Layered Perovskites L2(Pb1-xMnx)Cl4 to Mn Doped CsPbCl3

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Layered Perovskites L2(Pb1-xMnx)Cl4 to Mn Doped CsPbCl3 Perovskite Platelets SAMRAT DAS ADHIKARI, Anirban Dutta, Sumit Kumar Dutta, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00653 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Energy Letters

Layered Perovskites L2(Pb1-xMnx)Cl4 to Mn Doped CsPbCl3 Perovskite Platelets Samrat Das Adhikari, Anirban Dutta, Sumit Kumar Dutta and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India 700032

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Abstract: Doped perovskite nanocrystals are recently emerged as a new class of energy material for solar concentrators and solid state lighting device applications. Among these, doping Mn (II) in high band gap CsPbCl3 perovskite host nanostructures were extensively studied. However, going beyond their optical emissions, herein, the impact of dopant ions for tuning the doped platelet dimensions and retaining the monodispersity were reported. These were performed by designing appropriate compositions of layered perovskites, L2(Pb1-xMnx)Cl4, which on thermal treatment in presence of Cs(I) ions transformed to Mn doped CsPbCl3 platelets. Correlating the amount of Mn present in layered perovskites and retained in doped platelets, roles of Mn for the conversion of layered to doped perovskites were established. These doped platelets showed dominated Mn d-d emission and also Mn concentration dependent emission tuning. Even though several reports of Mn doped CsPbCl3 are reported; these findings add the new fundamental insights of designing dimension tunable doped perovskites from layered perovskites.


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Among the recent emerging energy materials, nanostructured lead halide perovskites remained in forefront for their efficient light emitting and photovoltaic device performances.1-13 Enormous efforts have been put forwarded for understanding the chemistry and physics of formation of these nanostructures and bringing their stability in ambient conditions.6, 12, 14-28 In addition, introducing dopants in the crystal lattice, the optical properties were further tuned.11, 29-39

Moreover, doped nanocrystals which were widely known for their minimum self-quenching

of emission possessed several opportunities to be implemented as efficient energy materials for solid state lighting, solar concentrators etc.40-41 Among these, the dopant Mn was extensively studied and its d-d emission with the host perovskite excitations was optimized.29-30, 33, 42-43

Not only for the emission; but also the dopant Mn is shown bringing stability to CsPbI3

nanocrystals indicating its strong influence in controlling the crystal growths of perovskite nanocrystals.9 Several synthesis protocols for the incorporate Mn(II) in the place of Pb(II) in the Lead halide perovskites including, use of excess dopant ions, use of hydrohalic acid, incorporating alkylamine salts, tuning the reaction parameters and also following post synthesis cation exchange approach were developed.34-35, 44-53 In addition, the exciton transfer process for evolving Mn d-d emission, the low temperature effect on the PL etc. were studied to understand the origin of the dopant emission.43, 50, 54-55 Beyond the dopant induced changes in optical properties, the crystal growths were strongly influenced with dopants in metal oxides and chalcogenides host nanocrystals; but not explored for perovskites. Keeping the importance of this doping chemistry in mind, herein, thermal conversion of layered perovskites to Mn doped CsPbCl3 perovskites platelets are reported. This led to Mn concentration dependent changes in the dimensions of the platelets. Using butylammonium 4 ACS Paragon Plus Environment

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along with oleylammonium ions as the spacer organic ligands, layered perovskites L2Pb1-xMnxCl4 were first prepared with appropriate amount of Pb and Mn precursors, and then with thermal treatment in presence of Cs precursor these were transformed to Mn doped CsPbCl3 platelets. Varying the Pb to Mn composition ratio, the monodispersity as well as dimensions of these platelets were controlled. Interestingly, the Mn concentration in the layer perovskites could controllably reduce the micron size undoped platelets to a few nanometer lengths of the square platelets. These nanostructures showed dominated Mn d-d emission with minimized host exciton emission. Correlating the Mn content in the layered perovskites to doped platelets, the role of Mn for the platelet formation and change in optical properties were established.

Figure 1. (a) Schematic presentation of formation of doped perovskites from layered perovskites L2(Pb1-xMnx)Cl4. L is the organic molecules n-butylammonium and oleylammonium ions. (b) Schematic shows Mn concentration in the reaction mixture to control the size of doped platelets. With increase in the amount of Mn in layered perovskites, the surface area of the platelets decreases.

