Aluminum Distearate–Modified CsPbX 3 (X = I, Br, or Cl/Br) Nanocrystals with Enhanced Optical and Structural Stabilities

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2020Aluminum Distearate–Modified CsPbX3 (X = I, Br, or Cl/Br) Nanocrystals with Enhanced Optical and Structural Stabilities Weinan Xue, Xiaoyan Wang, Wei Wang, Fangfang He, Wei Zhu and Yan Li Weinan Xue Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) , Xiaoyan Wang Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) , Wei Wang Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) , Fangfang He Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) , Wei Zhu Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) and Yan Li *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 (China) https://doi.org/10.31635/ccschem.020.201900063 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Colloidal all-inorganic perovskite nanocrystals (PNCs), possessing unique optical properties, have attracted considerable attention in the field of semiconductor nanocrystals (NCs), but their application is hindered by the stability issue resulting partly from dynamic capping ligand binding. Herein, we report a simple method for the synthesis of all-inorganic cesium lead-based (CsPbX3) NCs with enhanced structural stability and photoluminescence quantum yield. Aluminum distearate (AlDS) was introduced into the preparation of CsPbX3 NCs, on the basis that the surface defects of CsPbX3 NCs are passivated to form a protective layer on the CsPbX3 NC surface simultaneously. Benefiting from surface modification, the resistance of the CsPbX3 NC dispersion against ethanol, ultraviolet irradiation, and heat treatment was enhanced effectively. Moreover, the photoluminescence intensity and stability of the AlDS–modified NC-based films displayed functional superiority to those of pristine NC-based films. Download figure Download PowerPoint Introduction Owing to unique optical versatility, defect tolerance, and solution processability, colloidal perovskite nanocrystals (PNCs) recently arouse considerable scientific and technological interest as a new class of semiconductor nanocrystals (NCs).1–5 Despite their excellent prospects in photovoltaic light-emitting diode, photocatalysts, solar cells, and laser applications, their structural and optical instabilities hinder the commercial applications of PNCs at this stage.6 Hybrid organic–inorganic PNCs, although have low energy of formation, they tend to decompose into volatile organic halide and lead halide even under ambient conditions.7–9 All-inorganic cesium lead halide (X = I, Br, or Cl/Br) NCs are reported to be relatively more stable than hybrid organic–inorganic PNCs.10,11 Nevertheless, the disintegration of CsPbX3 NCs, especially, CsPbI3 NCs, could be accelerated by the exposure to light, heat, or polar environments because of their intrinsic ionic crystal nature and dynamic capping ligand binding character.12–14 Oleylamine (OAm) and oleic acid (OA) are usually used as typical surface ligands to synthesize PNCs, stabilizing the colloidal PNCs and passivating the surface defects of PNCs. Except that OAm and OA bind only loosely to the surface of PNCs and tend to detach from PNC surface, leading directly to the aggregation of PNCs.15–17 Besides, the resultant surface vacancy defects might deteriorate the photoelectrical performance and structural integration of PNCs.18–20 The coating with inert inorganic layers such as silicon and titanium dioxides (SiO2 and TiO2) has been confirmed to be a well-established approach for stabilizing traditional colloidal semiconductor NCs of metal chalcogenides.21–24 However, it is difficult to perform a controllable growth of inorganic shell layer on PNCs without sacrificing optical performance due to the structural lability of PNCs in the typical polar solvents applied.25,26 The physical mixing with hydrophobic polymers such as poly(lauryl methacrylate) and polystyrene