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Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications

Hui-wen LIU Dong YAO Yi LIU Hao ZHANG

刘慧雯, 姚栋, 刘轶, 张皓. 锰离子掺杂纯无机钙钛矿纳米晶及应用[J]. 中国光学, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
引用本文: 刘慧雯, 姚栋, 刘轶, 张皓. 锰离子掺杂纯无机钙钛矿纳米晶及应用[J]. 中国光学, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
LIU Hui-wen, YAO Dong, LIU Yi, ZHANG Hao. Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications[J]. Chinese Optics, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
Citation: LIU Hui-wen, YAO Dong, LIU Yi, ZHANG Hao. Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications[J]. Chinese Optics, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933

锰离子掺杂纯无机钙钛矿纳米晶及应用

doi: 10.3788/CO.20191205.0933
基金项目: 

中国国家重点研发计划 2016YFB0401701

国家自然科学基金 21773088

国家自然科学基金 51425303

吉林大学科技创新研究团队 2017TD-06

吉林省科技发展计划 20190103024JH

详细信息
  • 中图分类号: O631

Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications

Funds: 

the National Key Research and Development Program of China 2016YFB0401701

National Natural Science Foundation of China 21773088

National Natural Science Foundation of China 51425303

JLU Science and Technology Innovative Research Team 2017TD-06

the Jilin Province Science and Technology Research 20190103024JH

More Information
    Author Bio:

    LIU Hui-wen (1993-), Ph.D.candidate, State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun.Her research interests are on the synthesis of perovskite nanocrystals and their applications in LEDs.E-mail:liuhuiwenjlu@163.com

    ZHANG Hao (1976-), Professor, State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun.His research interests are on the synthesis and controllable self-assembly of photoelectric functional nanocrystals and polymer-based nanocomposites.E-mail:hao_zhang@jlu.edu.cn

    Corresponding author: ZHANG Hao, E-mail:hao_zhang@jlu.edu.cn
  • 摘要: 胶体锰离子掺杂的纯无机钙钛矿纳米晶由于其优异的光电性质,使其作为一种新兴的荧光发射材料,被研究者们广泛研究。不仅如此,纯无机钙钛矿纳米晶的锰离子掺杂行为也揭示了由于掺杂过程和掺杂剂本身引起的新的光学性质。通过不同的合成方法和选择不同的锰前驱体可以实现不同的掺杂行为,以及由此引发不同的荧光性质。在高带隙钙钛矿主体中进行锰离子掺杂时,其中激发能量由钙钛矿主体转移到掺杂锰离子位点的d态,进而产生橙黄色d-d发射荧光。研究者们一直致力于理解锰离子掺杂过程并由此设计高效掺杂的纳米晶。这些锰离子掺杂的钙钛矿纳米晶由于具有独特的电子和光学特性使其在发光二极管和太阳能电池等应用中发挥了巨大的作用。结合之前的相关工作和进展,本综述重点总结了锰离子掺杂的纯无机钙钛矿纳米晶的合成方法、发光来源、发光机理和潜在应用的最新进展,并提出了未来潜在合理的研究方向。
  • Figure  1.  Summary of various synthesis methods for the synthesis of Mn2+-doped CsPbX3 NCs. (a)Sketch of the hot-injection method. (b)Scheme of LARP synthesis approach. (c)Room temperature post-synthesis method. (d)Microwave-assisted synthesis method. (e)Solvothermal synthesis method

    Figure  2.  Summary of the selection of Mn sources for various synthesis methods of the Mn2+-doped CsPbX3 NCs. The most used MnCl2(a) and (b)MnBr2 with the aid of HBr(c) MnAc2 with the aid of HCl(d) Mn-stearate(e) manganese acetate, manganese acetylacetonate, and manganese halides with the aid of benzoyl halide(f) as the Mn sources participated in the reaction

    Figure  3.  (a) Sketch of 0D CsPbX3 QDs, (b) and (c)Mn2+-doped 0D CsPbX3 QDs, (d)sketch of 2D CsPbX3NSs, (e) and (f)Mn2+-doped 2D CsPbX3NSs or NPLs, (g-j) TEM images of NCs with different Mn2+ substitution ratios(color version please see in the journal website)

    Figure  4.  (a) The host CsPbX3 band gap and relative positions of Mn2+4T1 and 6A1 states, (b)PDOS of CsPbCl3, CsPb0.875Mn0.125Cl3 and CsPb0.75Mn0.25Cl3, respectively, (c)the synthesis scheme of CsPbxM1-xBr3 NCs by triggering Cs4PbBr6 NCs transformation with MnBr2 salts, (d-e)overview of enhanced stability in optical and structural properties of CsPbxMn1-xI3 NCs(color version please see in the journal website)

    Figure  5.  (a) and (b) Mn2+-doped CsPbX3 NCs as color-converting materials for LEDs. (c)LEDs with white color emission. (d)Mn2+-doped CsPbBr3 NCs for electroluminescent LED devices. (e) and (f) Mn2+-doped CsPbX3 NCs for solar cells

    Table  1.   Comparison of the performance parameters of PLEDs based on different Mn-substitution ratio

    Von
    (V)
    Max
    EQE(%)
    Max.CE
    (cd·A-1)
    Max.PE
    (lm·W-1)
    Device structure
    PLED-pure 3.6 0.81 3.71 0.70 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
    PLED-Mn2.6 3.5 0.95 4.33 0.72 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
    PLED-Mn3.8 4.2 1.49 6.41 1.14 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
    下载: 导出CSV

    Table  2.   Comparison of the performance parameters of PSCs based on different CsPbBrI2 films

