Pressure-Induced Optical Bandgap Transition in Methylammonium

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Pressure-Induced Optical Bandgap Transition in Methylammonium Lead Halide Perovskite Soghra Mirershadi, Farhad Sattari, Shahla Golghasemi Sorkhabi, and Amir Masoud Shokri J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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The Journal of Physical Chemistry

Pressure-Induced Optical Bandgap Transition in Methylammonium Lead Halide Perovskite

Soghra Mirershadia,b,*, Farhad Sattaric, Shahla Golghasemi Sorkhabid,e , Amir Masoud Shokric Department of Engineering Sciences, Faculty of Advanced Technologies, University of Mohaghegh Ardabili, Namin, Iran a

Department of Engineering Sciences, Faculty of Advanced Technologies, Sabalan University of Advanced Technologies (SUAT), Namin, Iran b

c Department

of Physics, Faculty of Sciences, University of Mohaghegh Ardabili, Ardabil, Iran

Research Institute for Applied Physics and Astronomy (RIAPA), University of Tabriz, 51666, Tabriz, Iran d

Université d’Angers/UMR CNRS 6200, MOLTECH-Anjou, 2 bd Lavoisier, 49045 Angers, France e

Abstract Organic-inorganic hybrid perovskites are bringing forth great excitement due to their distinguished optoelectronic properties, which lend them applications in high-efficiency solar cells and light-emission devices. Majority of the focused systematic studies on the regulation of the bandgap in the family of organolead halide perovskites have been on changing the compositions of halogens. But, mechanical compression provides a wider structural diversity without changing the composition. In this paper, Organic-inorganic hybrid perovskites (CH3NH3PbX3, X=Cl, Br and I) were fabricated. Crystal structure, electronic properties and optical bandgap energies of the synthesized perovskites were investigated. Also, the effects of changing the external pressure on the bandgap of *

E-mail: [email protected]

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CH3NH3PbX3, X=Cl, Br and I were studied. The results indicate an important progress in tuning the band structure and optoelectronic properties of organometal halide perovskites via pressure engineering, which brings forward substantial implications for practical device applications. Investigation of the optical band transition induced through pressure could provide a different perspective on studying the perovskite materials.

1. Introduction Organic-inorganic hybrid semiconductors emerge as unconventional semiconductors with promising optical and electronic properties in the '90s.1 In a very general sense, they comprise an inorganic frame and an organic part. As the organic and inorganic parts may give rise to Frenkel-type or Wannier-type excitonic bands, respectively2, they affect the optical properties of these systems. Recently, organometal halide perovskites (CH3NH3PbX3, X = Cl, Br and I) have attracted significant attention due to their innovation in solar energy conversion. Until very recently, few studies report a nearly 23.3% power conversion efficiency for these halide perovskites.3-5 This high power conversion efficiency is the outcome of perovskites’ high charge-carrier mobilities, very narrow photoluminescence with high quantum yield and hundreds of nanometers of electron/hole diffusion lengths.6-8 Moreover, the innate advantages including band-gap tenability, low solution-processing temperature, and a narrow band emission

9

make

halide perovskites the alternative candidate elements for optics and optoelectronic devices, such as LEDs and especially solar cells. This unique superiority not only makes


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hybrid perovskites interesting candidates for photovoltaic applications but also presents fascinating optoelectronic properties.10 The band structure of CH3NH3PbI3 films was calculated via W. Yin and coworkers11 and D. B. Mitzi et al.12 by which it was found that Pb and I are mostly responsible for the electronic structures around the band edges, while the role of organic molecules is insignificant in contribution to the band edges. However, the importance of the effect of the orientation of organic molecules on the CH3NH3PbI3 films was suggested by Motta et al., and theoretical studies propose that diverse orientation of organic molecules would lead to the alteration of bandgap from direct to indirect.13 Until now, no experimental report exists to support this theory. A comprehensive appreciation of the relationship between the electronic properties and physical structures of halide perovskites is needed, for their structural diversity to be fully leveraged for targeted properties. This relationship in as-synthesized perovskites and perovskites accessed through postsynthetic reactions 14, 15

have been investigated. Adam Jaffe and coworkers16 examined the systematical

modulation of the structures and electronic perspective of these compressible solids through lattice compression and the resulted induction of new photo-physical and transport properties. Recently, theoretical calculations predicted a weak indirect bandgap in CH3NH3PbI3.13 Brivio et al.17 computed the band structure of CH3NH3PbI3 using quasi-particle self-consistent GW theory and discovered that this slightly indirect bandgap is generated as the consequence of a Rashba-splitting of the conduction band. Also, other calculations have reported the similar relativistic effect.18-20 Recent experimental studies by Niesner et al.

