High stability modified CsPb(ClBr)₃@glass@PS for wide color gamut mini-LED backlight displays

Abstract

Perovskite quantum dots (QDs) were considered as a new generation of emitters for lighting and displays due to their high photoluminescence (PL) efficiency and pure color. However, their commercialization process was currently hampered by stability issues and challenges in mass production. Amorphous glass-protected CsPbX₃ perovskite nanocrystals (PNCs) have ultra-pure green light emission and excellent long-term stability. This work demonstrated that CsPb(Cl/Br)₃@glass PNCs were successfully deposited in germane–silicate glass, and CsPbBr₃@glass had bright green luminescence under ultraviolet irradiation, a narrow half-peak width (FWHM) and a high photoluminescence quantum yield (PLQY, 91.5%). The color gamut of the prepared WLED almost covered 123% of the NTSC 1953 standard and 91.8% of the Rec 2020 standard. It was proposed that a one-step mixed pressure film forming method using perovskite glass materials and PS materials can successfully obtain a high-quality, high-luminescence light conversion film, which can accelerate the commercialization of PQDs in display and lighting industries.

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1. Introduction

Solid-state lighting has become prevalent in almost every aspect of daily life due to its low energy consumption, high efficiency, and long lifetime compared to traditional incandescent and fluorescent light sources. Optoelectronic devices based on luminescent materials mainly include electroluminescence and photoluminescence. Although a QD-LED was still in the research and development stage, its composite materials had entered the display and lighting market. When combined with a backlight inorganic LED, it could provide higher color purity.^1–4 The photoluminescence process used a single light source with high energy to excite the luminescent material, so that the luminescent material was stimulated to produce the corresponding optical band gap energy luminescence.⁵ In the field of displays, the wide color gamut of the devices showed great potential to improve the visual performance.^6–9

All-inorganic metal halide perovskite (CsPbX₃, X = Cl, Br, I) NCs have excellent optical properties in display applications.^10–14 In view of the intrinsic defects and environmental instability of CsPbX₃ NCs,^15–18 this work has proposed to use inorganic glass to encapsulate perovskites. As an alternative material, an inorganic glass matrix has good mechanical stability, thermal stability and chemical stability. In the past few decades, glass has developed from the main silicate to borate, phosphate and tellurite.¹⁹ At present, various quantum dots (QDs) have been successfully prepared in glass,^20–24 and QD glass has good chemical stability and thermal stability. Although the preparation of halide perovskites in borogermanate glass has been reported, its PLQY varies from 19% to 81%. At present, no material with stable and excellent luminous efficiency has been prepared.^25–27 Glass forming metal–organic frameworks (MOFs) can effectively passivate perovskite interface defects and improve the stability of perovskites.^28,29 Some researchers^30–32 formed a composite material by using a polymer and a porous material to co-wrap the perovskite. Due to the double-layer protection of the two materials, the stability of the perovskite material was greatly improved. Optical film integration is a kind of packaging method that combines an optical film, prepared using a composite of luminescent and polymer materials, with other optical films to form a single unit. Compared with simple LED devices, the light conversion film in liquid crystal display (LCD) devices avoided the direct contact between CsPbX₃ NCs and LED backlight by combining CsPbX₃ NCs with polymer materials. On the other hand, there was a certain distance between the light conversion film and the backlight source. Under working conditions, the light radiation and thermal radiation of CsPbX₃ NCs were much lower than those of LED devices, which ensured the stability of CsPbX₃ NCs to a certain extent.³³

Herein, the CsPb(Cl/Br)₃ NC was prepared by in situ crystallization in the Ge–Si–Li–Al–K glass matrix. The prepared CsPb(Cl/Br)₃@glass showed excellent long-term stability and its luminescence wavelength was adjusted between 472 and 520 nm. Ultrapure green light was generated at ∼520 nm, the color purity was as high as 91%, and the PLQY reached 91.5%. By combining with the InGaN blue chip, green CsPbBr₃@glass, and the red commercial CASN:Eu²⁺ phosphor, its wide color gamut reached 123% of the NTSC 1956 standard and 91.8% of the Rec.2020 standard. The composite material of perovskite glass and polymers could greatly improve the stability of light conversion.

