Controlling the Property of Edges in Layered 2D Perovskite Single

Jun 27, 2019 - The crystal edges of layered 2D perovskites were found to play an important role ... However, their formation mechanism and whether the...
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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3950−3954

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Controlling the Property of Edges in Layered 2D Perovskite Single Crystals Chunyi Zhao,†,‡ Wenming Tian,*,† Jing Leng,† Yang Zhao,§ and Shengye Jin*,† †

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State Key Laboratory of Molecular Reaction Dynamics and the Dynamic Research Center for Energy and Environmental Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China S Supporting Information *

ABSTRACT: Two-dimensional (2D) hybrid perovskites have emerged as promising materials for optoelectronic devices owing to their improved stability. The crystal edges of layered 2D perovskites were found to play an important role in device performance by providing a pathway to dissociate bound excitons into longlived free charge carriers. However, their formation mechanism and whether they are controllable remain unclear. Herein, we report a photoluminescence (PL) imaging study on layered (BA)2(MA)n−1PbnI3n+1 (BA = CH3(CH2)3NH3+, MA = CH3NH3+) perovskite single crystals before and after treatment with butylammonium iodide (BAI) and methylammonium iodide (MAI) solutions. We find that the crystal edges with exciton dissociation ability are induced by the loss of BA ligands and can be removed by adding additional BA cations and regenerated by BA-to-MA cation exchange. This work presents a simple yet efficient method to develop and control the properties of crystal edges for better applications in 2D perovskite devices.

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perovskites in applications such as solar cells requiring exciton dissociation into long-lived free carriers.28 In a related work, Feng et al. reported that such edge states facilitated the collection and transport of photogenerated carriers in layered 2D perovskite single-crystalline nanowires for the demonstration of ultrasensitive 2D nanowire photodetectors.29 Recently, Shi et al. found that the formation of the edge states is triggered by moisture and demonstrated that the edge states in 2D perovskites are a chemically induced semiconducting phase with higher charge mobility, rather than a localized electronic state.30 It is therefore worth noting that the so-called edge state discussed here should be different than the electronic edge state observed in other materials such as layered graphene and chalcogenides.31−34 Nevertheless, as the unusual properties of crystal edges are a critical factor affecting the 2D perovskite devices, understanding their formation mechanism and realizing an effective control on these edges are crucial for the development of high-performance 2D perovskite devices. Herein, we reported a systematic photoluminescence (PL) imaging study on (BA) 2 (MA) n − 1 Pb n I 3 n + 1 (BA = CH3(CH2)3NH3+, MA = CH3NH3+) layered 2D perovskite single crystals before and after treatment with butylammonium iodide (BAI) and methylammonium iodide (MAI) solutions

wo-dimensional (2D) Ruddlesden−Popper hybrid perovskites have become an alternative to 3D perovskites for optoelectronic and photovoltaic applications because of their improved photo- and chemical stabilities.1−10 The structural formula of 2D perovskites is usually described as A2Bn−1MnX3n+1, where A and B are cations, M is metal, and X is halide. The integer n is the number of perovskite layers between two organic spacers (A) and is tunable through stoichiometric control in solution-processed fabrication. Due to the geometric effect and dielectric confinement effect, photogenerated electrons and holes in 2D perovskites form excitons with a binding energy of ∼400 to ∼100 meV for perovskites with n = 1−4,11−13 and a tunable exciton absorption/emission energy can be obtained by changing the n value via the quantum confinement effect. These properties make 2D perovskites an ideal candidate for light-emitting and other optoelectronic applications.14−21 The strongly bound excitons also lead to carrier dynamics in 2D perovskites that dramatically differ from those in their 3D counterpart. Excitons in 3D perovskites are weakly bound and can be readily dissociated into long-lived free carriers.22−27 In contrast, the high binding energy presumably results in stable and short-lived excitons at room temperature in 2D perovskites. However, it was recently reported that excitons in layered 2D perovskite single crystals underwent efficient dissociation through a lower-energy state located at the layered crystal edges.28 These so-called “edge states” were believed to be responsible for the improved performance of 2D © 2019 American Chemical Society

Received: April 26, 2019 Accepted: June 27, 2019 Published: June 27, 2019 3950

DOI: 10.1021/acs.jpclett.9b01193 J. Phys. Chem. Lett. 2019, 10, 3950−3954

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The Journal of Physical Chemistry Letters

