Highly Tunable Colloidal Perovskite Nanoplatelets Through Variable Cation, Metal, and Halide Composition

Transcription

1 Highly Tunable Colloidal Perovskite Nanoplatelets Through Variable Cation, Metal, and Halide Composition Mark C. Weidman, Michael Seitz, Samuel D. Stranks, William A. Tisdale * Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States * corresponding author: tisdale@mit.edu 1

2 Chemicals The following chemicals were purchased from commercial vendors, stored in a glovebox, and used without further purification. Several additional chemicals critical to the results were synthesized the details of which are given on the following page. solvents: N,N-dimethylformamide (DMF) (Sigma-Aldrich, anhydrous, 99.8%), dimethyl sulfoxide (DMSO) (Sigma-Aldrich, anhydrous, 99.9%), toluene (Sigma-Aldrich, anhydrous, 99.8%), toluene (Sigma-Aldrich, 99.5), acetone (Sigma-Aldrich, 99.5%), ethanol (VWR, 100%), diethyl ether (Sigma-Aldrich, 99.7) AX: cesium bromide (CsBr) (Sigma-Aldrich, %), cesium iodide (CsI) (Sigma-Aldrich, %), methylamine hydrochloride (MACl) (Sigma-Aldrich), methylammonium bromide (MABr) (Sigma-Aldrich), methylammonium iodide (MAI) (Sigma-Aldrich), formamidine acetate salt (Sigma-Aldrich, 99%) BX 2 : lead (II) chloride (PbCl 2 ) (Alfa Aesar, %), lead (II) bromide (PbBr 2 ) (Sigma-Aldrich, %), lead (II) iodide solution (PbI 2 ), 0.55M in DMF (Sigma-Aldrich), tin (II) iodide (SnI 2 ) (Alfa Aesar, %) LX: octylamine (Sigma-Aldrich, 99%), butylamine (Sigma-Aldrich, 99.5%), n-butylammonium iodide (BAI) (Sigma-Aldrich) acids: hydrochloric acid (HCl) (Sigma-Aldrich, 37%), hydrobromic acid (HBr) (Sigma-Aldrich, 48%), hydriodic acid (HI) (Sigma-Aldrich, 55%) 2

3 Precursor salt preparation The preparation of precursor salts were followed from procedures found in literature, when possible. The general preparation is as follows. The amine or cation species were added to a solvent, typically ethanol, in a single-neck round bottom flask. A water bath was placed around this flask. An equimolar (or slight excess) of acid was then added dropwise to the stirring solution using a dropping funnel. The reaction was allowed to proceed for 2 hours at room temperature under ambient conditions. The volatiles were then removed from the products using a rotary evaporator, leaving behind the solids. The solids were washed several times with diethyl ether (at least 3 washing cycles the precursors prepared from hydriodic acid were washed many times until they no longer had an orange color). The precursors were then recrystallized once (producing a white powder in all cases) and washed a final time with diethyl ether before being brought into an oxygen and water-free glovebox. Table S1 summarizes the synthetic details for each precursor salt. The formamidinium halide salts were prepared without a reaction solvent. In the case of mixed recrystallization solvents, such as acetone / ethanol, acetone was a poor solvent for the salt even near its boiling point, and so a small amount of ethanol was added to aid with dissolution. The octylamine and butylamine used for the synthesis were kept in a glovebox for storage. Table S1. Synthesis parameters for precursor salts. precursor salt reagent 1 reagent 2 reaction solvent recrystallization solvent reference butylammonium chloride (BACl) butylamine, 120 mmol hydrochloric acid, 130 mmol ethanol, 100 ml acetone / ethanol - butylammonium bromide (BABr) butylamine, 120 mmol hydrobromic acid, 130 mmol ethanol, 100 ml acetone Dou 1 octylammonium chloride (OACl) octylamine, 120 mmol hydrochloric acid, 130 mmol ethanol, 100 ml acetone / ethanol Pathak 2 octylammonium bromide (OABr) octylamine, 42 mmol hydrobromic acid, 45 mmol ethanol, 50 ml acetone Tyagi 3 octylammonium iodide (OAI) octylamine, 120 mmol hydriodic acid, 130 mmol ethanol, 100 ml acetone Pathak 2 formamidinium chloride (FACl) formamidinium acetate salt, 100 mmol hydrochloric acid, 100 mmol - ethanol - formamidinium bromide (FABr) formamidinium acetate salt, 100 mmol hydrobromic acid, 100 mmol - ethanol Eperon 4 formamidinium iodide (FAI) formamidinium acetate salt, 100 mmol hydriodic acid, 100 mmol - ethanol Eperon 4 3