    Jsc(mA/cm2) Voc/V FF/% PCE/% Jsc(EQE)(mA/cm2)
    MnCl2-0.5% 14.21 1.133 76.8 12.36 13.86
    MnCl2-1% 14.29 1.144 79.9 13.07 13.93
    MnCl2-2% 14.37 1.172 80.0 13.47 14.09
    下载: 导出CSV

    Table  3.   Key J-V parameters of PSCs with different coated layer thicknesses of CsPbCl3:0.1Mn QDs

    QDs(mg/mL) Jsc(mA/cm2) Voc/V FF/% PCE/%
    CsPbCl3-xMn 1 21.42 1.105 76.4 18.08
    CsPbCl3-xMn 5 22.03 1.105 76.3 18.57
    CsPbCl3-xMn 20 20.73 1.105 76.6 17.55
    下载: 导出CSV
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出版历程
  • 收稿日期:  2019-04-10
  • 修回日期:  2019-05-07
  • 刊出日期:  2019-10-01

Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications

doi: 10.3788/CO.20191205.0933
    基金项目:

    中国国家重点研发计划 2016YFB0401701

    国家自然科学基金 21773088

    国家自然科学基金 51425303

    吉林大学科技创新研究团队 2017TD-06

    吉林省科技发展计划 20190103024JH

    通讯作者: ZHANG Hao, E-mail:hao_zhang@jlu.edu.cn
  • 中图分类号: O631

摘要: 胶体锰离子掺杂的纯无机钙钛矿纳米晶由于其优异的光电性质,使其作为一种新兴的荧光发射材料,被研究者们广泛研究。不仅如此,纯无机钙钛矿纳米晶的锰离子掺杂行为也揭示了由于掺杂过程和掺杂剂本身引起的新的光学性质。通过不同的合成方法和选择不同的锰前驱体可以实现不同的掺杂行为,以及由此引发不同的荧光性质。在高带隙钙钛矿主体中进行锰离子掺杂时,其中激发能量由钙钛矿主体转移到掺杂锰离子位点的d态,进而产生橙黄色d-d发射荧光。研究者们一直致力于理解锰离子掺杂过程并由此设计高效掺杂的纳米晶。这些锰离子掺杂的钙钛矿纳米晶由于具有独特的电子和光学特性使其在发光二极管和太阳能电池等应用中发挥了巨大的作用。结合之前的相关工作和进展,本综述重点总结了锰离子掺杂的纯无机钙钛矿纳米晶的合成方法、发光来源、发光机理和潜在应用的最新进展,并提出了未来潜在合理的研究方向。

English Abstract

刘慧雯, 姚栋, 刘轶, 张皓. 锰离子掺杂纯无机钙钛矿纳米晶及应用[J]. 中国光学, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
引用本文: 刘慧雯, 姚栋, 刘轶, 张皓. 锰离子掺杂纯无机钙钛矿纳米晶及应用[J]. 中国光学, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
LIU Hui-wen, YAO Dong, LIU Yi, ZHANG Hao. Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications[J]. Chinese Optics, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
Citation: LIU Hui-wen, YAO Dong, LIU Yi, ZHANG Hao. Mn2+-doped CsPbX3 (X=Cl, Br and I) perovskite nanocrystals and their applications[J]. Chinese Optics, 2019, 12(5): 933-951. doi: 10.3788/CO.20191205.0933
    • Colloidal all-inorganic cesium lead halide(CsPbX3) perovskite nanocrystals(NCs) have become a subject of intense research in recent years due to their superb luminescence properties and facile chemical tunability of the bandgap, which have shown great potential for outperforming commonly used Ⅱ-Ⅵ and Ⅲ-Ⅴ NCs in their ability to harvest photons, create charge carriers, and efficiently generate photons from the recombination of charge carriers[1-13]. As an already-demonstrated strategy for controlling over the electronic and optical properties of semiconducting NCs, doping impurity ions into NC hosts has been applied in traditional Ⅱ-Ⅵ and Ⅲ-Ⅴ NCs[14-19]. Specifically, doping with transition metal ions has been extensively explored as a way to introduce the possibility of creating a charge and size imbalance center in the host lattice with new optical, electronic, and magnetic properties, making them much more functional than their host NCs[20-22]. A variety of impurity dopants, including Mn2+, Cu2+, Ag+, Co2+, and Eu2+, have been incorporated to improve their original properties[23-25]. For instance, Mn2+ can generate intense sensitized dopant luminescence and create a magnetically coupled excitonsstate[16-19]. These new properties of Mn2+-doped NCs result from the exchange coupling between the charge carriers of the host semiconductor and d electrons of the dopant, which opens new pathways of energy exchange or forms new coupled electronic states between the exciton and dopant[14-15]. The above properties make Mn2+doped NCs attractive for light-emitting diodes(LEDs), luminescent solar concentrators, and related photonic technologies[26-28].

      Based on the increased understanding of the doping mechanism, Mn2+ doping is now performed in CsPbX3(X=Cl, Br and I) perovskite NCs[29-31]. Mn2+ ions, occupying the substitution position, can also be doped in CsPbX3NCs stably with sufficiently strong exchange coupling between the charge carriers. This would preferably be incorporated during perovskite NCs formation, which introduces new optical, electronic and magnetic properties[32]. Importantly, the intrinsic ionic characteristics and flexibility of the perovskite crystal structure of CsPbX3 NCs allow for the strong possibility of Mn2+ ions being doped in hosts to tune their properties, which has already attracted extensive interest[33-34]. The first successful doping of Mn2+ ions in CsPbX3 NC host was demonstrated in CsPbCl3, which was achieved by a simple modification of the usual hot-injection synthesis method, i.e., adding MnCl2 as an additional reactant[29]. Subsequently, various other methods were developed to synthesize Mn2+-doped CsPbX3 NCs in a wider range of doping levels[35-38]. Similarly, the high-energy host emission is switched to Mn yellow-orange emission; however, they reveal different doping paths and several new findings[39-45]. Until now, synthesis of Mn2+-doping in CsPbX3 NCs remains one of the key issues for developing new optical and electronic properties, which attracts increasing attention[45-48].

