Solution-grown millimeter-scale Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals with enhanced photoluminescence and stability for light-emitting applications

Abstract

Metal halide perovskites are particularly emerging for optoelectronic applications in light-emitting diodes, photodetectors, and solar cells due to their flourishing photophysical properties. However, the poor stability of three-dimensional (3D) lead halide perovskite nanocrystals (NCs) significantly hampers their optoelectronics and photovoltaics applications. Embedding 3D perovskites into zero-dimensional (0D) perovskite crystals and doping ions of appropriate elements into host lattices provide effective approaches to improve the stability and optical-electronic performance. In this study, millimeter-scale Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ perovskite crystals were successfully fabricated by a one-step slow cooling method. We systematically investigated the effects of Mn²⁺ ion doping on the PL performance and stability of CsPbBr₃/Cs₄PbBr₆ crystals. Compared with undoped crystals, the existence of Mn²⁺ ions not only blue-shifted the PL peak but also improved the luminescence performance and stability of the prepared millimeter-sized crystals. Moreover, doping Mn²⁺ ions can increase the proportion of radiative recombination at low temperature, which may be because Mn²⁺ ions can effectively accelerate the decay of a dark exciton by the magnetic mixing of bright and dark excitons. In addition, green LED devices with high efficiency packaged as-grown crystals are explored, which promises further application in display backlights.

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1 Introduction

All-inorganic lead halide perovskites, such as three-dimensional (3D) CsPbX₃ nanocrystals (NCs), have emerged as new promising semiconductor photoelectric materials due to their high photoluminescence quantum yield (PLQY), tunable photoluminescence (PL) wavelength, narrow-band emission, high colour purity, and high carrier mobility.^1–6 In particular, perovskites fabricated by wet chemical approaches have potential applications in high-performance displays,⁷ light-emitting diodes,^8–11 photovoltaic cells,^12–18 solid-state lasers,^19,20 photodetectors,²¹ and solar fuel production.²² Compared with 3D perovskite NCs, large-sized zero-dimensional (0D) perovskite crystals typically show better stability and are more adaptable for practical applications, while showing relatively weak PL.²³ Therefore, the combination of 0D perovskite NCs and 3D perovskites has attracted wide attention and has been used in various types of optoelectronic devices.²³ Cesium-lead halide perovskites are normally classified as 3D CsPbBr₃, two-dimensional (2D) CsPb₂Br₅, and 0D Cs₄PbBr₆ based on the connectivity of the corner-sharing [PbBr₆]⁴⁻ octahedron.²⁴ The PLQY of the core/shell structure of CsPbBr₃/Cs₄PbBr₆ can reach up to 87.23%, compared with the single crystals.²⁵ Many methods have been developed to fabricate those kinds of composites. By solution-phase synthesis,²⁶ the preparation of Cs₄PbBr₆/CsPbBr₃ nanocomposite microcrystals with high PLQY (55%) can be achieved. An HBr-assisted slow cooling method (SCM) also provided a new way to prepare the centimeter-scale CsPbBr₃ NCs embedded in Cs₄PbBr₆ crystals, which showed a strong green PL emission with an absolute PLQY of up to 97%.²⁷ Although the PLQYs of CsPbBr₃/Cs₄PbBr₆ composites have been significantly enhanced via various strategies, the optical and thermal stabilities still lack significant improvements,^28,29 which hinder the practical applications of those kinds of composites.

Doping impurity ions into crystals has proven to be an effective way to enhance the environmental stability and adjust the optoelectronic properties of conventional colloidal II–VI and III–V semiconductor materials,³⁰ which also happens to perovskite NCs. For instance, Zou et al. reported that doping Mn²⁺ ions into the CsPbBr₃ NCs can stabilize the exciton emission and improve the thermal stability.³¹ Doping lanthanide into CsPbCl₃ NCs can significantly improve and expand the NCs’ optical properties.³² Moreover, theoretical studies of first-principles calculations have confirmed that CsPbBr₃ perovskites doped with Mn²⁺, Sr²⁺, Sn²⁺, Co²⁺, Zn²⁺ and Cd²⁺ ions have enhanced thermodynamic stability.^33,34 Many efforts have been dedicated to investigate the effect of doping ions on the luminescence properties of perovskite nanocrystals, while there are relatively few studies on large-scaled perovskite crystals. Recently, doping Na ions into the centimeter-sized CsPb₂Br₅ single crystals exhibited an interesting self-trapped exciton emission and an enhanced PLQY.³⁵ Although doping ions offers an effective way to improve the optoelectronic properties of perovskite materials, no attempts have been made to investigate the doping influence on the properties of large-scale 3D–0D perovskite crystals. In additional, doping ions typically exhibited a large change in the PL emission,^35,36 while the effect of a slight adjustment of the PL emission is relatively rare. Obtaining solution-grown ion-doped large-scale perovskite crystals is not only important for achieving a deeper understanding of the underlining doping mechanism and related photophysics, but also crucial for the practical applications of optoelectronic devices.