4 Synthesis details Figure S1. Schematic of non-solvent crystallization used for nanoplatelet synthesis. Syntheses were performed under ambient laboratory conditions, except for those involving tin, which were performed in a glovebox. Stock solutions were prepared by dissolving precursor salts (AX, BX 2, LX) in N,N-dimethylformamide (DMF), typically at concentrations of 0.1M. Stock solutions were prepared in a glovebox (using chemicals and anhydrous DMF from the glovebox) and then removed for use. In the case of CsBr, which was not soluble in DMF, the salt was dissolved in DMSO. Salts which did not readily dissolve in DMF (e.g. PbBr 2, PbCl 2 ) were heated to 80 C to ensure full dissolution in the DMF. For PbI 2 solutions, we have used a 0.55M solution of PbI 2 in DMF which is commercially available from Sigma-Aldrich, as it was difficult to completely dissolve solid PbI 2 in DMF at all concentrations desired. Cesium chloride (CsCl) did not dissolve in either DMF or DMSO and so L 2 [CsPbCl 3 ]PbCl 4 was not synthesized by this method. The stock precursor solutions were then mixed in proper proportions to obtain either n = 1 or n = 2 thickness nanoplatelets. The stoichiometry calls for a 2:1:0 ratio of LX:BX 2 :AX for n = 1 nanoplatelets and a ratio of 2:2:1 of LX:BX 2 :AX for n = 2 nanoplatelets. Experimentally, we use a ratio of 10:2:1 LX:BX 2 :AX for the n = 2 nanoplatelets to achieve better thickness homogeneity and colloidal stability (see Figure S8). The excess of the ligands, L, helps to prevent growth to thicker nanoplatelets and ensures only n = 2 is produced. In all cases we have used a 50/50 mixture of octylammonium (OA) and butylammonium (BA) as the ligand species L. Therefore, those solutions are 0.05M OA, 0.05M BA for a total ligand concentration of 0.1M. The precursor solution was added dropwise to 10mL of toluene undergoing stirring at room temperature. The nanoplatelets form immediately, as evidenced by the presence of photoluminescence. For this study, a single drop of precursor solution (~10µL) was added to the toluene phase. Larger quantities of nanoplatelets can be produced by adding additional precursor drops or using higher concentration precursor solutions (0.5M, 1.0M). However, dropwise precursor addition can red-shift the emission peak by a few nanometers with additional drops and so we used a single precursor addition in this work. These results indicate that partial growth in the confined dimension of existing nanoplatelets may be occurring during the addition of subsequent precursor amounts. While we do not observe significant changes in the lateral dimensions of the nanoplatelets with addition of precursor, the photoluminescence indicates that 4

5 slight growth in this thickness-confined direction of the nanoplatelets is occurring. These results pose a problem for scale-up of this synthesis, as the energy of emission is somewhat dependent on the scale of the crystallization. Further work is needed to address this issue; early signs show that it may be a synthesis which is best suited for continuous rather than batch fabrication. Figure S2. Solution and thin film photoluminescence for L 2 [FAPbBr 3 ]PbBr 4 nanoplatelets. Insets show photographs of samples under an ultraviolet lamp. Figure S3. Nanoplatelet suspensions in toluene under ambient lighting. Nanoplatelets were made using 0.5M precursor solutions except for the tin iodide nanoplatelets, which were made using 5M solutions. 5

