Cu Kα radiation, λ = 1.5418 Å) at room temperature in the air. For XRD measurements, the FAPbI 3 PQD solution was concentrated by flowing nitrogen and dropped on the sample holder. Transmission electron microscope (TEM) was recorded with JEM-2800 (JEOL, Japan). UV-visible absorption spectra (UV-Vis) were measured with a Cary 100 spectrometer (Varian, Palo Alto, CA, USA) in absorbance mode. Steady-state photoluminescence (PL) emissions and time-resolved photoluminescence (TRPL) were carried out by Edinburgh FS5 (Edinburgh Instruments Ltd., Livingston, UK) with a monochromatized Xe lamp as the source excitation and the QD samples were excited at 475 nm.

Results and Discussion
FAPbI 3 PQDs are commonly synthesized via a hot-injection method, as illustrated in Figure 1a. In detail, the FA-oleate precursor is prepared by mixing 0.521 g FAAc (5 mmol) with 10 mL OA and then the mixture is degassed under vacuum at 70 • C for 30 min. The temperature is subsequently increased to 110 • C and held to obtain a clear solution. Then, the FA-oleate precursor is maintained in nitrogen at 90 • C. Meanwhile, a mixture of 0.344 g PbI 2 (0.75 mmol) and 20 mL 1-ODE is degassed under vacuum at 120 • C for 30 min in a three-necked round-bottom flask. Then, 4 mL OA and 2 mL OAm are heated to 120 • C for 20 min and added into the mixture of PbI 2 and ODE. After PbI 2 is fully dissolved, the temperature of the mixture is reduced to 80 • C under N 2 . For air-synthesized PQDs, the FA-oleate precursor and the Pb source are prepared by directly increasing the temperature to 80 • C in ambient air. Lastly, a 5 mL FA-oleate precursor solution is swiftly injected into the PbI 2 mixture at 80 • C. After around 15 s, the reaction is quenched and cooled to room temperature using an ice water bath. For the purification of as-synthesized FAPbI 3 PQDs, 9 mL MeOAc is added to the reaction mixture, which is subsequently centrifuged at 8000 rpm for 30 min. The precipitate is dispersed in 9 mL hexane, re-precipitated with 10 mL MeOAc, and centrifuged at 8000 rpm for 10 min. The final precipitate is dispersed in 2 mL octane and stored at room temperature. Nanomaterials 2023, 13, x FOR PEER REVIEW 3 of 9

Results and Discussion
FAPbI3 PQDs are commonly synthesized via a hot-injection method, as illustrated in Figure 1a. In detail, the FA-oleate precursor is prepared by mixing 0.521 g FAAc (5 mmol) with 10 mL OA and then the mixture is degassed under vacuum at 70 °C for 30 min. The temperature is subsequently increased to 110 °C and held to obtain a clear solution. Then, the FA-oleate precursor is maintained in nitrogen at 90 °C. Meanwhile, a mixture of 0.344 g PbI2 (0.75 mmol) and 20 mL 1-ODE is degassed under vacuum at 120 °C for 30 min in a three-necked round-bottom flask. Then, 4 mL OA and 2 mL OAm are heated to 120 °C for 20 min and added into the mixture of PbI2 and ODE. After PbI2 is fully dissolved, the temperature of the mixture is reduced to 80 °C under N2. For air-synthesized PQDs, the FA-oleate precursor and the Pb source are prepared by directly increasing the temperature to 80 °C in ambient air. Lastly, a 5 mL FA-oleate precursor solution is swiftly injected into the PbI2 mixture at 80 °C. After around 15 s, the reaction is quenched and cooled to room temperature using an ice water bath. For the purification of as-synthesized FAPbI3 PQDs, 9 mL MeOAc is added to the reaction mixture, which is subsequently centrifuged at 8000 rpm for 30 min. The precipitate is dispersed in 9 mL hexane, re-precipitated with 10 mL MeOAc, and centrifuged at 8000 rpm for 10 min. The final precipitate is dispersed in 2 mL octane and stored at room temperature.  