      In this review, we mainly focus on the progress and challenges of Mn2+doped CsPbX3 perovskite NCs. Recent developments, applications, and setbacks of this new class of materials are summarized. Firstly, we give a brief description of the selection of Mn sources for dopant and preparation methods. Secondly, we emphasize their optoelectronic properties, doping and emission mechanisms and stability. Thirdly, the various applications of this new class of materials are reviewed. Finally, we summarize the existing challenges facing this research and give an outlook on probable ways to mitigate such challenges with our vision for the future of the Mn2+-doped perovskite NCs.

    • The most widely reported and most extendly used doping strategy in traditional semiconductor NCs is growth doping in which dopants are allowed to be absorbed onto host NCs during NCs growth[19, 49-50]. Another widely accepted doping protocol is diffusion doping performed mostly via thermal annealing, in which added dopant ions substitute host ions by ion exchange and reside in the crystal lattice[16, 51-52]. However, recently developed strategies for doping in perovskites suggest that a dopant precursor is required at the beginning of the reaction, which does not follow conventional nucleation doping[29-31]. In the case of doped perovskite NCs, the most widely used strategy is reportedly simultaneous formation[29-35]. Hence, Mn precursors are introduced along with Pb precursors at the beginning. Mn2+-doped CsPbX3 NCs can be achieved using various methods similar to the traditional semiconductor NCs, which are summarized as follows.

    • Colloidal synthesis of halide perovskite NCs at relatively high temperatures of 140-200 ℃ was firstly reported by Protesescu et al.[53]. This synthesis process is illustrated in Fig. 1(a). This method takes advantage of the ionic nature of chemical bonding in CsPbX3 compounds and the majority of crystal growth occurs within the first 3 seconds after the injection of Cs-oleate into the mixture of PbX2 and octadecene(ODE) owing to the fast nucleation and growth kinetics[53]. This can be completed by many groups, including our group. The synthesis of Mn2+-doped CsPbX3 NCs where MnX2, PbX2 and oleic acid(OA), oleylamine(OLA), and ODE were completely dissolved in crude solution with the subsequent injection of Cs-oleate solution[29-31]. Our group also achieved different Mn substitution ratios by manipulating the molar feed ratios and reaction temperatures, such as a high Mn substitution ratio of 46% with high photoluminescence(PL) efficiencies of 54%[31].

      Figure 1.  Summary of various synthesis methods for the synthesis of Mn2+-doped CsPbX3 NCs. (a)Sketch of the hot-injection method. (b)Scheme of LARP synthesis approach. (c)Room temperature post-synthesis method. (d)Microwave-assisted synthesis method. (e)Solvothermal synthesis method

    • The LARP at room temperature for the fabrication of halide perovskite NCs was developed by Zhang and co-workers(Fig. 1(b))[10]. Specifically, halide perovskite NCs were simply obtained by vigorously stirring a halide perovskite precursor solution including PbX2(X=Cl, Br, and I), CsX, DMF, and long-chain organic ligands such as n-octylamine and oleic acid in a poor solvent such as toluene or hexane[10]. In this way, an Mn-precursor will be added together into the DMF solution. In detail, CsX, MnX2 and PbX2, OA, and OAm are dissolved in DMF or DMSO, and toluene is dropped into the resulting mixture. The lower solubility of ions compared to DMF in the toluene induces rapid recrystallization and simultaneous Mn2+ doping into the NCs, where surface ligands control the size and morphology of NCs. Li and coworkers developed this room temperature synthesis method to achieve Mn2+-doped cesium lead halide quantum dots(QDs) with a high Mn substitution ratio. The low-temperature reaction prefers to occur in the metastable phase[54]. Furthermore, the as-prepared perovskite QDs exhibits bright orange emission owing to an ultrahigh level of Mn2+ doping[54].

    • Nag et al. developed a post-synthesis strategy for Mn2+ doping in colloidal CsPbX3 NCs(Fig. 1)[55]. Firstly, CsPbX3 NCs with the desired composition, size, and shape were prepared following reported protocols[53]. Then, a post-synthesis doping methodology using the precursor with MnBr2 dissolved in a mixture of acetone and toluene was used, employing a 1 minute reaction at room temperature[55]. This post-synthesis doping protocol provides a unique opportunity to achieve different dopant concentrations, which eliminates most of the synthesis-related inhomogeneity to make the study of the effect of dopant concentration more reliable. Besides, this 1 min post-synthesis doping procedure can probably be extended to other sets of dopants and perovskite NCs.

    • The microwave-assisted method plays an important part in the synthesis of various traditional semiconductor NCs, which is usually highly efficient but time- and energy-consuming(Fig. 1(d))[56-60]. Our group proposed this microwave-assisted method for the synthesis of Mn-doped CsPbCl3[61]. Cesium acetate (CsOAc), bis(2, 4, 4-trimethylpentyl) phosphinic acid(TMPPA), PbCl2, MnCl2, and ODE can be put in one beaker with certain microwave power to initiate the reaction. It should be known that the higher reaction activity of CsOAc plays an important role in the fast reaction rate of the microwave-assisted method[61]. The Mn substitution ratio can reach 27% with the microwave-assisted power of 100 W. Other groups also developed the microwave-assisted methods for achieving high PL efficiency CsPbX3 NCs covering the fully visible spectrum[62-64].

    • The solvothermal method has been considered as the most promising route for the preparation of various NCs due to its simple procedure, precise control over morphology, high crystallinity and easy reproduction[65-68]. Li′s group developed a facile solvothermal strategy to synthesize Mn2+-doped CsPbCl3 NCs, which shows better stability than those fabricated by hot injection(Fig. 1(e))[69].