21

mention a Rashba spin-orbit split band in

CH3NH3PbI3. There are many known halide perovskites with indirect and direct bandgaps, however, experimental evidence to confirm the theoretical predictions of the



band structure for the prototypical solar cell material, CH3NH3PbI3, or the dramatic effects of the charge carrier dynamics is lacking. Structural changes under externally applied pressure can alter the band structure of a semiconductor. To understand the structural changes and the recombination dynamics, pressure was applied to CH3NH3PbI3, by Adam Jaffe and coworkers.22 It was found that CH3NH3PbI3 goes through a phase transition at around 325 MPa. Orthorhombic and cubic crystal phases, after been subjected to debate, were assigned to high-pressure phase and the phase at ambient pressures is known to be tetragonal. In the GPa regime, a further phase transition is known to occur. Recently, Tianyi Wang and coworkers23 discovered an indirect bandgap of 60 meV, below the direct bandgap, for CH3NH3PbI3 in both emission and absorption spectra. The unusually long carrier lifetime is related to this indirect bandgap since the thermalized carriers are protected versus recombination with the fast-direct transition. Also, crystal structure and optical properties of CH3NH3PbI3 and the improvement of the photovoltaic efficiency in the presence of strain within tetragonal phase II was studied by Marek Szafrański and coworker24. However, a hypsochromic shift of the absorption edge is observed at the subsequent phase transitions and through the continuously compressed cubic phases IV and V. This shift was correlated with the distortions of the (PbI3)n framework. The electronic states responsible for the crystal absorption edge is significantly affected via the pressure-induced structural distortions, resulting in huge variations of the energy gap, a change much larger than the ones observed at the ambient pressure as a function of temperature in crystal phases I, II, and III. Furthermore, the gradual amorphization of phase V, triggered by an isostructural phase transition, was observed by Marek Szafrański and coworker that swiftly alters the trends in structural changes related to the crystal compression. Prior to this, to their knowledge, there was no evidence that an isostructural phase transition can lead to the 4 ACS Paragon Plus Environment

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amorphization process and that it was not necessarily continuous in its initial stages. The breakdown of local-symmetry while maintaining the average crystal symmetry nucleates the amorphization at the phase transition. This mechanism paves the road for obtaining materials susceptible or resistant to amorphization, often needed in manufacturing products in various areas of applications, such as photovoltaic cells. In this work, organic-inorganic hybrid perovskites precursor of various halide sources (CH3NH3PbX3, X=Cl, Br and I) were prepared and their morphology and optical band structure were investigated under hydrostatic pressure. Absorption, photoluminescence and diffuse reflectance spectra were investigated, while the band structure changes under the applied pressure. The results demonstrate that a small structural variation can notably improve relevant optoelectronic properties of organometal halide perovskites.

2. Experimental 2.1. Preparation of Materials Solution chemistry method was used to prepare the organic-inorganic hybrid perovskites precursor of various halide (CH3NH3PbX3, X=Cl, Br and I), with the CH3NH2/PbX2: 6/1 molar ratio. In our previous work, we found that the 6/1 molar ration for CH3NH3PbBr3 hybrid is more effective. The related experimental details for the preparation of hybrids are reported elsewhere.9 In summary, CH3NH3X (X=Cl, Br and I) was first prepared through the reaction of Methylamine (CH3NH2, 40% solution in water, from Merck) with a stoichiometric amount of either hydrochloric acid (37%) - or hydrobromic acid (48%) or hydroiodic (57%). In order to remove the heat of reaction, each aqueous solution put in the glass reactor, at 0oC. Afterward, a stoichiometric amount of CH3NH3X was added to the lead halide salt (PbCl2, PbBr2 or PbI2, Sigma Aldrich). Each synthesized material was 5 ACS Paragon Plus Environment


air dried at room temperature, and then annealed at 60oC during 7 days, for the organicinorganic lead halide perovskites to form. In the next stage, the synthesized perovskite powders were made into pellets, through hydrostatic pressure. To investigate the effects of mechanical tension on the energy gap in the structures of these perovskites, the pellets were subjected to various amounts of pressure. All acids were obtained from Merck Co. All the chemicals were used as obtained without any further purification.