The vacuum-free air-synthesized FAPbI 3 PQDs obtained without the addition of ZnI 2 are denoted by Zn:FAPbI 3 -0. When different amounts of ZnI 2 are added into the PbI 2 precursor with the ZnI 2 /PbI 2 molar percentage of 10% (0.075 mmol ZnI 2 ), 20% (0.15 mmol ZnI 2 ), 30% (0.225 mmol ZnI 2 ), 40% (0.3 mmol ZnI 2 ), and 50% (0.375 mmol ZnI 2 ), the air-synthesized FAPbI 3 PQDs prepared by the same synthetic method (Figure 1b) are denoted by Zn:FAPbI 3 -10, Zn:FAPbI 3 -20, Zn:FAPbI 3 -30, Zn:FAPbI 3 -40, and Zn:FAPbI 3 -50, respectively. After 20 days' storage, the Zn:FAPbI 3 PQDs show high colloidal stability, as demonstrated in Figure 1c. Furthermore, compared with the FAPbI 3 -vaccum/N 2 PQDs, the Zn:FAPbI 3 PQDs demonstrate brighter emission under UV illumination (395 nm), as observed in Figure 1c, which suggests that the introduction of ZnI 2 may lead to reduced non-radiative recombination. The higher colloidal stability of the air-synthesized FAPbI 3 PQDs can be attributed to the iodine-rich negatively charged surfaces and the decreased crystal size induced by the addition of ZnI 2 , as discussed below. To compare the production yield of PQDs, we add 5 µL of PQD solutions under different synthetic conditions into 3 mL hexane and measure their UV-Vis absorbance, as presented in Figure 1d,e. The absorbance of PQDs is increased by increasing the molar percentage of ZnI 2 /PbI 2 from 0% to 30%. When the molar percentage is further increased to 40% and 50%, a decrease in absorbance can be observed. To clearly demonstrate this trend, we plot the absorbance at 500 nm, 600 nm, and 700 nm for different molar ratios of ZnI 2 /PbI 2 in Figure 1e. It can be found that the Zn:FAPbI 3 -30 PQDs exhibit a higher production yield than other conditions, which results from their higher colloidal stability. However, the excessive addition of ZnI 2 might suppress the formation of [PbI 6 ] 4− octahedra by reducing the relative concentration of Pb 2+ ions during QD nucleation and growth. The improved production yield and stability are helpful in reducing the cost of PQDs and the susceptibility to ambient environment which can be seen as the main obstacles for the further industrialization of PQDs, respectively.
First, we investigate the effect of ZnI 2 additives on the morphologies of the FAPbI 3 PQDs. The TEM measurements are performed and used to characterize the mean size and size distribution of the PQDs. As presented in Figure 2, the mean size is decreased by increasing the amount of added ZnI 2 , while there are no observable changes in the crystal shape and size dispersion of the FAPbI 3 PQDs. Similarly, it has been reported for the CsPbX 3 QDs that the particle size is decreased with increasing the X-to-Pb ratio in the reaction mixture, where the excess halide ions, X − , are able to suppress the further growth of QDs [21,22]. Therefore, the introduced ZnI 2 can be seen as an excess source of I − , leading to the reduced PQD size. Nevertheless, it should be noted that when the added Zn salts have different halide ions than the PQDs, the over-all size changes may occur in an opposite trend. For example, during the synthesis of the CsPbI 3 PQDs, the increase of the ZnCl 2 additive amount results in a noticeable increase in crystal size, exhibiting a different influence of ZnI 2 additives on the QD nucleation/growth mechanism [23]. Interestingly, we also observed that the ZnI 2 -free air-synthesized FAPbI 3 PQDs (Zn:FAPbI 3 -0) demonstrate a smaller crystal size than the vacuum/N 2 -synthesized FAPbI 3 PQDs. This reduction of QD size might be attributed to the existence of the water in the reaction solution as an antisolvent that is not removed in the air-synthesis process, thus facilitating the supersaturation of the PQDs. The influence of the ZnI2 additive on the crystal structure of the FAPbI3 PQD is identified by the XRD measurements. In Figure 3, the vacuum/N2-synthesized FAPbI3 PQDs show the diffraction peaks at 13.7°, 27.9°, 31.1°, 39.9°, and 42.4° that can be assigned to (001), (002), (012), (022), and (003) plane of cubic α-phase perovskite, respectively [17]. The air-synthesized Zn:FAPbI3 PQDs do not show any new diffraction peaks (the left panel in Figure 3), while no significant peak shift is