In this work, the millimeter-scale Mn²⁺-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals were synthesized by one-step slow cooling method under an air atmosphere. Upon adding Mn²⁺ ions, a blueshift in the PL spectrum was observed, without changing the full-width-at-half-maximum (FWHM) of the PL spectra and generating a new excitonic emission. Compared with the undoped crystals, introducing Mn²⁺ ions can improve the proportion of radiative recombination when the temperature is below 130 K, while there is almost a negligible effect on the PL decay dynamics at higher temperature. Moreover, Mn²⁺ ions can improve the stability of the prepared millimeter-scale crystals. Based on the as-grown crystals, high efficiency and finely-tunable electroluminescence green LEDs were successfully fabricated, providing an alternative choice for display backlights.

2 Experimental section

2.1. Chemicals

All chemical materials and reagents were used directly as received without any further purification. PbBr₂ (99%) and MnBr₂ (99%) were commercially obtained from Shanghai Boer Chemical Reagent Technology Co., Ltd (Shanghai, China). CsBr (99.9%) was purchased from Shanghai Titan Technology Co., Ltd (Shanghai, China). HBr (48 wt% in water) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). N,N-Dimethylformamide (DMF, 99.8%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). Toluene (99.5%) was purchased from Titan Technology Co., Ltd (Shanghai, China).

2.2. Synthesis of millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals

Millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals were synthesized according to a one-step slow cooling method.²⁷ The synthesis process is described as follows. A mixture of PbBr₂ (2.5 mmol, 0.917 g) and CsBr (10 mmol, 2.130 g) were added at room temperature to a 100 mL round bottom flask, which contained 50 mL DMF and HBr (the volume ratio of DMF and HBr is 5 : 4). The temperature of the crystal growth was controlled using an external water bath, with a temperature accuracy of ± 0.01 °C. The mixture solution was first heated and stirred to 80 °C in a water bath. After it was completely dissolved into a transparent solution, it was taken out and cooled to room temperature. About three days later, the millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals were successfully fabricated at the bottom of the flask. Thereafter, the pure CsPbBr₃/Cs₄PbBr₆ crystals were obtained by washing and filtering with toluene. The schematic of the synthesis process flow chart is shown in Fig. 1.

Fig. 1 Schematic diagram of the process of synthesizing Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals by slow-cooling method.

2.3. Procedure for the synthesis of millimeter-scale Mn-Doped CsPbBr₃/Cs₄PbBr₆ crystals

Millimeter-scale Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals were also synthesized according to a one-step slow cooling method,²⁷ and made some improvement. Firstly, the mixture of PbBr₂ (2.5 mmol, 0.917 g), CsBr (10 mmol, 2.130 g) and MnBr₂ (1 mmol, 0.215 g) were added to a flask at room temperature. Then, 50 mL of the DMF/HBr solution (the volume ratio of DMF and HBr is 5/4) was subsequently mixed with the abovementioned mixture. The solution was then heated and stirred to 80 °C using an external water bath (an accuracy of ± 0.01 °C), and subsequently dissolved completely into a transparent solution and cooled to room temperature. About three days later, the millimeter-scale Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals were successfully grown at the bottom of the flask.

2.4. Fabrication of green LEDs

Green CsPbBr₃/Cs₄PbBr₆ crystals with and without Mn-doping and AB organic silicone (the A-to-B mass ratio of 1 : 2) were added to a mortar. The mixture was stirred fully until all bubbles disappeared completely, and then the obtained mixture was spread evenly. The mixture was coated onto the surface of the blue InGaN LED chip, followed by heating and curing in an oven at 120 °C for 2 hours. The green LED devices were successfully fabricated for further optical performance testing.

