Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 14865−14870
Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide Perovskite Single Crystals Published as part of The Journal of Physical Chemistry virtual special issue “Hai-Lung Dai Festschrift”. Fang Liu,† Feifan Wang,† Kameron R. Hansen, and X.-Y. Zhu* Department of Chemistry, Columbia University, New York, New York 10027, United States
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S Supporting Information *
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 affects 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 ≥ 0.13. These findings are manifestations of the structural complexity and phase instability of lead halide perovskites.
1. 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+ (methylammonium 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 perovskites1−3 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 PbX64−sublattice13−15 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 PbX64−octahedrons © 2019 American Chemical Society
and thus modify the bandgap. As an example, replacing the smaller 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 blue-shift in the absorption edge of FA1−xCsxPbI3 by 10 meV as x increases from 0 to 0.4.10 Here we studied the mixed-cation (Cs+ and MA+) lead bromide perovskite, CsxMA1−xPbBr3, model system to explore the correlation between alloying stoichiometry and electronic structure. We grew single crystals of CsxMA1−xPbBr3 (x = 0.05−0.33), with pure MAPbBr3 and CsPbBr3 as controls, using the antisolvent crystallization technique.19−21 Elemental analysis by energy-dispersive X-ray spectroscopy (EDS) confirmed the correlation of Cs concentrations in the crystal from those in the precursor solution (Supporting Information (SI), Figure S7). In the following, we used 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 were 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 Received: April 15, 2019 Revised: May 25, 2019 Published: May 25, 2019 14865
DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
Article
The Journal of Physical Chemistry C
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 homogeneous 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 crystals. The percentage corresponds to the mixing ratio of Cs/Pb in the precursor solution.
with PTFE 0.2 μm pore-size syringe filter. The crystallization was performed via the slow diffusion of methanol (antisolvent) 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 50× 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, the 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 lowfrequency Raman measurements using the same microscope
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.
2. EXPERIMENTAL SECTION 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 99.8%), and dichloromethane (DCM, Aldrich, ≥ 99.8%) are used in the synthesis. The synthesis was performed according to published procedures21 with slight modification. PbBr2, MABr, and CsBr precursors were dried in vacuum at 120 °C 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 h. For mixed-cation perovskites, the molar ratio of CsBr/PbBr2 was set at 5%, 9%, 13%, 17%, and 33%. The solution was filtered 14866
DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
Article
The Journal of Physical Chemistry C 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 Bragg grating (VBG) beamsplitter filter (NoiseBlockTM, Ondax, Inc.), and focused by a 50× objective lens onto the sample. The Raman shifted signal was collected by the same objective and sent through the 90/10 beamsplitter, which rejected 90% of the Rayleigh scattered HeNe light. The collected signal passed through two ultranarrowband VBG 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.
Figure 2. Low-frequency Raman spectra of the single-crystal mixedcation perovskite CsxMA1−xPbBr3. The corresponding images and PL spectra of the same samples are in Figure 1. Measurements were taken at 77 K with 632.8 nm excitation. Remaining Raleigh scattering at 0 cm−1 is removed for simplicity. The percentage corresponds to the mixing ratio of Cs/Pb in the precursor solution.
3. RESULTS AND DISCUSSION Figure 1a shows PL spectra from the Csx(CH3NH3)1−xPbBr3 single-crystal samples at 77 K. The PL peak of mixed-cation perovskite alloys synthesized with mixing ratio x ≤ 0.13 exhibits a characteristic PL of pure MAPbBr3 emission near 545 nm. However, as the mixing ratio x increases to above 0.13, the corresponding PL peaks of the mixed-cation perovskite alloys blue-shift to ∼530 nm, which resemble that of pure CsPbBr3. Parts b to h of Figure 1 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−xPbBr3 alloys, we characterized the phonons using Raman spectroscopy and crystal structures using SC-XRD. Figure 2 shows low-frequency Raman spectra at 77 K of the same samples used in PL measurement in Figure 1. The Raman spectra for pure MAPbBr3 and CsPbBr3 are consistent with those reported earlier, and the low-frequency phonons have been assigned to mainly PbBr64− 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 the 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. Because of 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 and 361 K, respectively, from the low-temperature orthorhombic phase to the intermediatetemperature tetragonal phase.24,25 Figure 3 shows the singlecrystal 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− 0.3, SI, Figure S6). Crystalline parameters obtained from the diffraction are listed in Table 1. Further insight into the structures of the mixed-cation perovskites comes from analysis of the fine structures of the 14867
DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
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The Journal of Physical Chemistry C
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 the indexing of each Bragg diffraction spot in each image.
