Constructing Gradient Energy Levels to Promote Exciton Energy

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Constructing Gradient Energy Levels to Promote Exciton Energy Transfer for Photoluminescence Controllability of Allinorganic Perovskite and Application in Single Component WLED Chao Luo, Wen Li, Ji Fu, and Weiqing Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01392 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

Constructing Gradient Energy Levels to Promote Exciton Energy Transfer for Photoluminescence Controllability of All-inorganic Perovskite and Application in Single Component WLED Chao Luo,† Wen Li,† Ji Fu‡ and Weiqing Yang*,† †

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, PR China ‡ The Graduate School of Information, Production and Systems, Waseda University, 2-7 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0135, Japan

ABSTRACT The photoluminescence tunability is exceedingly crucial to all-inorganic halide perovskite (CsPbX3, X=Cl, Br, I) quantum dots (QDs) for high resolution display. However, up to now, the wide-range color tunability could only be achieved by varying the halide ratio. A drawback in this approach is the color instability caused by inevitable halide ions exchange. Therefore, it is of great significance to exploit the new train of thought to tune the emission color in a wide range. Herein, we designed a novel exciton energy transfer strategy by introducing 1G4 energy level of Tm to construct the intermediate gradient energy levels between conduction band of host perovskite and the 4T1 energy level of Mn for efficiently promoting the exciton energy transfer from host perovskite to Mn. Thus the emission color could be accurately tuned from green to orange and the corresponding correlated color temperature (CCT) changed from 12400 to 1800 K. Most strikingly, the single component pure white light QDs with a record photoluminescence quantum yield (PLQY) of 54% was obtained based on this strategy. Additionally, white light-emitting diodes (WLED) with color rendering index (CRI) up to 91 and CIE color coordinate of (0.33, 0.34) are further fabricated. Therefore, we believe that this novel exciton energy transfer strategy will possibly open up a

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new avenue for efficient emission controllability of all-inorganic perovskite, promoting the higher PLQY of single component white light QDs and closer to practical application.

Introduction The all-inorganic halide perovskite have become a subject of intense research in recent years owning to their attractive optical properties, including the high photoluminescence quantum yields (PLQY), optical band gap tunability, solution processability and low cost.1-4 These unique and outstanding photophysical properties make them the most promising candidate materials for next generation high resolution displays, which require a rich color variety.5 At present, however, the wide-range emission color tuning of all-inorganic perovskite mainly depends on the variety of halide component and ratio.6,7 Nevertheless, the uncontrollable anions exchange in mixed halide could lead to peaks drift, which is fatal for display devices.8 Thus, it is extremely meaningful to excavate new mechanism to tune the emission color in a wide range, which could enrich and supplement the exploration of the optical properties of all-inorganic perovskite. The typical transition metal Mn2+ ions were usually employed to tune the emission color of all-inorganic perovskite based on the exciton energy transfer mechanism.9-13 Specifically, the exciton of host perovskite could be transferred to Mn, intrinsically resulting in an orange emission (d-d) around 600 nm. Traditionally, varying the Mn doping concentrations was applied to tune the peak intensity of Mn. Although this strategy could tune the emission color of all-inorganic perovskite in a modest range by excessive Mn doping concentration (up to 37%), the excessive Mn will lead to drastic reduction of PLQY.14-15

11, 16-21

At present, it is

still a great challenge to achieve wide range emission color tuning by Mn doping due to the inefficient exciton energy transfer, especially at low doping concentration.