6 Tin-based perovskites were synthesized in the same manner, however, all steps were performed in an oxygen and water free glovebox (oxygen < 10 ppm, water < 1 pmm), as the materials are highly sensitive to degradation by exposure to ambient conditions. We find that the tin-based nanoplatelets do not crystallize as readily as lead-based nanoplatelets and so typically a 1M or 5M basis was used for all of the precursor salt solutions. n = 1 (L 2 SnI 4 ) and n = 2 (L 2 [FASnI 3 ]SnI 4 ) tin-based perovskite nanoplatelets are dark red and black, respectively, which allows a straightforward determination of a successful synthesis. However, under several circumstances transparent, broad-emitting particles can form instead of the desired tin-based nanoplatelets. Undesirable conditions under which the formation of these transparent particles was observed are summarized here: 1. Using dilute stock solutions: 1M or more concentrated stock solutions should be used for the tin-based synthesis. 2. Using too many ligands (L) for the precursor mixture (LI:SnI 2 :FAI): For 1M and 5M stock solutions, no more than 3 or 5 parts of ligands (3:2:1 and 5:2:1), respectively, could be used for the synthesis of n = 2 nanoplatelets. 3. Dilution of successfully synthesized nanoplatelets in additional toluene: Addition of toluene often triggers the transition from nanoplatelets to the transparent particles. 4. Increased temperature during the synthesis by heating toluene with a hotplate. 5. Air exposure for some seconds or minutes. The transparent particles can form either directly when adding a precursor drop to toluene or after a successful synthesis (solutions turn transparent after being dark-red/black (Figure S4a,b)). Other than their transparency, the particles exhibit a characteristic orange fluorescence, as shown in Figure S4c,d. The exact origin and composition of the transparent particles is unclear. However, in bulk tin-based perovskites a self-doping from Sn 2+ to Sn 4+ has been reported by Takahashi et al. 5 Similarly, the loss of all excitonic feature in the optical absorption spectra has been reported for tin-based perovskite nanocrystals by Jellico et al. 6 and for L 2 SnI 4 layered perovskites by Xu et al. 7 6

7 Figure S4. (a) and (c) Photographs of tin-based nanoplatelets undergoing transformation to the transparent particle phase. (a) under ambient light, (c) under ultraviolet light. (b) Absorption spectra of n = 2 nanoplatelets as compared with the transparent particles. (d) Photoluminescence spectra of n = 1 and n = 2 nanoplatelets as compared with the transparent particle photoluminescence. 7

8 Stability of nanoplatelets The nanoplatelets synthesized here generally (excluding tin-based nanoplatelets) show stability over the course of several days. Cesium nanoplatelets are a notable exception, as they typically evolve into thicker nanoplatelets over several minutes to hours, which we attribute to the smaller size of the cesium cation and increased diffusivity. However, there are several other observations related to stability that we have made during the course of our experiments. The first is related to lead bromide nanoplatelets of thickness n = 2 (L 2 [APbBr 3 ]PbBr 4 ). We observe that the photoluminescence spectrum evolves after several days from one characteristic of n = 2 thickness (440 nm) to one characteristic of a mixture of thicker nanoplatelets ( nm). The effect is illustrated in Figure S5. TEM images of as-synthesized nanoplatelets compared to 3 days post-synthesis shows that the nanoplatelets have grown in lateral dimension and likely in thickness, as evidenced by the increased contrast of the nanoplatelets (and revealed by the photoluminescence spectrum). This effect is most pronounced for formamidinium-based nanoplatelets but also can occur in methylammonium-based nanoplatelets. Interestingly, the absorption remains relatively unchanged during the changes to the photoluminescence. Figure S5. Changes in the photoluminescence of L 2 [FAPbBr 3 ]PbBr 4 nanoplatelets after several days under ambient conditions and the corresponding TEM images of these samples. We also observe that evolution to thicker and bulk-like particles can be caused by high ultraviolet light intensity (~5-10 mw). Therefore, samples should not be exposed to excessive intensity while measuring photoluminescence (see Figure S6). Notably, the areas of a drop-cast film which have the thinnest coverage will photoluminesce green before the thicker areas, which continue to photoluminesce blue. We take this as an indication that exposure to oxygen and / or water is likely the driving force behind the transformation. 8