2. Results and discussion

A series of Ge–Si–Li–Al–K glasses with a fixed perovskite content and different heat treatment temperatures were prepared by a melt-quenching method, and the preparation of CsPb(Cl/Br)₃@glass nanocomposites was realized. The preparation process of CsPb(Cl/Br)₃@glass was detailed in the experimental part. The DSC curve is used to examine the thermal behavior of CsPbBr₃@glass without heat-treatment for precise determination of glass transition (T). T_g of CsPbBr₃@glass is located at 424.2 °C (Fig. S1, ESI†). By comparing the heat treatment conditions of CsPbBr₃@glass powder materials, it could be intuitively seen that their optimal heat treatment temperature was 470 °C, which could be inferred that the glass matrix prepared in this work was the optimal nucleation temperature of perovskites under 470 °C conditions (Fig. 1d). Fig. 1a shows the fluorescence diagram of different Cl–Br ratios at a temperature of 470 °C. With the increase of the Br content, the fluorescence peak gradually redshifts, and the luminescence wavelength is adjusted between 472 and 520 nm. Distinctly, Fig. 1b shows the typical absorption spectra of CsPb(Cl/Br)₃@glass, displaying a corresponding red shift as the bromine content increases, respectively. The red shift of absorption peaks was mainly caused by lattice expansion and grain size increase (Fig. S2a, ESI†). The formation of PNCs in the glass matrix included nucleation and crystal growth, which is a representative of the diffusion-controlled process. The increase of heat treatment temperature contributed to the growth and increase of grain size, resulting in the decrease of band gap (E_g) (Fig. 1f shows the electronic band structure of the CsPbBr₃ perovskite) of nanocrystals due to the quantum confinement effect. The regularity of the bandgap value is observed in Fig. S2b (ESI†); with an increase of temperature, the exciton characteristics in the absorption spectrum become more prominent due to the quantum size effect, and the band gap decreases from 2.41 eV to 2.32 eV, indicating that the band gap could be adjusted by adjusting the heat treatment temperature.³⁴

Fig. 1 (a) Fluorescence spectra of CsPb(Cl/Br)₃ glass powder samples at 470 °C heat treatment temperature. (b) UV-Vis absorption spectra and (c) PL decay curves of CsPb(Cl/Br)₃ glass powder samples at 470 °C heat treatment temperature. (d) The PL peak and FWHM and (e) the PLQY and PL intensity of CsPbBr₃@glass samples with different heat treatment temperatures. (f) The electronic band structure diagram of the CsPbBr₃ perovskite.

Time resolved photoluminescence (TRPL) was conducted to analyze the kinetics of exciton recombination, and the TRPL curve for CsPb(Cl/Br)₃@glass fitted is shown in Fig. 1c. The decay of the fluorescence signals could be well-fitted via a tri-exponential function and the corresponding lifetimes of the three components are summarized in Table S1 (ESI†). The average lifetime was shorter for the lighter halides (Cl > Br), ranging from 366.98 ns for CsPbBr_3.00@glass to 99.27 ns for CsPbCl_1.50Br_1.50@glass, which was consistent with previous reports.³⁵ In addition, as shown in Fig. 1d, the photoluminescence quantum yield (PLQY) results of the CsPbBr₃@glass sample showed that when the heat treatment temperature reaches 470 °C, the PLQY was about 91.5%. As shown in Fig. 1e, the PL peak of the CsPbBr₃@Glass sample was redshifted from 510 nm to 520 nm with the increase of the heat treatment temperature, indicating that different heat treatment temperatures would lead to tunable luminescence, and the full width at half maximum (FWHM) value fluctuates around 23 nm.

Fig. S3 (ESI†) reveals the crystallization mechanism of CsPbBr₃ PNCs in germane–silicate glass. In the high temperature molten state, the glassy mixture was treated by water quenching, and the internal glass network structure was in a compact state. With the increase of heat treatment temperature, the network structure inside the glass began to become loose. After the network was opened, ions began to migrate and diffuse, and Cs, Pb and X ions collided. As the heat treatment temperature decreased, the [PbX₆]⁴⁻ octahedrons were interconnected in three dimensions, and the Cs ions were in the gap between the octahedrons; finally, the luminescent CsPbX₃ PNC was formed. Transmission electron microscopy (TEM) images and particle size distribution plots confirmed that CsPbBr₃ PNCs with a size of several nanometers could be well identified inside the glass matrix (Fig. 2a and d). High-resolution TEM (HRTEM) showed that CsPbBr₃ PNCs had high crystallinity, and the particles had obvious interplanar spacings of 0.278 nm and 0.262 nm, which correspond to the (200) crystal plane and the (210) crystal plane, respectively (Fig. 2b and c and e and f). The PNCs in the amorphous glass matrix exhibited non-uniform nucleation, leading to the uneven grain size. Energy dispersive X-ray (EDX) data confirmed the existence of all glass and perovskite elements in the prepared samples (Fig. 2g and Fig. S4, ESI†), in which Cs, Pb and Br elements were evenly distributed in the germane–silicate glass matrix.