Figure 1a shows the schematic presentation of the formation of doped perovskites platelets from layered perovskites. Layered hybrid perovskites, L2(Pb(1-x)Mnx)Cl4, chosen here have two organic species, n-butylammonium and oleylammonium ions (L) as spacer cations and two 5 ACS Paragon Plus Environment


divalent cations (PbII and MnII) in different compositions. Heating these perovskites in presence of Cs(I) led to Mn doped perovskites. Figure 1b shows the Mn concentration dependent change in platelets dimensions. The ideal temperature for obtaining nearly monodisperse platelets was 230 oC though even at 100 oC slow formations of platelets were also observed. Hence, the synthesis of various size platelets were carried out at 230 oC; but for monitoring their formation from layered perovskites was set at lower temperature (100 oC). Layered perovskites L2(Pb(1-x)Mnx)Cl4 with variations of butyl ammonium and oleyl ammonium ions compositions were synthesized using pre-synthesized butylammonium chloride and oleylamine-oleic acid mixture. These were loaded along with required amount of PbCl2, MnCl2 and n-ocdadenece (ODE) in the reaction flask and the mixture was heated to 100oC for obtaining the desired layered perovskites. More details of the synthesis process were provided in supporting information. Figure 2a and 2b (Figure S1) show the transmission electron microscopic (TEM) images of the layered perovskites with Pb and Mn ratio taken 1:1 in the reaction system and these were observed like stacked layered structures.56-57 The characteristic absorption peak at 334 nm (Figure 2c and Figure S2) resembled with the layered perovskites similar to the previous report.56 Figure 2d presents the powder X-ray diffraction (XRD) pattern of these nanostructures which showed parallel peaks placed at a distance of ~2.2 (2θ). This corresponds to layer stacking of the long chain spacer oleylammonium ions in between two layers.56 Butylammonium ions being short chain ligands, these would not control the inter-layer distance even if these were present in L sites of the perovskites. The representation of typical layered perovskites is shown in Figure 1a.The use of butylammonium


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chlorides was aimed to trigger the formation of platelets and to facilitate the transformation of layered perovskites to doped perovskites at relatively lower temperature.

Figure 2. (a-b) TEM images of layered perovskites L2(Pb1-xMnx)Cl4 in different resolutions. (c) Absorption spectra of the layered perovskites. The peak at 334 nm is the typical characteristic of the mono-layered structures. (d) Powder X-ray diffraction pattern of the layered perovskites. The inter-peak spacing was 2.2 degree (2θ) which corresponds to ~4 nm.

Figure 3. (a) Typical absorption spectra and (b) PL spectra of Mn doped CsPbCl3 platelets obtained in a reaction with 1:1 ratio of Pb to Mn carried out at 230 oC. (c) Absorption and PL



spectra of the control reaction without Mn. (d) PLE and PL spectra of Mn doped CsPbCl3 platelets obtained in a reaction with 1:1, 2:1 and 4:1 ratio of Pb to Mn carried out at 230 oC. (e) PL lifetime decay plots for the Mn dopant emission of doped platelets obtained with various composition of Pb to Mn in the initial reaction mixture. (f) PL spectra for 1:1 ratio of Pb to Mn precursor with prolonged annealing. Excitation wavelength is 350 nm for PL and lifetime measurements, for PLE and lifetime measurements were carried out at Mn emission maxima. Heating these precursors after injecting calculated amount of Cs(I) stock solution evolved the intense Mn d-d emission centred ~ 600 nm. The optimized injection temperature was fixed at 100 oC and the upper temperature to collect sample was 230 oC. The appearance of orange emission was also observed soon after the Cs(I) injection though for complete transformation, the reaction was heated to higher temperature. These reactions were controlled varying the precursor ratio of Pb to Mn and also Pb to Cs. Figure 3a shows the typical absorption and Figure 3b presents corresponding Photoluminescence (PL) spectra of the doped platelets obtained with 1:1 molar ratio of Pb to Mn precursor ratio taken in reaction flask. In this case, Pb to Cs molar ratio was retained 2.5:1 which was typically considered for most optimized reactions of lead halide perovskite nanocrystals.14, 33 The respective absorption and PL spectra of the controlled undoped system is shown in Figure 3c. The dominated intense emission centred ~ 600 nm in Figure 3b reflected from the Mn d-d emission where the blue emission in Figure 3c and also in Figure 3b appeared from the host exciton. Varying Mn concentration (wrt Pb), the PL and the Photoluminescence Excitation (PLE) spectra of each case were also measured and shown in Figure 3d. Interestingly, the PLE and the exciton emission were at overlapping positions in all these cases suggesting the thickness of the platelets were 8 ACS Paragon Plus Environment

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ACS Energy Letters

remained almost same and within the quantum confinement regime (~ 5 nm). However, the Mn d-d emission tuned red though in a short range with increase of Mn content in the reaction system. This red tuning was expected due to more Mn incorporation in the nanostructures as reported in the literature.49-51 Figure 3e shows the excited state PL decay plots for the Mn d-d emissions, and the respective decay lifetimes were observed as 1.87, 1.76 and 1.49 ms for 4:1, 2:1 and 1:1 Pb to Mn concentration ratios respectively. The longer lifetime further supported these emissions originated from dopant Mn.35, 47 The quantum yield measured using integrated sphere for the dopant emission varied within 30 to 40% from one to other reaction (Figure S3) where the increase was observed with increase of Mn content in the doped system. However, the possibility of Mn leakage from the doped platelets during thermal annealing was ruled out as the emission intensity and the PL spectral nature (FWHM, PL positions etc.) remained unchanged even after prolonged annealing. Figure 3fpresents two PL spectra obtained from the sample at 230 oC after 5 min and 30 min of annealing and the intensity ratio of the dopant to host emission remained same in both cases. As above reactions were carried out with excess Pb2+ ions in comparison to Cs, it is expected that all the layered perovskites might not be decomposed under standard reaction condition. Hence, reactions were also carried out with with increasing the concentration of Cs precursor. Figure 4a shows the PL and Figure 4b presents the PLE of doped platelets with various ratios of Pb to Mn in the reaction obtainedwith 1:1 ratio of Pb to Cs presursor. Interestingly, the Mn d-d emission was observed almost at identical position (~ 600nm) and so also the exciton emission. For higher Mn concentration, the exciton emission intensities were also noticed almost negligible though the QY remained close to 40% ((33.5, 35 and 36 % with 9 ACS Paragon Plus Environment