2.5. Characterization

Microscopy photographs of the as-grown crystals were obtained using the SZM continuous zoom stereo microscope (Sunny Optical Technology Co., Ltd). PL spectra were recorded by fluorescence spectrometry (Edinburgh Instrument Spectrofluorometer FS5) equipped with a solid-state sample holder. A flashlamp (λ_ex = 405 nm) was used for gathering the data on the decay under time-correlated single-photon counting (TCSPC) mode. X-Ray diffraction (XRD) data were collected with 2θ from 10 to 60° using a Rigaku Ultima IV X-ray diffractometer (40 kV, 40 mA, sealed Cu X-ray tube). Transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) data of the as-grown crystals were obtained by a JEM-2100 high-resolution transmission electron microscope (HRTEM, JEOL, Tokyo, Japan). Time-resolved PL measurements were characterized at various temperatures using an Edinburgh FLS980 spectrometer. Excitation was provided by an Edinburgh EPL-365 picosecond pulsed diode laser and probed at 520 nm. Temperature-dependent PL experiments were performed using the FX4000-Ex high-resolution spectrometer (Shanghai Fuxiang Optics Co., Ltd) and the MS-H280-Pro heating table (Dalong Xingchuang Experimental Instrument (Beijing) Co., Ltd) with a temperature range of 300 to 400 K. Moisture stability experiments were performed by measuring the PL intensities of the prepared crystals as a function of time using the FX4000-Ex high-resolution spectrometer (Shanghai Fuxiang Optical Co., Ltd) and a humidifier. The humidifier and the crystals were placed together in a plastic box with a humidity control of 70%. The photostability was characterized by recording the corresponding PL intensities of the crystals under the laser irradiation (with a central wavelength of 455 nm and a power density of 5 W cm⁻²) generated by 100 W optical fiber output laser (Yantai Huachuang Intelligent Equipment Co., Ltd).

3 Results and discussion

3.1. Structure and optical properties

Fig. 2a shows the photographs of an as-grown single CsPbBr₃/Cs₄PbBr₆ crystal and a pure Cs₄PbBr₆ single crystal without further polishing under the microscope. The CsPbBr₃/Cs₄PbBr₆ crystals show good transparency and an apparent yellow-green colour under daylight with a size of 3.0 × 3.0 × 1.045 mm (see Fig. 2a and Fig. S1, ESI†), while a typical pure Cs₄PbBr₆ single crystal presents as white and transparent with a size of 2.0 × 2.0 × 1.018 mm (see Fig. 2b and Fig. S1, ESI†). Under UV light (395 nm) irradiation, the CsPbBr₃/Cs₄PbBr₆ crystals show bright green emission, while the pure Cs₄PbBr₆ single crystals do not emit any light (Fig. S1, ESI†). This indicates that the green emission is attributed to the existence of the CsPbBr₃ phase, which is in line with the literature.³⁷ The PL spectrum of the CsPbBr₃/Cs₄PbBr₆ crystals shows a PL peak at 518 nm with a narrow FWHM of 21 nm, which is comparable to that reported for the Cs₄PbBr₆ crystals with embedded CsPbBr₃ NCs with a CsPbBr₃ NCs average size of 2.85 nm.²⁷ The PLQY of the as-grown CsPbBr₃/Cs₄PbBr₆ crystals was 46.02% (as shown in Fig S4, ESI†). Moreover, the UV-Vis diffuse reflection spectrum (Fig. 2c) obtained from the grinding powder of the CsPbBr₃/Cs₄PbBr₆ crystals apparently shows two clear absorption peaks at about 310 and 513 nm, corresponding to the characteristic absorption of Cs₄PbBr₆ caused by the allowed optical transition of ³P₁ to ¹S₀ of the Pb²⁺ ions³⁸ and the CsPbBr₃ NCs, respectively. The existence of 3D CsPbBr₃ NCs in the powder XRD is almost undetectable. Minor XRD peaks corresponding to CsPbBr₃ can be found due to the low proportion of CsPbBr₃ in the composite, which has also been reported in previous studies.^39,40 However, the existence of the CsPbBr₃ phase can be clearly demonstrated from the comparison of the daylight photographs, as well as the absorption and PL spectra. In the following, we further confirm from the structure that the 3D CsPbBr₃ NCs are embedded in the Cs₄PbBr₆ matrices.