dependent PL measurements in the >710 nm spectral region show a nonlinear dependence with a power exponent of 1.2− 1.5 (Figure S3), indicating the presence of bimolecular recombination caused by exciton dissociation into free electrons and holes at these low-energy crystal edges.25,37 Similar crystal edges were observed in 2D crystals with n = 2 and 4 (Figure S4) but were absent in n = 1 crystals (Figure S11). The crystals edges with low-energy emissions exhibit consistent properties with the previous observations reported in the literature,28,29 where these edges were named “edge states”. However, the PL images of the crystals (Figures 1c and 2a) show that the low-energy edges are not continuous throughout the edges, implying that these edges should differ from the electronic edge states that typically have no spatial separations.31−34 In order to avoid the confusion with an electronic state, the observed edges with low-energy emission are described as low-energy edges in this work instead of using the word “edge states”. A previous report proposed that the formation of low-energy edges in 2D perovskite crystals might be caused by many factors, including distortion of the perovskite octahedra, dangling bonds, absorption of molecules, and exciton selftrapping.28,29 Shi et al. also found that the low-energy edges can be generated under the treatment of moisture.30 Indeed, the emission spectra (at ∼740 nm) and photophysical properties (e.g., longer carrier lifetime and exciton dissociation) of the low-energy crystal edges are similar to 3D (n = ∞) perovskites (emission centered at 780 nm). We also compared the PL spectra between a few crystals with the same n value (Figure S5) and found that the low-energy emission peak wavelength from crystal edges was not a constant value but ranged from 715 to 760 nm between different crystals. We speculate that the low-energy emission is possibly introduced by the stochastic loss of BA ligands, which might occur more likely at the crystal edges during fabrication and/or a crystal exfoliation process. Without the BA ligands, the local perovskite layers could partially lose the quantum confinement effect and exhibit a photophysical property similar to that of n ≈ ∞ (3D) perovskites. In order to examine our proposed mechanism, we first performed TEM structure characterizations on a typical n = 3 exfoliated 2D perovskite crystal collected from an interior site and a few crystal edge sites. The selected area electron diffraction (SAED) patterns (Figure S6a) from the crystal interior agree well with the crystal structure of (BA)2(MA)2Pb3I10 2D (n = 3) perovskites. We also found that the SAED patterns from some specific edge sites (Figure S6b) show a very different crystal structure with the n = 3 perovskite but agree well with the crystal structure of 3D perovskites. This result indicates that the n = 3 exfoliated 2D perovskite crystal has some specific local sites with 3D crystal structure, and these sites very likely correspond to the observed low-energy edges in the PL images. If the lower-energy edges are local sites with n ≈ ∞ (3D) structure, they could be regulated through cation exchange reaction. We therefore rinsed the perovskite exfoliated crystals by the BAI toluene/isopropanol (V:V = 1:5) mixed solution with a concentration of 0.5 mg/mL and dried the crystals under a flow of N2 gas. Figure 2a compares the PL intensity images of a typical 2D crystal (n = 3) collected at emission channels of 610−680 (interior) and >710 nm (low-energy edges) before (pristine) and after (+ BAI) the treatment with

by using a laser-scanned confocal imaging microscope. We found that in 2D perovskites the unusual property of crystal edges is associated with local structural impurity sites, where the perovskites lose some BA ligands in their 2D lattice (thus, the quantum confinement effect) and exhibit a photophysical property (e.g., exciton dissociation and long carrier lifetime) similar to n ≈ ∞ (3D) perovskites. These properties of crystal edges can thus be eliminated by adding BA cations. Further treatment of the crystals with MAI solution can rebuild the functional crystal edges with n ≈ ∞ through a BA-to-MA cation exchange. Our work indicates that the properties of crystal edges in layered 2D perovskite are controllable through a simple cation solution treatment. The (BA)2(MA)n−1PbnI3n+1 layered 2D perovskite single crystals with n = 1−4 were synthesized according to the reported method described in detail in the Supporting Information (SI).13,35 Their schematic crystal structure is shown in Figure 1a and was confirmed by the X-ray diffraction

Figure 1. (a) Schematic of the crystal structure of (BA)2(MA)n−1PbnI3n+1 2D perovskites. (b) UV−vis absorption and PL spectra of (BA)2(MA)n−1PbnI3n+1 2D perovskites with n = 1−4. (c) Microscopic PL spectra of a typical exfoliated 2D crystal with n = 3 and its PL images collected at emission channels of 610−680 (interior) and >710 nm (crystal edges). The scale bar is 5 μm.

(XRD) patterns shown in Figure S1. Figure 1b shows the absorption and PL spectra from an ensemble of 2D perovskite crystals with n = 1−4. The blue shift of the optical gap (the first exciton transition) from ∼1.92 (645 nm) to ∼2.42 eV (512 nm) from n = 4 to 1 is due to the quantum confinement effect.2,13,36 These absorption spectra indicate that the pristine 2D crystals are phase-pure in ensemble. For microscopic PL imaging measurements, mechanically exfoliated 2D crystals with a lateral dimension of >10 μm and a thickness of a few hundred of nanometers (corresponding to hundreds of perovskite layer units) were prepared and transferred on glass coverslips. The PL spectra and images from the exfoliated 2D crystals were collected by laser-scanned confocal imaging microscopy coupled with a time-correlated single-photon counting module (Figure S2). Figure 1c shows the PL spectrum and images from a typical exfoliated 2D crystal with n = 3. This spectrum exhibits an exciton emission peak at ∼620 nm in company with a low-energy emission centered at 740 nm. The PL images (insets in Figure 1c) collected at emission channels of 610−680 (excitons) and >710 nm identify that the low-energy emission originates from a portion of the crystal edges. Moreover, excitation-power3951

