To Exchange or Not to Exchange. Suppressing Anion Exchange in

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To Exchange or Not to Exchange. Suppressing Anion Exchange in Cesium Lead Halide Perovskites with PbSO-Oleate Capping 4

Vikash Kumar Ravi, Rebecca A Scheidt, Angshuman Nag, Masaru Kuno, and Prashant V. Kamat ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00380 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Energy Letters

To Exchange or Not to Exchange. Suppressing Anion Exchange in Cesium Lead Halide Perovskites with PbSO4-Oleate Capping Vikash Kumar Ravi1,2, Rebecca A. Scheidt1, Angshuman Nag2,3, M. K. Kuno1 and Prashant V. Kamat1* 1

Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States and 2

Department of Chemistry, 3Centre for Energy Science,

Indian Institute of Science Education and Research (IISER), Pune, 411008, India

*Address correspondence to this author: [email protected]

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2

Abstract The ease of halide ion exchange in metal halide nanocrystals offers an opportunity to utilize them in a layered or tandem fashion to achieve graded bandgap films. We have now successfully suppressed the halide ion exchange by capping CsPbBr3 and CsPbI3 nanocrystals with PbSO4-Oleate to create a nanostructure assembly that inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3, for several days. Furthermore, the effect of PbSO4-Oleate capping on the excited state dynamics has also been elucidated. The effectiveness of PbSO4-Oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid towards the tandem design of perovskite light harvesting assemblies.

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ACS Energy Letters

3 Mixed halide perovskite nanocrystals have attracted a lot of attention in recent years because of their ability to tune their bandgap through halide ion composition.1-6 By tailoring the ratio of Cl:Br and Br:I it is possible to modulate the absorption and emission properties of metal halide perovskites across the entire visible region.7 These unique composition-dependent optical properties have made them suitable candidates for tandem solar cells and display devices.8-11 Metal halide perovskites are ionic in nature and halide ion mobility within perovskite films has been studied in detail.9, 12-14 The migration of halide ions can also be visualized through spectral changes as the mixed halide films undergo phase segregation upon exposure to light.12, 15-17 The movement of these halide ions is also seen through the exchange of halide ions in nanocrystals. For example, CsPbBr3 nanocrystal suspensions are able to undergo halide ion exchange with lead-halide (or any halide source) salts to produce both mixed and completely exchanged lead halide nanocrystals depending on the concentration of salts added.7, 18-20 The ability to readily exchange halide ions has also allowed for the creation of gradient structures with CsPbBr3 on one side and CsPbI3 on the other side of the same film. 21 Upon excitation these gradient structures direct the flow of charge carriers to the lowest bandgap region (viz. the CsPbI3 region) in the film, which allows for efficient charge transfer.22 However, the ease of halide ion exchange also poses a problem to create a tandem structure with layers of metal halide perovskites of different compositions due to their high mobility.23 The obvious problem is keeping the lead halide perovskite nanocrystals intact without undergoing exchange of halide ions so that the original band structure is maintained. One way to suppress the exchange of halide ions is to cap the CsPbX3 nanocrystals with a nonhalide layer. Use of bidentate ligands or polymer capping can provide stability to these nanocrystals to some degree.24 The recently developed approach to cap nanocrystals with PbSO4-Oleate and align them in a linear fashion is an interesting strategy that offers a provision to suppress halide ion exchange.25-26 Since the absorption and emission behaviors of perovskite nanocrystals is dependent on the halide composition, one can use these spectral features to gauge the extent of halide ion exchange between CsPbBr3 and CsPbI3 nanocrystals. Results that establish the protective role of PbSO4-Oleate capping layer and its effectiveness in suppressing halide ion exchange in CsPbI3 and CsPbBr3 are discussed here.