    • Inspired by the developed CH2X2(X=Cl, Br) photo-induced anion exchange, Qiao et al. demonstrated that photo excitation results in the cation exchange and the formation of Mn2+-doped CsPbX3 NCs in the presence of a small amount of dissolved Mn acetate in CH2X2[70].

      Overall, the two-step standard hot-injection synthesis method has reached its maturity and can now be used to produce various monodisperse Mn-doped perovskite NCs with excellent control over the shape of the NCs. While the aforementioned hot-injection methods are particularly appropriate for producing Mn-doped perovskite NCs samples with a high degree of control, they have two main draw backs:the synthesis needs to be performed in air-free conditions and it is hard to employ them for large-scale production. These problems can be avoided by employing alternative synthesis routes, such as the LARP and the heat-up "related" approaches(microwave and solvothermal synthesis methods), which can yield NCs ingram scale even under air atmosphere. More in details, the heat-up, solvothermal, photo-induced and microwave techniques can easily be used to produce mainly Mn-doped CsPbX3 NC systems in large quantities and with high PLQYs.

    • In the synthesis of Mn2+-doped perovskite NCs, Mn precursor powders are usually introduced along with Pb precursor at the beginning of the procedure[29-31]. The proper Mn precursor seems to be essential for the doping process. Firstly, among various manganese(Ⅱ) salts, MnX2(X=Cl, Br, and I) is given priority in doping perovskite NCs due to its ability to easily break the Mn-X bond for further doping[29-35]. The MnX2, MnCl2 was proved to be the superior precursor for doping in CsPbCl3(Fig. 2(a)). Our group used MnCl2 to achieve the Mn2+doped CsPbCl3 NCs with cube-shaped tetragonal CsPbCl3 nanostructures(Fig. 2(b))[31, 61]. We also noticed that with higher doping efficiency, the crystalline shape of the host diminishes[31, 61]. Nevertheless, we also achieved a very high Mn:Pb precursor ratio(10:1) for 46% Mn2+ doping at 210 ℃[31]. Interestingly, Liu et al. reported that this can also be directly synthesized using MnBr2 and PbCl2 to achieve Mn:CsPbCl3-xBrx NCs because the weaker Mn-Br bond would more easily be broken compared to MnI2 and PbCl2[29]. On the other hand, direct doping was observed to be difficult for CsPbBr3 and CsPbI3 irrespective of using MnBr2 and MnI2 as a dopant precursor. At this point, Parobek et al. have achieved MnBr2-doped CsPbBr3 NCs, but did so with an excess of HBr to supply the amount of Br(Fig. 2(c))[71]. However, methods of directly doping the CsPbI3 NCs with MnI2 as dopant precursor have not yet been published. Rather, Br and I were incorporated via anion exchange on Mn:CsPbCl3 or the synthesis was carried out in mixed halides(discussed later). Secondly, Xu et al. synthesized the Mn2+-doped CsPbCl3 perovskite NCs utilizing MnAc2 as an Mn precursor. HCl was an important and necessary raw material to be added in the room-temperature reactions to effectively motivate the formation of Mn-Cl bond(Fig. 2(d))[72]. Thirdly, Lin et al. used Mn-stearate as the Mn-precursor for the doping of Mn2+ into perovskite CsPbCl3 QDs via a facile colloidal hot-injection approach(Fig. 2(e))[39]. Recently, high reaction activity halide precursors like benzoyl halide is used for the synthesis of high PL efficiency CsPbX3 NCs(Fig. 2(f))[73], and we found that high reaction activity halide sources make it possible for various Mn precursors(manganese acetate, manganese acetylacetonate, and manganese halides etc.) to participate in the doping process. These can also achieve high PL efficiencies and high Mn substitution ratios. The selection of the Mn precursor seems to play an important part in the synthesis of Mn2+-doped CsPbX3 NCs. Based on the above Mn2+-doped examples, as Mn substituted the Pb in the crystal lattice, it was evident that Mn-X bond strength in the Mn-precursor should be comparable to the Pb-X bond strength in CsPbX3 for successful doping. To this point, the Mn-Cl bond is most benefit to the bond broken and further diffused into perovskite NCs lattice. Based on the above discussion, excess chloride ions mostly facilitated Mn2+ doping in CsPbCl3 NCs and chloride ions were the key for promoting the insertion of Mn2+ in perovskite nanocrystals for room-temperature reactions. Therefore, if the Mn sources are chosen without MnX2, the extra halide ions should be added as the supplement to guarantee the Mn-doped process.

      Figure 2.  Summary of the selection of Mn sources for various synthesis methods of the Mn2+-doped CsPbX3 NCs. The most used MnCl2(a) and (b)MnBr2 with the aid of HBr(c) MnAc2 with the aid of HCl(d) Mn-stearate(e) manganese acetate, manganese acetylacetonate, and manganese halides with the aid of benzoyl halide(f) as the Mn sources participated in the reaction

    • Tremendous progress has been made in this method over the past two years. Various low-dimensional morphologies, including zero-dimensional(0D) morphologies, such as QDs and nanoparticles; one-dimensional(1D) morphologies such as nanowires and nanorods; and two-dimensional(2D) morphologies perovskite, such as nanoplatelets(NPLs) and nanosheets(NSs)(Fig. 3(a) and 3(d)) are developed by more and more researchers[74-75]. In this section, we discuss the Mn2+-doped perovskite NCs that have been reported recently with controlled morphologies, such as QDs, NPLs, NSs, etc.