observed when the ZnI2 additive is introduced The influence of the ZnI 2 additive on the crystal structure of the FAPbI 3 PQD is identified by the XRD measurements. In Figure 3, the vacuum/N 2 -synthesized FAPbI 3 PQDs show the diffraction peaks at 13.7 • , 27.9 • , 31.1 • , 39.9 • , and 42.4 • that can be assigned to (001), (002), (012), (022), and (003) plane of cubic α-phase perovskite, respectively [17]. The air-synthesized Zn:FAPbI 3 PQDs do not show any new diffraction peaks (the left panel in Figure 3), while no significant peak shift is observed when the ZnI 2 additive is introduced (the right panel in Figure 3). Since the ionic radius of Zn 2+ (74 pm) is smaller than that of Pb 2+ (119 pm), the replacement of Pb 2+ by Zn 2+ will bring a reduced lattice constant and thus higher-angle diffraction peaks [22,24,25]. However, if the added Zn 2+ resides in an interstitial lattice position, this will lead to a lattice expansion, demonstrating a lower-angle diffraction peaks [26]. Both circumstances have been reported and the differences are probably attributed to the introduced ions or the types of A-site cations. It will definitely be interesting to identify the dynamic and thermodynamic processes during the Zn salt-involved PQD synthesis. However, based on the XRD results here, we state that the introduced ZnI 2 would not induce the significant lattice distortion in the PQDs, rather only reside at the surface. Nanomaterials 2023, 13, x FOR PEER REVIEW 6 of 9 (the right panel in Figure 3). Since the ionic radius of Zn 2+ (74 pm) is smaller than that of Pb 2+ (119 pm), the replacement of Pb 2+ by Zn 2+ will bring a reduced lattice constant and thus higher-angle diffraction peaks [22,24,25]. However, if the added Zn 2+ resides in an interstitial lattice position, this will lead to a lattice expansion, demonstrating a lowerangle diffraction peaks [26]. Both circumstances have been reported and the differences are probably attributed to the introduced ions or the types of A-site cations. It will definitely be interesting to identify the dynamic and thermodynamic processes during the Zn salt-involved PQD synthesis. However, based on the XRD results here, we state that the introduced ZnI2 would not induce the significant lattice distortion in the PQDs, rather only reside at the surface. Lastly, we compare the optical and optoelectronic properties of these synthesized PQDs using different methods. The variations of PL emission intensity are presented in Figure 4a: Zn:FAPbI3-20 > Zn:FAPbI3-10 ≈ Zn:FAPbI3-0 > Zn:FAPbI3-30 > FAPbI3-vaccum/N2 > Zn:FAPbI3-40 > Zn:FAPbI3-50 PQDs. The PL emission intensity of the colloidal QDs is influenced by many factors: solution concentration, particle size, particle dispersity, ligand binding, and defect density. As observed in Figure 4a, the variation in PL intensity for different FAPbI3 PQDs is complex and is a result of competitions among these factors. Thus, we are only able to presume that the better colloidal stability and decreased iodine-vacancy states are helpful in enhancing the PL intensity, while the weakened ligand binding and formed impurities induced by excessive addition of ZnI2 result in a decrease in PL intensity. Therefore, the higher intensity of Zn:FAPbI3-0, Zn:FAPbI3-10, Zn:FAPbI3-20, and Zn:FAPbI3-30 than the pristine FAPbI3-vaccum/N2 PQDs may be due to suppressed non-radiative recombination and improved particle dispersity. However, when excessive ZnI2 is used, the Zn ions start to negatively impact the ligand binding with OA and OAm, inducing the defects on the PQD surface, further resulting in a decrease in PL intensity. In addition to intensity variation, the peak position shifts are also observed in Figure 4b. The blue shift can be attributed to the strong quantum confinement that is demonstrated by the FAPbI3 PQDs with a smaller crystal size (as illustrated in Figure 2). The smaller size of PQDs typically results in stronger quantum confinement