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Abstract
The large-scale and continuous production of CsPbBr3@PMMA composite film is realized by the in-situ ultrasonic spray coating method at room temperature. Through embedding CsPbBr3 nanocrystals into the hydrophobic polymer framework, the as-fabricated films (20 cm × 20 cm) exhibit uniform green emissions with a relatively high PLQYs of 76%, and could maintain 80% PL intensity after 3 months storage under ambient conditions. Assembling the green-emissive CsPbBr3@PMMA film and the red-emissive KSF@PMMA film with blue LED chip, a high-performance LCD is obtained, reaching a higher saturation with 126% and 94% color gamut of NTSC and Rec.2020, respectively. This work demonstrates that ultrasonic spray coating technique could be widely used in the large-scale fabrication of uniformly high-quality perovskite films for backlight application.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Among the current display, liquid crystal displays (LCDs) adopt blue light-emitting diodes (LEDs) combined with down-converters as the backlight [1–3]. In order to achieve a more realistic and ultrahigh definition presentation, the development of high-performance backlight with wide color gamut, high brightness and excellent color rendition is necessary. Perovskite nanocrystals (PeNCs) have attracted significant attention in the past few years due to their tunable bandgaps, narrow emission peaks and high photoluminescence (PL) quantum yields (PLQYs), as well as solution processability, low cost and large-scale fabrication capabilities [4–6]. These advantages make them a promising candidate for the next-generation quantum dots (QDs) based backlit display, which can achieve higher color gamut than traditional phosphor-based LCD [7]. However, owing to the strong ionic nature and low formation energy, PeNCs would be rapidly decomposed when they suffer from light, heat, oxygen and especially moisture [8,9]. Several encapsulation strategies have been proposed to address the instability of PeNCs, for instance, embedding PeNCs into the glass, silicon and polymer matrices [10–14]. Among these, polymers with compact molecular chains could effectively encapsulate PeNCs, passivate the surface defects and exhibit unique features when they exposed to water or moisture, giving the perovskite–polymer composites with superior environmental stability and enhanced PL performances [15–18].
The strategy of embedding PeNCs into polymers could be easily combined with many solution-processed deposition techniques to prepare large-area films for backlit display [19–21]. As early as 2016, Zhou et al. [22] first in-situ fabricated green-emissive MAPbBr3 (MA = CH3NH3+) embedded PVDF film with an area of 10 cm × 20 cm, and a wide color gamut of 121% national television system committee (NTSC) was exhibited by integrating the as-fabricated film and K2SiF6:Mn4+ (KSF) phosphor with InGaN chip. Li et al. [23] reported a supersaturated recrystallization method for in situ fabrication Cs4PbBr6/CsPbBr3@PMMA film with a higher fluorescence property and an enhanced stability, and the demonstrated LCD could display a higher saturation with 131% color gamut of NTSC. Adopting a modified hot-injection method, Tong et al. [24] prepared CsPbBr3@lauryl methacrylate (LMA) composite film by replacing the commonly used octadecene (ODE) with LMA, and the LCD panel assembled with the PeNCs−polymer composite film and KSF phosphor displays a color gamut of 115% NTSC. Despite many successful works have been reported to demonstrate the highly emissive PeNCs films with good stability, however, the major challenge associated with these techniques is the large-area film deposition with uniform characteristics and fewer defects. Spray coating technique, which shows advantages of deposition continuity, minimal loss of coating solutions and construct active devices on various substrates, has been widely used in the large-scale production of high-quality solar cells, but it was rarely explored for the preparation of perovskite luminescent films to date [25–28]. Typically, perovskite film with large grain size and less defects at grain boundaries is desirable for charge transport in solar cells. In contrast, nanocrystals with uniform size distribution and large exciton binding energy is critical for preparing highly luminescent films.
Here, the brightly luminescent CsPbBr3@PMMA composite film was prepared by using in-situ ultrasonic spray coating method. Two precursor solutions were simultaneously and continuously sprayed onto the substrate, yielding a large-area composite film, which exhibits uniform brightness and stable performance in ambient storage conditions. Moreover, the as-fabricated film with narrow emission peak and high PLQY was successfully applied in LCD backlight as a green-emissive film. The assembled LCD eventually produces a much wider color gamut and a more accurate color rendition.
2. Experimental section
2.1 Chemicals
Cesium carbonate (Cs2CO3, 99.9%), lead bromide (PbBr2, 99.99%), octanoic acid (OTAc, 99%), oleylamine (OAm, 80-90%) and tetraoctylammonium bromide (TOAB, 98%) were purchased from Macklin. Poly(methyl methacrylate) (PMMA, high flow grade) was purchased from Aladdin. Toluene (C7H8, 99.5%) was obtained from Damao Chemical Reagent. All the chemicals were used as received without further purification.
