Morphology, luminescence and the gravity sedimentation effect of flexible and highly stable CsPbBr₃ nanocrystal patterns by the inkjet printing method

Yang Li, Jiayue Xu*, Hui Shen*, Xia Shao, Yankai Gu, Jiahao Zhao, Yasheng Li and Yuan Gui
Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China. E-mail: xujiayue@sit.edu.cn; hshen@sit.edu.cn

First published on 22nd August 2024

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1. Introduction

Lead halide perovskites (LHPs), represented by CsPbX₃ (X = I, Br, Cl), have attracted enormous attention owing to their remarkable optoelectronic properties, including the wide color gamut, long carrier diffusion length, and highly efficient luminescence. These extraordinary characteristics endow them as fascinating candidates in the fields of X-ray detectors, photodetectors,^1–3 lasers,^4,5 light-emitting diodes (LEDs),^6–8 solar cells,^9,10 electro-chemical sensors,¹¹ photocatalysis,¹² etc. Due to the low melting temperature and solubility in many organic solvents, CsPbBr₃ crystals have been successfully grown by both of the solution and melting methods.¹³ Typically, the anti-solvent vapor-assisted crystallization (AVC) method has been applied to grow CsPbBr₃ single crystals, with a size of 42 × 5 × 3 mm³.¹⁴ The optimized molar ratio of PbBr₂ and CsBr was 1.5 and mixed antisolvent composition was 50% MeOH and 50% DMSO. Based on the difference of solubility between perovskites and lead halide in the corresponding halide acid, the counterdiffusion-in-gel crystallization method has also been employed to prepare high-quality lead halide single crystals.¹⁵ Moreover, the Bridgman method is also favorable to grow high quality CsPbBr₃ crystals, with an optimized temperature gradient and a solid–liquid interface.¹⁶ Nevertheless, bulk CsPbBr₃ crystals display band-edge emission with a small Stokes shift, causing strong self-absorption, limited luminescence efficiency and deteriorated light yield.¹⁷ In comparison, perovskite nanocrystals (NCs) display extremely high photoluminescence quantum yield, high color purity and tunable emissions, due to the quantum confinement effect. However, it is worth nothing that CsPbBr₃ suffers from serious instability with low formation energies and strong polarity, in which it is extremely sensitive to water, oxygen, light, heat, etc.¹⁸ This issue severely impedes the practical applications of halide perovskite, and enhancement of the stability has become a critical concern for the exploration of CsPbBr₃. Most recently, many effective strategies have been proposed to enhance the stability of CsPbBr₃, including polymer encapsulation, inorganic matrix protection (e.g., core–shell structure, silica coating, laser-assisted etched into glass, and oxide coating), device encapsulation and surface passivation of organic ligands.^19–21 For instance, the cellulose nanocrystal (CNC) was used to encapsulate MAPbBr₃ by using a vacuum filtration growth method, and the relative photoluminescence (PL) intensity of the composite remained at around 80% after exposure to an environment of 60% relative humidity (RH) and 20 °C for 8 months.²² Polypropylene/metal halide perovskites composite was fabricated in one single step by co-extrusion of the perovskite precursors with polypropylene, which was highly stable in air and resistant to water.²³ Due to the strong interactions between the pyrophosphate shell and perovskite surface, core@shell CsPbX₃@NH₄AlP₂O₇ retained most of the PL emission even after 400 days in aqueous solution, with significant improvement of irradiation and thermal resistance.²⁴

Fabrication of high-quality patterns is crucial to integrate functional layers into photoelectric devices.²⁵ Up to now, a range of techniques have been applied for patterning, such as mask-based photolithography, nanoimprinting, and inkjet printing.²⁶ The UV-exposed processes in photolithography may lead to fast degradation of perovskites, which is extremely sensitive to UV light. For nanoimprinting, one could not avoid the complicated processes and direct contact with substrates. Inkjet printing is a highly efficient patterning technique that enables the precise position of very small volumes of ink onto a target substrate, eliminating the requirement for patterning masks.²⁷ This approach also offers significant advantages, including a non-contact process, scalability, direct writing fabrication of diverse patterns and flexible substrate capability.²⁷ Zeng et al. proposed a universal ternary-solvent-ink (naphthene, n-tridecane, and n-nonane) strategy to prepare a highly dispersive and stable CsPbBr₃ perovskite quantum dot (QD) ink, leading to high quality QD thin film and a record peak external quantum efficiency (EQE) of 8.54% in inkjet-printed green perovskite quantum dot light-emitting diodes (QLEDs).²⁸ With optimized printing parameters, CsPbBr₃ polycrystalline perovskite microcavity arrays with green laser emission were inkjet printed on a substrate with low surface energies.²⁹ Meanwhile, flexible patterned laser arrays on a polydimethylsiloxane (PDMS) substrate were also demonstrated. It is known that the compositions of the ink directly modify the morphology of the printed patterns. For instance, the addition of long-chain PVP to a CsPbBr₃ precursor solution increased the viscosity and reduced the outward capillary flow, contributing to in situ crystallized CsPbBr₃–PVP nanocomposite microarrays. Due to the presence of metallic bonds with the CO groups in PVP and the spatial confinement of such a polymer, printed patterns with uniform size distribution were achieved by the inkjet printing method.³⁰ Moreover, incorporation of an interfacial PVP layer decreased the surface tension of the underlying hole transport layer (HTL) to enhance the perovskite crystallization. Uniform and high quality perovskite patterns with efficient electroluminescence (EL) emission were effectively achieved, and the insulating PVP layer suppressed the detrimental leakage current in inkjet-printed PeLEDs.³¹