Fig. 2 (a) and (b) Photographs of the as-grown CsPbBr₃/Cs₄PbBr₆ crystal and Cs₄PbBr₆ single crystal under the microscope under daylight. (c) Absorption (black curve) and PL spectra (red curve, excitation by CW laser wavelength of λ_ex = 365 nm) of CsPbBr₃/Cs₄PbBr₆ crystals. (d) The powder XRD pattern of the as-grown CsPbBr₃/Cs₄PbBr₆ crystals.

To further characterize the morphology of the as-grown CsPbBr₃/Cs₄PbBr₆ crystals, we conducted TEM measurements. Since TEM cannot be used to measure large crystals, the as-grown crystals are processed into small particles by ball-milling, which are then dispersed in toluene under sonication for TEM measurement. During this process, the PL emission remains consistent. Fig. 3 shows a high-resolution TEM (HRTEM) of a typical sample. As shown in Fig. 3a, the particles processed from the large-sized crystals have a lattice spacing of 6.3 Å, corresponding to the (110) phase of Cs₄PbBr₆. Fig. 3b and c show the corresponding enlarged TEM image of panel (a), where the lattice spacing is indicated to be 2.9 and 4.5 Å, respectively, which corresponds to the (200) and (110) phases of CsPbBr₃. Thus, from the TEM image shown in Fig. 3, it is directly confirmed that the CsPbBr₃ NCs are embedded in the Cs₄PbBr₆ crystals. Moreover, the HRTEM images of the CsPbBr₃/Cs₄PbBr₆ crystals and the corresponding FFT patterns are shown in Fig. S3 (ESI†), and match well with the standard diffraction patterns of the CsPbBr₃ and Cs₄PbBr₆ phases, further validating the existence of CsPbBr₃ NCs embedded in the Cs₄PbBr₆ crystals.

Fig. 3 (a) Typical TEM image of a particle processed from the as-grown CsPbBr₃/Cs₄PbBr₆ crystals. (b) The corresponding enlarged TEM image of panel (a) plotted with the lattice spacing of the CsPbBr₃ phase. (c) Another corresponding enlarged TEM image of panel (a) plotted with a lattice spacing of the CsPbBr₃ phase.

In order to further investigate the effect of ion doping on the luminescence performance and stability of the 3D–0D type perovskites, the Mn²⁺ ions were introduced into the CsPbBr₃/Cs₄PbBr₆ crystals by adding MnBr₂ during the one-step slow cooling process. The prepared CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystal with a size of 2.0 × 2.0 mm is shown in the inset of Fig. 4a displays a yellow-green color in daylight and green color under UV light (λ_ex = 395 nm), respectively. Fig. 4a shows the comparison of the PL spectra of the CsPbBr₃/Cs₄PbBr₆ crystals with and without Mn²⁺ doping. The PL peak of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals is blue-shifted by about 7 nm compared with the as-grown CsPbBr₃/Cs₄PbBr₆ crystals, without showing a new PL peak. This phenomenon is contradictory to that of the Mn²⁺-doped CsPbBr₃ system with a higher content of Mn²⁺ doping,⁴¹ while exhibiting a similar blue-shifted PL peak with lower content of Mn²⁺ doping.^31,41 As described in Fig. 2a–c, for the as-grown CsPbBr₃/Cs₄PbBr₆ crystals, the green PL emission comes from the CsPbBr₃ phase. Doping of Mn²⁺ in the CsPbBr₃/Cs₄PbBr₆ crystals alters the PL peak as shown in Fig. 4a, which indicates that the Mn²⁺ ions were successfully doped into the CsPbBr₃ phase. The PLQY was 65.43% (Fig. S5, ESI†), showing a large improvement compared with the CsPbBr₃/Cs₄PbBr₆ crystals (46.02%). This is also consistent with the PL spectra shown in Fig. 4a, indicating that Mn²⁺ doping can enhance the PL performance. The improved PL performance from doping the Mn²⁺ ions into the CsPbBr₃/Cs₄PbBr₆ crystals may be attributed to Mn²⁺ possibly passivating the surface or grain boundary defects of the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals.^41,42 To further confirm the success of Mn²⁺ doping, the crystal was characterized using powder XRD measurement. The comparison of the powder XRD spectroscopy of the CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals are shown in Fig. 4b. It can be seen that the powder XRD pattern of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals slightly shifts to a higher angle direction (∼0.04 degrees) compared to that of the CsPbBr₃/Cs₄PbBr₆ crystals, which may be caused by the lattice contraction due to the replacement of the larger Pb²⁺ with the relatively smaller Mn²⁺.³⁶ However, the observed shift in the angle is rather small, which may be due to the fact that it is more difficult to dope ions into those 3D–0D type perovskites. Thus, X-ray spectroscopy was applied to gain more insight into the incorporation of Mn²⁺ ions into the CsPbBr₃/Cs₄PbBr₆ crystals during doping via high-angle annular dark-field scanning transmission microscopy (HAADF-STEM) equipped with EDS element mapping. The corresponding results are shown in Fig. 4c. From Fig. 4c, the Mn²⁺ ions are homogeneously distributed among the particles, providing solid evidence verifying the successful doping of Mn²⁺ ions into the CsPbBr₃ phase. The content distribution of Mn elements in Fig. S8 and Table S1 (ESI†) shows the relatively small Mn-doping ratio, which is consistent with the rather small shifted angle observed in the XRD patterns and the undetectable new excitonic emission in the PL spectrum.