effects, showing a higher bandgap (a higher-energy PL emission) [27]. Zn:FAPbI3-30, Zn:FAPbI3-40, and Zn:FAPbI3-50 PQDs exhibit red shifts in a peak position (a lower-energy PL emission), which is likely caused by the weakened electronic coupling between QDs and the formation of impurities of related Zn salts on PQD surfaces owing to the excessive Zn 2+ [28]. Furthermore, TRPL measurements are carried out to examine the carrier lifetime, as Lastly, we compare the optical and optoelectronic properties of these synthesized PQDs using different methods. The variations of PL emission intensity are presented in Figure 4a: Zn:FAPbI 3 -20 > Zn:FAPbI 3 -10 ≈ Zn:FAPbI 3 -0 > Zn:FAPbI 3 -30 > FAPbI 3vaccum/N 2 > Zn:FAPbI 3 -40 > Zn:FAPbI 3 -50 PQDs. The PL emission intensity of the colloidal QDs is influenced by many factors: solution concentration, particle size, particle dispersity, ligand binding, and defect density. As observed in Figure 4a, the variation in PL intensity for different FAPbI 3 PQDs is complex and is a result of competitions among these factors. Thus, we are only able to presume that the better colloidal stability and decreased iodine-vacancy states are helpful in enhancing the PL intensity, while the weakened ligand binding and formed impurities induced by excessive addition of ZnI 2 result in a decrease in PL intensity. Therefore, the higher intensity of Zn:FAPbI 3 -0, Zn:FAPbI 3 -10, Zn:FAPbI 3 -20, and Zn:FAPbI 3 -30 than the pristine FAPbI 3 -vaccum/N 2 PQDs may be due to suppressed non-radiative recombination and improved particle dispersity. However, when excessive ZnI 2 is used, the Zn ions start to negatively impact the ligand binding with OA and OAm, inducing the defects on the PQD surface, further resulting in a decrease in PL intensity. In addition to intensity variation, the peak position shifts are also observed in Figure 4b. The blue shift can be attributed to the strong quantum confinement that is demonstrated by the FAPbI 3 PQDs with a smaller crystal size (as illustrated in Figure 2). The smaller size of PQDs typically results in stronger quantum confinement effects, showing a higher bandgap (a higher-energy PL emission) [27]. Zn:FAPbI 3 -30, Zn:FAPbI 3 -40, and Zn:FAPbI 3 -50 PQDs exhibit red shifts in a peak position (a lower-energy PL emission), which is likely caused by the weakened electronic coupling between QDs and the formation of impurities of related Zn salts on PQD surfaces owing to the excessive Zn 2+ [28]. Furthermore, TRPL measurements are carried out to examine the carrier lifetime, as demonstrated in Figure 4c. Two TRPL lifetimes, τ 1 and τ 2 , are plotted with different synthesis conditions of FAPbI 3 PQDs in Figure 4d, which are drawn from the fitting results using a two-exponential decay function. The fast decay component τ 1 and the slow decay component τ 2 are ascribed to a trap-assisted recombination at the QD surfaces and an intrinsic radiative recombination inside the QDs, respectively [4,29]. It should be noted that the average lifetime is not presented here, because the average lifetime basically has no real physical meaning. Both τ 1 and τ 2 for the air-synthesized FAPbI 3 PQDs are decreased, compared with the PQDs obtained using degassing (vacuum) and N 2 protection, which can be ascribed to the moisture and volatile contaminants in the reaction mixture inducing large amounts of defects in the PQDs. However, the lifetime of the photogenerated charge carrier, both τ 1 and τ 2 , can be prolonged by introducing the ZnI 2 additive, as illustrated in Figure 4d. The Zn:FAPbI 3 -30 PQDs exhibit 29.81 ns for τ 1 and 96.04 ns for τ 2 , comparable with, or slightly higher compared with, the pristine FAPbI 3 -vaccum/N 2 PQDs (28.05 ns for τ 1 and 92.54 ns for τ 2 ). The iodine-rich environment provided by the addition of ZnI 2 inhibits the formation of iodine-vacancy trap states in the FAPbI 3 PQDs, which is also beneficial to the structural stability [4]. Nevertheless, when the percentage of ZnI 2 is increased to 40% and 50%, the increased surface-to-volume ratio (due to the decrease in size) and the abundant Zn ions that can be seen as new contaminants, lead to a new defect state formation inside and on the PQD surfaces, which is responsible for the decreased TRPL lifetimes shown in Figure 4d. For practical device applications, the enhanced carrier lifetime of PQDs may lead to increased current density in solar cells, increased external quantum efficiency in LEDs, and improved responsivity in photodetectors, etc.  