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4 Capping of CsPbBr3 and CsPbI3 Nanocrystals with PbSO4-Oleate. We prepared CsPbBr3, CsPbBr1.5I1.5 and CsPbI3 nanocrystals in hexane using the hot injection method as described previously.7, 21, 27

(Detailed synthetic steps are provided in the supporting information.) These nanocrystal suspensions

were treated with PbSO4-Oleate cluster (prepared in chloroform) to obtain a capping of PbSO4–Oleate on the nanocrystals. Following centrifugation and washing, the nanocrystals were suspended in hexane. The TEM images of pristine (uncapped) CsPbBr3 (11.3 nm) and CsPbI3 (11.9 nm) nanocrystals and their size distributions are presented in the supporting information (see Figures S1 and S2). The TEM (Figures 1A and B) shows the PbSO4-Oleate capping of individual CsPbBr3 and CsPbI3 nanocrystals, respectively, followed by linear alignment on the TEM grid. As shown previously, the capping of PbSO4-Oleate serves as a template to induce linear alignment of these nanocrystals.25-26 The TEM image (Figure 1C) was obtained by mixing the suspensions of CsPbBr3 and CsPbI3 capped nanocrystals. The mixture of these two

Figure 1. TEM images of PbSO4-Oleate treated (A) CsPbBr3 (B) CsPbI3 and (C) mixed CsPbBr3 and CsPbI3 nanocrystals. TEM images of pristine (uncapped) nanocrystals are shown in the supporting information.

nanocrystals shows no significant change in morphology except for the size. Compared to the pristine nanocrystals the capped nanocrystals show different size particles. It is likely that the PbSO4-Oleate capping process distorts the particle size. Given the wide size distribution of these two capped nanoclusters, it is not possible to identify the individual nanoparticles in the mixture. It is interesting to note that the capping layer keeps the nanocrystals well-dispersed with minimal inter-particle interactions. CsPbBr3 nanocrystals are extremely sensitive to the surrounding environment and can undergo quick halide ion exchange7, 18-20 and/or transform into 2D CsPb2Br5 crystallites.5, 28 The instability of CsPbBr3 and CsPbI3 nanocrystals in reactive chemical environments limits their use in light energy harvesting applications. Hence, it is important to check whether the capping with PbSO4-Oleate can provide enough chemical stability to overcome halide exchange to provide the desired protection for these perovskites.


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ACS Energy Letters

5 Excited State Properties of Capped Nanocrystals. Figure 2 shows absorption and emission spectra of three different perovskite nanocrystals with and without capping of PbSO4-Oleate. As prepared CsPbBr3, CsPbBr1.5I1.5 and CsPbI3 nanocrystals in hexane exhibit characteristic excitonic absorption at 500 nm, 565 nm, and 665 nm and emission maxima at 518 nm, 592 nm, and 686 nm, respectively. These features correspond to previously reported bandgaps of the pristine bulk materials (viz. 2.3 eV for CsPbBr3, and 1.57 eV for CsPbI3, respectively).29-31 The mixed halide nanocrystals with Br:I ratio of 1.5:1.5 exhibit a bandgap that lies between the two pristine systems. This shift in the bandgap is evident from the emission band at 592 nm. It is interesting to note that the absorption and emission features are retained following the capping with PbSO4-Oleate. This confirms that the capping layer does not introduce any noticeable distortion in the band energy or the optical properties of these nanocrystals. The emission lifetimes of the three perovskite nanocrystals with and without PbSO4-Oleate capping are shown in Figure 3. The decay parameters were analyzed through biexponential fit and are presented in Table S1 in the supporting information. Inherently, the CsPbI3 has longer lifetime than CsPbBr3 and CsPbBr1.5I1.5 nanocrystals. It is evident that the capping with PbSO4-Oleate increases emission lifetime for all three systems employed in this investigation. For example, the average lifetime of CsPbBr3

Figure 2. (A) Absorbance spectra and (B) photoluminescence (PL) spectra of CsPbBr3 (a,a’), CsPbBr1.5I1.5 (b,b’) and CsPbI3 (c,c’) nanocrystals without (a’,b’,c’) and with (a,b,c) PbSO4-Oleate capped nanocrystals in hexane. The PL spectra were recorded using 370 nm excitation. Panel B inset shows photographs of the nanocrystal dispersions in hexane under UV light.



