Air-Resistant Lead Halide Perovskite Nanocrystals Embedded into Polyimide of Intrinsic Microporosity

Advanced Membranes and Porous Materials Center (AMPMC) & KAUST Catalysis Center (KCC), Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia Advanced Membranes and Porous Materials Center (AMPMC), Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia KAUST Catalysis Center (KCC), Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

Among the aforementioned strategies, polymer encapsulation has recently gained great attention, as perovskitepolymer composites exhibit high environmental stability. In addition, the surface defects of PNCs could be passivated by polymers, achieving a significant enhancement in PL intensity [46,51]. Due to the compatibility between polymers and perovskites, various composites have been fabricated and studied [49][50][51]. For instance, Snaith et al. mixed assynthesized inorganic PNCs with polystyrene (PS) and polymethyl methacrylate (PMMA) [47] in order to prevent anion exchange and increase air stability. Kovalenko et al. studied multiple polymer effects through single CsPbBr 3 PNC light emission on optical stability. This study suggested that the proper choice of polymers in perovskite film preparation could help improve device performance [52]. After that, Yang et al. applied the thick polymer poly(maleic anhydride-alt-1-octadecene) on CsPbBr 3 PNCs as a protection layer. The PL intensity of the film remains at 90% from the original after 144 h, and the film has been applied as white LEDs with high performance [53]. In addition, Lin and coworkers applied a copolymer nanoreactor strategy for crafting perovskite nanocrystals composited with the polymer poly(acrylic acid)-block-polystyrene, which is able to protect nanocrystals from air and water [54]. Note that polymers that have been used to enhance NC stability are nonporous polymers or porous polymers synthesized together with NCs. There are no reports about posttreatment polymers with intrinsic microporosity (PIMs) to boost NC stability and performance.
Herein, we propose and test a postsynthesis strategy to embed CsPbBr 3 PNCs into porous polymers to enhance the surface stability and maintain the optical properties of PNC films. Steady-state and time-resolved spectroscopy results confirm the improvement in NC stability after treatment. The selected polymer, an intrinsically microporous polyimide (6FDA-TrMPD), was synthesized via the polycondensation reaction of 4,4′ (hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,4,6-trimethyl-m-phenylenediamine (TrMPD). This polyimide is able to accommodate perovskite NCs and act as a shell surround, leading to much fewer surface defects and improved air stability. This approach can improve the chemical stability of perovskite and other sensitive materials and make their everyday applications more feasible.
CsPbBr 3 PNCs were synthesized by the hot-injection method. First, 0.203 g of Cs 2 CO 3 was loaded into a 50 ml round bottom flask with 10 ml of ODE and 0.625 ml of OA. The solution was stirred and degassed at 120°C under vacuum for 1 h until all Cs 2 CO 3 was dissolved to obtain a clear solution as a cesium oleate precursor. PbBr 2 (350 mg) was loaded into a flask with 25 ml of ODE, and the mixture was dried under vacuum for 1 hour at 120°C. OA (2.5 ml) and OAm (2.5 ml) were injected with purging N 2 , and the solution was stirred until PbBr 2 was fully dissolved. The temperature was raised to 180°C, and 2 ml of the cesium oleate precursor was quickly injected. After 5 seconds of reaction, the flask was cooled in an ice-water bath. After the icewater bath, the solution was centrifuged at 10000 rpm for 5 min, washed with TOL, and centrifuged again. The PNCs were collected by dispersion in 3 ml of TOL.