Fig. 4 (a) The PL spectra of CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals (irradiated by 410 nm continuous laser (CW) laser). The inset shows the dimensions of the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals under UV light (395 nm) and daylight. (b) XRD patterns of the Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals. The inset is the enlarged XRD patterns. (c) High-angle annular dark-field scanning transmission microscopy image and EDS elemental mapping (Br, Cs, Pb and Mn) of the particles from the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals by ball-milling.

Consequently, from the above results, it can be confirmed that the millimeter-scale Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals can be easily obtained via one-step slow cooling. In the following section, we will investigate the impact of doping Mn²⁺ ions on the CsPbBr₃/Cs₄PbBr₆ crystals.

3.2. The effect of Mn²⁺ ions on the PL dynamics of the CsPbBr₃/Cs₄PbBr₆ crystals

To investigate the change of the photophysical properties upon doping with Mn²⁺, the time-resolved PL decay of the CsPbBr₃/Cs₄PbBr₆ crystals with and without doping Mn²⁺ ions was carried out at different temperatures under pulsed 365 nm excitation. Fig. 5a and b show the experimental and fit curves of the PL decays of both crystals at various temperatures, ranging from 80 to 300 K. All of the measured PL decay traces can be well fitted.where A₁ and τ₁ refer to the fast component of the decay and the corresponding amplitude, respectively. A₂ and τ₂ refer to the slow component of the decay and the corresponding amplitude, respectively. The fast component τ₁ is typically attributed to trap-mediated nonradiative recombination, and the slow component τ₂ is assigned to the lifetime of radiative recombination.^43,44 The PL lifetimes (τ) and the corresponding amplitudes (A) as a function of temperature are plotted in Fig. 5c–f. For both CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals, both τ₁ and τ₂ vary with temperature and have nearly the same trend. Changing the temperature has a significant influence on τ₂ (varying from 5.5 to 39.9 ns for the CsPbBr₃/Cs₄PbBr₆ crystals, and from 3.35 to 35.1 ns for the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals), while the fast component τ₂ remains almost consistent (varying from 2.3 to 7.2 ns for the CsPbBr₃/Cs₄PbBr₆ crystals, and from 1.6 to 6.5 ns for the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals). For the CsPbBr₃/Cs₄PbBr₆ crystals, the proportion of the trap-mediated nonradiative recombination (A₁) as a function of temperature is nearly in line with the temperature-dependent integrated PL emission intensity reported by M. Chen et al.⁴⁵ There exists some difference between the A₁/A₂ ratios of the Mn-doped and undoped crystals. At 300 K, the ratios of A₁/A₂ equal 1.5 for both Mn-doped and undoped crystals, indicating that the effect of doping Mn²⁺ ions on the relaxation rates of trap-mediated recombination and radiative recombination is negligible, even though the PL lifetime of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals is shorter than that of the CsPbBr₃/Cs₄PbBr₆ crystals. However, at 80 K, the ratios of A₁/A₂ become 1.6 and 3.1 for the Mn-doped and undoped crystals, respectively, indicating that doping Mn²⁺ ions can enhance the proportion of radiative recombination. At low temperatures, the lack of thermal excitation leads to the trapped carriers decaying through nonradiative pathways.⁴⁶ Meanwhile, doping Mn²⁺ ions can effectively accelerate the decay of a dark exciton due to the magnetic mixing of bright and dark excitons.⁴⁷ Thus, to some extent, it also can improve the proportion of radiative recombination. Therefore, doping Mn²⁺ ions has little influence on the ratio of radiative and nonradiative recombination at the temperature range from 180 to 300 K, while it enhances the proportion of radiative recombination at low temperatures (below 130 K).