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)a
MAPbBr3 7.9930 8.5730 11.8256 810.3 1.2249
± ± ± ± ±
0.0024 0.0050 0.0045 0.5 0.0008
5%Cs 8.0186 8.5008 11.847 807.5 1.2189
± ± ± ± ±
0.0015 0.0015 0.003 0.3 0.0005
9%Cs 8.0049 8.5701 11.8520 813.08 1.2241
± ± ± ± ±
13%Cs
0.0011 0.0012 0.0016 0.19 0.0003
8.2516 8.2259 11.752 797.7 1.2010
± ± ± ± ±
0.0019 0.0015 0.002 0.3 0.0005
17%Cs 8.151 8.2547 11.7686 791.9 1.1923
± ± ± ± ±
0.0018 0.0016 0.0014 0.2 0.0003
33%Cs 8.199 8.2551 11.573 783.3 1.1793
± ± ± ± ±
0.002 0.0018 0.003 0.3 0.0005
CsPbBr3 8.0159 8.3192 11.6763 778.64 1.1723
± ± ± ± ±
0.0009 0.0010 0.0014 0.16 0.0002
a
Vm is the molar volume.
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). The electronic characteristics of the VBM and CBM band structure is dominated by the orbital overlap between Pb and Br atoms.14 In lead halide perovskites with ABX3 structure, the Pb atom is located at the center of an octahedra formed by six Br atoms while the A site cation is coordinated by eight octahedra units. The band gaps are dominated by the conformation of PbX63− sublattice. 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 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
Bragg diffraction spots. Parts c to j of Figure 3 show 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 transitions.28,29 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 14868
DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
Article
The Journal of Physical Chemistry C
To summarize, we have investigated the single-crystal mixedcation 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 MACsPbI 3 33 and CsFAPbI3,10 the bandgap change in CsxMA1−xPbBr3 singlecrystal 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.
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 CsxMA1−xPbBr3 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 precursor solution.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03536. Steady-state PL for the mixed-cation perovskite single crystals at room temperature, two-component fitting of low-frequency Raman spectra of the single-crystal mixedCs 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 (PDF)
increases the orbital overlap.30−32 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 pseudocubic structure.30 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. While the results presented above are for spatially homogeneous CsxMA1−xPbBr3 alloy crystals, we also find that, in some cases, the antisolvent synthesis method also leads to domain segregated core−shell single-crystal 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, Figures 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 (SI, Figure S5); the core domain has a much higher Cs/Pb ratio than the shell domain. It suggests that in the initial antisolvent 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 a 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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Fang Liu: 0000-0002-1467-8328 Feifan Wang: 0000-0002-3228-9932 X.-Y. Zhu: 0000-0002-2090-8484 Author Contributions †
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Louis E Brus for providing the essential equipment and lab space for low-frequency Raman and photoluminescence spectroscopy measurements. X.Y.Z. acknowledges support by the US Department of Energy, Office of Energy Science, grant DE-SC0010692. F.W. 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) for the DOE. ORISE is managed by Oak Ridge Associated Universities 14869
DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
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The Journal of Physical Chemistry C
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(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.
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DOI: 10.1021/acs.jpcc.9b03536 J. Phys. Chem. C 2019, 123, 14865−14870
