Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide

Subscriber access provided by UNIV OF SOUTHERN INDIANA

Article

Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide Perovskite Single Crystals Fang Liu, Feifan Wang, Kameron R Hansen, and Xiaoyang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03536 • Publication Date (Web): 25 May 2019 Downloaded from http://pubs.acs.org on May 25, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide Perovskite Single Crystals Fang Liu*, Feifan Wang*, Kameron R. Hansen, X.-Y. Zhu† Department of Chemistry, Columbia University, New York, NY 10027, USA

ABSTRACT

Alloying inorganic cations such as Cs+ or Rb+ into hybrid lead halide perovskites has been shown to provide long-term material stability critical to the implementation in solar cells, but little is known about how cation alloying affect the electronic properties. Here we study single crystals of mixed cation lead bromide perovskite, Csx(CH3NH3)1-xPbBr3, in a range of cation mixing ratios, x = 0.05-0.3. In contrast to the continuous bandgap tunability known for mixed anion alloys, we find that the bandgaps of Csx(CH3NH3)1-xPbBr3 single crystals adopt a bimodal distribution, with bandgaps similar to those of pure CH3NH3PbBr3 for x ≤ 0.13 and pure CsPbBr3 for x > 0.13. Single crystal X-ray diffraction reveals a structural origin of this abrupt change in bandgap: with increasing Cs concentration, there is a ~3% lattice shrinkage and appearance of twin splitting at x

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 22

≥ 0.13. These findings are manifestations of the structural complexity and phase instability of lead halide perovskites.

INTRODUCTION Solution processed hybrid organic inorganic lead halide perovskites (HOIPs) are among the most promising materials for solar cells and light emission devices, enabling low manufacturing cost, room temperature processing, and high conversion efficiencies. These materials adopt the ABX3 perovskite structure, where A = CH3NH3+ (methyl-ammonium or MA+), HC(NH2)2+ (formamidinium or FA+), or Cs+; B= Pb2+, and X = I-, Br- or Cl-. Alloying different cations and halide anions into the lead halide perovskite structure has been demonstrated as an effective means not only to tune the optoelectronic properties of perovskites,1–3

but also to achieve high

efficiencies with improved long-term stability.2,4–7 Examples for the latter include, among others, solar cells with high power conversion efficiency and long term stability from the incorporation of a small amount of Cs+ or Rb+ into the HOIP crystal structures.6,8,9 The incorporation of inorganic cations may increase entropy and their smaller effective radii than that of MA+ or FA+ may stabilize the perovskite phases.10 In most lead halide perovskite alloys investigated to date, the mixing of cations or anions has also been shown to continuously vary the bandgap of the alloy, thus tuning the absorption edge or photoluminescence peak energy.2,11,12 The valence and conduction bands of HOIPs are determined by the PbX63- sub-lattice13–15 and, as a result, mixing different halogen anions (Br- vs. I- or Cl- vs. Br-) has large effects on the band structures and can be used to tune the bandgap in the entire visible down to the near-IR region.12,16,17 In comparison, the effect of the A+ cation on the band structure is secondary. Cations with different sizes can distort the PbX63- octahedrons and thus modify the bandgap. As an example, replacing the smaller