      Figure 3.  (a) Sketch of 0D CsPbX3 QDs, (b) and (c)Mn2+-doped 0D CsPbX3 QDs, (d)sketch of 2D CsPbX3NSs, (e) and (f)Mn2+-doped 2D CsPbX3NSs or NPLs, (g-j) TEM images of NCs with different Mn2+ substitution ratios(color version please see in the journal website)

    • The first synthesis of cesium lead halide(CsPbX3, X=Cl, Br, I) perovskite QDs was reported by Kovalenko et al. in 2015 based on the traditional hot-injection methods in the presence of OLA and OA as ligands[53]. Inspired by this, Mn2+ ions doping NCs are mainly focused on the 0D perovskite QDs, usually obtained by the solution phase synthesis methods. Parobek′s group, Liu′s group, and our group almost simultaneously reported colloidal Mn2+-doped CsPbCl3 nanocubes, and the morphologies of as-prepared NCs showed no difference after Mn doping (Fig. 3(b) and 3(c))[29-31]. Totally, Mn2+-doping into CsPbX3 QDs exhibited strong dopant luminescence characteristic in the d-d transition of Mn2+ ions resulting from the exciton-to-Mn energy transfer, which is similar to the case of Mn2+-doped Ⅱ-Ⅵ QDs[32-33].

    • Perovskite materials have been included in the class of 2D semiconductor materials, mainly in the form of NPLs. However, unlike other materials of this type, which are covalent semiconductors, these 2D morphologies perovskites are also ionic materials, endowing them with special properties. Because of this, developing the Mn2+-doped 2D morphologies perovskites has the potential for unexpected energy or electron transfer owing to 2D morphologies perovskites, which can enhance fluorescence emission decay rates and higher exciton binding energies. The fabrication of perovskite NPLs through both solution-phase synthesis and vapor phase deposition techniques have been reported. For Mn2+-doped NCs, the methods mostly used are solution-phase synthesis. Nag et al. reported the colloidal Mn2+-doped cesium lead halide perovskite NPLs, which was the first report of Mn2+ doping in a CsPbX3 host exhibiting strong quantum confinement of charge carriers[76]. The fluorescent properties are similar to the Mn2+-doped perovskite nanocubes(Fig. 3(e))[76]. These Mn2+-doped CsPbX3NPLs are suitable candidates for exploring the effects of quantum confinement on dopant-carrier exchange interaction and exhibiting interesting magneto-optic properties[76]. Kundu et al. reported Mn2+-doped 2D morphologies perovskites could be a suitable material to tune dopant-exciton exchange interactions and further explore their magneto-optoelectronic properties[77]. The successful production of these perovskite NPLs has introduced a new family of 2D semiconductors for nanoscale and printable optoelectronic devices[77]. Also, Pradhan et al. reported the dimension tunability governed by the amount of Mn composition retained in the layered structure(Fig. 3(f))[78]. Totally, as Mn2+ doping in perovskites have opened up a new window of tuning the optical properties for perovskites, the discussed new physical process of doping and the evolution of the dopant emission will certainly help better understand doping and its mechanism.

    • In all the above-discussed reports of Mn2+-doped CsPbX3 nanocubes, the samples all show a broad PL with a peak at around 580 nm because of the Mn2+ d-d transition, along with the excitonic PL of CsPbCl3 with a peak at about 405 nm(Fig. 3(b)). Subsequently, we are most concerned about the doping and emission mechanism of Mn2+-doped CsPbCl3 NCs. This Mn2+ d-d emission is a spin-forbidden 4T1 to 6A1 transition with microsecond to millisecond lifetimes[32-33]. The host NC absorbs UV excitation light to form photo-generated excitons, which then transfer their energy to the dopant ions (Mn2+ ions) forming the excited 4T1 state of Mn2+ d-electrons, followed by 4T1 to 6A1 de-excitation process, emitting orange-red emission light[32-33]. Li et al. further illustrates the respective partial density of state(PDOS) of CsPbCl3, CsPb0.875Mn0.125Cl3 and CsPb0.75Mn0.25Cl3, which shows that the conduction band and the upper valence band are mainly dominated by the electrons of the Pb(4p) and Cl(3p) orbits, while Cs seems to have no apparent contribution. This may explain the band-edge emission of CsPbCl3 QDs(Fig. 4(b))[54]. It also shows that the contribution of Mn(d) orbits is obvious to both the conduction band and the upper valence band. Additionally, the energy of the d-d transition in Mn2+ ions is lower than the energy gap of CsPbCl3, causing energy transfer between excitons and Mn2+ ions[54]. The calculation results imply that the dual-color emission is most likely caused by the band-edge emission of CsPbCl3 QDs combined with the d-d transition in Mn2+ ions, which is consistent with the results of the PL spectra.

      Figure 4.  (a) The host CsPbX3 band gap and relative positions of Mn2+4T1 and 6A1 states, (b)PDOS of CsPbCl3, CsPb0.875Mn0.125Cl3 and CsPb0.75Mn0.25Cl3, respectively, (c)the synthesis scheme of CsPbxM1-xBr3 NCs by triggering Cs4PbBr6 NCs transformation with MnBr2 salts, (d-e)overview of enhanced stability in optical and structural properties of CsPbxMn1-xI3 NCs(color version please see in the journal website)

      Energy transfer(ET) from the exciton to Mn2+ is orders of magnitude slower than that in Mn2+-doped traditional semiconductor NCs(CdS/ZnS:Mn2+). Even for high Mn2+ doping concentrations, exciton emission from the perovskite host is still present. The inefficient ET has been attributed to the more ionic character of the perovskite NCs and the weaker confinement which reduces ET. To further understand the exciton-to-Mn2+ ET process, Xu et al. investigate the evolution of the exciton-to-Mn2+ ET efficiency as a function of composition and temperature in CsPbCl3-xBrx:Mn2+ NCs, which shows a strong dependence of the transfer efficiency on Br- content[41]. An initially fast increase is followed by a decrease for higher Br- concentrations, which are explained by a reduced exciton decay rate and faster exciton-to-Mn2+ ET upon Br-substitution. Further addition of Br- makes back-transfer from Mn2+ to the CsPbCl3-xBrx host possible and lead to a decrease in Mn2+ emission[41]. The full understanding of the ET transfer dynamics of Mn2+-doped perovskite NCs will aid to optimize the ET and Mn2+ luminescence efficiency.

