Letter pubs.acs.org/JPCL
Colloidal Organolead Halide Perovskite with a High Mn Solubility Limit: A Step Toward Pb-Free Luminescent Quantum Dots Paulraj Arunkumar,† Kyeong Hun Gil,† Seob Won, Sanjith Unithrattil, Yoon Hwa Kim, Ha Jun Kim, and Won Bin Im* School of Materials Science and Engineering and Optoelectronic Convergence Research Center, Chonnam National University, 77, Yongbong-ro, Buk-Gu, Gwangju 61186, Republic of Korea S Supporting Information *
ABSTRACT: Organolead halide perovskites have emerged as a promising optoelectronic material for lighting due to its high quantum yield, color-tunable, and narrow emission. Despite their unique properties, toxicity has intensified the search for ecofriendly alternatives through partial or complete replacement of lead. Herein, we report a roomtemperature synthesized Mn2+-substituted 3D-organolead perovskite displacing ∼90% of lead, simultaneously retaining its unique excitonic emission, with an additional orange emission of Mn2+ via energy transfer. A high Mn solubility limit of 90% was attained for the first time in lead halide perovskites, facilitated by the flexible organic cation (CH3NH3)+ network, preserving the perovskite structure. The emission intensities of the exciton and Mn were influenced by the halide identity that regulates the energy transfer to Mn. Homogeneous emission and electron spin resonance characteristics of Mn2+ indicate a uniform distribution of Mn. These results suggest that low-toxicity 3DCH3NH3Pb1−xMnxBr3−(2x+1)Cl2x+1 nanocrystals may be exploited as magnetically doped quantum dots with unique optoelectronic properties.
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transition of octahedral Mn2+, which is largely insensitive to the physical and electronic structure of the host.13 This forbidden transition is sensitized by the optically excited host, leading to emission with a longer lifetime (milliseconds). The introduction of Mn2+ in the Pb-based perovskites, especially 3D perovskite, is a promising approach for developing low-toxicity and inexpensive color emitters. Recently, efforts have been devoted to synthesize Mn2+-doped 3D-CsPbX3 exhibiting dual emissions originating from an exciton (blue emission) and Mn2+ (580−600 nm), arising via exciton-to-dopant energy transfer.12−15 However, synthesis of Mn-doped 3D-CsPbX3 requires high temperature and seriously limits the introduction of higher Mn concentration into its structure.12,14,15 Liu et al.12 reported yellow-emitting (∼580 nm) CsPbCl3 with the highest possible Mn substitution of 46%. To the best of our knowledge, Mn substitution above 50% has not been reported in any kind of inorganic or organic lead-based halide perovskites. In this work, we demonstrate Mn2+-substituted hybrid 3D perovskite QDs with a highest Mn substitution of 90%, for the first time. Despite the reduction of toxic Pb, the synthesized QDs retains their unique optoelectronic properties of excitonic emission, with additional orange emission (600 nm) from Mn2+. The strong Mn-sensitized orange emission occurs through energy transfer between the exciton and dopant via exchange coupling. Thus, nontoxic organometal perovskites
ybrid organolead halide perovskites (CH3NH3PbX3, X = Cl, Br, I) have emerged as next-generation materials for efficient solar cells and light-emitting diodes (LEDs).1,2 Recently, colloidal perovskite quantum dots (QDs) have received great attention as a potential color-emitting component, owing to their high photoluminescence (PL) quantum yield, narrow emission, and wide color gamut over the entire visible spectral range by controlling the halide identity and size.3,4 Its unique color-tunable property surpasses other QDs in the application of selective narrow emitting LEDs and electroluminescent devices.5 The most promising halide perovskites are three-dimensional (3D) CH3NH3PbX3, which are extended to lower-dimensional homologues of 2D-A2PbX4.6 The terms 3D and 2D refers to the coupling and decoupling of PbX6 octahedra in the crystal, respectively, with distinct excitonic properties.7 Nevertheless, the main perovskite constituent Pb is highly toxic, which must be addressed before any market viability of perovskite-based lighting applications.8 Thus, to deal with the toxicity concern, elemental substitution could be a promising strategy to replace the Pb with nontoxic multivalent cations in the perovskites.9 Such elemental substitution in the semiconductors incorporating transition metals, namely, Cu2+, Ag+, and Mn2+, imparts properties of dopant emission, tunable exciton emission, and magnetic behavior.10,11 The dopant introduces new optical, electronic, and magnetic properties realizing multifunctional properties of the perovskite QDs.12 Mn2+ substitution has been investigated to bestow an intense dopant-sensitized orange emission, arising from the 4T1 → 6A1 © XXXX American Chemical Society
Received: June 7, 2017 Accepted: August 11, 2017
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Figure 1. Rietveld refinement of X-ray diffraction and crystal structures of (a,b) 3D-MAPb0.35Mn0.65Br0.7Cl2.3 and (c,d) 2D-MA2MnCl4, respectively. The red, orange, green, white, black, and magenta spheres represent Mn, Pb, Br/Cl, (CH3NH3) units, carbon, and nitrogen, respectively, (e) Structural map with t vs μ factors representing the formation of perovskite structure in the synthesized 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1 QDs.