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6 increases from 13.3 ns to 16.9 ns upon capping with PbSO4Oleate. Reduction in surface defects through capping is likely to increase the emission lifetime due to a decrease in the possible number of trap states. Table 1 compares

the

emission

quantum yield, radiative, and non-radiative lifetimes. The details

Figure 3. Emission decay of (a’, b’, c’) pristine and (a, b, c) PbSO4Oleate capped nanocrystals in hexane: (a,a’) CsPbBr3, (b,b’) CsPbBr1.5I1.5 and (c,c’) CsPbI3 nanocrystals. The excitation wavelength was 370 nm and the emission was monitored at the peak wavelength for each sample.

of

quantum

yield

measurements

including

absorbance

matching

conditions and emission traces are presented in the supporting information (Figure S3). We see a small decrease in the emission yield when nanocrystals are

capped with PbSO4-Oleate. It is likely that the scattering effects of the capped perovskite nanocrystal suspension at the selected absorbance wavelength could introduce error in emission yield measurements. We have not made any effort to exclude scattering effects in these calculations. However, increased emission lifetimes of capped nanocrystals show that radiative decay rate constants become smaller upon capping with PbSO4-Oleate. In general, we can conclude that the capping layer stabilizes the excited state by remediating the defects at the nanocrystal surface.


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ACS Energy Letters

7

Table 1 Excited state characteristics of PbSO4-Oleate capped and pristine cesium lead halide nanocrystals Quantum

τavg

kr

knr

Yield (Φ )

ns

107s-1

107s-1

CsPbBr3/PbSO4-Oleate

0.73

16.9

4.32

0.16

CsPbBr3

0.95

13.3

7.14

0.04

CsPbBr1.5I1.5/PbSO4-Oleate

0.12

37.6

0.32

2.3

CsPbBr1.5I1.5

0.25

29.3

0. 85

2.6

CsPbI3/PbSO4-Oleate

0.06

74.1

0. 08

1.3

CsPbI3

0.09

60.2

0. 15

1.5

Sample

τavg =1/( kr+ knr) and kr=(Φ /τavg)

Halide Ion Exchange between CsPbBr3 and CsPbI3 Nanocrystals. It is well known that the halide ions in perovskite nanocrystals and films are readily exchanged when CsPbBr3 materials are exposed to iodide (I-) ions or vice versa for CsPbI3 materials.7, 18, 32 Depending upon the concentration of halide ions in solution and exposure time, it is possible to control the composition of mixed halide in the nanocrystals.21 The same phenomenon of halide ion exchange occurs when solutions of separately synthesized CsPbBr3 and CsPbI3 nanocrystals are mixed together. The halide ion exchange process can be seen from the changes in the absorption and emission spectra over time following the mixing of the two nanocrystal suspensions (Figures 4A-D). The concentrations of the nanocrystals were determined from their absorbance spectra (see Figure S4) and using the extinction coefficient values of 3.8×106 M-1cm-1 for CsPbBr3 and 2.4×106 M-1 cm-1 for CsPbI3 nanocrystals at the corresponding excitonic peak.31 Suspensions of CsPbBr3 and CsPbI3 that had concentrations of approximately 40 nM were mixed in hexane and absorption changes were recorded with time. At t=0 s, we observe two separate exciton peaks corresponding to CsPbBr3 (500 nm) and CsPbI3



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8 (665 nm). With increasing time, the absorption peak corresponding to CsPbBr3 shifts to lower energies while that of CsPbI3 shifts to higher energies as the halide ion exchange occur between the two. Finally, after about 3 hours, the two peaks merge to produce a single absorption peak around 565 nm and thus confirms the completion of halide ion exchange. Interestingly, this peak is similar to the peak observed for CsPbBr1.5I1.5 nanocrystals in Figure 2A.

Figure 4: (A) Absorption and (B) Emission spectra of pristine CsPbBr3 nanocrystals and CsPbI3 nanocrystals in hexane (~40 nM each) recorded immediately after mixing. (C) Absorption and (D) Emission spectra of PbSO4-Oleate capped CsPbBr3 nanocrystals and PbSO4-Oleate capped CsPbI3 nanocrystals (~60 nM each) recorded immediately after mixing. The spectra were recorded at fixed time intervals for 300 min (5 hr). The spectra ‘a’ to ‘e’ were recorded with 15 min intervals while spectra from ‘f’ to ‘m’ were recorded with 30 min intervals.