The demands for wearable and flexible optoelectrical devices are rapidly increasing, and the stability of the printed patterns is also critical for the long-term performance of the devices. To satisfy the requirement of flexible devices, Zhong et al. fabricated perovskite quantum diode (PQD) patterns by printing a precursor solution onto several polymeric layers (e.g., polymethyl methacrylate (PMMA), polyvinyl chloride (PVC)), demonstrating bright photoluminescence with a quantum yield up to 80%.³² MAPbBr₃ perovskite inks have also been directly inkjet-printed into the liquid PDMS precursor to in situ form the self-encapsulated perovskite single-crystal-embedded PDMS structure, which is transparent, flexible, and stable in the ambient environment.³³

The crystalline morphology of inkjet printed patterns is relatively sophisticated, which is greatly dependent on many factors, including the physical parameters of the ink solution (viscosity, surface tension, etc.), the evaporation, nucleation and crystallization of the liquid droplets, the nature of the substrate and the post-treatment process.^34,35 Thus, the modulation of crystalline morphology and the related mechanism deserve systematic investigation. Furthermore, the intrinsic instability and crystal friability impede the exploration of flexible and wearable devices. Herein, inspired by the combined effects of PVP long-chain polymers, including the spatial confinement, adjustment of the surface tension, and improvement of the perovskite crystallization, a series of CsPbBr₃ precursor inks have been designed and printed into a viscous PVP layer by the inkjet printing technology, forming PVP-encapsulated perovskite patterns. The crystalline morphology and related mechanism were comprehensively discussed. The photoluminescence is highly associated with the crystalline morphology, which is ascribed to the enhanced exciton characters of the perovskite nanocrystals. The printed perovskite patterns display intensive green emission under an excitation of 365 nm and 480 nm. More importantly, due to the gravity sedimentation effect of CsPbBr₃ crystals within the soft PVP layer, the stability of the fluorescent 2D perovskite patterns is significantly strengthened. The printed flexible patterns show superior ambient stability, and the relative fluorescence intensity was maintained around 82% of its original value at an ambient environment for 210 days of storage. Furthermore, this work provides an efficient strategy to prepare flexible and stable perovskite patterns, with great potential for road traffic indication, night vision indication, wearable LED devices, etc.

2. Experimental

2.1 Chemicals and Reagents

Cesium bromide (CsBr, 99.9%), lead(II) bromide (PbBr₂, 99%), phenylethylammonium bromide (PEABr, 99%), 18-crown-6 (≥99%), polyvinylpyrrolidone (PVP, average Mw ∼ 40000 g mol⁻¹), dimethylsulfoxide (DMSO, 99.9%), and isopropanol (IPA, ≥99.7%) were purchased from Shanghai Titan Scientific Co., Ltd. Polyethylene glycol terephthalate (PET) film was purchased from Suzhou Dongxuan Plastic Products Co., Ltd. All these materials were used without further purification.

2.2 Preparation of perovskite ink and PVP precursor solution

Stoichiometric amounts of CsBr and PbBr₂ were dissolved into DMSO, and the solution was fully stirred at 80 °C for 3 hours until the reaction was completed. The concentration of the CsPbBr₃ solution was optimized to be around 0.4 mol L⁻¹, which is preferable for the rational control of crystallization. Both faster or slower crystallization rates are undesirable for the uniformity of crystalline morphology. PEABr was chosen as the additive, and the molar ratio of PEABr and CsPbBr₃ was set as 10%, 20%, 30% and 40%. The preparation process was the same as that of the pure perovskite precursor solution, except that different amounts of PEABr were introduced. For the 18-crown-6 added solution, the molar ratio between CsBr, PbBr₂ and 18-crown-6 was maintained at 1:1:0.07. Prior to inkjet printing, all perovskite precursor solutions were filtered using a polytetrafluoroethylene filter with a 0.22 μm pore size to obtain the transparent perovskite solution. The PVP precursor solution was prepared by dissolving PVP into isopropanol and stirring vigorously for 6 hours.