Figure 2. (a) XRD of 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1 and 2D-MA2MnCl4 (2D-Mn) and (b) SEM and (c) TEM of 3D-MAPb0.35Mn0.65Br0.7Cl2.3, with an inset containing cube particles with a scale bar of 200 nm.
could be a suitable candidate for QD-based lighting devices due to their unique optoelectronic properties and an exceptional probe to investigate the host−dopant interactions. The 3D-CH3NH3Pb1−xMnxBr3−(2x+1)Cl2x+1 QDs (x = 0− 0.90; hereafter CH3NH3 is denoted as MA and the x values described here correspond to the 3D perovskite structure) were synthesized by a modified ligand-assisted reprecipitation technique.2 Detailed synthesis conditions are provided in the Supporting Information. Mn-doped QDs prepared using
chloride precursors, namely, CH3NH3Cl and MnCl2, in the presence of PbBr2 only exhibited Mn2+ emission, which may be attributed to the necessity of a desirable band gap of the host and/or the pre-existence of a Mn−Cl bond in the perovskite.13 The precise formula of 3D-MAPb 1−x Mn x Br 3−(2x+1) Cl 2x+1 described here was derived from inductively coupled plasma− optical emission spectroscopy results (Table S1). The Rietveld refinement of MAPbBr3, MAPbBr2Cl, and MAPb 0.70 Mn 0.30 Br 1.4 Cl 1.6 (30% Mn) (Figure S1), and 4162
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The Journal of Physical Chemistry Letters MAPb0.35Mn0.65Br0.7Cl2.3 (65% Mn) QDs converged with a cubic 3D-structure (Pm3̅m), as presented in Figure 1a,b. In the 3D perovskites, the PbX6 octahedra share all the corners, with a stacking sequence of ···PbX2−MAX···. Complete replacement of Pb with Mn resulted in the formation of 2D(CH3NH3)2MnCl4 (hereafter represented as 2D-MA2MnCl4) perovskite with orthorhombic structure, in which PbCl6 are separated by layers of MACl cations with a stacking sequence of ···MACl−PbCl2−MACl··· (Figure 1c,d).16 The refined atomic parameters are presented in Table S2−S6. The probability of perovskite (ABX3) formation in the synthesized QDs was estimated from the Goldschmidt tolerance (t) and octahedral factor (μ). The octahedral factor (ratio of ionic radii of the B cation to X anion) elucidates the stability of octahedral BX6, where μ < 0.40 may destabilize the octahedral unit (octahedral tilting) and the perovskite structure.17 The synthesized QDs are within the limits of the ideal cubic 3D perovskite structure with a tolerance factor of 0.8 < t < 1.1 and octahedral factor of 0.40 ≤ μ < 0.90 (Table S7).17−19 3D perovskite formation was further confirmed from the structural map where the samples lie within the suitable t−μ region (Figure 1e).17 The lowest octahedral factor of 0.40 for the x = 0.90 (90% Mn) suggests that any further inclusion of Mn would destabilize the 3D perovskite structure. Our attempt to completely replace the Pb with Mn (100% Mn) resulted in the formation of 2D-MA2MnCl4 (Figure 1c). Because interesting optoelectronic properties are the attributes of the cubic perovskite structure due to the high degree of ionic bonding, the cubic 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1 are the most favored compared to CsPbX3 QDs, where the cubic phase is the least favored phase due to octahedral tilting.12,14,15,20,21 In agreement with refinement, the MAPb1−xMnxBr3−(2x+1)Cl2x+1 with x = 0−0.90 formed a cubic 3D perovskite structure (Figure 2a). This indicates high dispersibility of Mn in the organolead perovskite, forming an excellent solid solution with high Mn substitution of 90%, retaining the structure of the perovskite host. High Mn solubility may be favored by the flexible organic cation (CH3NH3+) network that rotates freely along octahedral cages for allocating the incoming Mn ion.22 Such a high Mn content is intolerable in the CsPbX3 QD due to its low symmetry and octahedral tilting, causing