might improve the stability of PNCs in polar and ambient environments, but after polymer covering, leaving the original surface defect sites intact is always a challenge.27,28 The fact that PNCs degradation is initiated generally from the defect sites at the surfaces or grain boundaries, some organic compounds have been applied to chemically passivate surface defects in order to enhance the stability of PNCs.29–32 The use of phosphine ligands to bind Pb2+ ions on the surface of CsPbX3 NCs more tightly than OA and OAm have been attempted, and found to endow better passivation and protection of the PNCs,32 and the effectiveness of this surface passivation was confirmed by applying 2,2′-iminodibenzoic acid with dual carboxyl groups, able to bind the Pb2+ ions on CsPbI3 NCs.33 Through the stabilizing mechanisms of physical covering and chemical passivation, it is anticipated that an even better PNC stability would be obtained by the application of ligands with both multiple coordination sites and long hydrocarbon branches.34 Herein, we introduced aluminum distearate (AlDS) with a branched-chain structure in organic solvents into the preparation of CsPbX3 NCs to synthesize AlDS-capped CsPbX3 NCs, as shown in Scheme 1. With the simultaneous surface defect passivation of multiple coordination sites and physical protection by hydrophobic hydrocarbon branches, AlDS-capped CsPbX3 NCs displayed bright light emission and enhanced the chemical and phase stabilities. Furthermore, the modified CsPbX3 NCs exhibited much better resistance against polar solvents, UV irradiation, and heat treatment. Scheme 1 | Schematic illustration of the synthesis of CsPbX3@Al-n NCs and AlDS structure. Download figure Download PowerPoint Experimental Methods Materials Lead iodide (PbI2, 99.9%) and lead chloride (PbCl2, 99.999%) were purchased from Aladdin (Beijing, China). Lead bromide (PbBr2, 99.999%), 1-octadecene (ODE, 90%), and oleylamine (OAm, 95%) were purchased from Sigma-Aldrich (Shanghai, China). Oleic acid (OA, 90%) and aluminum tristearate (AlTS, tech grade) were obtained from Alfa Aesar Chemical (Beijing, China). Aluminum monostearate (AlMS, 75% of Al) and aluminum distearate (AlDS, CP) were purchased, respectively, from Macklin and Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Cesium carbonate (Cs2CO3, 99%) was purchased from Vetec (Singapore, China). Methyl acetate (99%, super dry) and hexane (97.5%, super dry) were received from J&K Scientific (Hong Kong, China). Toluene (99%) was obtained from Guangzhou Chemical Reagent (Tianjin City, China). All chemicals were used as received without further purification. Synthesis of CsPbI3@Al-n, CsPbI3@AlMS-n, CsPbI3@AlTS-n, and pristine CsPbI3 NCs CsPbI3@Al-n NCs were synthesized by the hot-injection method reported in the literature35,36 with minor modification. Briefly, 0.1628 g of Cs2CO3, 0.5 mL of OA, and 8 mL of 1-octadecene (ODE) were placed into a 50 mL three-neck flask, followed by degassing for 1 h at 120 °C. After refilling with atmospheric N2, the system was heated to 150 °C under N2 and kept at this temperature until all Cs2CO3 reacted with OA. The cesium (Cs)oleate (Cs-OA) obtained was stored at ambient conditions (room temperature with adequate ventilation). It should be noted that the Cs-OA precursor needed to be preheated to 100 °C, as Cs-OA precipitates out of ODE at room temperature. We synthesized CsPbI3@Al-n NCs by adding 0.083 g of PbI2 and a certain amount of AlDS (0.027–0.220 g) into a 50 mL three-neck flask containing 5 mL of ODE, 0.7 mL of OAm, and 0.7 mL of OA. The system was heated to 120 °C under vacuum and degassed at this temperature for 1 h. Then the mixture was heated to 170 °C under N2 flow, followed by a rapid injection of 0.4 mL of the Cs-OA precursor. After reacting for 5 s, the reaction system was cooled to room temperature with an ice-water bath. The CsPbI3@Al-n NCs obtained were precipitated by centrifugation at 12,000 rpm for 5 