2.3 Preparation of CsPbBr₃ patterns by inkjet printing

The PVP precursor was spin-coated on the cleaned PET film to form a viscous liquid substrate. Various CsPbBr₃ perovskite inks were printed on the liquid PVP layer by the inkjet printing method, with a high precision printer (BP4000) equipped with a 30 μm diameter nozzle (the liquid output is 22 pL). The related inkjet printing parameters are controlled as follows: (1) printing voltage: 9.9 V; (2) temperature value: room temperature; (3) preheating time: 360 ns; (4) printing height: 1 mm; (5) spray delay (time interval of each printing): 1000 μs. The pattern of the printed film was designed using a computer software control terminal and the distance between the luminous dots was set as 400 μm.

2.4 Characterization

The optical images of perovskite crystals and their fluorescence emission were obtained using a fluorescence microscope (MX6R). A high vacuum ion sputtering coater (Q150T ES PLUS) was used to spray gold on the samples for scanning electron microscopy. The morphology images of these printed perovskite patterns were characterized by field emission high resolution scanning electron microscopy (Tescan Mira 3 XH). XRD patterns were measured by D/Max-2550V diffractometer with Cu Kα radiation (X-ray wavelength of 1.54 Å) over 2θ range of 20–50°. The graphic fluorescence images were obtained using UV lamps and a digital camera, which was conducted at a dark box. The PL spectra of the fluorescent perovskite patterns were obtained using a laser scanning confocal microscope (Leica SP8), excited by 480 nm laser.

3. Results and Discussion

3.1 Crystalline morphology and related mechanism

In this work, varied compositions of perovskite precursor solutions were designed, in which the concentration of the pure CsPbBr₃ solution was set as 0.4 mol L⁻¹ (solution no. 1). Additive PEABr was added to the perovskite precursor solution, in which the molar ratios of PEABr and CsPbBr₃ was changed from 10%, 20%, 30%, to 40% (solution no. 2–no. 5), respectively. Solution no. 6 was formed by adding additional 18-crown-6 to solution no. 5, with the molar ratio of 18-crown-6 and CsPbBr₃ as 7%. PEABr is expected to hinder the crystallization of perovskite crystals, reduce the crystal size and improve the crystal uniformity.³⁶ However, PEABr is prone to self-crystallization and aggregation, with further increasing concentration. With the assistance of 18-crown-6, the aggregation of PEABr can be effectively inhibited and the growth of perovskite crystals is also suppressed.³⁶

Fig. 1 shows a schematic illustration of the experimental procedures, including the spin coating of a PVP layer on the PET film, inkjet printing of CsPbBr₃ droplets on the PVP substrate and annealing process. With a chemical formula of (C₆H₉NO)_n, PVP is a synthetic polymer consisting of linear 1-vinyl-2-pyrrolidone groups. It possesses distinctive physical and chemical properties, including chemical stability, biocompatibility, plastic deformability, non-toxicity, good solubility in water and many organic solvents, and affinity to both hydrophobic and hydrophilic substances. First, the PET film was ultrasonically cleaned by deionized water, acetone, isopropanol, ethanol, and deionized water. Thereafter, the PVP layer was spin-coated on the PET film, with a concentration of PVP solution of 250 mg mL⁻¹. Second, a series of CsPbBr₃ perovskite solutions (no. 1 to no. 6) were printed on the liquid PVP substrate by the inkjet printer with a 30 μm diameter nozzle. Under the same conditions, the printing times of these solutions were set as 10, 50, 100, and 200. After printing, all the samples (no. 1 to no. 6) were annealed at 50 °C in air for 2 minutes.

Fig. 2 shows the OM images of the CsPbBr₃ samples with varied printing times of (a) 10, (b) 50, (c) 100, (d) 200 and increased annealing time (20 s, 40 s, 60 s, 80 s) at 50 °C. Under the same conditions, five droplets were printed and annealed for 0 s, 20 s, 40 s, 60 s, and 80 s, respectively. The image on the top is the liquid droplet just printed on the PVP substrate. The evolutions of the crystalline morphology with annealing time (20 s, 40 s, 60 s, 80 s) are displayed at the bottom of Fig. 2(a–d). For the samples printed for 10 times (Fig. 2(a)), due to the low volume of the solution, the crystallization was mainly distributed at the edge of the droplet, with increasing annealing time. With printing times of 50 (Fig. 2(b)), the droplets were quickly crystallized within 40 s and a large area of fine CsPbBr₃ crystals was observed in the middle region. The crystallization patterns are homogeneous and almost unchanged with further increase of annealing time from 60 s to 80 s. For the samples printed for 100 times (Fig. 2(c)), during 20–60 s, due to faster evaporation rate of the solution at the edge of the droplet, the CsPbBr₃ crystals were firstly observed in this region, while the solution in the center region was not fully crystallized in time. The droplet was completely crystallized at 80 s, and the crystallization area was larger than that of printing 50 times. With printing times of 200 (Fig. 2(d)), at the beginning (20 s), some large crystalline particles are randomly distributed within the droplet, which is ascribed to the large volume of the solution and partial aggregation of the solutes. When the annealing time was increased to 40 s, more and more large crystals were formed at the edge of the droplet. With the annealing time from 60 s to 80 s, the residual solutions were gradually crystallized for the whole region, forming many fine crystals.