Fig. 5 (a) and (b) Time-resolved PL decay of millimeter-scale CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals at different temperatures, respectively. (c) and (d) The corresponding PL lifetimes in panels (a) and (b) as functions of temperature. (e) and (f) The corresponding amplitudes in panels (a) and (b) as functions of temperature.

3.3. Stability of the Mn²⁺ doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals

The stability of the perovskite materials, especially the thermal stability, hinders their practical applications. Intuitively, the observed luminescence (under 395 nm UV irradiation) of the as-grown CsPbBr₃/Cs₄PbBr₆ crystals stored in a sealed centrifuge tube under an air atmosphere almost remains the same after one year (see Fig. S2, ESI†), indicating that the millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals show high air stability. In order to further quantitatively confirm the long-term air stability, the PLQYs were obtained for CsPbBr₃/Cs₄PbBr₆ crystals with and without doping Mn²⁺ that had been directly placed under an air atmosphere for three months. The results show that the PLQY of the undoped crystals was 42.95% (Fig. S6, ESI†), while that of the Mn-doped crystals was 61.49% (Fig. S7, ESI†). Compared with the just-prepared undoped and doped crystals (the PLQYs are 46.02 and 65.43%, respectively, for the undoped and doped crystals), doping Mn ions can slightly enhance the air stability, even though both crystals show long-term stability under air atmosphere. Those results further indicate that the introduction of Mn²⁺ can enhance the luminescence of the crystals and improves the air stability of the crystal.

Moreover, the thermal stability, moisture stability and photostability of the Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals were carried out. Fig. 6a shows the intensities of the PL peak of the Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals as functions of temperature, ranging from 300 to 400 K, respectively. The corresponding detailed PL spectra are shown in Fig. S9 (ESI†). It is clearly seen that upon increasing the temperature, the PL intensity of both Mn²⁺ doped and undoped crystals was significantly quenched. However, in comparison with the undoped crystals, the decrease in the intensities of the PL peak of the Mn-doped crystals was slightly slower. For instance, at 350 K, the PL intensity of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals remained at around 25% of the initial intensity, while that of the undoped crystals only remained at 20% of the initial intensity. These results indicate that the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals exhibit relatively higher thermal stability than the pure CsPbBr₃/Cs₄PbBr₆ crystals.

Fig. 6 (a) Comparison of the PL peaks of the temperature-dependent PL spectra of the Mn²⁺ doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals. (b) Intensities of the PL peaks as a function of time of the Mn²⁺ doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals under a humidity of 70%. (c) Comparison of the PL peaks of the Mn²⁺ doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals under 455 nm laser irradiation with a power density of 5 W cm⁻².

Fig. 6b shows the intensities of the PL peak of the CsPbBr₃/Cs₄PbBr₆ crystals and Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals at a humidity of 70% in the time range of 0 to 60 min. The corresponding detailed PL spectra are also shown in Fig. S10 (ESI†). The PL intensity of both Mn²⁺ doped and undoped crystals decreases under a humid condition. However, compared with undoped crystals, the PL peak intensity of the Mn-doped crystals decreases much more slowly. After 10 min, the PL intensity of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals remains at about 77% of the initial intensity, while that of the undoped crystals is only maintained at 50%. In addition, the PL intensity of the Mn-doped crystals still remains at 36% of the initial intensity after 60 min, while that of the undoped crystals decreased to 24% of the initial intensity. Those results show that the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals exhibited higher humidity stability than the pure CsPbBr₃/Cs₄PbBr₆ crystals.

Next, we examined the photostability of the as-grown crystals by applying a 455 nm laser irradiation with a power density of 5 W cm⁻². Fig. 6c shows the variations of intensity of the PL peak of both crystals under laser irradiation, and the corresponding specific PL spectra are shown in Fig. S11 (ESI†). It is clear that the PL intensities of the Mn²⁺ doped and undoped crystals decrease slowly with increasing illumination time. After two hours, the PL intensity of both samples decreased to about 70% of the initial intensity. Upon increasing the laser irradiation time, the intensity of the PL peak of the Mn-doped crystals decreased relatively slowly. After 6 hours, the PL intensity of the CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals remained at about 64% of the initial intensity, while that of the undoped crystals stayed at 60% of the initial intensity. Therefore, the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals exhibit relative better photostability than the pure CsPbBr₃/Cs₄PbBr₆ crystals.