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Based on exciton energy transfer mechanism, the Mn2+ ion is one of the most promising candidate dopants for achieving single component white light perovskite QDs.12, 16, 22-23 At present, to obtain white light emission, three combinations of device structures have been developed. (I) The blue LED with a mixture of red and green phosphors or a yellow phosphor. (II) The UV LED with three primary colors (blue, green, and red) monochromatic phosphors. (III) The UV LED with a single component white phosphor.24-26 The latest approach III is the most promising one, as it is capable of simplifying device structure, eliminating the selfabsorption and color instability of multiple color combination. Thus it is of great value to practical application.27-28 However, the traditional method controlling exciton energy transfer by varying the superfluous Mn2+ doping concentrations to obtain single component white light QDs always have relatively low white light PLQY owing to the low exciton energy transfer efficiency. In this regard, Bi3+/Mn2+ ions co-doped all-inorganic perovskite was employed to make great efforts to improve the white light PLQY, but the PLQY was still as low as 4.2%.29 Similarly, even for the mature host ZnSe, the white light PLQY of Mn: ZnSe was still less than 17%,30 which is far below the requirements of commercial applications. Therefore, it remains a ubiquitous impediment for obtaining single component white light QDs with high PLQY. In this work, we introduced rare earth Tm3+ and Mn2+ ions into all-inorganic perovskite to synthesize Tm3+/Mn2+ co-doped CsPbBr2.2Cl0.8 QDs (TMCQ). The unique 1G4 energy level of Tm as intermediate gradient energy levels, ultra-highly efficiently promoting the exciton energy transfer from host perovskite to doped Mn2+. Specifically, the wonderful combination of ultra-low and fixed Mn2+ doping concentration of only 0.2% and just varying the states density of Tm3+ was novelty designed to control the exciton energy transfer and further tune the emission color of TMCQ from green to orange as well as the CCT from 12400 to 1800 K precisely, covering most of the visible spectral region. Except for the wide range of color tunability, this strategy can also obtain the single component white light QDs with an ACS Paragon Plus Environment


excellent white light CIE color coordinate of (0.34, 0.33) and a PLQY of 54%, representing the highest value among single component white light perovskite QDs.16 Moreover, based on 365 nm LED chips, the as-prepared white light-emitting diodes (WLED) array desk lamp exhibited standard white light emission with CIE color coordinate of (0.33, 0.34) and a color rendering index (CRI) up to 91, which is especially attractive for constructing efficient white light solid-state illumination and high resolution display. Besides, the doping Tm3+ could significantly enhance the air and thermal stability of TMCQ simultaneously. Such designed exciton energy transfer strategy will not only be meaningful for exploring new wide-range color tunability mechanism of perovskite, but also enlightening to obtain single component white light QDs with high PLQY

Results and discussion The deep insights into microstructural properties are essential to evaluate the crystalline quality of as-synthesized TMCQ. In Figures 1a-c, the transmission electron microscope (TEM) images reveal the QDs possess excellent crystallinity and uniformity. Although the QDs preserve the similar cubic shape, the average sizes of QDs decrease as the Tm3+ doping concentrations increase. Moreover, as presented in Figures 1d-f, high-resolution TEM images (HTEM) demonstrate that the lattice constants reduce slightly from 0.32 to 0.31 nm when the Tm3+ doping concentrations increase from 0% to 1.7%, evidently indicating the crystal lattice contraction was caused by doping. In addition, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were executed to characterize the composition and crystal structure of TMCQ. The XPS survey images clearly reveal the signals of Cs, Pb, Br, and Cl, while the relatively weak peaks of Tm and Mn also appear (Figures 1g and S1), evidently confirming the existence of Tm and Mn in TMCQ. Besides that, the XRD pattern proves that the crystal