9 Figure S6. Evolution of L 2 [MAPbBr 3 ]PbBr 4 nanoplatelets to thicker nanoplatelets and bulk-like phases under constant laser illumination (10 mw, 365 nm). The changes occur over the course of approximately 15 minutes. Lastly, the purification of nanoplatelets, as has been noted previously in several studies, can be challenging. The nanoplatelets are susceptible to re-dissolution in polar solvents, like those typically used in purification for colloidal destabilization. In this work, we have avoided the use of polar solvents and instead used longer centrifugation times in order to precipitate the nanoplatelets from the as-synthesized solution. We typically find that colloidal stability and photoluminescence brightness are lower after centrifugation and redispersal. We note, however, that the non-solvent crystallization method outlined here should produce fairly pure nanoplatelets as-synthesized. Unlike high-temperature syntheses, there are no high boiling point solvents used in the synthesis or large excesses of ligand species. The products synthesized here consist of: nanoplatelets, toluene, and very small quantities of DMF and ligands. We believe that DMF is the main impurity to consider, as it has the potential to redissolve the nanoplatelets. 9

10 Additional figures Figure S7. Perovskite nanoplatelets of varying thicknesses note the shoulders present in n > 2 thickness spectra, which result from the difficulty of controlling nanoplatelet thickness homogeneity. Figure S8. Synthesis of n = 2 nanoplatelets (in this case, L 2 [FAPbBr 3 ]PbBr 4 ) using different amounts of excess ligands (LBr, L = OA/BA mixture). The stoichiometry of the nanoplatelets dictates a 2:2:1 ratio, which produces n = 2 nanoplatelets (PL peak ~ 440 nm), but also produces thicker nanoplatelets with redshifted emission. To produce only n = 2 nanoplatelets, an excess of at least 5:2:1 of ligands is required. 10

11 Figure S9. The peak of n = 2 nanoplatelets can be shifted by using larger excesses of the ligand species (LBr, L = OA/BA mixture). The spectrum for the 100:2:1 ratio is noisy, however, this is likely due to fewer nanoplatelets being formed because of dilution rather than an indication that the photoluminescence quantum yield is less in these nanoplatelets. Figure S10. Absorption spectra from lead chloride nanoplatelets: n = 1 (L 2 PbCl 4 ) and n = 2 (L 2 [APbCl 3 ]PbCl 4 ) where A was methylammonium or formamidinium. 11

12 Figure S11. TEM image of micron sized nanoplatelet with formula L 2 [MAPbBr 3 ] 2 PbBr 4. Figure S12. TEM showing impurities on nanoplatelet surfaces. Nanoplatelets are L 2 [FAPbBr 3 ] 2 PbBr 4. 12

13 Figure S13. SEM images showing clusters of L 2 [FAPbBr 3 ] 2 PbBr 4 nanoplatelets. 13

14 Figure S14. PL spectra of n = 1 nanoplatelets with mixed halide compositions. From left to right: 100%I, 10%Br / 90%I, 20%Br / 80%I, 30%Br / 70%I, 100%Br. 14

15 Table S2. Mixed halide L 2 PbX 4 properties. halide composition absorption emission (nm) (ev) (nm) (ev) FWHM (mev) 100% Cl % Cl / 10% Br % Cl / 20% Br % Cl / 30% Br % Cl / 40% Br % Cl / 50% Br % Cl / 60% Br % Cl / 70% Br % Cl / 80% Br % Cl / 90% Br % Br % Br / 10% I % Br / 20% I % Br / 30% I % Br / 40% I % Br / 50% I % Br / 60% I % Br / 70% I % Br / 80% I % Br / 90% I % I