destabilization of the perovskite structure.12 The XRD peak shifts (15.20°) to higher 2θ, indicating the incorporation of smaller Mn into the lattice sites of Pb of 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1.23 The morphology of 3D-MAPb0.35Mn0.65Br0.7Cl2.3 was a cubic-shaped particle of size 5−200 nm (Figure 2b,c), similar to the undoped and other Mn-doped QDs, while 2D-MA2MnCl4 exhibited micronsized platelets (Figure S2). The undoped (3D-MA2PbBr2Cl) QD exhibits an absorption maximum at ∼400 nm (Figure 3a) and blue emission at 447 nm (Figure 3b), which is consistent with the earlier reports.24 The Mn-doped 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1 QDs display dual emission bands corresponding to the band-edge of the host (≤450 nm) and Mn2+ (600 nm).13 As the Mn content increases (accompanied by the increase in Cl content because MnCl2 was used as the Mn precursor), a variation in the PL intensities of the host and Mn2+ was observed. The ratio of the PL intensity of Mn to the exciton increases with increasing Mn content (Figure 3c). This is a clear indication of an energy transfer process from the exciton of the host to Mn2+, signifying the presence of strong exchange coupling between the charge carriers of the host and d electrons of Mn2+. A progressive blue
Figure 3. Optical properties of 3D-MAPb1−xMnxBr3−(2x+1)Cl2x+1 and 2D-MA2MnCl4 (2D-Mn) QDs. (a) UV−vis spectra normalized at 295 nm and (b) normalized PL spectra excited at 365 nm, except for 2DMA2MnCl4 where λex = 308 nm. The highest emission intensity among the exciton and Mn2+ was used for the normalization. For x ≤ 0.6, exciton emission peaks were used for the normalization, while for x > 0.6, Mn2+ emissions were used. The 2D-MA2MnCl4 emission was normalized at its emission maximum of 410 nm. (c) PL ratio of Mn to exciton (PLMn/PLexciton) vs Mn and Cl content, (d) exciton emission wavelength vs Cl content, (e) decay curves of 3DMAPb1−xMnxBr3−(2x+1)Cl2x+1 by monitoring the emission at 420 nm, except x = 0 (emission monitored at 440 nm), and (f) image of QDs in the visible (left side) and 365 nm UV light (right side), respectively.
shift in the exciton emission (447−407 nm for x = 0−0.90) with increasing Mn/Cl content was attributed to the halide identity (Cl) as the composition changes from less electronegative bromide to highly electronegative chloride (Figure 3d and Table S8), rather than a quantum confinement effect, as no apparent change in the particle size was noticed (Figure S3). The emissions of Mn2+ and exciton with increasing Mn (Cl) concentration were influenced by two factors; one is the change in the absorption of the host (Cl content), and the other is the enhanced energy transfer from the exciton to Mn2+ (Mn content). This may explain the origin of exciton and Mn2+ emission with increasing Mn, especially at higher concentration. A dramatic blue shift in the absorption of the host below 400 nm with an increase in the host absorption at the excitation wavelength of the current study (365 nm) for higher Mn2+ samples, x = 0.75 and 0.90, is evident from the excitation spectra measured at 600 nm (Figure S4a). This suggests an enhanced energy transfer from the exciton to Mn2+ at higher Mn concentration resulting in the decrease and increase of excitonic and Mn2+ emissions, respectively. Therefore, the origin of Mn 2 + and excitonic emissions in 3D4163
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Figure 4. XPS of (a) Pb 4f and (b) Mn 2p of 3D-MAPb0.35Mn0.65Br0.7Cl2.3, (c) X-band low-temperature EPR spectrum of frozen 3DMAPb1−xMnxBr3−(2x+1)Cl2x+1 (x = 0.20, 0.30, 0.75, 0.90) and 2D-MA2MnCl4 (2D-Mn) dispersion in toluene, measured at 173 K, and (d) schematic illustration of emission properties of 3D perovskites of MAPbBr2Cl and MAPb0.1Mn0.9Br0.2Cl2.8.