min. The precipitate was dispersed in toluene, and an equal volume of methyl acetate was added to precipitate NCs again. After redispersion of the latter precipitate in toluene, it was collected by centrifugation at 12,000 rpm for 5 min, and finally, the supernatant was collected for further characterization. To synthesize CsPbI3@AlMS-n and CsPbI3@AlTS-n, AlMS and AlTS were added in the reaction system in the same way as AlDS, respectively. As a reference, CsPbI3 NCs were also prepared in the absence of AlDS, using the same procedure and reaction conditions as CsPbI3@Al-n NCs. Synthesis of CsPbBr3@Al-n and CsPbCl1.5Br1.5@Al-n NCs CsPbBr3@Al-n and CsPbCl1.5Br1.5@Al-n NCs were prepared through the same synthesis procedure and reaction conditions as CsPbI3@Al-n NCs, except that PbI2 was replaced by PbBr2 and PbCl2. Characterization Methods UV–Vis spectroscopy (Shimadzu UV-2600, Beijing, China) and fluorescence spectroscopy (Varian Cary Eclipse, Beijing, China) were employed to evaluate the optical properties of the samples using a quartz cuvette (1 cm × 1 cm) at room temperature. X-ray diffraction (XRD; Rigaku D/MAX-2550, Shanghai, China) analysis was used to obtain information about the crystalline phase. Transmission electron microscopy (TEM; JEM-2100, JEOL USA Inc., Peabody, MA) and high-resolution TEM (HRTEM) were performed to obtain the morphology and interlayer spacing information of the samples at an accelerating voltage of 200 kV. Inductively coupled plasma atomic emission spectroscopy (ICP-AES; 725ES, Agilent, Beijing, China) was used to confirm the elemental composition of the modified CsPbX3, with the samples prepared by dissolving the purified NCs in aqua regia. Also, X-ray photoelectron spectra (XPS) of NCs were obtained using the Thermo Scientific ESCALAB 25 Xi XPS system (Shanghai, China). The absolute photoluminescence quantum yields (PLQYs) of the NC dispersions were detected using a fluorescence spectrometer (C9920-02; Hamamatsu Photonics, Japan) with an integrated sphere, and excited with 450 nm light-emitting diode. Further, time-resolved fluorescence spectroscopy was performed using the FLS 980 (FS5 Spectrofluorometer, Edinburgh Instruments Ltd., Shanghai, China) to evaluate temperature dependence of the dynamics of the NCs fluorescence decay. Result and Discussion Synthesis of AlDS-capped CsPbX3 NCs According to a similar method first reported by Kovalenko and co-workers,35,36 a series of CsPbX3 NCs modified by AlDS were synthesized by the injection of Cs-OA into a PbX2 ODE solution containing OA, OAm, and AlDS at 170 °C, as shown in Scheme 1. The NCs obtained were denoted as CsPbX3@Al-n NCs (n = 0.25, 0.5, 1, 1.5, or 2), and n is the molar ratio of Al to PbX2 added. Following the same synthesis procedure for CsPbX3@Al-n NCs, AlMS and aluminum tristearate (AlTS) were also applied to synthesize CsPbX3 NCs. For comparison, pristine CsPbX3 NCs were also prepared without introducing AlMS, AlDS, or AlTS into the PbX2 ODE solution. All the modified NCs obtained were purified by crystallizing in methyl acetate (MeOAc) to remove residual surface ligands, and finally, dispersed in hexane or toluene. CsPbI3 NCs were chosen as representative samples to investigate the effect of modification of AlMS, AlDS, or AlTS on the properties and stability of CsPbX3 PNCs in our current work, and compared with the pristine CsPbI3 NCs. Our results showed that both AlDS- and AlMS-modified CsPbI3 NCs exhibited enhanced optical performance at ambient conditions. However, the AlDS-modified CsPbI3 NCs exhibited superior phase stability, compared with the AlMS-modified CsPbI3 NCs. By contrast, only marginal photoluminescence (PL) and stability enhancements were observed on AlTS-modified CsPbI3 NCs (photographs and PL spectra are shown in , respectively). We found that AlMS and AlDS were in the form of a one-dimensional chain structure, but AlTS showed a monomeric framework (as shown in ).37,38 We deduced that