As displayed in Fig. 2 and 3, the coffee ring effect was observed for the printed patterns, which commonly appeared in the drying of a droplet containing nonvolatile solutes.^37,38 Here, the deposited CsPbBr₃ solutes were mainly located along the periphery of the droplet, leaving a ring-like pattern. Generally, the diffuse space at the edge of the droplet is larger than the central region, giving rise to a higher evaporation rate of the solution at the periphery.³⁹ Correspondingly, the solution at the center region is flowing outward to replenish the evaporation loss at the edge of the droplet, which is driven by the capillary force in the precursor droplets. Eventually, the nonvolatile solutes were carried to the edge, forming a thicker layer during the drying of the droplets.³¹

Typically, the formation of the coffee ring involves a non-volatile solute, the outward capillary flow and contact line pinning.³⁸ Actually, it is the pinning of the contact line of the drying droplet that contributes to the outward flow of the solution from the center part to the edge region. As seen in Fig. 2, with the same printing times (10, 50, 100, 200), the diameter of the droplets remains almost constant with increasing annealing time from 20 s to 80 s, which is also originated from the pinning of the contact line. It is known that the pinning of the contact line is closely related to contact angle hysteresis. The phenomenon of contact angle hysteresis can be explained by the Young's equation, which is expressed as .⁴⁰ It describes the relationship between the free energy γ_SG (solid–gas), γ_SL (solid–liquid), γ_LG (liquid–gas) and the contact angle (θ). According to this equation, the equilibrium contact angle is determined by the interface energy between the solid–vapor, solid–liquid and liquid–vapor interfaces. When θ reaches θ_e in the equilibrium state, the three-phase contact line will be fixed. Many researchers have found that the three phase contact line can be fixed not only at θ = θ_e, but also in a limited range of θ_rec < θ_e < θ_adv. θ_rec is the recessive contact angle, and θ_adv is the advancing contact angle. The difference between θ_adv < θ_rec is called contact angle hysteresis.⁴¹ The contact angle hysteresis is largely dependent on the complex interactions between the substrate and droplets, including the surface wettability, the roughness of the substrate, and the nature of the droplets (the viscosity, the surface tension, etc.).³⁸ For flexible and soft substrates, an additional vertical component of the surface tension possibly leads to the pinning of the contact line and the contact angle hysteresis.⁴² Here, as a typical soft and high-viscosity polymeric substrate, PVP provides uneven structures and resistance to the receding of the contact line. Therefore, a coffee ring is formed for CsPbBr₃ perovskite droplets sedimentated on the soft PVP layer.

Fig. 3 presents the OM images of CsPbBr₃ samples from solutions no. 1 to no. 6 with printing times of 10, 50, 100, and 200 and the crystallization is completed for all the samples. For the pure CsPbBr₃ (solution no. 1) sample printed for 10 times (i of Fig. 3(a) and (b)), due to the small volume of the solution, the crystallization only occurred at the edge of the droplet, and no luminescence was observed from the fluorescence image. The droplet printed for 50 times (ii of Fig. 3(a) and (b)) exhibited uniform crystallization, except for one large crystal. Accordingly, strong green fluorescence was observed for the entire crystalline region, while the large crystal showed no emission. When the droplets were printed for 100 times (iii of Fig. 3(a) and (b)), compared with the poor crystallization at the central region, more uniform and fine crystals were presented at the edge region. As shown in the fluorescence image, the luminescence intensity of the edge region was largely higher than that of the center region. Furthermore, for the droplet printed for 200 times (iv of Fig. 3(a) and (b)), the diameter of the printed pattern was enlarged, due to increased volume of the solution. The uniform crystalline particles at the edge region demonstrated a much stronger luminescence than those interconnected crystals at the central region. Similar behavior was also observed for samples no. 2 to no. 6, as seen in Fig. 3(c)–(l). These results directly prove that the luminescence of perovskite crystals is strongly correlated with the crystalline quality and particle morphology. It also possibly indicates that the defect density of the crystals at the edge differs from that at the center of the droplet, which is intimately associated with the luminescence performance. Moreover, with increasing printing times, the volume of the solutions is augmented, resulting in a larger diameter of the droplets. Meanwhile, due to the varied temperatures and evaporation rates between the edge and the center part, the solution at the edge firstly begins to crystallize to form homogeneous and dense particles. The crystallization of the solution at the center part is retarded because of the higher temperature and the encapsulation effect of PVP, contributing to the imperfect crystalline particles. Obviously, some large crystals are obtained for the printing times of 100 and 200, which is attributed to the aggregation of the solutes. Overall, for all these samples, the crystallization of the droplets printed for 50 times is much finer, denser and homogeneously dispersed, with better crystalline quality.