The enhanced stability of the Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals can be mainly due to the Mn²⁺ ions replacing a portion of the Pb²⁺ ions, and results in an increase of the absolute value of the formation energy of each Pb²⁺ (or Mn²⁺).^31,48 We note that the content of Mn²⁺ in our paper is smaller than the 2.08 mol% value mentioned in ref. 31. Furthermore, the substitution of the Pb²⁺ ion (ionic radius of Pb²⁺ is 1.33 Å) with the smaller Mn²⁺ ion (ionic radius of Mn²⁺ is 0.97 Å) in the lattices of the CsPbBr₃/Cs₄PbBr₆ crystals might induce lattice contraction and improve the stability. This phenomenon was also found in the previous literature.³¹ Accordingly, those results suggest that doping Mn ions into the CsPbBr₃/Cs₄PbBr₆ crystals can effectively enhance the luminescence performance and stability.

3.4. Applications

Saturated green PL emission with good thermal stability makes the Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals a promising candidate for display backlighting. The packaging experiments were carried out by combining the green emission Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals with blue-emitting GaN chips to fabricate green LEDs. Fig. 7a shows the device configuration of the green LED. Fig. 7b presents the EL spectra of the CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals obtained at a current of 60 mA. The insets of Fig. 7b are the corresponding pictures of the green LED. Fig. 7c shows the CIE 1931 chromaticity diagram of the CsPbBr₃/Cs₄PbBr₆ crystals and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals. The color coordinates of the CsPbBr₃/Cs₄PbBr₆ and CsPbBr₃/Cs₄PbBr₆:Mn²⁺ crystals are (0.0949, 0.6917) and (0.0697, 0.7114), respectively. From the color coordinates, it can be seen that the crystals doped with Mn²⁺ have a further shift, which is identical to that for the previous PL spectra. Fig. S12 (ESI†) further proves that the introduction of different amounts of Mn²⁺ ions can finely adjust the PL of the CsPbBr₃/Cs₄PbBr₆ crystals, which is promising for achieving applications in finely-tunable high color quality display backlighting.

Fig. 7 (a) Schematic diagram of the configuration of the prototype device. (b) EL spectra of the prototype green LED devices driven at a current of 60 mA. (c) CIE color coordinates of the green LED for the CsPbBr₃/Cs₄PbBr₆ crystals (black spots) and Mn-doped CsPbBr₃/Cs₄PbBr₆ crystals (red spots).

4 Conclusions

In summary, we successfully synthesized millimeter-scale Mn-doped and undoped CsPbBr₃/Cs₄PbBr₆ crystals by the one-step slow cooling method. The effect of doping Mn²⁺ ions into the millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals on the PL dynamics and stability were investigated. Compared with undoped crystals, the existence of Mn²⁺ ions blue-shifted the PL peak, although there was no new excitonic emission and the narrow FWHM of the PL spectra was retained. Doping Mn²⁺ ions can significantly enhance the proportion of radiative recombination at low temperature, while has little influence on the ratios of radiative and nonradiative recombination at the temperatures ranging from 180 to 300 K. In the presence of Mn²⁺ ions, the stability of the prepared millimeter-scale crystals can also be improved. Moreover, green LED devices packaged with both crystals were explored, and were promising for further display backlighting applications. Consequently, those results indicate that the addition of Mn²⁺ ions into millimeter-scale CsPbBr₃/Cs₄PbBr₆ crystals provides effective strategies to improve the optoelectronic performance and stability. We hope that the large size crystals could also be a promising candidate for investigations of photophysical processes dynamics and optoelectronics practical applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (Grant No. 2021YFB3501700), the National Natural Science Foundation of China (Grant No. 12104311, 12004239), the Shanghai Chenguang Program (Grant No. 22CGA74), Shanghai Science and Technology Committee (STCSM) Science and Technology Innovation Program (Grant No. 22N21900400, 23N21900100), and the Science and Technology Talent Development Fund for Young and Middle-aged Teachers of Shanghai Institute of Technology (Grant No. ZQ2022-3).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp04371a

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