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9 We also tracked the spectral changes associated with the emission spectra upon mixing CsPbBr3 and CsPbI3 nanocrystals in hexane (Figure 4B). The samples were excited at a constant wavelength of 370 nm. Initially we observe two separate emission peaks, at 518 nm and 688 nm, corresponding to pristine CsPbBr3 and CsPbI3 nanocrystals. As these halide ions undergo mixing we see gradual shift of the two emission peaks until they finally merge as a single peak at 586 nm. The shift seen in the emission peaks with increasing time parallels the shift seen in the absorption spectra to yield mixed halide nanocrystals. The majority of the halide ion exchange is completed in about 3 hours. The changes seen after 3 hours were minimal thus ensuring no further changes in the optical properties of these mixed nanocrystals. The halide ion exchange of nanocrystals with the halide ions in solution has been topic of many recent investigations.7, 19, 21, 33-34 In a recent study the internal hetero-structure of anion-exchanged nanocrystals (NCs) probed using variable energy hard X-ray photoelectron spectroscopy provides structural insights into the exchange process.32 We repeated the above experiments with PbSO4-Oleate capped CsPbBr3 and CsPbI3 nanocrystals in hexane solution (concentration around 60 nM each). Both absorption and emission spectra were recorded after mixing the two suspensions (Figures 4C, D). The mixed suspension was kept under slow stirring between the measurements to ensure suspension of the capped perovskite structures. The absorption spectra show a small decrease in absorption with no noticeable shift in the peaks. The mixture was allowed to stand for 18 hours and no appreciable change was detected over this time period. Mixing of the species showed some influence on the emission behavior. Although the emission spectra show a small shift (~30 nm) in the emission peak along with a small decrease in intensity, the observed effects are not as significant as we see in the case of mixing pristine CsPbBr3 and CsPbI3 nanocrystals. The small shift may arise from the halide ion exchange between poorly capped or uncapped nanocrystals or from the deterioration of the PbSO4-Oleate capping layer under stirring. (Note: Vigorous stirring results in larger shifts in the emission peaks. See supporting information Figure S5.) Additionally, we recorded absorbance and emission spectra for the nanocrystal samples where one of them was capped (CsPbBr3/PbSO4-Oleate) and another one was uncapped (pristine CsPbI3). Anion exchange occurred but at a relatively slower rate (see supporting information Figure S6).



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10 The emission of mixed halide perovskites is dependent on the ratio of halide ions. For example, if CsPbBr3 nanocrystals or films are exposed to iodide we see a gradual shift of the absorption and emission peaks towards red.7, 18 Thus, we can use the emission peak as a measure to gauge the exchange of halide ions between the nanocrystals. The changes in the emission peak with time following the mixing of pristine CsPbBr3 and CsPbI3 nanocrystals are shown in Figure 5A. The rate of change in the emission peak of CsPbBr3 from 2.4 eV to 1.8 eV after 3 hours matches well with the change in the CsPbI3 peak from 1.8 eV to 2.1eV. A mono-exponential fit for the shift in emission peak of CsPbBr3 gives a lifetime for the halide

Figure 5: The changes in emission peak energy with time of (A) pristine CsPbBr3 and CsPbI3 nanocrystals and (B) PbSO4-Oleate capped CsPbBr3 and CsPbI3nanocrystals after their mixing in hexane. The two peak wavelengths corresponding to CsPbBr3 and CsPbI3 emission (Figure 4B, 4D) were monitored at different times. Insets show photographs of the nanocrystals under UV light at various mixing times.

exchange to be 65±4 min. The first order analysis of the shift in the CsPbI3 peak also gives a similar rate constant. It should be noted that this rate constant value should be considered as an apparent value since it is dependent on the concentration of nanocrystals and/or presence of residual halide ions present in the system. The similarity between the two kinetics suggests the halide ion exchange between the two nanocrystals occurs simultaneously. Conversely, the capped nanocrystals show a very small change in the emission peak during the same period, further confirming the protective role of PbSO4-Oleate (Figure 5B). The effect of capping can be visualized from the photographs of the samples under UV irradiation included