2.2 Synthesis of CsPbBr3@PMMA and KSF@PMMA composite films
Firstly, lead precursor was prepared by mixing PbBr2 (0.4 mmol), TOAB (0.8 mmol) and PMMA (1.0 g) in 10 ml of toluene with continuous stirring at room temperature for 2 h. Cesium precursor was prepared by dissolving Cs2CO3 (0.4 mmol), OTAc (1.5 ml) and OAm (0−1.5 mmol) in 10 ml of toluene for 30 mins. Then, the lead and cesium precursor solutions were delivered by two syringe pumps (SPlab01) with the solution flow rates of 40 and 5 ul/min, respectively, and sequentially sprayed on a glass or a polyethylene terephthalate (PET) substrate. Finally, CsPbBr3@PMMA composite film was obtained by evaporating excess toluene in air. The spray nozzle (UCA501, 120khz) was programmed to move in a line over the substrate at a speed of 250 mm/min with the nozzle-to-substrate distance of 80 mm (flow gas: air, pressure: 1 MPa). KSF@PMMA film was prepared by blade-coating 8 ml of toluene solution containing 1.0 g of commercial KSF red phosphor and 2.0 g of PMMA.
2.3 Fabrication of LCD backlight based on CsPbBr3@PMMA and KSF@PMMA composite films
The as-fabricated CsPbBr3@PMMA film (green-emissive layer) and KSF@PMMA film (red-emissive layer) were located above the blue-emissive light guide plate (460 nm) to replace the original backlight of a commercial 7-inch LCD. The LCD was connected with a drive board to realize picture presentation under operational voltage of 5 V.
2.4 Fabrication of white LED based on CsPbBr3@PMMA and KSF@PMMA materials
KSF@PMMA solution was dropped on the blue LED chip and dried for 2 h at room temperature. Then, CsPbBr3@PMMA composite was in situ spray-coated on the top of KSF layer following the same procedure described above.
2.5 Fabrication of green-emissive active matrix based on CsPbBr3@PMMA
The blue mini-LED active matrix devices were used as the substrates for spray coating CsPbBr3@PMMA with 8 cycles.
2.6 Characterization
Ultraviolet–visible (UV–vis) absorption spectra were collected by Hitachi U4100 UV-vis spectrophotometer. PL spectra were measured on a Hitachi F-7000 fluorescence spectrometer with the excitation wavelength of 405 nm, and PLQYs were recorded on a Hamamatsu Quantaurus-QY C11347-11 spectrometer. Time-resolved photoluminescence (TRPL) spectra were collected by Edinburgh Instruments FLS 980 spectrometer. Electroluminescence (EL) spectra were measured with an Ocean Optics USB 2000 spectrometer. The crystal structures of the samples were recorded on X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the samples were collected by scanning electron microscope (SEM, FEI Nova Nano SEM450) and the fluorescence microscope images were examined by OLYMPUS IX53-RFAC inverted microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were carried out with a JEOL-2100 Plus microscope operating at a voltage of 200 kV.
3. Results and discussion
In a typical process for fabricating the CsPbBr3@PMMA composite films, as shown in Fig. 1(a), through an ultrasonic spray coating of the PbBr2/TOAB/PMMA toluene solution (S1) at a flow rate of 40 ul/min in air, accompanying by spraying the Cs2CO3/OTAc/OAm toluene solution (S2) at a flow rate of 5 ul/min, Cs(OTAc) react quickly with (C8H17)4NPbBr3 in the spray nozzle [29,30], giving the formation of CsPbBr3 PeNCs. After the rapid crystallization, the spray solution can continuously deposit the well-dispersed PeNCs together with PMMA molecules onto the substrate, allowing a uniform wet film throughout the coating process. The subsequent drying leads to the formation of CsPbBr3@PMMA composite film over the large-area (max. area: 20 cm × 20 cm) substrate with a consistent microstructure.
Fig. 1. (a) Schematic illustration of the in-situ fabrication of CsPbBr3@PMMA composite films. (b)-(e) Photographs of the as-fabricated film on PET (10 cm × 16 cm) and glass substrate (20 cm × 20 cm) under the ambient light and 365 nm UV lamp irradiation, respectively. (f) Top-view SEM image and (g) water contact angle of the CsPbBr3@PMMA film. (h) Cross-sectional SEM image of the film and the corresponding EDX images of C, Br, Pb, Cs, respectively. Fluorescence microscopic images of the CsPbBr3@PMMA film under (i) ambient light and (j) 365 nm UV lamp irradiation, respectively.