AlMS and AlDS having chain structures should adsorb firmly on the surface of CsPbI3 NCs. Furthermore, AlDS with two stearate groups should be capable of forming a denser coating layer on the surface of CsPbI3 NCs than AlMS with one stearate group. Therefore, AlDS was selected as the coating agent to modify CsPbI3 NCs, with a view of optimizing the optical properties and stability of the NCs hereafter. Optical properties, morphology, and structure of CsPbI3@Al-n NCs The ultraviolet–visible (UV−vis) absorbance and PL spectra of both pristine- and AlDS-modified CsPbI3 NCs are shown in Figure 1d. Pristine CsPbI3 NCs possess a first excitonic peak at 678 nm and a PL emission peak at 686 nm, following literature reports.39,40 The coating of AlDS resulted in a blueshift of both first excitonic absorption and PL emission peaks of CsPbI3 NCs, and the degree of blueshift increased along with the increasing amount of AlDS introduced. Such a result could be attributable to the decreasing CsPbI3 size,41,42 which was confirmed by the TEM images shown in Figure 1a,b. Both pristine CsPbI3 and CsPbI3@Al-1.5 NCs presented cubic morphologies with the average side lengths of 14.9 ± 4.2 nm and 7.8 ± 1.2 nm, respectively. Compared with pristine CsPbI3 NCs, we observed that CsPbI3@Al-1.5 NCs had a smaller and narrower size distribution. Figure 1 | TEM images of CsPbI3 NCs (a) and CsPbI3@Al-1.5 NCs (b), with insets showing corresponding HRTEM images. XRD patterns (c), UV–vis and PL spectra (d) of CsPbI3 and CsPbI3@Al-n NCs. Download figure Download PowerPoint According to the high-resolution TEM (HRTEM) images inset shown in Figure 1a,b, both pristine CsPbI3 and CsPbI3@Al-1.5 NCs have well-defined crystalline structures with a lattice fringe space of 0.31 nm, as per the "200" crystal face of a cubic phase.40,43 Besides, the XRD patterns, shown in Figure 1c indicate a high crystallinity of both pristine CsPbI3 and CsPbI3@Al-n NCs in the cubic phase structure. Regardless of the difference in the amount of AlDS introduced, the XRD peak positions of CsPbI3@Al-n NCs are identical to those of pristine CsPbI3 NCs. The coating of AlDS, thus, did not influence the lattice structure of CsPbI3 NCs, which eliminated the possibility of doping Al atoms into the lattice of CsPbI3 NCs.42 To verify the coating of AlDS on CsPbI3 NCs, ICP-AES measurements were conducted; the data are listed in Table 1. The content of Al in CsPbI3@Al-n NCs prepared increased synchronously with an increased amount of added AlDS, demonstrating the presence of Al in the modified CsPbI3 NCs. In terms of XRD and ICP-AES data, we expected Al atoms to be on the surface of the CsPbI3 NCs in the form of AlDS rather than inside of the structure as doping atoms. Table 1 | Relative Molar Ratios of Al to Pb in CsPbI3@Al-n NCs Samples Molar Ratio (Al∶Pb) CsPbI3@Al-0.25 0.181∶1 CsPbI3@Al-0.5 0.217∶1 CsPbI3@Al-1 0.969∶1 CsPbI3@Al-1.5 1.655∶1 CsPbI3@Al-2 1.910∶1 Interactions between AlDS and the surface of CsPbI3 NCs To further confirm the existence of AlDS on the NC surface, X-ray photoelectron spectroscopy (XPS) characterization on CsPbI3@Al-1.5 and pristine CsPbI3 NCs was performed. The results shown in Figure 2 reveal a peak of N 1s at an identical position on the spectra of both pristine- and AlDS-modified CsPbI3 NCs, suggesting the presence of OAm ligands on both NCs. An Al 2p peak associated with oxidized Al appears on the spectrum of CsPbI3@Al-1.5 NCs at 74.62 eV, demonstrating the existence of AlDS on the surface of CsPbI3 NCs.42,44 Compared with the spectra of pristine CsPbI3 NCs, the Pb 4f and I 3d peaks of CsPbI3@Al-1.5 NCs both presented higher binding energy. Meanwhile, no apparent shift of Cs 3d peaks appeared. Thus the shift of Pb 4f peaks noted from the Al3+ modified NCs might result from the coordination between Pb2+ and stearate groups, whereas the shift of I 3d peaks could be attributable to ionic interaction between Al3+ and I−.45 With this coordination