Fig. 2 (a) and (d) TEM images of CsPbBr₃@glass, and particle size distribution plots of CsPbBr₃@glass. (b and c and e and f) HRTEM images of CsPbBr₃@glass. (g) EDX elemental mappings of CsPbBr₃@glass. The CsPbBr₃@glass sample was heat treated at 470 °C. (h) FTIR spectra and (i) XRD patterns of CsPb(Cl/Br)₃@glass at 470 °C heat treatment temperature.

Fourier transform infrared spectroscopy (FTIR) revealed the vibration of B–O, Si–O, Ge–O and other structures in the glass network, as shown in Fig. 2h. In the samples with different Cl–Br ratios (or CsPbBr₃@glass with different heat treatment temperatures in Fig. S5, ESI†), we could observe that about 450 cm⁻¹ was the bending vibration of Al–O–Si, and Al polyhedrons with various coordination numbers can be used as network forming agents or modifiers in glass structures.^36–38 The absorption peak at about 1020 cm⁻¹was related to the vibration of the Ge–O–Si bond,³⁶ the peak at 1000–1100 cm⁻¹ was the bending vibration of the Si–O–Si bond in the [SiO₄] tetrahedral unit³⁹ and the tensile vibration of the [BO₄] unit,⁴⁰ and the peak at 1300–1550 cm⁻¹ was the B–O tensile vibration of the [BO₃] triangle.⁴⁰ There was an absorption peak at a high frequency of about 3550 cm⁻¹, which was due to the vibration of OH⁻and Ge bonding, which was a characteristic of GeO₂ glass.⁴¹ In addition, the abundant alkaline earth metal oxides in the glass composition caused the absorption peak to shift to a lower frequency. As shown in Fig. 2i, the typical cubic CsPbCl₃/CsPbBr₃ crystal diffraction peaks were superimposed on the amorphous halo in the sample after heat treatment, and the samples heat-treated with different temperatures had obvious perovskite characteristic peaks (Fig. S6, ESI†). The additional XPS spectra (Fig. S7, ESI†) proved the existence of perovskites in the glass matrix.

In further experiments, the thermal stability of CsPbBr₃@glass was systematically studied. As shown in Fig. 3a and e, when the temperature gradually increased from 20 °C to 200 °C, the PL emission intensity of CsPbBr₃@glass decreased. As shown in the mechanism diagram of Fig. 3i, when the temperature continues to increase, the network structure inside the germane–silicate glass gradually became loose. The surface defects of perovskite nanocrystals were thermally activated due to the loss of dense glass network coating, which led to permanent damage or destruction of the nanocrystal structure, so-called fluorescence thermal quenching phenomenon.³⁸ Then, we conducted heating and cooling cycle experiments. The luminescence mechanism is shown in the second half of Fig. 3i. When the temperature decreased, Cs, Pb and Br ions spontaneously combine to form perovskite structure luminescent materials. The specific experimental procedure was to gradually increase the temperature of CsPbBr₃@glass to 120 °C, then to gradually cool to room temperature, and to measure the PL emission intensity at each temperature. The untreated sample recovered about 70% of the original PL emission intensity after cooling, as shown in Fig. 3b. In order to improve its thermal recovery stability, we would investigate the different pretreatment methods of the sample. Among them, the luminescence intensity of the sample was restored to about 80% of the original luminescence intensity after 2–3 times water washing–heat treatment (Fig. 3c and d). The washing mechanism diagram (Fig. 3j) showed that there were some PNCs on the surface of the glass powder that were not coated by the glass matrix. During the washing process, due to its poor water stability, the exposed PNCs on the surface decomposed in water, resulting in fluorescence quenching, and the remaining ones were coated by the glass matrix. Then, the subsequent heating treatment was carried out, and all the elements of the perovskite were in the glass matrix. During the cooling process, it could be recombined to form a perovskite, which could reduce the loss of its fluorescence intensity. Subsequently, we treated with different concentrations of K₂CO₃ solution. Among them, the PL intensity of the sample treated with 5% and 10% concentrations of K₂CO₃ solution recovered to about 82% of the original PL intensity (Fig. 3f–g). The mechanism of surface treatment is shown in Fig. 3k. After K₂CO₃ was added to the aqueous solution to form KOH alkaline solution, the surface of germane–silicate glass would be corroded, so that the exposed perovskite would lose the protection of the glass matrix and directly undergo fluorescence quenching. After heat treatment at 300 °C, K ions combined with the exposed silicate ions on the surface of the glass to form K₂SiO₄ coating on the surface of the original germane–silicate glass, and it was this additional coating that stabilized the PNCs. Fig. 3h compares the light recovery data of heating and cooling cycles under five pretreatment conditions, and it was obvious that the in situ thermal stability of the pretreated samples was better than that of the unpretreated sample. Chen et al.⁴² synthesized CsPbX₃ PNCs in fluorine-doped borosilicate glass. After 1440 h (60 days) blue light (40 W m⁻²) irradiation, the green glass maintained 85.6% of the initial PL. In order to more clearly compare the effects of different pretreatments on CsPbBr₃@glass stability, under the harsh conditions of a blue light of 3500 W m⁻², we investigated the photostability of five samples. It could be clearly seen from Fig. S8 (ESI†) that the PL intensity of all samples decreased gradually with the increase of irradiation time under this harsh condition. However, the pretreated samples were better than the untreated samples. Among them, the samples treated with 10% K₂CO₃ solution exhibited the best performance, and the PL intensity was nearly 80% of the original after 5 h. Above all, the results of the heating and cooling cycle processes and the blue-light stability both showed that the pretreatment of the glass surface could improve the stability of the PNCs glass to a certain extent. This indicated that the pretreated sample could also be more stable under mild environmental conditions.