2

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MA+ by the larger FA+ expands the perovskite lattice and decreases the bandgap of MA1-xFAxPbI3 continuously by as much as 0.15 eV as x increases from 0 to 1.18 Similarly, the smaller Cs+ induces blueshift in the absorption edge of FA1-xCsxPbI3 by 10 meV as x increases from 0 to 0.4.10 Here we study the mixed-cation (Cs+ and MA+) lead bromide perovskite, CsxMA1-xPbBr3, model system to explore the correlation between alloying stoichiometry and electronic structure. We grow single crystals of CsxMA1-xPbBr3 (x = 0.05-0.33), with pure MAPbBr3 and CsPbBr3 as controls, using the anti-solvent crystallization technique.19–21 Elemental analysis by energydispersive X-ray spectroscopy (EDS) confirms the correlation of Cs concentrations in the crystal from those in the precursor solution (Supporting Information, Fig. S7). In the following we use the Cs concentration in precursor solution to label the mixing ratio of Cs in the resultant perovskite alloys. We carried out PL and Raman characterizations on freshly cleaved surfaces of single crystals at 77 K. Similar results are obtained for measurements at room temperature. In contrast to previous reports in other alloyed HOIP systems, we find that the CsxMA1-xPbBr3 single crystals exhibit a bimodal distribution in bandgap energies (Eg), as revealed by photoluminescence (PL) spectroscopy at low temperatures (~ 77 K), with Eg close to that of MAPbBr3 for x ≤ 0.13 and increasing by ~70 meV to that of CsPbBr3 for x > 0.13. Single crystal X-ray diffraction at low temperatures (120 K) reveals the increase in bandgap at x = 0.13 is accompanied by a shrinkage in unit cell volume of ~3% in the orthorhombic phase.

EXPERIMENTAL 2.1 Synthesis of the CsxMA1-xPbBr3 single crystals Lead bromide (PbBr2, Aldrich, ≥98%), methylammonium bromide (MABr, Dyesol, 98%), cesium bromide (CsBr, Aldrich, 99.999%), N,N-dimethylformamide (DMF, Aldrich, anhydrous


3


Page 4 of 22

99.8%), dichloromethane (DCM, Aldrich, ≥99.8%). The synthesis was according to published procedures21 with slight modification. PbBr2, MABr and CsBr precursors were dried in vacuum at 120 oC overnight. For the synthesis of MAPbBr3 single crystals, 0.2 M MABr and 0.2 M PbBr2 in DMF were mixed and stirred continuously at room temperature for 12 hours. For mixed-cation perovskites, the molar ratio of CsBr/MABr was set at 5%, 9%, 13%, 17%, and 33%. The solution was filtered with PTFE 0.2 um pore-size syringe filter. The crystallization was performed via the slow diffusion of methanol (anti-solvent) into the precursor solution. 2.2. Photoluminescence The continuous wave photoluminescence (PL) spectra were obtained on a home-built inverted microscope setup (Nikon, Eclipse TE300 inverted microscope). The excitation at 2.75 eV was output from a diode laser (CNI laser, MDL-III-450-1W). The excitation laser passed through an ND filter and a 50X objective, which focused the beam with a diameter ~ 1 µm at an energy of 60 W∙cm-2 at the sample surface. The emission collected from the same objective was focused into a spectrograph (SPEX 270M) and detected by a liquid-N2 cooled CCD array detector (RS Roper Scientific CCD-512-TKB). In all measurements, sample was in a cryostat under vacuum (10-6 torr), and the cryostat was mounted on a motion controller (Newport ESP300) for mapping experiments. 2.3. Low-frequency Raman We carried out low-frequency Raman measurements using the same microscope and spectrograph setup described above for PL. The laser light (632.8 nm He-Ne laser, CVI Melles Griot 25-LHP-991-249) was reflected from a dichroic 90/10 volume holographic grating (VHG) beamsplitter filter (NoiseBlockTM, Ondax, Inc.), and focused by a 50X objective lens onto the sample. The Raman shifted signal was collected by the same objective and sent through the 90/10


4

Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


beamsplitter which rejected 90% of the Rayleigh scattered HeNe light. The collected signal passed through two ultra-narrowband VHG notch filters (SureBlockTM, Ondax, Inc., ND >4.0, estimated system transmission efficiency of >80%), and detected by the spectrometer and liquid nitrogen cooled CCD. 2.4. Single crystal X ray diffraction The single crystal X-ray diffraction (SC-XRD) data was collected on a diffractometer (Agilent SuperNova) with sample cooling-heating (Oxford-Diffraction Cryojet). Each single crystal sample in SC-XRD measurement was cut from the individual crystal used in PL and Raman measurements. The sample was cooled gradually from room temperature to 120 K and maintained at the constant temperature during measurement. We carried out data collection and analysis using the CrysAlis Pro software package. Each crystal lattice was refined with a tolerance of 10%. 2.5. Scanning electron microscopy The scanning electron microscope (SEM) images were collected on a Zeiss Sigma VP SEM. The energy-dispersion X-ray spectroscopy (EDS) results were obtained from a Bruker EDS through the Zeiss SEM. Each sample was cut from the same crystal used in PL and Raman measurements and cleaved to expose a fresh surface for SEM measurement.