DOI: 10.1021/acs.jpclett.9b01193 J. Phys. Chem. Lett. 2019, 10, 3950−3954

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Figure 2. (a) PL images of a pristine exfoliated 2D perovskite crystal (n = 3) and after rinsing with BAI solution (+ BAI) and subsequently with MAI solution (+ BAI + MAI). These images are collected at emission channels of 610−680 (from interior, upper panels) and >710 nm (from crystal edges, lower panels), showing the elimination of crystal edges by BAI solution treatment and regeneration by MAI solution treatment. The scale bar is 5 μm. (b) Comparison of PL spectra of the 2D crystal in panel (a) before and after the solution treatment. (c,d) Time-resolved PL kinetics collected in the emission channels of 610−680 (interior) and >710 nm (crystal edges) from the 2D crystal in panel (a) before (pristine) and after the treatment with BAI and MAI solution (+ BAI + MAI).

interior excitons. Furthermore, a nonlinear dependence with a power exponent of 1.27−1.46 in the plot of PL intensity as a function of excitation intensity from the regenerated lowenergy edges (Figure S10) suggests their capability to dissociate the excitons into free carriers as in the pristine crystals.25,28−30,37 We concluded that the regeneration of low-energy crystal edges is due to the BA-to-MA cation exchange reaction, forming a local region with n ≈ ∞. Similar reversible change between 2D and 3D perovskite films through BA and MA cation exchange reactions was also reported previously.38 Our results also suggest that this cation exchange in layered 2D perovskites should occur along the BA ligand layer plane rather than in the out-of-plane direction crossing the perovskite layers. The similarity between the regenerated low-energy crystal edges and those in the pristine crystals further confirms that these unusual edges in the pristine crystals are local sites with n ≈ ∞, caused by the loss of BA ligands. This formation mechanism can also explain the absence of low-energy edges in n = 1 perovskite crystals (see Figure S11) because the loss of BA ligand does not lead to the formation of the n ≈ ∞ region due to the lack of MA cations in n = 1 perovskites. However, according to the formation mechanism, the low-energy crystal edges in n = 1 perovskites can also be produced by the treatment with MAI solution. The PL images (Figure 3a) and emission spectra (Figure 3b) of a typical n = 1 crystal show the generation of low-energy edges after rinsing with MAI solution (+ MAI) and the elimination of these edges by further rinsing with BAI solution (+ MAI + BAI). The time-resolved PL kinetics (Figure 3c) and the nonlinear dependence of PL

BAI solution. These images show that the addition of a BA cation does not significantly change the morphology and the interior emission of the crystal but almost completely removes the low-energy emission from crystal edges, as also evidenced in the PL spectra (Figure 2b). The control experiments shown in Figure S7 exclude influence of the toluene/isopropanol solvent without BAI. These results imply that the low-energy crystal edges are local structural impurity sites losing the BA ligands in the lattice and thus are repairable with the addition of BA cations. The same effect of BAI solution treatment was also observed in 2D crystals with n = 2 and 4 (Figures S8 and S9). After treatment with the BAI solution, the same 2D crystals were further rinsed by the MAI toluene/isopropanol (V:V = 1:5) mixed solution with a concentration of 0.2 mg/mL. More interestingly, the PL image of the crystal after the treatment with MAI solution (+ BAI + MAI) (see the right panel in Figure 2a for an n = 3 crystal and Figures S8 and S9 for n = 2 and 4 crystals) shows the regeneration of low-energy edges with much higher density than that in the pristine crystals. Consistently, their PL spectra show the regenerated low-energy emission peak. In order to further confirm that these regenerated low-energy crystal edges exhibit the same photophysical properties as those in the pristine crystals, we collected and compared the time-resolved PL kinetics from the pristine crystals (Figure 2c) and those after the treatment with BAI and MAI solutions (Figure 2d). The PL kinetics from the regenerated low-energy edges is very similar to that in the pristine crystals, both exhibiting a slow rising component due to the exciton collection process to the low-energy edges followed by a slower carrier recombination than that for the 3952

DOI: 10.1021/acs.jpclett.9b01193 J. Phys. Chem. Lett. 2019, 10, 3950−3954

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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.9b01193. Experimental details of sample synthesis, the X-ray diffraction, PL and UV−vis absorption spectra measurements, experimental setup, and additional results of these measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shengye Jin: 0000-0003-2001-2212 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.J. acknowledges funding support from the MOST (2018YFA0208704, 2016YFA0200602) and NSFC (21725305). W.T. acknowledges funding support from the NSFC (21703241) and Youth Innovation Promotion Association CAS (2019188).

Figure 3. (a) PL images of a typical n = 1 perovskite crystal after rinsing with MAI solution (+ MAI) and subsequently with BAI solution (+ MAI + BAI). The scale bar is 5 μm. (b) Corresponding PL spectra of the crystal in panel (a) after the solution treatment. (c) Time-resolved PL kinetics collected in the emission channels of 490− 550 (interior) and >710 nm (crystal edges) from the crystal in panel (a) after the treatment with MAI solution.

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