Fig. 4 depicts the fluorescence microscopy images, SEM images and particle size distribution diagrams of CsPbBr₃ perovskite samples no. 1 to no. 6 with printed times of 50. The SEM images verify that rectangular CsPbBr₃ perovskite crystals are well dispersed for all of these samples. The size and distribution of crystallization of these printed samples are uniform, while several agglomerated grains are also observed. With the introduction of PEABr, the average grain sizes for sample (no. 1) are largely reduced from 90 nm to 59.5 nm for sample no. 2. This behavior possibly implies that PEABr is effective to suppress the growth of the perovskite crystals, which is ascribed to the strong interaction of hydrogen atoms of PEABr and PbBr₆ halide atoms in CsPbBr₃.³⁶ Nevertheless, with further increase of PEABr concentration to 40 mol% in CsPbBr₃, the average crystal sizes are enlarged to approximately 82.6 nm (no. 3), 97 nm (no. 4), and 99 nm (no. 5), respectively. The aggregation and self-crystallization of PEABr may account for the increased crystallite size of samples no. 3 to no. 5. Moreover, the average grain size of sample no. 6 is around 93 nm, which is slightly smaller than that of sample no. 5. Thus, 18-crown-6 is proven to inhibit the aggregation of PEABr and restrain the growth of CsPbBr₃ crystals, due to the its interaction with Cs⁺ and Pb²⁺.³⁶

3.2 Luminescence of CsPbBr₃ nanocrystal films

Fig. 4 also presents the fluorescence microscopy images of CsPbBr₃ samples with printing times of 50 for solution no. 1 to no. 6, respectively. All the samples exhibit intensive and uniform green emissions. For printed samples no. 1 to no. 3, the fluorescence intensity of sample no. 2 is the strongest, while the fluorescence intensities of sample no. 1 and sample no. 3 are almost equivalent. Furthermore, compared to samples no. 1 to no. 3, relatively weaker fluorescence is observed for samples no. 4 to no. 6. Meanwhile, comparable fluorescence is demonstrated for samples no. 4 to no. 6. Along with the variation of crystalline sizes in Fig. 4, the change of fluorescence intensities directly elucidates that the luminescence of CsPbBr₃ perovskite crystals is strongly associated with the crystalline size. More specifically, crystals with reduced sizes are characterized with much enhanced luminescence, as evidenced in sample no. 2. Perovskite crystals with smaller sizes commonly display higher fluorescence quantum yields.^30,43 Particularly, the large bulk CsPbBr₃ generally show a low photoluminescence quantum yield (PLQY), while CsPbBr₃ NCs display strong emission with PLQY of up to 90% due to the quantum confinement effect.⁴⁴ It is noticed that the average crystalline size of our samples is greatly larger than the Bohr diameter (∼7 nm) of CsPbBr₃, implying that the quantum confinement effect is not the main mechanism for enhanced luminescence.

CsPbBr₃ is a direct band gap semiconductor, and its band-edge emission is primarily originated from excitons. Upon light absorption, electrons are transferred from the valence band to the conduction band, leaving holes at the valence band. Excitons are formed due to the electrostatic Coulomb interactions between electrons and holes.⁴⁴ These excitons decay radiatively to produce PL emission of CsPbBr₃ crystals. The strength of Coulomb interactions is denoted as the exciton binding energy (E_b), which determines the formation and recombination rate of excitons after photoexcitation.^36,43 The results in this work suggest that the reduced crystal size favors the formation of excitons and the excitonic recombination. Similarly, compared with polycrystalline samples (∼1 μm), the excitonic feature is enhanced for micro-crystals (50 ± 25 nm) and nanocrystals (9.1 ± 1.6 nm) with the size reduction.⁴³ It is mainly attributed to the increased oscillator strength of the radiative recombination, due to the dominant excitonic character of CsPbBr₃. Furthermore, excitions with a larger exciton binding energy is also expected to suppress the non-radiative recombination process. The exciton dissociation probability is negligible due to the higher binding energy, and consequently the emission probability is significantly increased.⁴⁵ In the absence of the quantum confinement effect within the crystal, introduction of organic additives may also induce changes in the dielectric environment, enhancing the excitonic characters. Here, PEABr and perovskite possess distinct dielectric constants, leading to the formation of effective dielectric confinement. This results in more pronounced excitonic characteristics and higher exciton binding energy after light excitation, facilitating enhanced formation of excitons and higher luminescence intensity.^36,43