RESULTS AND DISCUSSIONS


5


Page 6 of 22

Figure 1a shows PL spectra from the Csx(CH3NH3)1-xPbBr3 single crystal samples at 77 K. The

Figure 1. (a) Typical steady state PL for the mixed cation perovskite single crystals. Peak positions are labeled with colored dots on the top. (b)-(h) Steady state PL map of the mixed cation CsMA perovskite single crystal samples with homogenous PL distribution across the surface. The PL is collected at 77 K with 450 nm excitation. The color bar illustrates relative contribution of the 515-535 nm portion to the entire spectrum. The small inset pictures are optical images of the corresponding perovskite crystal. The percentage corresponds to the mixing ratio of Cs/Pb in the precursor solution. ACS Paragon Plus Environment

6

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PL peak of mixed cation perovskite alloys synthesized with mixing ratio x ≤0.13 exhibit a characteristic PL of pure MAPbBr3 emission near 545 nm. However, as the mixing ratio x increases to above 0.13, the corresponding PL of the mixed cation perovskite alloys peaks blue shift to ~ 530 nm, which resembles that of pure CsPbBr3. Figure 1b-h illustrate the contribution of the 530 nm feature to the entire PL spectrum mapped at different positions across the cleaved crystal surface.

The PL emission

spectra are homogeneous across the entire cleaved crystal surface. Similar bimodal PL spectra from these samples are also observed at room temperatures (SI, Figure S1) To understand the structural origin of the bimodal bandgaps in CsxMA1-

Figure 2. Low frequency Raman spectra of the single

alloys, we characterize the

crystal mixed cation perovskite CsxMA1-xPbBr3. The

phonons using Raman spectroscopy

corresponding image and PL of the same samples are

and crystal structures using SC-XRD.

in Figure 1. Measurements were taken at 77 K with

Figure 2 shows low frequency Raman

632.8 nm excitation. Remaining Raleigh scattering at

spectra at 77 K of the same samples

0 cm-1 is removed for simplicity. The percentage

used in PL measurement in Fig. 1. The

corresponds to the mixing ratio of Cs/Pb in the

Raman spectra for pure MAPbBr3 and

precursor solution.

xPbBr3


7


Page 8 of 22

CsPbBr3 are consistent with those reported earlier and the low frequency phonons have been assigned to mainly PbBr63- lattice distortions, with coupling to cation motions.22,23 As the Cs mixing ratio increases, the corresponding low frequency Raman spectra gradually evolve from that characteristic of MAPbBr3 to that of CsPbBr3. Importantly, each spectrum of the CsxMA1-xPbBr3 alloy is not simply a superposition of the two spectra from MAPbBr3 and CsPbBr3 (SI Figure S2). The spectra show gradual shifts in peak positions and the appearance of new features with increasing Cs concentration, confirming the alloyed nature of the CsxMA1-xPbBr3 crystals. Consistent with the homogeneity of PL map, the low frequency Raman spectra also exhibit identical features as the probe laser is mapped across the entire cleaved surface. These results together with the PL mapping eliminate the possibility that the crystallization process leads to bimodal growth with only MA rich or Cs rich domain structures. The widths of the Raman peaks for CsxMA1-xPbBr3 alloys are broader than those of MAPbBr3 or CsPbBr3; the broadening can be attributed to the microscopic disorder in the mixed cation crystal. Structural characterization by SC-XRD was carried out at sample temperatures of 120 K. Due to instrument limitations, we were unable to reach 77 K. However, all the crystals probed are in the low temperature orthorhombic phase at 120 K and there are no further phase transitions as sample temperature is lowered to 77 K. In MAPbBr3 and CsPbBr3 phase transitions occur at 149 K and 361 K respectively, from the low-temperature orthorhombic phase to the intermediatetemperature tetragonal phase.24,25 Figure 3 shows the single crystal X-ray diffraction patterns at 120 K for CsxMA1-xPbBr3 (x = 0 – 1). The diffraction patterns of pure MAPbBr3 and CsPbBr3 are consistent with the low-temperature orthorhombic phases, with extracted lattice parameters in excellent agreement with previous reports.26,27 The diffraction patterns of the CsxMA1-xPbBr3 alloys also reveal an orthorhombic phase in the entire concentration range investigated (x = 0.05