amount of Cl content for obtaining Mn2+ emission. However, in the current study, Mn2+ emission appeared in the MAPb1−xMnxBr3−(2x+1)Cl2x+1 only when the Cl content reached 53% (x = 0.3), which is far higher than that for CsPbCl3 (Figure S5). This suggests that Mn2+ emission in the perovskite is principally governed by the band gap of the host, rather than the simple presence of Cl. Hence, it is proposed that the appropriate band gap of the host and its energy difference with the Mn2+ band are prerequisites for Mn2+ emission in the halide-based perovskites. The highest quantum yield for the exciton was ∼51% for the pristine, and that for Mn2+ (orange) emission was ∼11% for x = 0.90. The undoped and Mn-doped QDs display biexponential decay with respect to host emission, as shown in Figure 3e. The undoped QDs (monitored at 440 nm) exhibited two lifetimes of 5.1 and 13.3 ns, where the shorter one corresponds to the exciton lifetime and the longer one to carrier recombination through surface traps.26 With increasing Mn content, the lifetimes of the host emission decrease (Table S9), further confirming the energy transfer process from the host (donor) to Mn2+ (acceptor), which also agrees with the optical results (Figure 3b). The color of the MAPbBr2Cl solution is pale yellow, which turned to colorless with increasing Mn content (Figure 3f), which substantiates the blue-shifted absorption spectra (Figure 2a). Under 365 nm UV light, the pristine exhibited blue emission, which changed to orange for the highest Mn sample with x = 0.9. X-ray photoelectron (XPS) spectra of MAPb0.35Mn0.65Br0.7Cl2.3 present two minor peaks at 138.4 and 143.9 eV corresponding to Pb2+ (Figure 4a).27 Additional dominant peaks at 140.9 and 136.0 eV are assigned to elemental Pb0. This indicates that during the drying process the partial Pb2+ gets reduced and enriches the QD surface as Pb0, and conversely, Pb2+ is dominant at the bulk, which might have facilitated the effective incorporation of Mn2+ into the bulk of the perovskite structure. The metallic Pb on the particle surface
MAPb1−xMnxBr3−(2x+1)Cl2x+1 QDs are explained by the synergistic effect of change in the host absorption by chlorine content and an enhanced energy transfer from the exciton to Mn with increasing Mn content. The 2D-MA2MnCl4 exhibited a broad blue emission, with a maxima at 410 nm under λex = 308 nm, which is the first of this kind, to the best of our knowledge (Figure S4b). The Mn-doped QD exhibited a blue shift in the absorption edge with increasing Cl content from x = 0.40 to 0.90 (60−93% Cl), which could be related to the Cl identity that also finely increases the host band gap (Figure 3a).13 This feature is supported by the blue shift in the Mn2+ excitation spectra monitored at 600 nm (Figure S4a). It implies that the absorption spectrum of Mn2+ is the characteristic feature of the perovskite host, which further substantiates the Mn-doping. A significant change in the absorption feature was apparent for the 2D-MA2MnCl4 than other 3D perovskite QDs (Figure 3a). Two absorption bands at 308 and 320 nm were observed for 2D-MA2MnCl4, and its absorption edge was limited to ≤350 nm, which clearly suggests an increase in the band gap owing to its 2D structure compared to 3DMAPb1−xMnxBr3−(2x+1)Cl2x+1.25 The presence of Cl in the host was expected to play a vital role in deciding the Mn2+ luminescence in terms of the preexisting Mn−Cl bond or the suitable band gap of the host that is controlled by Cl content for an effective energy transfer to Mn2+. Liu et al.13 obtained a series of mixed halide (Br/Cl) compositions by performing anion exchange on the CsPbCl3:Mn QD with PbBr2 as a bromide precursor to produce CsPbClxBr3−x:Mn, and this study is critical to understand the minimum Cl content required for obtaining the Mn2+ emission. Liu et al.’s results suggest that a minimum of ≥15% chlorine in the total halide content is essential for Mn2+ emission in the CsPbCl3 or mixed halide CsPb(Cl/Br)3. In addition, results of their inverse anion exchange in the CsPbBr3:Mn QD with PbCl2 precursor also suggests the same 4164