Porous Polymer Synthesis.
Intrinsically microporous polyimide 6FDA-TrMPD was prepared via a polycondensation reaction at high temperature, as previously reported [55]. 4,4 ′ -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,4,6-trimethyl-m-phenylenediamine (TrMPD, 96%) were mixed in equimolar amounts (0.665 mmol each) in m-cresol and heated to 80°C under continuous nitrogen flow. A catalytic amount of isoquinoline was added, and the reaction was conducted at 200°C for a few hours. The highly viscous polymer solution was cooled down and poured into MeOH. The polymer powder was further purified by reprecipitation from its CHCl 3 solution to MeOH. The final off-white powder (0.36 g, yield = 90%) was dried at 150°C under vacuum for 24 hours. The polymer exhibited good solubility in chloroform, dichloromethane, tetrahydrofuran, dimethylformamide, dimethylacetamide, and N-methyl-2-pyrrolidone. 1 Scheme 1: Synthesis of the 6FDA-TrMPD polymer by the polycondensation of 6FDA and TrMPD in m-cresol.

Time-Resolved Photoluminescence Lifetime.
To understand the polymer coating effect on the PNC luminescent properties, we performed PL lifetime measurements on films using the time-correlated single-photon counting technique. The samples were excited at 405 nm with a ps-pulsed. The PL signal was monitored at 510 nm using a bandpass filter. The interpulse duration was 10 MHz, and the intensity of the pulses was adjusted to detect less than 1% of excitation events. The time resolution of the system is 120 ps. A detailed description of the system can be found in the Supporting Information.
2.5. Femtosecond Transient Absorption. The phenomena taking place early upon PNC excitation were obtained through femtosecond transient absorption (fs-TA) spectroscopy. For this purpose, the samples were excited with 480 nm pulses generated with an optical parametric amplifier pumped by an amplified Ti:sapphire laser (800 nm, 100 fs, 1 kHz). The pump fluence (0.5 μJ cm -2 ) was adjusted to prevent the generation of multiple charge carriers. The probe pulses (white light) were generated by passing another fraction of the 800 nm beam through a 2 mm-thick CaF 2 crystal. The white light was split into two beams (signal and reference). The excitation pump pulses spatially overlapped with the probe pulses on the samples after passing through a synchronized mechanical chopper (500 Hz), which blocked alternative pump pulses. The obtained signal was sent to the detector through an optical fiber. A detailed description of the system can be found in the Supporting Information.

Results and Discussion
Porous polyimide (6FDA-TrMPD) was synthesized by a onepot high-temperature polycondensation reaction (see Scheme 1) and used as an incubator to enhance nanocrystal stability without inducing any structural changes. The total conversion of poly(amic) acid to polyimides was achieved at 200°C. The conversion was confirmed by the absence of 1 H NMR peaks above 10 ppm ( Figure S1) [55].
The polyimide structure was also confirmed using 1 H NMR and FTIR (see Figure S1 and Figure S2). It is worth pointing out that 6FDA-TrMPD displays high porosity, a Brunauer-Emmett-Teller surface area of 480 m 2 g -1 , and high thermal stability, with a 5% degradation temperature of 503°C (see Figure S3 and Figure S4). 6FDA-TrMPD has   Energy Material Advances a pore size distribution ranging from 0.5 nm to 120 nm, with significant porosity ranges in the mesoporous area for pore sizes between 40 nm and 120 nm, which is suitable for accommodating PNCs. The large BET surface area, high thermal stability, and suitable pore size make 6FDA-TrMPD a promising material for embedding and protecting NCs.
To understand the interaction between PNCs and 6FDA-TrMPD during the mixing process, after preparing PNCs