8

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


– 0.3, Figure S6 in Supporting Information). Crystalline parameters obtained from the diffraction are listed in Table 1. Table 1. Lattice Parameters and Space Groups of CsxMA1-xPbBr3, MAPbBr3 and CsPbBr3 Crystals at 120 K. Parameter

a (Å)

b (Å)

c (Å)

V(Å3)

Vm(10-4 m3)*

MAPbBr3

5%Cs

9%Cs

13%Cs

8.0186

8.0049

8.2516

± 0.0015

± 0.0011

± 0.0019

8.5008

8.5701

8.2259

± 0.0050

± 0.0015

± 0.0012

± 0.0015

± 0.0016 ± 0.0018

± 0.0010

11.8256

11.847

11.752

11.7686

11.573

11.6763

± 0.0045

± 0.003

± 0.0016

± 0.002

± 0.0014

± 0.003

± 0.0014

810.3

807.5

813.08

797.7

791.9

783.3

778.64

± 0.5

± 0.3

± 0.19

± 0.3

± 0.2

± 0.3

± 0.16

1.2249

1.2189

1.2241

1.1923

1.1793

1.1723

±0.0008

±0.0005

±0.0003

±0.0003

±0.0005

±0.0002

7.9930 ± 0.0024 8.5730

11.8520

1.2010 ±0.000 5

17%Cs

33%Cs

CsPbBr3

8.151

8.199

8.0159

± 0.0018

± 0.002

± 0.0009

8.2547

8.2551

8.3192

* Vm is the molar volume.


9


Page 10 of 22

Figure 3. Low temperature (120 K) single crystal X-ray diffraction patterns of MAPbBr3 (a) and CsPbBr3 (b). Zoomed-in images of sample diffraction spot of CsxMA1-xPbBr3 in reciprocal space with x = 0 (c, d), 0.05 (e, f), 0.09 (g), 0.13 (h), 0.17 (i), and 0.33 (j). Note that indexing of each Bragg diffraction spot in each image. Further insight into the structures of the mixed-cation perovskites comes from analysis of the fine structures of the Bragg diffraction spots. Fig. 3c-j shows zoom-in images of selected Bragg spots. For CsxMA1-xPbBr3 alloys with x = 0.05 – 0.33, the Bragg spot splits into two or more in each case, suggesting each single crystal is crystallographically twined in the low symmetry orthorhombic phase. Twining is known to occur in perovskite crystals when they go through phase


10

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transitions28,29. In the low temperature orthorhombic phase, the perovskite lattice can be locked into different orientations and the distorted unit cell gives rise to twin structures. We find that the extent of lattice distortion depends on the relative concentration of the smaller size Cs+ incorporated into the MAPbBr3 lattice. For lower mixing ratio x ≤ 13%, splitting of the Bragg spot due to twinning was the [010] or [001] direction, while additional splitting direction occurred at mixing ratio x > 17%. The change of twinning behavior can be attributed to the increased strain as the Cs mixing ratio increases in the alloy structure. This increasing strain is reflected in the change in mean unit cell volume. Figure 4 plots the unit cell volume as a function of mixing ratio x, along with the measured PL peak position. There is an abrupt shrinkage in the crystalline lattice by ~ 3% at a critical Cs concentration of x ~ 0.15. This shrinkage in unit cell volume correlates well with the increase in bandgap obtained from the PL emission peaks (open squares, right axis).