Except for the influence of the crystal sizes on the luminescence of CsPbBr₃ crystals, as displayed in Fig. 3 and 4, the luminescence of the crystals is also significantly correlated with the crystalline morphologies. Fig. 5(a) depicts the SEM image of the regular fine crystals located in the central region of sample no. 2 printed for 50 times (ii of Fig. 3(c)). These rectangle crystals with a dimension of about 60 nm present intensive green emissions. In comparison, many irregular and dendritic grains, with sizes around 5 μm, are observed in Fig. 5(b). These crystals are located in the central region of sample no. 2 printed for 100 times (iii of Fig. 3(c)), and small luminous grains also grow along with these irregular larger crystals. Similar behavior was also observed for sample no. 2 printed for 200 times (iv of Fig. 3(c)). Furthermore, with the further increase of printing times and solution volume, several long and large particles were crystallized for sample no. 2 printed for 200 times, as seen in Fig. 5(c). The corresponding XRD patterns of regular luminescent crystals printed for 50 times and irregular non-luminescent crystals printed for 100 times are illustrated in Fig. 5(d). All of these diffraction peaks are in good accordance with the standard pattern of CsPbBr₃ (JCPDS No. 54-0751), suggesting that these printed patterns are the pure CsPbBr₃ phase. And luminescent crystals display the prominent (002) crystalline planes with single crystal characteristics.

Along with the discussion in Fig. 4, the excitons are predominant for the luminescence of CsPbBr₃ crystals. The excitonic character is effectively strengthened with decreasing crystal size.⁴³ Besides the size effect, the luminescence of CsPbBr₃ crystals is also highly influenced by the crystalline morphologies. Obviously, these rectangular crystals in Fig. 5(a) indicate controlled crystal growth, facilitating excellent crystalline qualities. These irregular and large crystals in Fig. 5(b) and (c) are usually featured with imperfect crystallites, which is possibly attributed to the aggregation of the solutes with increasing printing times. Additionally, the size and spatial distribution of these irregular crystals are random, resulting in the formation of crystals with poorly controlled microstructures. With uncontrolled crystal growth, these large and dendritic crystals are possibly characterized with higher defect densities, which serve as trapping sites for the excitons, enhancing the non-radiative recombination processes and degrading the luminescence efficiencies.⁴⁶ Moreover, due to the low formation energy, perovskite crystals are prone to various defects, including lead or cesium vacancies, interstitials, and antisites, which significantly promote charge trapping capability and affect the luminous characteristics.^46,47

The crystal size and morphology exert significant influence on the PL intensity of CsPbBr₃. In this work, six groups of CsPbBr₃ luminescent patterns have been designed and inkjet printed on the viscous PVP substrate, with printing times of 10, 50, 100, and 200. Overall, fine, regular and homogeneously dispersed nanocrystals are readily formed for the droplets printed for 50 times, exhibiting much enhanced luminescence performance. With increasing printing times to 100 and 200, continuous crystallizations are more likely to occur on the nuclei sites, giving rise to irregular and larger crystals with deteriorated luminous efficiency. Moreover, for the six samples printed for 50 times (Fig. 4), appropriate amounts of PEABr and 18-crown-6 are partly helpful to inhibit the crystal growth, in which sample no. 2 (10 mol% PEABr) is characterized with the smallest crystal size and most pronounced luminescence intensity. Hence, precise control over the crystallization process is imperative to achieve controlled crystal growth, thereby improving crystal quality, uniformity, and luminous efficiency.