between Pb2+ and stearate groups, we deduced that the coating of AlDS on CsPbI3 NCs could passivate the dangling bond of Pb2+ on the NC surface. We confirmed our deduction by PLQY and PL lifetime characterization based on the optical properties of pristine- and AlDS-modified CsPbI3 NCs. Figure 2 | XPS spectra: Cs 3d (a), Pb 4f (b), I 3d (c), and N (d) 1s in CsPbI3 and CsPbI3@Al-1.5 NCs. (e) Al 2p in CsPbI3@Al-1.5 NCs. Download figure Download PowerPoint According to the data shown in , CsPbI3@Al-1.5 NCs have a PLQY of 78%, which is higher than that of the pristine ones of 68%. At the same time, the values of the PL decay times, τ1 and τ2, of CsPbI3@Al-1.5 NCs approached those of pristine CsPbI3 NCs. Nevertheless, CsPbI3@Al-1.5 NCs presented a shorter average of τ1 and τ2 (τave) with a higher proportion of τ1 than pristine CsPbI3 NCs. The suppressed trap state recombination and the promoted exciton recombination of CsPbI3@Al-1.5 NCs indicated the passivation of surface trap states of NCs by AlDS.46,47 Optical and structural stabilities of CsPbI3@Al-n The exposure of lead halide PNCs with ionic crystalline nature to polar environments likely accelerate the degradation of the crystals and, consequently, deteriorate the performance of CsPbX3 NCs.48,49 AlDS has a chain structure with long hydrophobic side branches on the main chain. As a result, the coating of AlDS on NCs supposedly, isolate CsPbX3 NCs from polar solvents effectively, improving the optical and phase stabilities of NCs. Figure 3 shows the evolution of PL spectra of colloidal NC dispersions stored at ambient conditions in which the enhanced optical stability of AlDS-coated CsPbI3 NCs is confirmed. For the CsPbI3@Al-1.5 NC dispersion, bright emission was still visible even after 45-day storage. What is more, there was no formation of the yellow precipitates, which is typical with pristine NCs. In comparison, the emission of the pristine CsPbI3 NC dispersion was quenched completely after 3-day storage. The appearance of yellow precipitates suggests the transition from the cubic phase to the orthorhombic phase.50 The optical stability enhancement by CsPbI3 NCs via AlDS modification was validated by the evolution of PLQY of NCs. As shown in , the CsPbI3@Al-1.5 NCs maintain 54% of PLQY even after 45 days of storage at an ambient condition. By contrast, the PLQY of pristine-based prototype NCs almost decreases to 0 after merely 3 days of storage. Figure 3 | (a) PL spectra of CsPbI3 NC toluene dispersions before and after 3-day storage at ambient condition. (b) PL spectra of CsPbI3@Al-1.5 NC toluene dispersions before and after 45-day storage at ambient condition. Insets show corresponding photographs, taken under sunlight and 365 nm UV light. Download figure Download PowerPoint We verified the contribution of AlDS to the improved stability of modified CsPbI3 NCs in polar environments by performing PL spectrometry, recording the PL spectra before and after the addition of a small quantity of ethanol into the dispersions of CsPbI3@Al-n and pristine CsPbI3 NCs in toluene. As shown in Figure 4, the emission of pristine CsPbI3 NCs was quenched almost entirely upon the addition of ethanol, and because of the NC aggregation and phase transition, the dispersion becomes opaque and yellow.30 By contrast, only a slight decrease in the PL intensities of CsPbI3@Al-n (n = 1, 1.5, and 2) NC dispersions was apparent, which is also reflected by the almost identical emission color before and after ethanol addition. It is noticeable that PL intensities of CsPbI3@Al-0.25 and CsPbI3@Al-0.5 NCs show a rapid decline with the addition of ethanol. That is to say, a minimum change in the ratio of AlDS to PbI2, higher than 1.0, as observed here, is necessary for the adequate protection of NCs from the intrusion of polar solvents. Polar solvents induced the lattice distortion and triggered the dipole moment of the cubic phased CsPbI3 NCs, by which the self-assembly of