Figure 4. (black) Molar volume of mixed perovskite alloys CsxMA1-xPbBr3 with x ranging from 0.05 to 0.5, along with the pure MAPbBr3 (x=0%) and CsPbBr3 (x=100%). The unit cell volume is determined from single crystal XRD, normalized to one PbBr3 unit. (blue) Bandgap of the CsxMA1xPbBr3

perovskites.

The

peak

positions

are

determined from fitting of the corresponding spectra in Figure 1 to Lorentzian function. The Cs concentration corresponds to the ratio of Cs/Pb in

The valance band maximum (VBM)

precursor solution.

and conduction band minimum (CBM)


11


Page 12 of 22

of lead halide perovskites are known to be tuned by the nature of cations, mainly by variations of the lattice structure with different ionic radii30–32. The Pb atom is located at the center of an octahedra formed by 6 Br atoms. The A site cation is coordinated by 8 octahedra units. Occupation of small cations such as Cs+ in the A site leads to increased distortion of the octahedra angle and decrease of the Pb-Br bond length. Correlated with the octahedrally tilted structures is the shrinkage of unit cell volume. The electronic characteristics of the VBM and CBM band structure is dominated by the orbital overlap between Pb and Br atoms14. Distortion of the metal-halidemetal bond angles has been found to reduce the orbital overlap, while the decrease of Pb-Br bond length increases the orbital overlap30–32. While the orbital overlap is usually correlated with the bandgap, the two competing effects of octahedra tilting and Pb-Br bond length is accompanied by the strength of spin-orbital coupling, which decreases as the unit cell further distorts from the pseudo-cubic structure30. We can attribute the sudden increase in bandgap as the Cs concentration x is increased above 0.13 to the initial 2.5% reduction in unit cell volume and the accompanying decrease in orbital overlap. However, further decrease in unit cell volume by additional 1.8% for x > 0.17 leads to no further increase in bandgap, likely a result of the compensation of the competing effects discussed above. The electronic characteristics of the VBM and CBM band structure is dominated by the orbital overlap between Pb and Br atoms14. In lead halide perovskites with ABX3 structure, the Pb atom is located at the center of an octahedra formed by 6 Br atoms while the A site cation is coordinated by 8 octahedra units. The band gaps are dominated by the conformation of PbX63- sub-lattice. Mixing different halogen anions (Br- vs. I- or Cl- vs. Br-) has a direct effect on the band structure with a tunable band gap over a broad region12,16,17 In comparison, the effect of the A+ cation on band structure is only secondary, and is believed to be mainly by variations of the lattice structure


12

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with different ionic radii.30–32 Occupation of small cations such as Cs+ in the A site leads to shrinkage of unit cell volume, with increased distortion of the PbX63- octahedral tilt angle and decrease of the Pb-Br bond length. Distortion of the metal-halide-metal bond angles has been found to reduce the orbital overlap, while the decrease of Pb-Br bond length increases the orbital overlap30–32. While the orbital overlap is usually correlated with the bandgap, the two competing effects of octahedra tilting and Pb-Br bond length is accompanied by the strength of spin-orbital coupling, which decreases as the unit cell further distorts from the pseudo-cubic structure30. We can attribute the sudden increase in bandgap as the Cs concentration x is increased above 0.13 to the initial 2.5% reduction in unit cell volume and the accompanying decrease in orbital overlap. However, further decrease in unit cell volume by additional 1.8% for x > 0.17 leads to no further increase in bandgap, likely a result of the compensation of the competing effects discussed above. The lattice contraction effect is expected to occur for other mixed cation perovskites. The valance band maximum (VBM) and conduction band minimum (CBM) of lead halide perovskites are known to be tuned by the nature of cations, mainly by variations of the lattice structure with different ionic radii30–32. The Pb atom is located at the center of an octahedra formed by 6 Br atoms. The A site cation is coordinated by 8 octahedra units. Occupation of small cations such as Cs+ in the A site leads to increased distortion of the octahedra angle and decrease of the Pb-Br bond length. Correlated with the octahedrally tilted structures is the shrinkage of unit cell volume. The electronic characteristics of the VBM and CBM band structure is dominated by the orbital overlap between Pb and Br atoms14. Distortion of the metal-halide-metal bond angles has been found to reduce the orbital overlap, while the decrease of Pb-Br bond length increases the orbital overlap30–32. While the orbital overlap is usually correlated with the bandgap, the two competing effects of octahedra tilting and Pb-Br bond length is accompanied by the strength of