3.3 Gravity sedimentation effect of CsPbBr₃ embedded in soft PVP matrix

The printed CsPbBr₃ patterns were subjected to the cryogenic treatment in liquid nitrogen to induce brittle fracture and obtain the cross-sectional samples. Fig. 6(a) to (c) present the cross-sectional SEM images of sample no. 2 with printing times of 50. In Fig. 6(a), the upper thin layer is PVP with a thickness around 8 μm. The thick layer at the bottom represents the rigid glass substrate with a thickness of about 1 mm, which is used for the cryogenic treatment. To ensure optimal control of the PVP substrate, it is crucial to maintain an appropriate thickness for the PVP layer. Excessive thickness is not conducive to the transparency and flexibility of the printed patterns. If the PVP layer is too thin, it is difficult to realize the complete encapsulation of CsPbBr₃ nanocrystals within the PVP matrix. In our experiment, the thickness of the PVP layer is optimized to be around 8 μm. During printing, the inkjet printer sprayed perovskite precursor droplets repetitively (50 times) at the same location with a rate of 1 time/ms, resulting in gradual aggregation of solutions into larger droplets. Fig. 6(b) provides the enlarged view of the PVP layer shown in Fig. 6(a), revealing the presence of CsPbBr₃ crystals exposed in the cross section and showing that the sedimentation depth of CsPbBr₃ crystal grains in PVP layer is about 5 μm. Combining with the SEM image in Fig. 6(a), it demonstrates that all fine CsPbBr₃ grains are completely encapsulated by the PVP layer. Furthermore, Fig. 6(c) presents the partial enlargement of Fig. 6(b), in which a large number of well-defined rectangular grains are fully embedded within the soft PVP layer. These fine grains are surrounded by some larger crystals which are exposed to the cross-sectional surface. Obviously, these larger crystals are possibly ascribed to the accumulation of the solutes.

Although patterned preparation of perovskite crystals within the polymer layer has been reported for improved stability in ambient environments, the related crystallization behavior and embedding mechanism are still unclear and deserve comprehensive illustrations.^30,32,33 This study elucidates the primary mechanism underlying the encapsulation of CsPbBr₃ crystals into the soft PVP layer. Here, the gravity sedimentation effect is supposed as the main cause of the encapsulation of CsPbBr₃ crystals embedded in the PVP matrix, which refers to the downward sedimentation of crystalline particles driven by natural gravity, and also involves the conformation changes of PVP during the drying process and proper interactions between the crystalline grains and the PVP substrate.

When the CsPbBr₃ solution was printed on the flexible PVP layer, CsPbBr₃ solution was dissolved in the viscous PVP layer. During the drying process, with the rapid evaporation of the solvents, CsPbBr₃ nanocrystals were gradually grown and moved slowly downward due to the gravity sedimentation effect of CsPbBr₃ in the soft PVP matrix. During this process, the evaporation of the solvent also induced the shrinkage of PVP, in which in situ growth of these CsPbBr₃ nanocrystals was effectively regulated and encapsulated within the PVP layer. Generally, the gravity sedimentation effect is promoted by larger crystalline particles, while it is restrained by the curing and shrinkage of PVP chains. More specifically, at the beginning of the solvent evaporation, the concentration of PVP solution was relatively low, with elongated PVP polymer chains.^48,49 At this stage, the gravity sedimentation effect was pronounced, facilitating the downward movement of CsPbBr₃ crystals. In Fig. 6(c), the accumulation of the solutes may occur, leading to the growth of some relatively larger crystals in the central and lower regions of the PVP layer. With further evaporation of the solvents and increase of the concentration of PVP, the molecular chains of PVP began to overlap and entangle with each other, inducing enhanced shrinkage of PVP. Accordingly, the gravity sedimentation effect was gradually restrained. The crystallization of CsPbBr₃ was continuously confined at limited sites, which was called the space confinement effect. Consequently, tightly embedded CsPbBr₃ nanocrystals were distributed within the PVP layer. Ultimately, this process yielded a stable composite film based on CsPbBr₃–PVP patterns.

For the gravity sedimentation effect of CsPbBr₃ nanocrystals in the PVP matrix, the terminal settling velocity of particles for viscous-flow conditions is expressed by Stokes' law:⁵⁰

	(1)

This equation illustrates that the settling rate of particles under viscous-flow conditions is proportional to the square of the particle size (d) and to the difference between effective particle density (ρ_p) and suspension medium density (ρ_l), and inversely proportional to liquid viscosity (η). And g is the gravity acceleration constant. Here, PVP with an average molecular weight of 40000 was dissolved in IPA to prepare a solution with a concentration of 250 mg mL⁻¹. Based on the correlation between viscosity and the average molecular weight of PVP, the viscosity (η) was roughly estimated to be 4.63 mPa s. With these parameters for eqn (1), including particle density of CsPbBr₃ (ρ_p = 4.84 g cm⁻³), suspension medium density (ρ_l = 1.14 g cm⁻³), gravity acceleration (g ≈ 9.8 m s⁻²), and suspension medium viscosity (η = 1430 mPa s), the settling velocity of CsPbBr₃ particles (d ≈ 60 nm) in the PVP substrate is calculated to be around 0.002 μm s⁻¹. In this case, it takes about 40 minutes to obtain a sedimentation depth of 5 μm for CsPbBr₃ nanocrystals, without any annealing process.