    • Samanta′s group concluded that, as indicated by the electron paramagnetic resonance(EPR) spectra, the distribution of Mn2+ in doped CsPbCl3 NCs is uniform up to a Mn-concentration of 2% and for the higher Mn-content(> 15.5%), doped Mn2+ remains inside the NCs rather than on the surface[79]. In this regard, our group also gets the same conclusion in the characterization of XRD analysis[31]. When the Mn2+ doping concentration is higher, Mn2+ ions may also diffuse into the CsPbCl3 lattice and further occupy the Pb sites. Besides, by combining the EPR pattern and Mn2+ PL decay behavior, it can be concluded that in addition to the crystal field effect, the Mn-Mn exchange interaction also contributes to the red-shift of the Mn PL band at higher Mn2+ dopant concentrations[79].

    • The PLQY is an important factor in determining the performance of NC-based devices. Defects and traps in NCs can act as nonradiative recombination centers to reduce the PLQYs. Luminescence decay curves can provide insights into the PLQYs, because nonradiative recombination from the excited state shortens the luminescence lifetime, and inhomogeneities or other features may result in multiexponential decay. Meijerink et al. found that elongated Mn2+ PL decay and a change from multiexponential decay to single exponential decay have been observed upon growth of CsPbCl3 shells around the Mn2+:CsPbCl3 NCs, which separates the Mn2+ ions from the NC surfaces[41].

      As to the influence of PLQYs of Mn2+-doped perovskite NCs, both the Mn2+ concentration and temperature have an impact on the emission of Mn2+:CsPbCl3 NCs. The PLQYs of Mn2+ emission in Mn2+ doped CsPbX3 NCs has been described in several studies and all the studies found that Mn2+ concentration will influence the PLQYs. Klimov′s group shows the PLQYs were increased to a maximum value of 27% by increasing doping levels to a B-site Mn2+ concentration of 9.6%[29]. Our group also reported that Mn2+:CsPbCl3 NCs showed a peak Mn2+ PLQYs of 54% at 27% for Mn2+ concentration, but NC crystallinity was found to deteriorate at higher Mn2+ loading while the PLQY decreased[31]. So, proper Mn2+ concentration plays an important part in the contribution of the doped perovskite NCs to the PLQYs. Secondly, temperature is an important variable of PL. Variable-temperature experiments can elucidate fundamental features of an NCs excited-state dynamics. Meijerink et al. reported that upon raising the temperature, the peak intensity of the Mn2+ doped CsPbCl3 NCs shows a continuous decrease, losing nearly 50% of peak intensity at 383 K[41]. Cui's group also shows that the increased temperature from 278 to 323 K leads to gradual PL quenching(as a result of the thermally activated trapping of charge carrier) and emission red-shift due to the thermal expansion of the crystalline lattice[44].

    • Though Mn2+ doping is successful in nanocubes and NPLs of CsPbCl3, it was found to be difficult in other halides counterparts like CsPbBr3 and CsPbI3. Consequently, the anion exchange process is employed for converting Mn2+-doped CsPbCl3 NCs to other halide systems with partial success. The doping mechanism of Mn2+ in different halide compositions of CsPbX3 NCs is also a key issue to be solved. The exciton energy transfer to Mn2+ d-states typically depends on the host band gap and relative positions of Mn2+ 4T1 and 6A1 states[32-33]. As shown in Fig. 4(a), CsPbCl3 is ideal for accomplishing Mn2+ emission. Br incorporation, to some extent, red-shifts the absorption, but it would still be possible for Mn2+ ion energy transfer[33].

      Band gap and optical properties of CsPbX3 perovskites are typically tuned with halide ion exchange. Hence, doping Mn2+ is also extended to CsPbBr3 and CsPbI3 NCs, and the possible exciton energy transfer inducing Mn2+ emission in these nanostructures is investigated. For Br systems, Parobek et al. achieved directly MnBr2-doped CsPbBr3 NCs through an excess of HBr to supply an efficient amount of Br[71]. It can also be established by the anion exchange reaction from CsPbCl3 NCs, whose ions exchange from Cl to Br with retaining Mn2+ in host NCs and also reflects the change in PL spectra showing dual emission during this process. Importantly, as to the MnBr2-doped CsPbBr3 or CsPbClxBr1-x NCs, the QY of dopant emission was also observed to be significantly lower than in the Cl system[29-31, 71]. In comparison to CsPbCl3, the energy differences remain at a minimum between the excited stated of CsPbBr3 and Mn2+. This might be one of the reasons the exciton energy is not efficiently transferred to Mn2+ states, leading to poor dopant emission. On the other hand, the exciton emission intensity is significantly enhanced in Mn2+doped CsPbBr3. Klimov et al. analyzed this energy gap and correlated the possibility of both forward and backward transitions between the host and dopant states. Similar observations were also recorded for I- exchange where dopant emissions were finally quenched. These results suggest that the confined lead chloride is the most appropriate perovskite host for transferring the exciton energy to the dopant state which results in intense dopant emission.