As demonstrated in Figs. 1(b)−(e), all the samples exhibit bright green emissions under the 365 nm UV lamp irradiation, achieving a high PLQY of 76%. Besides, a cross-linking framework of the polymer is observed on the film surface (see top-view SEM image in Fig. 1(f)), which is caused by the PMMA polymerization during fast solvent evaporation. With the aid of PMMA deposition, the film surface became hydrophobic with a water contact angle of 85° (Fig. 1(g)). The cross-sectional SEM image and the corresponding elements distribution (Fig. 1(h)) indicate that CsPbBr3 NCs are uniformly packaged in the polymer film with a thickness of ∼10 um. Figures 1(i-j) show the fluorescence microscopic images of CsPbBr3@PMMA composite film in a micron scale. We can find that the samples are yellowish−green films under ambient light and show strong green-light emission under UV photoexcitation.
XRD patterns in Fig. 2(a) reveals that all the composite films fabricated by adding different amounts of OAm adopt orthorhombic structure of CsPbBr3 (PDF#18-0364). Besides, CsPbBr3 PeNCs in the composite film with a small addition of OAm ligand (0.8 mmol) exhibits an improved morphology with clear cubic edge than those without OAm, as demonstrated in Fig. 2(c) and Figure S1a. HRTEM and the corresponding fast Fourier transform (FFT) images further confirm the well-crystallized CsPbBr3 with orthorhombic phase. Further increasing addition amount of OAm to 1.5 mmol, several prominent XRD peaks appear along with the characteristic peaks of CsPbBr3, indicating the formation of Cs4PbBr6 (PDF#73-2478), which is matches well with the hexagonal PeNCs shown in Fig. 2(d). The phenomenon of adding amine ligand to trigger the transformation from three-dimensional (3D) CsPbBr3 to zero-dimensional (0D) Cs4PbBr6 perovskite structure has also been demonstrated in the synthetic process of recrystallization and hot injection [31–33]. The generation of Cs4PbBr6 PeNCs is mainly owing to that the strong binding of amines would leach of PbBr2 from original CsPbBr3 nuclei, bringing about a rapid phase transformation to Cs4PbBr6 (4CsPbBr3 → Cs4PbBr6 + 3PbBr2) [34]. Another reason is probably the steric hindrance effect occurring at high concentration of amine ligand so that the formation of [PbX6]4− octahedral 3D structural framework is hard to achieve [35]. The optical spectra and the corresponding photographs of these samples are shown in Fig. 2(b) and Figure S2, respectively. With the increased amount of OAm, the initial increase of PL intensity (PLQY ≈ 76%) indicates the suppressed non-radiative recombination due to the effective defect passivation with OAm, and a subsequent decrease of PL intensity (PLQY ≈ 39%) stems from the formation of non-emissive Cs4PbBr6 [36–38]. It is also found that the average crystal size gradually decreases from 16.6 to 8.4 nm (Figure S1b), and the observation of blue shift in the absorption and emission peaks gives a clear evidence of the quantum confinement effect. To sum up, an excess of OAm ligand not only triggers the phase transformation from CsPbBr3 to Cs4PbBr6, but also restricts the crystal growth of CsPbBr3.
Fig. 2. (a) XRD patterns, (b) UV-vis and PL spectra of films prepared by adding with 0, 0.8, 1.5 mmol of OAm ligand. TEM images of the PeNCs prepared by adding (c) 0.8 mmol and (d)1.5 mmol of OAm ligand, respectively, together with the corresponding HRTEM image and fast Fourier transform for the marked region. (e) XRD patterns, (f) UV-vis and PL spectra of films prepared with different solution flow rates. The flow rate ratios of S1 to S2 (from top to bottom) are 10:1, 8:1, 5:1, 3:1 and 1:1, respectively.
A similar result was observed in Fig. 2(e) and Fig. 2(f), which correspond to the films fabricated by adjusting the flow rate ratios of S1 to S2. With increasing the flow rate of S2 (i.e., decreasing the flow rate of S1), a larger amount of amine ligand mediates the crystallization process involving more Cs+ and less Pb2+, which could result in the lead-poor rhombohedral Cs4PbBr6 PeNC with a large size over 100 nm (Figure S1c), along with a very weak emission and low PLQY of the composite film (Figure S3). The strong absorption peak at 314 nm originates from the 6s1/2 − 6p1/2 transition of Pb2+ ions of the decoupled [PbX6]4− octahedrons in Cs4PbBr6 [33,34], and the weak PL at 512 nm is ascribed to a very small fraction of fluorescent CsPbBr3 PeNCs.