13


Page 14 of 22

spin-orbital coupling, which decreases as the unit cell further distorts from the pseudo-cubic structure30. We can attribute the sudden increase in bandgap as the Cs concentration x is increased above 0.13 to the initial 2.5% reduction in unit cell volume and the accompanying decrease in orbital overlap. However, further decrease in unit cell volume by additional 1.8% for x > 0.17 leads to no further increase in bandgap, likely a result of the compensation of the competing effects discussed above. While the results presented above are for spatially homogeneous CsxMA1-xPbBr3 alloy crystals, we also find that, in some cases, the anti-solvent synthesis method also leads to domain segregated core-shell single crystals structures. The core and shell domains are revealed by PL (SI Figure S3) and low frequency Raman mapping. The corresponding PL and Raman spectra of the core and shell domain resembles that of CsPbBr3 and MAPbBr3 perovskites respectively (SI Figure S3 and S4). The observed domain structure remains the same after several heat-cool cycles and/or under continuous laser illumination, thereby eliminating the possibility of temperature induced or photoinduced domain separations. Energy dispersive spectroscopy (EDS) mapping under the scanning electron microscopy (SEM) reveals different elemental spatial distribution in the two domains (Figure S5) the core domain has a much higher Cs/Pb ratio than the shell domain. It suggests that in the initial anti-solvent crystallization process, a faster growing Cs+ rich seed is formed initially, inducing growth of a core mixed cation crystal with CsPbBr3 band structure. After the formation of core, the crystal subsequently grows into larger mixed perovskite crystal that adapts the band structure of MAPbBr3. A similar process have been reported in synthesis of mixed halide perovskite Br/I, in which bromine nucleates first and induces crystallization.11 The domain segregation can be observed even with low Cs mixing ratio down to only 5%, a typical ratio of Cs widely adapted in alloyed perovskite solar cells.


14

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To summarize, we have investigated the single crystal mixed cation perovskites CsxMA1-xPbBr3 using a range of spectroscopic techniques. Doping of smaller size Cs+ cation into MAPbBr3 shrinks octahedral crystal lattice and increases the bandgap in the alloyed crystal. In contrast to the reported gradual shift of bandgap with increasing alloying concentration in the mixed perovskites including MACsPbI3 33 and CsFAPbI3 10, the bandgap change in CsxMA1-xPbBr3 single crystal appears bimodal in a broad range of Cs+ mixing ratios x = 0.05-0.33 in the precursor solution. The bandgaps are either close to those of pure CH3NH3PbBr3 for x ≤ 0.13 or close to pure CsPbBr3 for x > 0.17. The increase in bandgap by ~70 meV is accompanied by shrinkage of unit cell volume by ~3% at the critical mixing ratio of x = 0.17. These results illustrate the complex correlation of bandgap to unit cell structure in lead halide perovskites.

Supporting Information. Steady state PL for the mixed cation perovskite single crystals at room temperature, twocomponent fitting of low frequency Raman spectra of the single crystal mixed Cs MA cation perovskite, steady state PL map of the mixed cation perovskite single crystal samples with inhomogeneous PL distribution across the surface, low frequency Raman spectra of the core and shell region of the mixed cation perovskite single crystal samples with inhomogeneous PL distribution across the surface, EDS mapping of the mixed cation perovskite crystals, single crystal X-ray diffraction patterns of CsxMA1-xPbBr3 at 120 K. EDS spectra of homogeneously mixing perovskites, power dependent PL intensity.