    • Zhao′s group developed a novel high concentration doping method based on the transformation from Cs4PbBr6 NCs, when reacted with divalent metal bromide MBr2(M=Mn, Zn, and Eu), to CsPbxM1-xBr3 NCs(Fig. 4)[80]. This work achieves a high M doping concentration and endows perovskite NCs with new magnetic and electron features by inserting various divalent and even trivalent metal ions.

    • Song et al. were first to report dual ion Bi3+/Mn2+ co-doped CsPbCl3 perovskite NCs through the hot injection method, in which the doping concentrations for the Bi3+ and Mn2+ ions were carefully controlled and measured[81]. When the Bi3+ and Mn2+ ion doping concentrations were set at 8.7% and 2.5%, respectively, white light emission was achieved[81]. It shows great potential of single-component perovskite NCs in lighting and displays, especially for white light emission. Furthermore, co-doped CsPbX3 NCs can also endow new optical-electronic properties, which paves the way for further study of metal-doped CsPbX3 NCs.

    • Manna et al. reported fluorescent alloy CsPbxMn1-xI3 perovskite NCs with high structural and optical stability. We all know the fact that CsPbI3 NCs are still limited in their further application because of their phase instability as they can easily degrade into the yellow non-emitting δ-CsPbI3 phase within a few days(Fig. 4(e))[82]. Methods with Mn2+-doped into the CsPbI3 have essentially the same optical features and crystal structure as the parent α-CsPbI3 system but they are stable in films and in solutions for periods of over a month. Manna et al. noticed that the stabilization stemming from a small decrease in the lattice parameters slightly increases the tolerance factor combined with an increase in the cohesive energy[82]. The improvement of stability is undoubtedly a major break through in the development of CsPbX3 NCs, which shows the considerable advantages of Mn sources as the dopants in the perovskite NCs.

    • Chen et al., first investigated the effect of O2 on the luminescence of Mn:CsPbCl3 NCs and the degree of the host-dopant energy transfer process in NCs(Fig. 4(d))[83]. Importantly, the near-surface Mn2+ dopants are the sensitive sites whose ligand field can be temporarily disturbed by O2 and thereby influences the Mn2+ emission(4T16A1). This interesting sensing phenomenon can be seen to stimulate many new properties for Mn-doping perovskite NCs.

    • In this section, we will summarize different applications using colloidal Mn2+ doped perovskite NCs. Interestingly, compared with the PLQYs of undoped CsPbCl3 NCs, Mn2+-doped CsPbCl3 NCs shows higher PLQYs. Besides, exciton confinement and exciton energy transfer to the Mn2+ ion state endows NCs with extraordinary electronic and optical properties.

    • Our group prepared LEDs based on hot-injection-processed colloidal Mn2+ doped CsPbCl3 NCs, which were used as the color-converting material(Fig. 5(a) and 5(b))[31, 61]. We made a down-conversion LED by coating a mixture of curable resin and Mn2+-doped CsPbCl3 NCs on top of the UV(365 nm GaN-based) LEDs[31, 61]. Furthermore, various perovskite composites were produced to improve the stability and extra photoelectric property of doped-perovskite NCs. Zhang′s group synthesized Mn2+-doped CsPbCl3 embedded in a cage of Zeolite-Y as a new composite phosphor for the white light-emitting diode(WLED), which significantly improved the resistance to both elevated temperature and water(Fig. 5(c))[84]. The device possesses a CIE coordinate of (0.34, 0.36), a correlated color temperature of 5 336 K and a color rendering index of 81. Furthermore, other coated materials, such as SiO2, polydimethylsiloxane(PDMS), SiO2/Al2O3 monolith(SAM), etc, show higher stability both in the solution and as an LED color-conversion materials. The PL emission spectra show nearly no variation after 24 hours of operation[35, 37]. Interestingly, Zhang et al. proposed that strong Mn2+ emission using high Br- concentrations. Cs(Pb1-x-zZnz)(ClyBr1-y)3:xMn2+ perovskite NCs were first realized through ion exchange reaction with the aid of ZnBr2[84]. As a result, white light-emitting perovskite NCs could be obtained using ion exchange engineering and be used as new color conversion materials in the LED prototypes.

      Figure 5.  (a) and (b) Mn2+-doped CsPbX3 NCs as color-converting materials for LEDs. (c)LEDs with white color emission. (d)Mn2+-doped CsPbBr3 NCs for electroluminescent LED devices. (e) and (f) Mn2+-doped CsPbX3 NCs for solar cells

    • Benefitting from greatly improved thermal stability and optical performance through effective Mn2+ substitution strategies, Zeng′s group fully utilized Mn2+-doped CsPbX3 QDs as efficient light emitters toward the fabrication of high-performance perovskite LEDs(PLEDs), which show high EQE of 1.49% and CE of 6.40 cd/A, in comparison to devices using pure CsPbX3 QDs as light emitters(Fig. 5(d))[85]. The improved stability along with higher device performance reveals that such a Mn2+-substitution strategy might eventually open up a new avenue for the fabrication of efficient optoelectronic devices with excellent long-term stability. Summary of the performance of perovskite-based LEDs with different Mn-substitution ratio is shown in Tab. 1.