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phase of CsPbBr2.2Cl0.8 is still maintained after doping (Figure 1h). The diffraction peaks move slightly to higher angles as the Tm3+ doping concentrations increases, indicating the lattice contraction of TMCQ ascribed to the doping of Tm3+ and Mn2+. Further, the energydispersive X-ray spectroscopy (EDS) elemental mapping have proven that the Tm and Mn elements have been also involved in the TMCQ (Figures 1i-n, S2 and Table S1). As revealed in the high-resolution XPS spectra (Figure S3), the binding energy of Cs+ 3d3/2 and Cs+ 3d5/2 exhibited almost no change as Tm3+ ions were introduced, while the binding energies of Pb2+ 4f5/2 and Pb2+ 4f7/2 as well as Cl− 2p1/2 and Cl− 2p3/2 shifted slightly to a higher angle. This is attributed to the lattice contraction and the change of chemical environment of Pb2+ and Clcaused by doping. These results indicate that Tm3+ ions have successfully doped into the perovskite host lattice and most probably replace the Pb2+ sites.31The specific doping content was detected by ICP-OES (Figure S4). As we know, Mn-doped CsPbX3 QDs show two bands emission. Except for host perovskite emission, there is a broad PL emission peak near 600 nm, which attributes to the 4T -6A 1 1

transition of Mn2+.18 Almost all previous reports followed the same rule: varying the

excessive Mn doping concentrations (up to 37%)14 to control the exciton transfer from host perovskite to Mn directly, and then further tune the PL intensity of Mn.10-11, 17, 27, 29, 32-33, This method always has low exciton efficiency and can hardly achieve wide range color tunability of all-inorganic perovskite. Worst of all, the excessive Mn will lead to drastic reduction of PLQY.14-15 To break this restriction, we speculate that the energy level difference between the conduction band of host perovskite and the 4T1 energy level of Mn is likely to be responsible for the low efficiency energy transfer. Therefore, the strategy of constructing intermediate gradient energy levels between conduction band of host perovskite and the 4T1 energy level of Mn to improve energy transfer efficiency was conceived. The lanthanides were considered first due to their rich energy level structure. Among them, Tm was selected, as the 1G4 energy level of Tm is the most perfect candidate for the formation of gradient levels. In fact, the ACS Paragon Plus Environment


experimental results confirm the above design. As shown in Figures 2, the PL intensity of Mn could be efficiently tuned by a large margin at a fixed and ultra-low Mn2+ doping concentration (0.2%) for the first time. The PL intensity of Mn increase with the increase of Tm3+ doping concentration. Obviously, for the 0% Tm3+ doped sample, the PL intensity originate from Mn is negligible in comparison with host perovskite. While, for the sample with 3.1% Tm3+, the PL intensity of Mn is 4.5 times more than host perovskite. The specific intensity comparison of Mn and host perovskite with different Tm3+ concentrations is revealed in Figure S5. Based on this unique exciton energy transfer mechanism, the emission color of all-inorganic perovskite could be tuned from green to orange and the CCT also could be tuned from 12400 to 1800 k (Figure 3a,b). The halide ratios were kept constant to make the emission wavelengths of host perovskite as unchanged as possible (Experimental section for details), as evidenced by the almost unchanged absorption spectra in Figure S6. Although the peaks of the host perovskite shift slightly (Figures 3c), it is not enough to cause large color changes and the wide range of color regulation is mainly attributed to the variety of relative intensity of host perovskite and Mn peaks. Such a wide range of color tunability is inaccessible by doping either Mn2+ or rare-earth ions alone.10-11, 31, 34-36 Therefore, this strategy based on new exciton energy transfer mechanism provides a novel train of thought for tuning the emission color of perovskite QDs. To exclude the above effect was caused by halide ions in TmCl3, the YbCl3 was employed to dope CsPbBr2.2Cl0.8 under the same experimental parameters (Figure S7). The results show that YbCl3 could not play a similar effect as TmCl3, which further proves the unique role of Tm3+ in exciton energy transfer. Moreover, as shown in Figure 3f, the PL spectra of only Tm3+ doped CsPbBrxCl3-x QDs were measured to exclude peaks near 600 nm were from the intrinsic emission of Tm3+. To further prove it, a nearinfrared-triggered photon upconversion system was designed (Figure S8) to examine the intrinsic emission spectrum of Tm3+. The core-shell structure NaGdF4:Yb,Tm@NaGdF4 was obtained through high-temperature co-precipitation method. In this system, the Yb3+ could ACS Paragon Plus Environment