16 Table S3. Mixed halide L 2 [FAPbX 3 ]PbX 3 properties. halide composition absorption emission (nm) (ev) (nm) (ev) FWHM (mev) 100% Cl % Cl / 10% Br % Cl / 20% Br % Cl / 30% Br % Cl / 40% Br % Cl / 50% Br % Cl / 60% Br % Cl / 70% Br % Cl / 80% Br % Cl / 90% Br % Br % Br / 10% I % Br / 20% I % Br / 30% I % Br / 40% I % Br / 50% I % Br / 60% I % Br / 70% I % Br / 80% I % Br / 90% I % I

17 Figure S15. XRD patterns for n = 1 nanoplatelets showing periodicity commensurate with the average distance between stacked nanoplatelets in the film. Diamonds indicate peaks at the intervals listed in Table S4. 17

18 Figure S16. XRD patterns for n = 2 nanoplatelets showing periodicity commensurate with the average distance between stacked nanoplatelets in the film. Diamonds indicate peaks at the intervals listed in Table S4. 18

19 Figure S17. XRD patterns for lead bromide-based n = 2 nanoplatelets with either methylammonium or formamidinium. The peaks marked with diamonds are reflections from the nanoplatelet stacks, while the vertical black lines indicate the peaks from the unit cell of MAPbBr 3 which have previously been observed as strong reflections in nanoplatelets. 3 19

20 Figure S18. XRD patterns for (a) n = 1 and (b) n = 2 nanoplatelets which were deposited either by drop cast or by embedding with silica particles to disrupt the nanoplatelet stacking effects. The nanoplatelets were centrifuged and the precipitate mixed with the silica particles ( µm average size) in toluene to create a viscous slurry followed by vacuum-assisted evaporation of the toluene. The silica embedded samples still show some periodic reflections due to nanoplatelet stacking, but have stronger relative contributions from atomic plane reflections, which have been indexed on the plots. 20

21 Table S4. XRD reflections from stacked nanoplatelets. formula n 2θ periodicity ( ) d-spacing (nm) L 2 PbBr L 2 PbI L 2 SnI L 2 [MAPbBr 3 ]PbBr L 2 [FAPbBr 3 ]PbBr L 2 [MAPbI 3 ]PbI L 2 [FAPbI 3 ]PbI L 2 [FASnI 3 ]SnI Using the periodicity of the reflections measured in Figures S15 and S16, it is possible to extract the average distance between stacked nanoplatelets in the films used for XRD. For the leadbased nanoplatelets, we find the n = 1 nanoplatelet stacks are typically separated by ~1.7nm. The metal-halide octahedron is typically nm in these materials, 2, 8 which means that there is approximately 1.1nm occupied by the ligands between the nanoplatelet stacks (see Figure 3b). The ligands are a mixture of octylammonium (~1nm) and butylammonium (~0.5nm), which is consistent with the observed spacing values. For the n = 2 nanoplatelets, we calculate a spacing of 2.3nm for lead-based nanoplatelets. Assuming the same ligand spacing as the n = 1 nanoplatelets (1.1nm), this indicates the perovskite part is 1.2nm, which is 1 unit cell greater than the n = 1 nanoplatelets, or approximately the thickness of 2 metal halide octahedra layers (see Figure 3c). 21

22 Figure S19. Example of the three spectra recorded for a PLQY measurement, performed using an integrating sphere. In this case the sample was L 2 [FAPbBr 3 ]PbBr 4 nanoplatelets. The sample was suspended in toluene in a 1cm pathlength cuvette and diluted such that the optical density at 405nm (laser excitation wavelength) was less than 0.1. The in beam measurement was with the laser passing through the cuvette, the out of beam was with the laser bypassing the cuvette and striking the wall of the integrating sphere, and the blank is the laser passing through a cuvette of toluene. The PLQY is then calculated as the ratio of photons emitted to photons absorbed for the first pass of the laser through the sample. 9 22

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