      Table 1.  Comparison of the performance parameters of PLEDs based on different Mn-substitution ratio

      Von
      (V)
      Max
      EQE(%)
      Max.CE
      (cd·A-1)
      Max.PE
      (lm·W-1)
      Device structure
      PLED-pure 3.6 0.81 3.71 0.70 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
      PLED-Mn2.6 3.5 0.95 4.33 0.72 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
      PLED-Mn3.8 4.2 1.49 6.41 1.14 ITO/poly-TPD orPVK/QDs/TPBI/LiF/Al
    • Liu et al. focused more on applications in perovskite solar cells utilizing Mn2+-doped CsPbX3 NCs. The group firstly developed interstitial Mn2+ ions doped in a CsPbI2Br film providing a passivating effect. The champion device shows an increased open circuit voltage of 1.172 V with an overall power conversion efficiency(PCE) of 13.47%[86]. Detailed PSCs parameters are shown in Tab. 2. Subsequently, Liu′s group further applied the as-prepared Mn2+ doped CsPbCl3 NCs as the light conversion materials onto the front side of the perovskite solar cells by converting UV light to visible light(Fig. 5(e))[87]. The results show that Mn2+-doped CsPbCl3 NCs would respectively increase the PCE to 3.34% of the device, which is the best recorded enhancement. Meanwhile, the stability of perovskite solar cells under UV irradiation has also been improved[87]. The above work shows Mn2+-doped CsPbX3 NCs have significant potential to applications in photovoltaics. The key parameters of the perovskite solar cells coated with different concentrations of CsPbCl3:0.1Mn QD layers are summarized in Tab. 3. Liang and coworkers also developed all-inorganic perovskite solar cells(PSCs) based on CsPb1-xMnxI1+2xBr2-2xfilms, and when the doping concentration x was 0.005, PSC based on CsPb0.995Mn0.005I1.01Br1.99 film displayed the highest PCE of 7.36%(Fig. 5(f))[88]. Based on the above discussion about Mn2+-doped perovskite NCs applied in the solar cells, it can be seen that there is enhanced PCE and stability in the PSC devices, which is mainly ascribed to small bandgap and ideal band structure of Mn2+-doped NCs. After effective doping, the NCs firstly exhibited more uniform morphology and better crystallinity providing a smooth path for charge-transfer in PSCs, improved light harvesting ability and reduced energy loss in hole transfer[88]. It not only reveals the potential for achieving high photovoltaic properties in the Mn2+-doped perovskite NCs, but also opens the door for further studies on other dopants in perovskite materials.

      Table 2.  Comparison of the performance parameters of PSCs based on different CsPbBrI2 films

      Jsc(mA/cm2) Voc/V FF/% PCE/% Jsc(EQE)(mA/cm2)
      MnCl2-0.5% 14.21 1.133 76.8 12.36 13.86
      MnCl2-1% 14.29 1.144 79.9 13.07 13.93
      MnCl2-2% 14.37 1.172 80.0 13.47 14.09

      Table 3.  Key J-V parameters of PSCs with different coated layer thicknesses of CsPbCl3:0.1Mn QDs

      QDs(mg/mL) Jsc(mA/cm2) Voc/V FF/% PCE/%
      CsPbCl3-xMn 1 21.42 1.105 76.4 18.08
      CsPbCl3-xMn 5 22.03 1.105 76.3 18.57
      CsPbCl3-xMn 20 20.73 1.105 76.6 17.55
    • Here we discussed the progress made in synthesis, optical properties, fluorescence mechanisms and applications(electronic, optical, and optoelectronic) of colloidal Mn2+-doped CsPbX3 NCs. The emerging new findings on Mn2+ ions doping in perovskite nanostructures provide several new fundamental insights for understanding the doping process, exciton confinement and exciton energy transfer to a dopant state. Furthermore, Mn2+-doped CsPbX3 NCs endows higher stability and more excellent fluorescent properties, which shows outstanding properties in LED and solar cell applications. However, further investigation of Mn2+ doping chemistry and physics is still required.

      Firstly, new doping methods for efficient Mn2+ doping are required, which would be derived from more ideas on the synthesis methods of various traditional semiconductor NCs. More investigation of the various doping processes is also required to understand more about doping mechanisms. Secondly, the magneto-optic properties of Mn2+-doped CsPbX3 NCs is an interesting point to be studied. It will endow the doped perovskite NCs with more excellent properties, which tailors the magnetic properties in perovskite systems. Thirdly, highly toxic lead component is a key issue restricting its applicability, so many efforts have been devoted to the lead-free perovskite NCs. Similarly, Mn2+-doped non-lead perovskite NCs have also made some progress. Nag et al. and Manna et al. both reported the Mn2+-doped Cs2AgInCl6 double perovskite[89-90], but the Mn2+ doping ratio and PLQYs are relatively low at only 16%, which urges us to pay more attention to designing better methods of synthesis to achieve excellent fluorescent properties. Other lead-free perovskite NCs, such as Cs2NaBiCl6 and Cs2NaBi1-xInxCl6, can also be suitable doping host. Fourthly, while Mn2+-doping into the 0-dimension, 2-dimension and 3-dimension perovskite NCs have been intensively explored, extending the doping into 1-dimension perovskite NCs is also a key breakthrough. Fifthly, the development of the synthesis of doped perovskite NCs with pure dopant emission without any excitonic emission is also required, which is possible for more confined NCs in which the entire energy can be transferred to the dopant state and would result in highly intense dopant emission. Sixthly, Xu et al. have predicted Co-doped and Fe-doped CsPbBr3 perovskites could be a promising candidate for CO2 reduction from DFT calculation, which paves the way for designing a new doped CsPbBr3 system[91]. This simulation opens new ideas for further doped perovskite NCs with more catalytic applications. Seventhly, further improving the EQE of the electroluminescent LEDs is also a key issue needed to be paid attention. Accordingly, we first notice that postligand engineering is used to exchange or remove the ligand of the NCs after the synthesis process but this can also affect the stability of NCs, which seems to be an excellent way to further improve the EQE of LEDs. In this regard, replacing long ligands(here, oleylamineand oleic acid) with shorter ligands and reducing the concentration of surface ligands using ligand postengineering are worthwhile ventures. Furthermore, the fabrication of uniform NCs polycrystalline layers, in situ preparation of Mn2+-doped NCs thin films, suppression of luminescence quenching inside NCs layers and defect passivation also require further attention in research, which can hopefully improve EQE of LEDs greatly.

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