15


Page 16 of 22

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Fang Liu 0000-0002-1467-8328 Feifan Wang 0000-0002-3228-9932 Xiaoyang Zhu 0000-0002-2090-8484 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. *These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank Dr. Louis E Brus for providing the essential equipment and lab space for low frequency Raman and photoluminescence spectroscopy measurements. XYZ acknowledges support by the US Department of Energy, Office of Energy Science, grant DESC0010692. FW acknowledges support by the Office of China Postdoctoral Council. This research was supported in part by the Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Solar Energy Technologies Office administered by the Oak Ridge Institute for Science and Education (ORISE)


16

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-SC00014664. All opinions expressed in this paper are the author's and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

REFERENCES (1)

Zhou, Y.; Zhou, Z.; Chen, M.; Zong, Y.; Huang, J.; Pang, S.; Padture, N. P. Doping and

Alloying for Improved Perovskite Solar Cells. J. Mater. Chem. A 2016, 4, 17623–17635. (2)

Ono, L. K.; Juarez-Perez, E. J.; Qi, Y. Progress on Perovskite Materials and Solar Cells

with Mixed Cations and Halide Anions. ACS Appl. Mater. Interfaces 2017, 9, 30197–30246. (3)

Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V; Trinh, M. T.;

Jin, S.; Zhu, X. Y. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636–642. (4)

Correa-Baena, J.-P.; Saliba, M.; Buonassisi, T.; Grätzel, M.; Abate, A.; Tress, W.;

Hagfeldt, A. Promises and Challenges of Perovskite Solar Cells. Science 2017, 358, 739-744. (5)

National

Renewable

Energy

Laboratory

Best

Research-Cell

Efficiencies.

https://www.nrel.gov/pv/assets/images/efficiency-chart.png. (6)

Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Mohammad K., N.;

Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; et al. Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989–1997.


17


(7)

Page 18 of 22

Kulbak, M.; Gupta, S.; Kedem, N.; Levine, I.; Bendikov, T.; Hodes, G.; Cahen, D. Cesium

Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. J. Phys. Chem. Lett. 2016, 7, 167–172. (8)

Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.;

Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; et al. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206– 209. (9)

McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Hörantner, M.

T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B.; et al. A Mixed-Cation Lead Mixed-Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151–155. (10) Yi, C.; Luo, J.; Meloni, S.; Boziki, A.; Ashari-Astani, N.; Grätzel, C.; Zakeeruddin, S. M.; Röthlisberger, U.; Grätzel, M.; Graetzel, C.; et al. Entropic Stabilization of Mixed A-Cation ABX3 Metal Halide Perovskites for High Performance Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 656–662. (11) Xie, L.-Q.; Chen, L.; Nan, Z.-A.; Lin, H.-X.; Wang, T.; Zhan, D.-P.; Yan, J.-W.; Mao, B.W.; Tian, Z.-Q. Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals. J. Am. Chem. Soc. 2017, 139, 3320–3323. (12) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. Il. Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764–1769.


18

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Butler, K. T.; Frost, J. M.; Walsh, A. Band Alignment of the Hybrid Halide Perovskites CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3. Mater. Horizons 2015, 2, 228–231. (14) Meloni, S.; Palermo, G.; Ashari-Astani, N.; Grätzel, M.; Rothlisberger, U. Valence and Conduction Band Tuning in Halide Perovskites for Solar Cell Applications. J. Mater. Chem. A 2016, 4, 15997–16002. (15) Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104. (16) Kulkarni, S. A.; Baikie, T.; Boix, P. P.; Yantara, N.; Mathews, N.; Mhaisalkar, S. BandGap Tuning of Lead Halide Perovskites Using a Sequential Deposition Process. J. Mater. Chem. A 2014, 2, 9221–9225. (17) Zhang, T.; Yang, M.; Benson, E. E.; Li, Z.; van de Lagemaat, J.; Luther, J. M.; Yan, Y.; Zhu, K.; Zhao, Y. A Facile Solvothermal Growth of Single Crystal Mixed Halide Perovskite CH3NH3Pb(Br1−xClx)3. Chem. Commun. 2015, 51, 7820–7823. (18) Pellet, N.; Gao, P.; Gregori, G.; Yang, T.-Y.; Nazeeruddin, M. K.; Maier, J.; Graetzel, M. Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar-Light Harvesting. Angew. Chemie Int. Ed. 2014, 53, 3151–3157. (19) Konstantakou, M.; Perganti, D.; Falaras, P.; Stergiopoulos, T. Anti-Solvent Crystallization Strategies for Highly Efficient Perovskite Solar Cells. Crystals 2017, 7, 291. (20) Rybtchinski, B.; Kirmayer, S.; Tidhar, Y.; Edri, E.; Weissman, H.; Zohar, D.; Hodes, G.; Cahen, D.; Rybtchinski, B.; Kirmayer, S. Crystallization of Methyl Ammonium Lead Halide