Fig. 3 (a) and (e) The corresponding temperature-dependent PL emission spectra of CsPbBr₃@glass. (b) Heating–cooling cycle data of the unpretreated sample. (c) and (d) The heating–cooling cycle data of the samples after water washing–heat treatment twice and three times. (f) and (g) The heating–cooling cycle data of the samples after treating with 5% and 10% concentrations of K₂CO₃ solution. (h) Comparison of light recovery data of heating and cooling cycles under five pretreatment conditions (corresponding to b, c, d, f, and g, respectively). (i) The mechanism diagram of CsPbBr₃@glass heating and cooling cycle treatment. (j) The mechanism diagram of water washing treatment. (k) The mechanism diagram of K₂CO₃ solution treatment.

Based on the excellent optical properties of CsPbBr₃@glass, we studied the application of the composite in the field of white light emitting diodes (WLED). The WLED in this experiment was constructed using the commercial CASN:Eu²⁺ phosphor, CsPbBr₃@glass, and 460 nm blue InGaN chip (as shown in Fig. 4e). The PL spectra of blue chips, green CsPbBr₃@glass composites and the red CASN:Eu²⁺ phosphor are shown in Fig. 4a–c. The InGaN blue chip was identified at a working current of 20 mA, and the corresponding color coordinates were (0.1496, 0.0338), (0.1768, 0.7622), and (0.6960, 0.3034), respectively. As a comparison, the NTSC 1953 standard and the Rec 2020 standard are shown in Fig. 4d. Through the area calculation, the color gamut of the prepared WLED almost covered a NTSC 1953 standard of 123% and a Rec 2020 standard of 91.8%, which was attributed to the ultrapure color emission of CsPbBr₃@glass (Table S2, ESI†). In Fig. 4g, we compared the effects of different currents on the correlated color temperature (CCT) and color purity of CsPbBr₃@glass. When the current gradually increased, the CCT and color purity of CsPbBr₃@glass gradually decreased, but its color purity was always at a high level, which laid a foundation for subsequent backlight display. Subsequently, we studied the influence of different currents on WLEDs. Obviously, as shown in Fig. 4f, as the current increased, the color rendering index gradually increased. In Fig. 4h, the CCT of WLEDs gradually decreased, but its color purity gradually increased. This showed that the red CASN:Eu²⁺ phosphor was of great help to the overall color rendering of the device.

Fig. 4 (a)–(c) The PL spectra of blue chips, green CsPbBr₃@glass composites and the red CASN:Eu²⁺ phosphor, and insets included blue, green and red light, respectively. (d) Color gamut of the NSTC 1953 standard (black line), the Rec. 2020 standard (red line) and this work (yellow line) in the CIE diagram, and the inset was white light. (e) Structural schematic of WLEDs. (f) EL spectra of WLEDs with different IF. (g) and (h) The correlated color temperature (CCT) and color purity of CsPbBr₃@glass and WLEDs, respectively.