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with and without polymer, they were also characterized and tested to confirm their structural and optical properties. To investigate the structural change of PNCs after polymer treatment, high-resolution transmission electron microscopy (HR-TEM) was performed. As shown in Figures 1(a) and 1(b), the HR-TEM images reveal that PNCs have a cubic shape, which is consistent with previous reports [6,7,12]. Also, the size distribution of PNCs was 10:5 ± 1:5 nm after polymer treatment, which was comparable to that of PNCs without treatment, with a size distribution of 10:9 ± 1:2 nm (see Figures 1(c) and 1(d)). This result suggests that the polymer does not interact or modify the morphology or size of the CsPbBr 3 PNCs, making it a suitable coating material for PNCs. It should be noted that the length of the ligand (oleic acid and oleylamine) is~2 nm, and the average size of the PNCs is~11 nm. Considering that the average pore size of the polymer is more than 40 nm, it will be able to capture at least one or two PNCs per pore.
X-ray diffraction (XRD) patterns were also used to determine the PNC structure before and after polymer treatment (Figures 1(e) and 1(f)). The XRD patterns of the films of as-synthesized PNCs and PNCs mixed with the polymer were compared with standard CsPbBr 3 XRD patterns. In comparison, they exhibited exactly the same peaks. It should be noted that nanocrystal samples have different orientations and different environments, which could lead to a change in the relative intensity of XRD peaks. This indicates that the porous polymer did not change or modify the structure or the dimensionality of the CsPbBr 3 NCs. These results again confirm that the polymer only provides a coating for the PNCs. Therefore, the polymer treatment does not affect the NC structure or size.
We further investigated the stability of CsPbBr 3 PNC films by performing steady-state and time-resolved spectroscopic measurements. The steady-state PL spectra and time-resolved PL decay of as-synthesized and polymertreated PNCs were collected within 168 h at 40% humidity and 23°C following 375 nm excitation. The as-synthesized PNC films without and with polymer exhibit 63.58% and 61.34% PLQY, respectively. It would mean that there is no immediate effect on the PLQY by the polymer encapsulation. As shown in Figure 2, the film with as-synthesized PNCs lost 70% of its initial PL intensity. On the other hand, the film with PNCs and the porous polymer lost only 20% of its initial PL intensity (Figures 2(a) and 2(b)). It is important to mention that both samples display their peak maxima at 510 nm, confirming that the polymeric coating does not alter the main PNC structure. Interestingly, the film with PNCs treated with PMMA was also tested to compare it with porous polymer to study the importance of porous structure, in which the nanocrystal lost 40% of its original PL intensity ( Figure S6). This observation highlights the importance of the pore size of 6FDA-TrMPD, which can provide better protection for PNCs and enhance PNC stability.
Furthermore, the PL lifetimes of the CsPbBr 3 PNC film with and without the polymer after 168 h are drastically different, as observed in Figure 2(c). After 168 h, the PL lifetime at 510 nm of the PNCs with the porous polymer is much longer than that of the PNCs without the polymer. More specifically, for the film without the polymer, the lifetime dropped from 5:80 ± 0:12 ns to 3:67 ± 0:07 ns, and for the film with the polymer, the lifetime decay exhibited no significant variation, changing from 5:60 ± 0:10 ns to 5:86 ± 0:13 ns (see Figure 2(c)). This observation indicates that the porous polymer successfully preserved the optical properties of the CsPbBr 3 PNCs in an air environment after 168 h. The last results confirm that our method can serve as a promising way to protect PNCs in films and effectively enhance their chemical resistance while maintaining their desired luminescent behavior.
After confirming the notable effect of polymer encapsulation on the enhancement in the luminescent stability of the CsPbBr 3 PNC film, a further examination was performed through fs-TA in order to understand the changes in PNC excited-state dynamics and the effect of the polymer on the charge carrier recombination of the PNC film. Femtosecond time-resolved laser spectroscopy has been widely used to convey detailed information on the relaxation of the excited state in photoactive materials. Therefore, the effect of the 6FDA-TrMPD polymer on the perovskite optical properties was studied by the transient absorption technique . Figures 3(a)  6 Energy Material Advances with and without the polymer obtained in response to 480 nm optical excitation. A persistent ground state bleaching signal appears at 512 nm, which can be attributed to the effect of band filling, which agrees well with the optical bandgap (2.3 eV) and the result obtained from steadystate optical