CsPbI3 nanocubes into single-crystalline nanowires and the transition from cubic phase into orthorhombic phase occurred.51 The enhanced stability of AlDS-modified CsPbI3 NCs in polar environments is attributable to the dense protective AlDS layer covering of the NCs. With a large number of hydrophobic hydrocarbon branches presented on the surface, the diffusion of polar solvent molecules through the AlDS coating layer and the subsequent adsorption on the NCs are avoided effectively. Figure 4 | PL spectra of CsPbI3 and CsPbI3@Al-n NC toluene dispersions before and after ethanol treatment. Insets show corresponding photographs, taken under sunlight and 365 nm UV light. Download figure Download PowerPoint CsPbI3 NCs with suitable narrow bandgap and high PLQY present great potential application in the field of photovoltaics and light-emitting devices, both of which require high photostability of the NCs.52–54 We investigated the influence of AlDS on the photostability of CsPbI3 NCs by exposing the dispersions of pristine- and AlDS-modified NCs to 365 nm UV light under ambient conditions, and their PL spectra were recorded over time. As shown in Figure 5a, the PL intensity of all the samples declined at varying degrees under constant UV irradiation for 12 h and recovered slightly after storage in the dark for another 12 h. The migration of iodide ions from the lattice of CsPbI3 NCs to the surroundings under UV irradiation resulted in vacancies and PL loss, while the reverse process in the dark led to the partial recovery of PL.55 The PL intensity of pristine CsPbI3 NCs reduced drastically, maintaining merely less than 10% of their initial intensity after 3 irradiation cycles (one irradiation cycle means 12 h of continuous UV light irradiation and subsequent 12 h of dark storage). By contrast, CsPbI3@Al-n NC samples all exhibited improved photostability. Exceptionally, CsPbI3@Al-1.5 NCs maintained ∼80% of the initial PL intensity after 3 irradiation cycles. PNCs isolated from water and air are reported to exhibit high photostability, since the presence of water, O2, and CO2 synergistically leads to the decomposition of NCs and the generation of surface trap states.56 Accordingly, the improved photostability of CsPbI3[email protected]n NCs is attributed to the coating of AlDS, preventing adsorption of water from causing subsequent damage to NCs. Figure 5 | (a) Relative PL intensities as function of UV irradiation time for CsPbI3 and CsPbI3@Al-n NC toluene dispersions. Relative PL intensities of CsPbI3 and CsPbI3@Al-n NC toluene dispersions performed at varying temperatures (b) and treated at a fixed temperature of 85 °C, but at varying times (c). (d) Fluorescence images for CsPbI3 and CsPbI3@Al-n NC films on a hotplate set at 85 °C at varying times. Download figure Download PowerPoint In addition to photostability, the thermostability of CsPbI3 NCs is also critical for performance in devices like photovoltaic devices, since such applications are operated at high temperatures up to 85 °C.57 We investigated the effect of coating AlDS on the thermostability of CsPbI3 NCs, the pristine- and AlDS-modified NC toluene dispersions were heated from 30 °C to 110 °C under N2 atmosphere, during which aliquots were sampled at various temperatures. Their corresponding PL spectra are shown in Figure 5b. The PL intensities of the whole series of CsPbI3@Al-n and pristine CsPbI3 NCs increased first at varying degrees with the increased temperature and then decreased at higher temperatures. The initial increase in the relative PL intensity could be ascribed to the decrease in the defect concentration in the NCs during annealing.58–62 The reduction of defects led to a decrease in the nonradiative recombination centers, enhancing PL intensity accordingly. For pristine CsPbI3 NCs, only a slightly enhanced PL intensity was observable at 40 °C. At the boiling point of toluene at 110 °C, the PL intensity of pristine CsPbI3 NCs almost