19


Page 20 of 22

Perovskites: Implications for Photovoltaic Applications. J. Am. Chem. Soc. 2014, 136, 13249– 13256. (21) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519–522. (22) Yaffe, O.; Guo, Y.; Tan, L. Z.; Egger, D. A.; Hull, T.; Stoumpos, C. C.; Zheng, F.; Heinz, T. F.; Kronik, L.; Kanatzidis, M. G.; et al. Local Polar Fluctuations in Lead Halide Perovskite Crystals. Phys. Rev. Lett. 2017, 118, 136001. (23) Guo, Y.; Yaffe, O.; Paley, D. W.; Beecher, A. N.; Hull, T. D.; Szpak, G.; Owen, J. S.; Brus, L. E.; Pimenta, M. A. Interplay between Organic Cations and Inorganic Framework and Incommensurability in Hybrid Lead-Halide Perovskite

CH3NH3PbBr3. Phys. Rev. Mater. 2017,

1, 042401. (24) Chen, C.; Hu, X.; Lu, W.; Chang, S.; Shi, L.; Li, L.; Zhong, H.; Han, J.-B. Elucidating the Phase Transitions and Temperature-Dependent Photoluminescence of MAPbBr3 Single Crystal. J. Phys. D. Appl. Phys. 2018, 51, 045105. (25) Hirotsu, S.; Harada, J.; Iizumi, M.; Gesi, K. Structural Phase Transitions in CsPbBr3. J. Phys. Soc. Japan 1974, 37, 1393–1398. (26) Poglitsch, A.; Weber, D. Dynamic Disorder in Methylammoniumtrihalogenoplumbates (II) Observed by Millimeter‐wave Spectroscopy. J. Chem. Phys. 1987, 87, 6373–6378. (27) Rodová, M.; Brožek, J.; Knížek, K.; Nitsch, K. Phase Transitions in Ternary Caesium Lead Bromide. J. Therm. Anal. Calorim. 2003, 71, 667–673.


20

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019–9038. (29) T. J. White J. C. Barry and J. L. Hutchison, R. L. S. Twin Boundaries in Perovskite. Acta. Cryst. 1985, 41, 93-98 (30) Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. Cation-Induced Band-Gap Tuning in Organohalide Perovskites: Interplay of Spin– Orbit Coupling and Octahedra Tilting. Nano Lett. 2014, 14, 3608–3616. (31) Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Steric Engineering of Metal-Halide Perovskites with Tunable Optical Band Gaps. Nat. Commun. 2014, 5, 5757. (32) Kieslich, G.; Sun, S.; Cheetham, A. K. Solid-State Principles Applied to Organic–inorganic Perovskites: New Tricks for an Old Dog. Chem. Sci. 2014, 5, 4712–4715. (33) Choi, H.; Jeong, J.; Kim, H.-B.; Kim, S.; Walker, B.; Kim, G.-H.; Kim, J. Y. CesiumDoped Methylammonium Lead Iodide Perovskite Light Absorber for Hybrid Solar Cells. Nano Energy 2014, 7, 80–85.


21


224x146mm (225 x 225 DPI)


Page 22 of 22

Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide

Recommend Documents