To illustrate the emission uniformity as well as PL stability of the large-area CsPbBr3@PMMA composite film, four parts of the sample (10 cm × 10 cm) were cut to be equivalent pieces, and tested together periodically (Fig. 3(a)). The four samples exhibit almost the same PL characteristics and slight difference of their thickness (Table S1), suggesting a uniform emission of the film. Meanwhile, these samples could retain 80% of their initial PL intensity after exposing in ambient condition with relative humidity of ∼ 60% for 90 days (Fig. 3(b)). The lifetime decay curve of CsPbBr3@PMMA film can be well fitted by a triexponential function with an average lifetime of 11.5 ns (Fig. 3(c)). As listed in Table S2, all the four samples exhibit the fitting parameters with a large account of short lifetime and a small fraction of medium and long lifetimes, implying an ideal intrinsic excitonic feature with fewer surface trap states [39–42]. The high ratio of radiative recombination to non-radiative recombination is agreement with the high PLQY achieved in the as-fabricated CsPbBr3@PMMA film, indicating a promising potential as color converter in LCD backlight.
Fig. 3. Variation of (a) PL intensity, (b) peak wavelength and FWHM for the different parts of CsPbBr3@PMMA film (10 cm × 10 cm) stored in ambient conditions for a period of 90 days. The left and right inserts in (a) are the photograph of the film marked with four parts and their corresponding PL spectra after 90 days. (c) TRPL decay of the film.
For concept demonstration, a 7-inch display device was prepared by using a commercial LCD panel backlighted by the dual emissive layers in combination with a blue-emissive light guide plate, as shown in Fig. 4(a). A white LED device was constructed by spray coating CsPbBr3 and KSF materials on a blue LED chip to collect EL spectrum (Fig. 4(b)). With a bias voltage of 3 V, the device produces a combined white-light luminescence with tricolor narrowband emissions of 460 (blue LED chip), 520 (CsPbBr3@PMMA) and 630 nm (KSF@PMMA). The color gamut estimated from EL spectrum, reaches 126% of NTSC and 94% of Rec. 2020, respectively, as illustrated in Fig. 4(c). In comparison with the normal LCD applied a traditional white backlight, the screen assembled with the demonstrated backlight exhibits more details of object colors and accurate color rendition (Fig. 4(d)). The minimum and the maximum luminance obtained from different parts of the demonstrated backlight are 118.3 and 146.1 cd/m2, respectively, reaching the luminance uniformity of 81%. The color uniformity is calculated to be 1.4 × 10−2, which is the largest difference of two color coordinates in the CIE [7]. In addition, the color conversions (Fig. 4(e)-(f)) are investigated by in-situ spraying CsPbBr3@PMMA on a blue mini-LED matrix with different cycles, and the corresponding EL spectra are shown in Figure S4. The complete green color conversion at 520 nm with a FWHM ∼22 nm was achieved after spray coating CsPbBr3@PMMA with 8 cycles. The high performance indicates a great potential of CsPbBr3@PMMA composite film as color converter for next-generation display technology.
Fig. 4. (a) Schematic diagram of the assembled LCD. (b) EL spectrum of the white LED based on CsPbBr3@PMMA and KSF@PMMA films. (c) Color gamut of the white LED (dashed line), NTSC 1953 (black line) and Rec.2020 standard (yellow line) in the CIE diagram. (d) Display performance of the LCD screen with normal backlight unit (left) and CsPbBr3@PMMA film backlight unit (right). (e) Display performance of the rigid mini-LED device before (top) and after (bottom) spraying with CsPbBr3@PMMA. (f) Display performance of the flexible mini-LED device.
4. Conclusions
In summary, we report a facile one-step ultrasonic spray coating strategy to prepare large-scale CsPbBr3@PMMA composite film for backlit display. Two precursor solutions were simultaneously and continuously sprayed onto the substrate, embedding the PeNCs into PMMA polymer matrix. It is found that a larger amount of amine ligand would mediate the crystallization process involving more Cs+ and less Pb2+, which results in non-emissive Cs4PbBr6 PeNC. Through adjusting the parameters, pure orthorhombic-phase CsPbBr3 with improved defect passivation was obtained in the composite film, which exhibits uniform PL properties with an average FWHM of ∼ 23 nm and a high PLQY of 76%, and retains 80% of its initial PL intensity after exposing in ambient condition for 90 days. The composite film further demonstrates a higher color conversion ability in backlight with a wide color gamut covering up 126% of NTSC and 94% of Rec.2020, respectively, suggesting the possibility of CsPbBr3@PMMA film to be practically applied in the next-generation displays. More importantly, the one-step spray coating at room temperature without inert gas protection enable it to demonstrate great potential in the wide color gamut and high definition display on a large scale.
Funding
Natural Science Foundation of Hebei Province (A2021201038).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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