quenched entirely. On the contrary, the whole series of CsPbI3@Al-n NCs exhibited better performance in terms of thermostability. It is noteworthy that the best thermostability was observed with CsPbI3@Al-1.5 NCs with simultaneously improved resistance in polar solvents and photostability under UV irradiation. The PL intensity of CsPbI3@Al-1.5 NCs at 60 °C was ∼1.5 times that of the initial value and was maintained above 60% at 110 °C of the initial value. The photographs displayed in reveal that the dispersion of CsPbI3, CsPbI3@Al-0.25, and CsPbI3@Al-0.5 NCs become opaque and dark after heat treatment. In the meantime, the dispersions of CsPbI3@Al-1, CsPbI3@Al-1.5, and CsPbI3@Al-2 NC are kept clear and reddish. That is to say, higher coverage of AlDS might prevent the NCs from aggregation at elevated temperatures. To further illustrate the thermostability of NCs at photovoltaic cells' performance temperature, the NCs prepared were dispersed in the toluene kept at 85 °C. The PL spectra of the aliquots sampled at a fixed time interval are shown in Figure 5c. For the pristine CsPbI3 NCs, a monotonic decrease in PL intensity together with the appearance of yellow precipitates occurred with the extension of treatment time. After 100 min of heat treatment, almost no light could be observed. The formation of yellow precipitates was indicative of the transition of NCs from the cubic phase to the orthorhombic phase.63 By contrast, the PL intensity of the CsPbI3@Al-1.5 NC dispersion, first, increased remarkably, reaching a maximum value at a treatment time of 10 min, which resulted mainly from a reduction of defects in CsPbI3@Al-1.5 NCs. After that, the PL intensity decreased gradually but remained above 70% of their initial value at the treatment time of 100 min. According to the photograph of , there was no appearance of yellow precipitates, and the color of the dispersion always maintained the red coloration. NC films were further fabricated by depositing NC dispersions on glass substrates via the drop-cast method. At a temperature of 85 °C, and under ambient conditions, the PL emission of NC films under UV light was recorded over time; the results are shown in Figure 5d. Also, the corresponding recorded PL spectra are shown in . Before and after heat treatment at 85 °C, the whole series of CsPbI3@Al-n NC films always exhibited brighter PL emission than the pristine CsPbI3 NC film under UV light. When heated at 85 °C for 4 h, the light could no longer be observable on the pristine CsPbI3 NC film. The CsPbI3@Al-n (n = 0.5, 1, and 1.5) NC films exhibited bright PL emission after 8 h of heat treatment, and the CsPbI3@Al-n (n = 1 and 1.5) NC films displayed weak emission, even after 16 h of heat treatment. XRD spectra were recorded to investigate the possible phase transition of NC films after heat treatment. As shown in , a transition of the pristine CsPbI3 NC film from the cubic phase to the orthorhombic phase occurred, similar to the observation made in the dispersion experiments. After 16 h of heat treatment, the orthorhombic phase was observable on the CsPbI3@Al-n NC films. Nevertheless, the cubic phase was partly reserved, confirming the protection of AlDS to CsPbI3@Al-n NCs. Heat treatment might cause halogen vacancy and the detachment of OAm and OA ligands loosely binding to the surface of CsPbI3 NCs, thereby, generating surface defects on CsPbI3 NCs.64–66 The resultant surface defects might become the nonradiative recombination centers and accelerate the degradation of NCs, which, in turn, reduce the PL intensity of NCs.19,67 According to the results shown in Figure 5b–d and , the detachment of capping ligands from the CsPbI3 NC surface is suppressed effectively by coating AlDS with chain structure. We investigated the universality of AlDS for stabilizing PNCs, a series of CsPbBr3@Al-n and CsPbCl1.5Br1.5@Al-n NCs were prepared using the same synthesis procedure for CsPbI3@Al-n NCs, except that P