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ACS Energy Letters

11 as insets in Figure 5. While pristine nanocrystal mixture changes from bright green (due to the dominant emission of CsPbBr3) to yellow-orange emission indicative of mixed halide nanocrystals, the emission of mixture of PbSO4-Oleate capped nanocrystals remains unchanged even after six hours. Transient Absorption Measurements. The individual identity of the capped nanocrystals in mixed solutions was also confirmed through transient absorption spectroscopy. The time-resolved transient spectra of pristine CsPbBr3 and PbSO4-Oleate capped and CsPbI3 nanocrystals in deaerated hexane under vacuum recorded following 387 nm laser pulse excitation are shown in Figure S7 and Figure S8 (see supporting information). As seen in the absorption and emission spectra, PbSO4-Oleate capping has no noticeable effect on the shape and position of the spectral features in the transient absorption spectra. Both PbSO4-Oleate capped and pristine nanocrystals show spectral features corresponding to CsPbBr3 and CsPbI3 nanocrystals with no shift in the wavelength of the ground state bleach. Fitting the ground state bleach recovery kinetics with biexponential fits shows that recovery time becomes slightly longer upon capping with PbSO4-Oleate, which is consistent with the increase in lifetime seen in time correlated single photon counting (TCSPC) measurements. The bleaching recovery analysis is presented in the supporting information, Table S2. We also recorded transient absorption spectra of pristine CsPbBr3 and CsPbI3 nanocrystals after mixing the solution for 3 hours. The transient spectra recorded following 387nm laser pulse excitation of pristine and PbSO4-Oleate capped samples (after mixing) are shown in Figure 6. The pristine CsPbBr3 and CsPbI3 nanocrystals, upon mixing, undergo halide exchange and, when excited with 387 nm laser pulse, we observe only one ground state bleach that indicates only one species present in solution. The single bleaching maximum in the transient spectrum at 575 nm confirms the formation of mixed halide perovskite. The details of the excited state behavior of mixed halide perovskite are discussed elsewhere.21, 35



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12 Interestingly, PbSO4-Oleate capped nanocrystals of CsPbBr3 and CsPbI3 upon mixing still retained the individual spectral characteristics. The transient spectra (Figure 6B) showed two separate bleaching maxima thus confirming the suppression of anion exchange between the two different nanocrystals. The possibility exists for the nanocrystals lying within close proximity to undergo excited state energy or electron transfer. However, the kinetics analysis of bleaching recovery showed that the recovery of the two bleached absorption are independent of each other. This behavior further rules out any excited state interactions between the two different nanocrystals under present experimental conditions. The lifetime

Figure 6: Difference absorbance spectra of mixtures of (A) pristine CsPbBr3 and CsPbI3, and (B) PbSO4Oleate capped CsPbBr3 and CsPbI3 nanocrystals in deaerated hexane following 387 nm laser pulse excitation. The spectra were recorded three hours after mixing the two nanocrystal suspensions. (C and D) Representative kinetics recorded at the ground state bleach maxima for the pristine (575 nm) and PbSO4-Oleate capped nanocrystals (542 and 684 nm), respectively.


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ACS Energy Letters

13 analysis of the bleaching recovery of traces in Figures 6C, D is presented in the supporting information (Table S3). In summary, the results presented here show a convenient way to protect cesium lead halide nanocrystals with PbSO4-Oleate capping. This strategy of capping offers a new approach to keep the anions of cesium lead halide nanocrystals from undergoing anion exchange reactions. Capping strategy thus enables employment of a wide array of tunable nanocrystals for harvesting light energy while retaining the optical properties of perovskite nanocrystals with different optical properties. Efforts are underway to deposit these nanocrystals as tandem layers to engineer effective harvesting of light energy. ASSOCIATED CONTENT Supporting Information. Experimental methods including nanocrystal preparation and spectroscopic measurements are presented in the Supporting Information. AUTHOR INFORMATION Corresponding Author *Address correspondence to this author: [email protected] twitter: @kamatlabND Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT V.K.R acknowledges the Department of Science and Technology (DST), Govt. of India and Indo-U.S. Science and Technology Forum (IUSSTF) for the Bhaskara Advanced Solar Energy (BASE) internship and IISER Pune for graduate research fellowship. R.A.S. acknowledges the support of King Abdullah University of Science and Technology (KAUST) through Award OCRF-2014-CRG3-2268. P.V.K. acknowledges the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through award DE-FC02-04ER15533. M.K. acknowledges the support of the Division of Materials Sciences and Engineering Office of Basic Energy Sciences of the U.S. Department of Energy through Award DE-SC0014334 for carrying out the analysis and discussion of results. This is contribution number NDRL No. 5204 from the Notre Dame Radiation Laboratory.



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ACS Energy Letters

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To Exchange or Not to Exchange. Suppressing Anion Exchange in

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