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function as an energy acceptor to absorb 980 nm photons, and then emit multiplep photons to excite Tm3+, thus realizing the intrinsic spectrum emission. As a result, there was no emission peak near 600 nm in its intrinsic spectrum. Therefore, it could be reasonably concluded that the peak near 600 nm indeed originated from Mn2+ rather than Tm3+. Moreover, Tm3+ doping can effectively enhance the PLQY of TMCQ from 70% to 93% (Figure S9). Although the concentrations of Mn remained extremely low and constant, the PLQYs of Mn improved greatly to a extraordinary high value with the increase of Tm doping concentrations, which further embodied the superiority of constructing gradient energy levels to promote exciton energy transfer. The aforementioned results and the process of exciton energy transfer can be understood more clearly from the schematic Figure 3d, e. The externally radiated energy promotes the electron transition from the ground states to excited states, upon absorption of photons, exciton is formed. And then CsPbBr2.2Cl0.8 host will release these exciton by radiative recombination and emit a 491 nm light at the same time. While one part of the photoinduced exciton will be transfered from CsPbBr2.2Cl0.8 host to the 1G4 energy level introduced by Tm3+.31, 35 Then these exciton will be transfered downward to the lower energy state 4T1 of Mn2+ rather than 3F3 energy level of Tm3+ due to the smaller energy level difference.31, 35 At last, the exciton will be released by radiative recombination with a emission spectra of about 600 nm. It is noteworthy that the energy levels of Tm3+ play an transitional role in the process of exciton transfer, making it easier and more efficient to transfer exciton from CsPbBr2.2Cl0.8 host to Mn2+.35 Therefore, as the Tm3+ doping concentration increase, the promotion effect will be enhanced accordingly. So, more exciton will be transferred to Mn2+ and then released, accompanying with more photons emission. As a result, the PL intensity of Mn2+ is expected to increase progressively. Owing to this novel design makes exciton energy transfer much more efficient, the wide range of color tunability could be easily realized.



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To shed more light on the effect of new states in the process of exciton energy transfer from dynamics, the time resolved PL decay spectra were performed to measure the PL dynamics of undoped CsPbBr2.2Cl0.8 QDs (sample 1), only 0.2% Mn2+ doped CsPbBr2.2Cl0.8 QDs (sample 2), and 1.0% Tm3+, 0.2% Mn2+ co-doped CsPbBr2.2Cl0.8 QDs (sample 3). The corresponding PL decay spectra are fitted by double exponential function, the detailed fitting information is shown in Table S2, and the average lifetimes (𝜏𝑎𝑣𝑒) are calculated by the following equation:16 𝜏𝑎𝑣𝑒 =

∑𝑖𝐴𝑖𝜏𝑖

(1)

∑𝑖𝐴𝑖

Where Ai are the constants from fitting, 𝜏 is decay time. These QDs exhibit two lifetimes from Mn2+ (𝜏' ) and host perovskite (𝜏). As shown in Figure 4a, the 𝜏'2 and 𝜏'3 are the Mn2+ lifetimes from sample 2 and sample 3. Mn2+ has a long lifetime (0.8 ms) consistent with spinforbidden nature of the transition and the doping of Tm3+ has little effect on it. As presented in Figure 4b, 𝜏1 , 𝜏2 and 𝜏3 represent the lifetimes of host perovskite from sample 1, sample 2 and sample 3 respectively. Although only Mn2+ doping can make the decay lifetime of host perovskite slightly faster (5 to 4.8 ns), Tm3+/Mn2+ co-doping can accelerate the decay more greatly from 5 to 1.8 ns. The shortened decay lifetime could be attributed to that the energy transfer from host perovskite to Mn2+ accelerates the depletion of the excitonic population, thus inducing a decrease in the lifetime of excitonic transitions.37-38 The effect of Tm3+ in the process of exciton energy could be further explored by combining the above results and theoretical analysis, which can be expressed as follows:16 1