In fact, the gravity sedimentation effect involves many factors. For instance, the printed CsPbBr₃ patterns were annealed at 50 °C for 2 minutes in air to enhance the crystallization speed. During the annealing process, the printed patterns were rapidly crystallized to obtain nanocrystals, which may facilitate the gravity sedimentation of these crystals. Thereafter, the viscosity of the PVP substrate was continuously increased due to the evaporation of solvents. In this case, the gravity sedimentation of CsPbBr₃ nanocrystals was gradually retarded until the balanced state was achieved, obtaining transparent and flexible CsPbBr₃ nanocrystal patterns.

3.4 Stability of flexible CsPbBr₃ patterns

The inkjet printing technique is proficient to prepare various complicated patterns. A series of flexible CsPbBr₃ luminescence films have been directly inkjet-printed on the soft and transparent PVP substrate, and the stability was also systematically evaluated. Fig. 7(a) illustrates the patterned arrow and its luminescence images under UV light and fluorescence microscopy. The arrow is 3 mm in length and exhibits prominent and uniform green emission on a macroscale when excited at 365 nm and 480 nm. The flexible feature of the printed patterns is clearly demonstrated. Furthermore, the arrow pattern was stored under ambient conditions (25 °C, 60–70% relative humidity) for 210 days. As shown in Fig. 7(b) (iii), CsPbBr₃ nanocrystal patterns exhibit excellent ambient stability, in which around 82% of the PL intensity is remained after storing under ambient conditions for 210 days. The fluorescence intensity of the in situ prepared perovskite array encapsulated with PDMS showed no apparent decay after 60 days of storage under ambient conditions (25 °C, 50–60% relative humidity).³³ The quasi-two-dimensional perovskite–polymer composite structure, fabricated via inkjet printing, exhibited a fluorescence intensity that remained at 80% of its original value after being stored in ambient air for 50 days.⁵¹ Compared to the aforementioned studies, the stability of the luminescent pattern in this work is still considerable. These findings notably confirm that the printed patterns are characterized with excellent stability while retaining the flexibility. As discussed above, significant improvement of the stability of the CsPbBr₃–PVP composite film is realized by the combination of the gravity sedimentation effect and space confinement effect of the CsPbBr₃ crystals in the soft PVP matrix. Additionally, fluorescent 2D patterns of macro letters “SIT”, heart, and star are also precisely designed and printed (as depicted in Fig. 7(c)), demonstrating the compatibility of inkjet printing technology with flexible materials and intricate designs. In Fig. 7(d), the flexible printed patterns were subjected to bending tests for over 50 times. No cracks or degradation of the luminescence were observed, displaying excellent mechanical stabilities of the luminescent patterns under continuous bending conditions. All of these patterns were transparent, flexible, and highly stable in the ambient environment. The investigation of functional patterns with high resolution and tunable color is still going on to fulfill the great potential for wearable light-emitting devices and flexible panel displays.

4. Conclusions

To summarize, CsPbBr₃ nanocrystals embedded in a transparent film are prepared by inkjet printing CsPbBr₃ solutions into the liquid PVP precursor substrate. The evolution of the crystalline morphologies and the related mechanism are systematically discussed, regarding varied growth parameters such as printing times, the addition of crystallization inhibitors (PEABr and 18-crown-6) and the annealing process. Well-defined rectangular nanocrystals are effectively induced with precisely controlled crystal growth. Moreover, rectangular crystals with reduced sizes are characterized with much enhanced luminescence, which is ascribed to the strengthened excitonic characters. When excited at 365 nm and 480 nm, pronounced green emissions are observed for these flexible patterns. Due to the gravity sedimentation effect and space confinement effect, CsPbBr₃ nanocrystals are effectively embedded within the transparent and soft PVP film with superior stability, in which over 80% of fluorescence intensity is maintained for 210 days of storage under ambient conditions. Importantly, this study combines inkjet printing with a polymer substrate for in situ preparation of perovskite luminescent patterns, thereby improving uniformity and resolution of patterns with enhanced long-term stability. Moreover, the inkjet printing method is a facile approach to fabricate well-designed patterns due to its easy combination of direct writing, high throughput, low cast and self-assembly of nano/micro structures, making it highly competitive for generating luminous patterns. These results prove both the compatibility of flexible substrates and the effectiveness of complex patterns using inkjet printing. This work also gives in-depth understanding of flexible inkjet-printing CsPbBr₃ nanocrystal films in the aspects of droplet formation and crystallization dynamics, facilitating the great potential for wearable and flexible optical devices, including light-emitting devices and panel displays.

Data availability

All data generated or analyzed during this study are included in this published article.

Conflicts of interest

There is no conflict of interest in this manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51972213) and the NSAF of China (No. U2130124).

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