Figure 4c. Two TRPL lifetimes, τ1 and τ2, are plotted with different synthesis conditions of FAPbI3 PQDs in Figure 4d, which are drawn from the fitting results using a two-exponential decay function. The fast decay component τ1 and the slow decay component τ2 are ascribed to a trap-assisted recombination at the QD surfaces and an intrinsic radiative recombination inside the QDs, respectively [4,29]. It should be noted that the average lifetime is not presented here, because the average lifetime basically has no real physical meaning. Both τ1 and τ2 for the air-synthesized FAPbI3 PQDs are decreased, compared with the PQDs obtained using degassing (vacuum) and N2 protection, which can be ascribed to the moisture and volatile contaminants in the reaction mixture inducing large amounts of defects in the PQDs. However, the lifetime of the photogenerated charge carrier, both τ1 and τ2, can be prolonged by introducing the ZnI2 additive, as illustrated in Figure 4d. The Zn:FAPbI3-30 PQDs exhibit 29.81 ns for τ1 and 96.04 ns for τ2, comparable with, or slightly higher compared with, the pristine FAPbI3-vaccum/N2 PQDs (28.05 ns for τ1 and 92.54 ns for τ2). The iodine-rich environment provided by the addition of ZnI2 inhibits the formation of iodine-vacancy trap states in the FAPbI3 PQDs, which is also beneficial to the structural stability [4]. Nevertheless, when the percentage of ZnI2 is increased to 40% and 50%, the increased surface-to-volume ratio (due to the decrease in size) and the abundant Zn ions that can be seen as new contaminants, lead to a new defect state formation inside and on the PQD surfaces, which is responsible for the decreased TRPL lifetimes shown in Figure 4d. For practical device applications, the enhanced carrier lifetime of PQDs may lead to increased current density in solar cells, increased external quantum efficiency in LEDs, and improved responsivity in photodetectors, etc.

Conclusions
In summary, an easy and effective synthetic method by introducing ZnI2 additives is developed to achieve high-quality FAPbI3 PQDs without degassing and N2 protection.

Conclusions
In summary, an easy and effective synthetic method by introducing ZnI 2 additives is developed to achieve high-quality FAPbI 3 PQDs without degassing and N 2 protection.
Compared with the pristine vacuum/N 2 -FAPbI 3 PQDs, the air-synthesized Zn:FAPbI 3 PQDs exhibit higher structural and colloidal stability, as well as higher production yield. As the amount of added ZnI 2 in the reaction mixture is increased, the crystal size of the FAPbI 3 PQDs is decreased, while no significant changes in the crystal structure, shape, and size dispersion are observed. The introduced ZnI 2 would not induce the significant lattice distortion in the PQDs, but may only reside at the surfaces. Thus, the excessive addition of ZnI 2 induces the formation of impurities of related Zn salts on PQD surfaces, negatively impacts the ligand binding with OA and OAm, and results in defects as well as weakened electronic coupling between QDs. However, importantly, with assistance of an optimal amount of ZnI 2 additives,~30 mol%, these air-synthesized FAPbI 3 PQDs demonstrate slightly longer carrier lifetimes than vacuum/N 2 -assisted FAPbI 3 PQDs. This vacuum-free air-synthesis route offers an industrial-friendly method for producing low-cost colloidal FAPbI 3 PQDs for high-performance PV applications.