𝜏1 = 𝐾𝑟 + 𝐾𝑛𝑟

(2)

1

(3)

1

(4)

𝜏2 = 𝐾𝑟 + 𝐾𝑛𝑟 + 𝐾1𝐸𝑇 𝜏3 = 𝐾𝑟 + 𝐾𝑛𝑟 + 𝐾2𝐸𝑇


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where 𝐾𝑟, 𝐾𝑛𝑟, and 𝐾2𝐸𝑇 are the irradiative decay rate constant, non-irradiative recombination rate constant via host perovskite emission, and the exciton energy transfer rate constant from host perovskite to Mn2+ respectively. According to Eqns (2), (3) and (4), 𝐾𝐸𝑇 can be represented as: 𝜏1 ― 𝜏2

(5)

𝐾1𝐸𝑇 = 𝜏1 × 𝜏2

𝜏1 ― 𝜏3

(6)

𝐾2𝐸𝑇 = 𝜏1 × 𝜏3

where 𝐾1𝐸𝑇 and 𝐾2𝐸𝑇 are the exciton energy transfer rate constant from sample 2 and sample 3. As a result, the 𝐾1𝐸𝑇 is as low as 0.0083, while the 𝐾2𝐸𝑇 is up to 0.36, revealing that about 36% exciton will be transfered from host perovskite to Mn2+.16 In summary, the exciton energy transfer efficiency of the Tm3+/Mn2+ co-doping CsPbBr2.2Cl0.8 QDs is about 43 times than that of Mn: CsPbBr2.2Cl0.8 QDs without Tm3+. It is clear that Tm3+ plays a stimulative role in the exciton energy transfer process and make it ultra-highly efficient. It is well known that the stability is a bottleneck for perovskite that needs to be broken urgently.39 According to Zeng et al.'s reports, the lattice contraction will lead to an increase in the defect formation energies of all inorganic perovskite, which can further enhance the thermal and air stability.

17, 32, 34-35

It can be seen from TEM (Figures 1a-f) results that the

doping of Tm3+ results in the lattice contraction, encouraging us to further characterize thermal and air stability of TMCQ (Support information for details). As shown in the degradation PL spectra over time with different concentrations of Tm3+ (Figures 5a-c), it is obvious that the sample without Tm3+ doping exhibits much more rapid degradation after exposing to air for 60 h. While the PL emission spectrum of the sample with 1.0% Tm3+ hardly degradate after 60 h, unambiguously vindicating that the doping Tm3+ can significantly boost the air stability of TMCQ. The thermal stability is also crucial for perovskite because heat is inevitably generated during device operation. To assess the thermal stability, the TMCQ samples were stored in a vacuum oven at 80℃ for 24 h. As presented in Figures 5d-f, ACS Paragon Plus Environment


the PL emission peak intensity of host perovskite without Tm3+ has almost disappeared entirely after 24 h, presenting exceedingly poor thermal stability. This is attributed to the crystal lattices were severly destroyed after heat treatment due to the low formation energy. The octahedron formed by Mn2+ will separate from host perovskite,40 thus the emission of Mn will be weakened. Moreover, the doping concentrations of Mn were extremely low (0.2%), so it was possible that the Mn peaks almost completely disappeared. While the lattices were not thoroughly destroyed, so the peaks of host perovskite still existed. The XRD characterization of two comparative samples with 0% and 1.7% Tm doping after heat treatment further validated the above analysis (Figure S10). It is obvious that the crystal lattice and phase of the sample without Tm3+ doping was seriously damaged after heat treatment. While, in sharp contrast, the PL emission peak intensities of host perovskite and Mn2+ from the sample with 1.7% Tm3+ maintain about 100% and 90% of the initial intensity after 24 h. Thus, it is certain that Tm3+ doping could indeed dramatically enhance the air and thermal stability simultaneously. Except that, the illumination stability was further characterized by simultaneously putting the two contrast samples under a 405 nm laser beam (440 Wcm−2) and the PL spectra were measured every three hours (Figure S11). Astonishingly, we found that TMCQ only showed a decrease in PL intensity, but no phase separation was observed. While the pure CsPbBr2.2Cl0.8 exhibited obvious phase separation after six hours under laser illumination. The single peak of CsPbBr2.2Cl0.8 was separated into two peaks and there was a tendency to gradually move closer to CsPbBr3 and CsPbCl3 phases. A significant fraction of global electricity demand is for lighting, it is always desirable to fabricate high-performance white-light devices, as their extremely high energy and economic value.41 The single component white light phosphors as single layer color conversion media for WLED are highly promising in practical application. To obtain the pure white light emission, HBr was employed to auxiliarily tune the emission spectra of TMCQ. As shown in Figures 6a and b, the color of TMCQ was tuned from pink to green when adding different ACS Paragon Plus Environment