4:1, 2:1 and 1:1 ratio of Pb to Mn). PLE for all cases had overlapping bandedge (Figure 4b) positions suggesting the quantum confined dimensions (typically widths) of the platelets formed were almost equal. The excited state decay lifetimes were within close proximity for all these three cases(0.91 to 1.43 ms) though trend of higher Mn for shorter lifetime was mentained (Figure 4c).50

Figure 4. (a) PL spectra of Mn doped CsPbCl3 platelets with various Pb to Mn compositions in 1:1 Pb to Cs precursor ratio reactions. (b) Corresponding PLE obtained at 600 nm emission. (c) PL decay lifetime plots for all three emissions. The excitation wavelength for the PL spectra and lifetime measurements is 350 nm and PLE and lifetime measurements were carried out at Mn emission maxima. The most intriguing observation here was noticed in transmission electron microscopic (TEM) images. Figure 5a and 5e(Figure S4-S5) present the undoped platelets with ~ 250-300 nm and ~500-700 nm length respectively obtained from 2.5:1 and 1:1 Pb to Cs ratio of precursors. In both cases, introduction of Mn was seen drastically reduced their planner dimensions (Figure 5b-5d, Figure S6-S8 and Figure 5f-5h, Figure S9-S11) and this decrease was related to increase in Mn content in layered perovskites. The size distribution histograms for all cases are shown in


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Figure 6a-6b. While all the above discussions were focused with Pb to Mn or Pb to Cs ratios of the precursors taken for the reaction, their amount retained in these nanostructures were also measured. As energy disperse spectra (EDS) gives the composition in small area, this was considered as the idea tool for observing the amount of different elements in the final products. Table 1 presents the composition ratios of different precursors rations taken during reactions, elemental ratios obtained from EDS of the purified samples and also planner square dimensions of the doped platelets obtained in different conditions of reactions (Figure S12-14). All these results confirmed that the amount of dopant Mn taken for the layer perovskite formations indeed controlled the Mn intakes of the nanostructures and also their dimensions.

Figure 5. TEM images of (a) undoped and (b-d) Mn doped CsPbCl3 platelets in different Pb to Mn ratio carried out in the reaction with Pb to Cs precursor ratio 2.5:1 at 230 oC. TEM images of (e) undoped and (f-h) Mn doped perovskite platelets obtained in different Pb to Mn ratios and carried out with 1:1 molar ratio of Pb to Cs precursor.



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This also explained the PL tunability where higher Mn content in the final nanostructures red tuned the Mn emission. This was pronounced more for less Cs content of the system; but the narrow difference of Pb to Mn ratio for Pb:Cs 1:1 systems, did not show the wide tuning of this dopant emission. This also further correlates the shortening of the lifetime with increase of Mn content in the nanoplatelets.50 Table 1. Precursor-dopant concentration ratios and the platelet dimensions

Dopant to host precursor ratios (Pb:Mn)

Layered Perovskites (Pb:Mn)

Final nanoplatelets (Pb : Mn)

Final nanoplatelets size (nm)

With Pb:Cs= With Pb:Cs = With Pb:Cs = 2.5:1 1:1 2.5:1 1:0 270 ± 60 4:1 1 : 0.399 1 : 0.02 1 : 0.021 40 ± 7 2:1 1 : 0.696 1 : 0.031 1 : 0.07 27 ± 5 1:1 1: 1.02 1 : 0.052 1 : 0.082 17 ± 3 Blue = Precursor ratios taken for reaction, Red = Ratios obtained from EDS analysis.

With Pb:Cs = 1:1 580 ± 100 120 ± 40 37±6 20 ±2

Further, understanding the formation mechanism, the crystal phases of these platelets in different compositions were also measured. Figure 6c-6d present the powder X-ray diffraction patterns for different compositions of precursor and almost in all cases peaks of cubic CsPbCl3 were visible. However, for less Cs content reactions, additional peaks related to layered perovskites and possibly some from byproducts were noticed though prolonged heating only perovskite peaks were dominantly retained (Figure S15). For larger sheets (Figure 6d), (001) and (002) peaks were prominently visible. However, depending on Mn content, these peaks were broadened and other peaks of cubic phase of CsPbCl3 were appeared. The gradual broadening the peak patterns also reflected for size reductions of the nanostructures. Similar


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observations were also noticed for Figure 6c. However, the reaction with excess Cs (Pb:Cs

Layered Perovskites L2(Pb1-xMnx)Cl4 to Mn Doped CsPbCl3

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