absorption spectra (see Figure S7). As shown in Figures 3(a)-3(c), the ground-state recovery time is much longer for CsPbBr 3 PNC films treated with the polymer than for untreated ones, which is consistent with PL experiments and further supports the significant role of the polymer in protecting the surface PNCs and subsequently reducing the surface trapping centers. The bleach recovery kinetics of the CsPbBr 3 film with the polymer can be fitted by a biexponential function with time constants of 212 and 1670 ps, whereas the NCs without the polymer need an additional component to be fitted appropriately; the time constants obtained were 45.7, 264, and 1860 ps. The new sub-100 ps time constant (45.7 ps) for the CsPbBr 3 film without the polymer takes half of the amplitude of the total signal with picosecond timescale, which can be assigned to a trapping process not observed for the polymer-treated film. The absence of short-lifetime components in PNC films with 6FDA-TrMPD makes the lifetime longer, and the origin of these short components might be caused by surface defects formed after ligand detachment. Different surface passivation enhancing the lifetime and having long lifetime components have been applied to PNCs by previous researchers [56,57]. Similarly, 6FDA-TrMPD was able to accommodate the PNCs, preserve the surface, and prevent surface defects from forming during 168 h of storage. This significant difference again shows the dramatic effect of the polymer in PNCs on the early excited-state dynamics and suggests the essential role of the polymer in protecting the PNC surface. Thus, we can conclude that the 6FDA-TrMPD polymer enhanced the optical stability of the PNC film under environmental conditions, and polymer treatment could prevent ligand detachment, which is responsible for decreasing the overall photoluminescence quantum yield. Scheme 2 illustrates the mechanism of the enhancement in the optical stability for the PNC film; mesoporous 6FDA-TrMPD with a suitable pore size (more than 40 nm) can accommodate the CsPbBr 3 PNCs to enhance their stability, similar to other polymers. Similar to PMMA, 6FDA-TrMPD can protect the PNCs from air environments. After protection, PNCs were embedded into porous polymer, which maintains the surface of PNCs and reduces ligand detachment during film preparation, which causes less degradation of PL intensity. In addition, polymers with pore size greater than 40 nm can capture one or two PNCs in a single pore. This feature can significantly prevent NC aggregation and can preserve the surface of PNCs. The ligands of CsPbBr 3 nanocrystals have nonpolar end chains, and all nonpolar and carbonyl groups of the polyimide are not activated to interact with other sites. Polymer pore size as 40 nm is larger than PNCs with around 15 nm, meaning they do not form a compact core-shell structure. The porous polymer may not fully isolate all of PNCs, so the film might be conductive for further application as optical devices. Based on these two factors, 6FDA-TrMPD shows better stability enhancement than PMMA and maintains the optical properties in the film phase.
In summary, a polyimide with intrinsic microporosity, 6FDA-TrMPD, has been developed as a new treatment approach to significantly enhance the optical properties and photostability of CsPbBr 3 perovskite nanocrystal films. The charge carrier dynamics and the optical stability of PNC films were studied through time-resolved PL and fs-transient absorption measurements. The results demonstrated that the porous polymer coating could prevent ligand detachment and subsequently preserve the surface stability. Moreover, we find that the polymer also plays a fundamental role in inhibiting surface trapping deactivation processes. This method enables porous polymers to significantly enhance the optical stability and chemical resistance of perovskite nanocrystals for their use as potential optoelectronic devices, including LEDs, scintillators, and lasing devices. The approach proposed here is a step contributing to overcoming the persistent challenge of stability in inorganic perovskite materials and can be used as a starting point to increase the commercial use of these highly efficient luminescent materials.

Data Availability
The authors declare that the main data supporting the findings in this study are available within the article and its supplementary information. Additional data are available from the corresponding authors upon reasonable request.

Conflicts of Interest
The authors declare no competing financial interest. C.C. carried out the TEM characterizations. culated using the relative pressure range between 0 and 0.3. Figure S5: pore size distribution of 6FDA-TrMPD obtained from ASAP 2020 using NLDFT. Figure S6: the PL intensity of CsPbBr3 perovskite nanocrystals with PMMA treatment decreased after 168 h. Figure S7: PL decay during 168 h and steady-state absorption with and without the polymer. (Supplementary Materials)