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amounts of HBr to the samples with 1.7% Tm3+ and 0.2% Mn2+. With the increase in the molar ratio of Br-:Cl-, the CsPbBrxCl3-x host would compete with the energy transfer process and quench the emission of Mn2+. Therefore, as HBr increases, the emission peak of Mn gradually decreases, and then decreases the energy gap of 4T1-6A1,

17, 42-43

so the emission

peaks shift were observed. Among them, the pure white light TMCQ was successfully realized with CIE color coordinate of (0.34, 0.33) when added 25µl HBr to the reaction solution (Figure 6h). It is different from the traditional method of synthesizing white light perovskite QDs by varying the Mn2+ concentrations with low exciton energy transfer efficiency29 and PLQY,12,

22, 44-45

this pure white light TMCQ based on the ultra-highly

efficient exciton energy transfer strategy has a record white light PLQY up of 54% at the excitation wavelength of 365 nm. In addition, based on 365 nm LED chips and white light TMCQ phosphors (Figures 6f, g), the WLED (Figure 6e) and array table lamps device (Figures 6c,d) was successfully developed, which exhibit standard white light emission wih CIE color coordinate of (0.33, 0.34), as shown in Figure S12. Moreover, The CRI value of WLED is 91, which means that it has super high color fidelity and is very close to standard solar white light (Figure S13). Compared with the single component WLED developed in recent years in Table 1, the above excellent characteristics are extremely competitive and it will be beneficial to promote the practical application of perovskite-based white solid-state lighting and display. Conclusions In conclusion, a novel exciton energy transfer strategy was designed by constructing the intermediate gradient energy levels to effectively tune the photoluminescence properties of TMCQ, which enriches the research on the tunability of optical properties of all-inorganic perovskite. Based on it, the single component TMCQ with pure white light emission was obtained, which has a PLQY up to 54%. Further, the WLED with CRI up to 91 and CIE color ACS Paragon Plus Environment


coordinate of (0.33, 0.34) are fabricated. Moreover, this strategy can also remarkably improve the air and thermal stability of TMCQ. Therefore, we believe that this work not only blazes a new avenue toward tuning the photoluminescence properties of perovskite QDs and understanding the underlying mechanisms of exciton energy transfer more deeply, but also provides a new strategy to controllably synthesize single component white light perovskite QDs and further promote its practical application.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental methods, XPS survey analysis, HAADF-STEM images, the element ratio from EDS mapping, actual atoms concentration, the intensity comparison of Mn and host perovskite, the spectra of Mn2+/Yb3+ co-doped CsPbBr2.2Cl0.8, the PLQY of TMCQ, the schematic diagram of near-infrared-triggered photon upconversion system, the XRD results of two comparative samples after heat treatment, the illumination stability characterization, the average lifetimes fitting information, the The CIE of WLED. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Weiqing Yang: 0000-0001-8828-9862 Notes The authors declare no competing financial interest. Conflicts of interest There are no conflicts to declare. ACKNOWLEDGMENTS