In this work, a method of one-step mixing pressure film formation using CsPb(Cl/Br)₃@glass materials and PS materials was proposed. Using this method, a large-mass, high-luminescence light conversion film could be successfully obtained. The mechanism diagram of the film prepared by powder mixing and high temperature extrusion and the sample photographs of the prepared CsPb(Cl/Br)@glass@PS film under ultraviolet light (365 nm) irradiation are shown in Fig. 5a. The excellent optical properties and stability of CsPbBr₃@glass provided a good foundation for the realization of the direct-down backlight unit. We prepared the CsPbBr₃@Glass@CASN:Eu²⁺@PS film (Fig. 5b) to match the mini-Blue LED screen to achieve white backlight. On this basis, we explored the stability of the CsPbBr₃@glass@PS film in different solvents. It could be seen from Fig. S9 (ESI†) that the CsPbBr₃@glass@PS film still maintained bright green luminescence after soaking in H₂O and 95% ethanol for 42 days, and its luminescence intensity decreased by less than 10%, which indicated that the double coating of the germane–silicate glass matrix and the PS film could improve the stability in polar solvents of CsPbBr₃. Fig. 5c shows the basic composition of the backlight board, Fig. 5d and e show the spectrum and picture of the green CsPbBr₃@Glass@PS film and the red CASN:Eu²⁺@PS film matched with the mini-Blue LED screen, respectively. Fig. 5f shows the photographs of mini-Blue LEDs and CsPbBr₃@Glass@CASN:Eu²⁺@PS and Fig. 5g shows the spectrum of the white CsPbBr₃@glass@CASN:Eu²⁺@PS film matched with the mini-Blue LED screen. We know that the matching of the monochromatic film had a deviation from the color display of the image, resulting in a poor visual effect on the human eye. Fig. S10 (a)–(d) (ESI†) show the spectral data and pictures of different red–green ratio films matching mini-blue LEDs. When the red film was kept unchanged, the concentration of the green film gradually increased, and the color coordinates were gradually shifted to the green direction (Table S3, ESI†). For example, if green visual perception is lower, the display could be adjusted to a higher green-light concentration; if red sensitivity is lower, the display can increase the red-light concentration, which has certain reference significance for future health displays. When combined into white light based on a wide color gamut (Fig. 5g), this feature significantly benefits human observation. This adjustable backlight display surface could be adapted to meet the visual needs of users in different regions. The backlight device continued to be used for 12 h and the 24 h spectral data were not significantly reduced as shown in Fig. S11 (ESI†), indicating that the stability of the film combined device was good and could potentially make it suitable for future public use.

Fig. 5 (a) A schematic diagram of the mechanism of the film prepared by mixing the powder and the photographs of the CsPb(Cl/Br)₃@glass@PS sample under ultraviolet (365 nm) irradiation. (b) A schematic diagram of the preparation procedure of the white CsPbBr₃@glass@CASN:Eu²⁺@PS film. (c) A schematic structure of the backlit LCD device. BEF represented the brightness enhancement film. (d) and (e) PL spectra of green and red images recorded on the LCD screen. (f) The photographs of mini-Blue LEDs and CsPbBr₃@glass@CASN:Eu²⁺@PS. (g) White PL radiation spectrum on the LCD screen.

3. Conclusions

In summary, we had prepared CsPbBr₃ PNCs in a germane–silicate glass matrix. The prepared CsPbBr₃@glass could produce bright green luminescence at ∼525 nm, with a color purity of more than 91% and a PLQY of more than 90%. Thanks to the tight glass network, CsPbBr₃ PNCs were effectively isolated from the external environment. In addition, the InGaN blue chip was combined with CsPbBr₃@glass and the CASN:Eu²⁺ phosphor to achieve a wide color gamut of 123% of the NTSC 1956 standard and 91.2% of the Rec.2020 standard. And the LCD display prepared using the CsPbBr₃@glass@PS film and CASN:Eu²⁺@PS film could meet the requirements of the color display. This work was conducive to the adjustment of the optical properties of future CsPbBr₃@glass composites and was suitable for the practical application of backlight displays. A one-step mixed pressure film forming method using perovskite glass and PS materials could obtain a large mass and high luminescent light conversion film, which could accelerate the commercialization process of CsPBr₃@glass in display and lighting industries.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (5207021296 and 1237040868) and the Doctoral Innovation Foundation of Wenzhou University (Grant No. 3162024001003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03904a

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