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acting as surface traps may have contributed to the larger decay component (Figure 3e).27 Negligible Mn2+ on the particle surface of MAPb0.35Mn0.65Br0.7Cl2.3 was further confirmed from the low-intensity and undefined Mn XPS spectra (Figure 4b). The Mn peak at 641.1 eV with broad shoulders at lower and higher binding energies hints at the presence of Mn2+ in the 2D-MA2MnCl4, as assigned from the large width of the multiplet structure (Figure S6a).28 The low-temperature electron paramagnetic resonance (EPR) spectrum of frozen liquid samples in toluene at 173 K exhibited strong lines with hyperfine splitting, arising from high-spin (d5) Mn2+ in a cubic ligand field (Figure 4c). Six wellresolved spectral lines corresponding to the 55 Mn nucleus (I = 5/2) and five different overlapping electron spin transitions were apparent for the samples with x ≤ 0.75. Further, the g factor of 2.0015 and the average isotropic hyperfine splitting constant (Aiso) of 9.50 mT indicate the presence of Mn2+ in the cubic lattice sites of perovskites, as reported in the literature.29,30 The Aiso value increases marginally to 9.60 mT, along with the spectral line width from 47.5 to 48.0 mT, with increasing Mn concentration from x = 0.20 to 0.90.31 At higher Mn concentrations with x = 0.90 and 2DMA2MnCl4, less resolved signals with broad resonance were observed due to enhanced magnetic dipolar interactions between neighboring Mn centers, resulting in an increase of the spectral line width.32 It is noteworthy that EPR signals were absent for liquid samples at room temperature due to averaging of spectral lines by a rapid tumbling motion effect. However, single lines corresponding to Mn2+ were obtained for the powder samples at room temperature (Figure S6b).33 These results suggest a uniform dispersion of Mn2+ and a well-defined doping environment, supporting the presence of Mn2+ in lattice sites of 3D organolead perovskites. On the basis of the above results, an emission model for the synthesized QDs is illustrated in Figure 4d. Under 365 nm excitation, the perovskite host absorbs energy and emits in the blue region (447−407 nm) through radiative recombination of excitons. The Mn substitution creates a new exciton recombination pathway through energy transfer of a photoinduced exciton to Mn2+ resulting in an intense orange emission at 600 nm (x = 0.90). Relative PL intensities of the host and Mn2+ depend on the energy difference between the host band gap and the Mn2+ transition. In conclusion, low-toxicity Mn2+-doped 3DMAPb1−xMnxBr3−(2x+1)Cl2x+1 hybrid perovskites were synthesized by replacing ∼90% of lead for the first time and retaining its unique optoelectronic properties of excitonic emission with an additional Mn-sensitized orange emission. A high Mn solubility limit of 90% in the organolead halide perovskites was attributed to the presence of a flexible organic cation network. Intense Mn2+ luminescence under host excitation occurs via an energy transfer process from the exciton to Mn2+. The diffraction, lifetime, and EPR spectra suggest homogeneous Mn2+ substitution (90%) displacing Pb, resulting in the formation of a stable 3D perovskite structure. Therefore, further efforts are required for the development of nontoxic halide perovskites for safer QD-based technologies that could sustain the unique optoelectronic properties of lead-based 3D perovskites.
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01440. Experimental and characterization methods, Rietveld refined XRD pattern, lattice parameters, atomic coordinates, tolerance and octahedral factor values, UV− visible, excitation and emission spectral results, decay time results, ICP-OES results for estimating the precise elemental contents of compositions, SEM results, structural map using octahedral and tolerance factors of pristine and Mn-doped perovskite QDs, XPS, and roomtemperature EPR results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Paulraj Arunkumar: 0000-0002-9308-085X Sanjith Unithrattil: 0000-0001-9072-7163 Won Bin Im: 0000-0003-2473-4714 Author Contributions †
P.A. and K.H.G. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (2017R1A2B3011967, 2016R1E1A2020571).
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DOI: 10.1021/acs.jpclett.7b01440 J. Phys. Chem. Lett. 2017, 8, 4161−4166