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This work was financially supported by the National Natural Science Foundation of China (No. 51602265), the Independent Research Project of State Key Laboratory of Traction Power (Nos. 2017TPL_Z04 and 2016TPL_Z03), and the Fundamental Research Funds for the Central Universities of China (Nos. 2682016CX074, 2682017CY06, 2682016ZY01, 2682017ZDPY01 and 2682017CX071). The authors gratefully acknowledge ceshigo (www.ceshigo.com) for providing testing services. The authors gratefully acknowledge the Analysis and Testing Center of Southwest Jiaotong University. REFERENCES 1. Wu, Y.; Wei, C.; Li, X.; Li, Y.; Qiu, S.; Shen, W.; Cai, B.; Sun, Z.; Yang, D.; Deng, Z.; Zeng, H. In Situ Passivation of PbBr64– Octahedra toward Blue Luminescent CsPbBr3 Nanoplatelets with Near 100% Absolute Quantum Yield. Acs Energy Lett. 2018, 3, 20302037. 2. Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, B.; Shan, Q.; Han, B.; Zeng, H. 50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29. 1603885. 3. Wu, H.; Wang, S.; Cao, F.; Zhou, J.; Wu, Q.; Wang, H.; Li, X.; Yin, L.; Yang, X. Ultrastable Inorganic Perovskite Nanocrystals Coated with a Thick Long-Chain Polymer for Efficient White Light-Emitting Diodes. Chem. Mater. 2019, 31, 1936-1940. 4. Palazon, F.; Di Stasio, F.; Akkerman, Q. A.; Krahne, R.; Prato, M.; Manna, L. Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White ColorConversion Layers in LEDs. Chem. Mater. 2016, 28, 2902-2906. 5. Zhao, X. F.; Ng, J. D. A.; Friend, R. H.; Tan, Z. K. Opportunities and Challenges in Perovskite Light-Emitting Devices. Acs Photonics 2018, 5, 3866-3875. 6. Aamir, M.; Adhikari, T.; Sher, M.; Khan, M. D.; Akhtar, J.; Nunzi, J. M. Cesium Lead Halide Perovskite Nanostructures: Tunable Morphology and Halide Composition. Chem. Rec. 2018, 18, 230-238. 7. Bi, C.; Wang, S.; Wen, W.; Yuan, J.; Cao, G.; Tian, J. Room-Temperature Construction of Mixed-Halide Perovskite Quantum Dots with High Photoluminescence Quantum Yield. J. Phys. Chem. C 2018, 122, 5151-5160. 8. Yoon, H. C.; Kang, H.; Lee, S.; Oh, J. H.; Yang, H.; Do, Y. R. Study of Perovskite QD Down-Converted LEDs and Six-Color White LEDs for Future Displays with Excellent Color Performance. ACS Appl. Mater. Interfaces 2016, 8, 18189-18200. 9. Guria, A. K.; Dutta, S. K.; Adhikari, S. D.; Pradhan, N. Doping Mn2+ in Lead Halide Perovskite Nanocrystals: Successes and Challenges. Acs Energy Lett. 2017, 2, 1014-1021. 10. Liu, H.; Wu, Z.; Shao, J.; Yao, D.; Gao, H.; Liu, Y.; Yu, W.; Zhang, H.; Yang, B. CsPbxMn1–xCl3 Perovskite Quantum Dots with High Mn Substitution Ratio. ACS nano 2017, 11, 2239-2247. 11. De, A.; Mondal, N.; Samanta, A. Luminescence tuning and exciton dynamics of Mndoped CsPbCl3 nanocrystals. Nanoscale 2017, 9, 16722-16727.



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Figures and Table:

Figure 1. Morphology and structural characterization. (a-c) TEM images and (d-f) HRTEM images of TMCQ with different Tm3+ doping concentrations but the Mn2+ concentrations is fixed at 0.2%. (g) XPS survey images of TMCQ with 1.7% Tm3+ and 0.2% Mn2+. (h) XRD patterns of the TMCQ with different Tm3+ doping concentrations but the Mn2+ concentrations is fixed at 0.2%. (i-n) EDS mapping images of TMCQ with 1.7% Tm3+ and 0.2% Mn2+.



Figure 2. Photoluminescence properties modulation. (a-h) PL spectra of the samples with different Tm3+ doping concentrations but the Mn2+ concentrations is fixed at 0.2%.

Figure 3. Principle of photoluminescence properties modulation. (a) CIE color coordinate and the corresponding colloidal solution under 365 nm ultraviolet lamp with different Tm3+ doping concentrations. (b) Crystal structure and multicolour films under 365 nm ultraviolet lamp. (c) PL spectra of a series of samples with different concentrations of Tm3+. (d, e) Photophysical processes and principle of intermediate gradient energy level. (f) The PL spectra of only Tm3+ doped CsPbBrxCl3-x QDs, there are no other additional peak except for the blue shift caused by the increase of Cl- proportion. Therefore, it safely to exclude intrinsic emission of Tm3+ at about 600 nm.


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Figure 4. (a) Mn2+ decay lifetimes from sample 2 and sample 3. (b) Host perovskite decay lifetimes from sample 1, sample 2 and sample 3.

Figure 5. Air and thermal stability. Degradation PL spectra of TMCQ with different Tm3+ doping concentrations after exposing to air for 60h (a-c), and storing in 80℃ vacuum atmosphere for 24h (d-f).



Figure 6. Single component white light TMCQ. (a, b) The PL spectra and CIE color coordinate of TMCQ with different concentrations of HBr. (c-e) Array table lamps and WLED made by introducing white light TMCQ phosphors as single layer color conversion media. (f, g) The white TMCQ phosphors under visible light and ultraviolet light. (h) PL spectrum and colloid of TMCQ with pure bright white emission.


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Table 1. Parametric comparison of single component perovskite WLED developed in recent years Excitation

White Light Emitting Materials

UV-LEDs UV-LEDs UV-LEDs Blue-LEDs UV-LEDs UV-LEDs UV-LEDs UV-LEDs UV-LEDs Blue-LEDs UV-LEDs UV-LEDs UV-LEDs UV-LEDs UV-LEDs UV-LEDs

Mn:CsPb2ClxBr5−x CsPb0.76Zn0.014Cl0.93Br2.06:0.08Mn2+ α-(DMEN)PbBr4 CsPbBr3/CsPbBrxI3-x (N-MEDA) PbBr4 EA4Pb3Br0.5Cl9.5 (PEA)2PbCl4 (N-MEDA)PbCl0.5Br3.5 CsPb0.81Mn0.19Cl3 Tb3+/Eu3+:CsPbBr3 CsPb(Br/I)3@anthracene (C4H9NH3)2PbCl4 (OA: MA)PbX3 C4N2H14Pb0.992Mn0.008Br4 CsPb(Br/I)3@anthracene Tm3+/Mn2+:CsPbBr2.2Cl0.8

CIE (0.35, 0.32) (0.35, 0.30) (0.33, 0.33) (0.35, 0.38) (0.36, 0.41) (0.33, 0.33) (0.37,0.42) (0.31, 0.36) (0.34, 0.32) (0.33, 0.34) (0.36, 0.36) (0.37, 0.40) (0.33, 0.36) (0.37, 0.37) (0.33, 0.33) (0.33, 0.34)


CRI

White Range

Reference

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Cold Cold Natural Natural Cold Natural Natural Cold Cold Natural Warm Warm Natural Natural Natural Natural

16 22 45 46 47 48 49 47 50 35 51 52 41 53 51 This work

Constructing Gradient Energy Levels to Promote Exciton Energy

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