2.2. Characterization techniques X-ray diffraction (XRD) with 2θ, within the range of 2 to 70º, by PANalytical model X’Pert Pro MPD, was used to characterize the structures and Bragg reflections of organometal halide perovskites. The scanning speed and step intervals were 1º/min and 0.02º, respectively. Ultraviolet-visible (UV/vis) optical absorption spectra were measured via Shimadzu UV-2450 spectrophotometer, in the range of 200 to 800 nm. Also, photoluminescence (PL) spectra were measured by JASCO FP-6200 spectrofluorometer at room temperature. In addition, CH3NH3PbI3 organic-inorganic perovskite was illuminated by pumping 437 nm laser diode and collected by fiber spectrometer (model USB-4000, Ocean Optics) in order to measure the photoluminescence spectra. Plus, Ultraviolet-visible diffuse reflectance spectra (UV/vis DRS) was used to investigate the properties of the energy bandgap, via Sinco S4100 spectrophotometer.

3. Results and discussion Figure 1 shows the comparison between the typical X-ray diffraction patterns of CH3NH3PbCl3, CH3NH3PbBr3, and CH3NH3PbI3 organic-inorganic hybrid perovskites. 6 ACS Paragon Plus Environment

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The X-ray diffraction pattern of CH3NH3PbCl3 perovskite (Figure 1A) corresponds with (100), (110), (111), (200), (210), (211) and (221) lattice planes. For CH3NH3PbBr3 perovskite (Figure 1B), the X-ray diffraction presents strong peaks at 2θ values of 14.8°, 21.0°, 30.0°, 33.8°, 42.9°, 45.7°, which correlate well with lattice planes (100), (110), (200), (210), (220) and (300), respectively. The keen peaks shown in the X-ray diffraction pattern of CH3NH3PbI3 perovskite (Figure 1C) correlate with (100), (112), (211), (202), (310), (312), (224) and (314) lattice planes. These results corroborate that the prepared samples are in fact organic-inorganic hybrid perovskites and are in agreement with previous studies.25-27 Figure 2 presents the typical Ultraviolet-visible absorption spectra of CH3NH3PbX3 (X=Cl, Br and I) organic-inorganic perovskites, at room temperature. For CH3NH3PbCl3 hybrid perovskite, the exciton absorption wavelength is around 405nm. Substituting X=Cl by Br and I, the exciton absorption for CH3NH3PbX3 is shifted from 405nm to 522nm and 656nm, respectively. Thus, the absorption spectrum envelopes a large range of the Ultraviolet region of the solar spectrum. It is perceivable based on these results that expanding the absorption range of organic-inorganic halide perovskites, through replacing the halogen atoms, and enhancing the Stokes shift can be used to further increase the efficiency of perovskites doped Luminescent Solar Concentrators.28-30 Additionally, the optical diffuse reflectance spectra were used to determine the optical bandgap energy values of the hybrid perovskites. Figure 3-a presents the DRS of the organic-inorganic hybrids, as a function of wavelength. The results affirm the valence to conduction band transport of electrons, as a result of the absorption of the incident photon that leads to a reduction of the intensity of light at the relevant wavelength. Consequently, the relative percentage of transmission to reflectance is diminished. As can be seen in 7 ACS Paragon Plus Environment


Figure 3-a, in the CH3NH3PbCl3 perovskite, by reducing the wavelength of the incident photons from 593nm to 515nm, the reflectance is decreased. Moreover, for CH3NH3PbBr3 perovskite, this reduction is occurred by decreasing the incident photons’ wavelength from 580nm to 515nm, and from 795nm to 550nm for the CH3NH3PbI3 perovskite. These outcomes indicate that the bandgap energy in the perovskites’ structures is decreased by changing the halogen type from Cl to Br and I. The reflectance technique was used to estimate the bandgap energies by applying the Mott and Davis theory.31 To do so, the linear region of the curves was extrapolated to the zero absorption at (αhν)2 = 0, for direct allowed transitions. For a direct allowed transition of the synthesized perovskites, Figure 3-b displays the (αhν)2 as a function of photon energy (hν). The optical bandgaps for the synthesized perovskites CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3 were calculated as 2.33, 2.31 and 1.77eV, respectively. Figure 4 shows the photoluminescence spectra of the CH3NH3PbCl3 perovskite under pressure. According to Figure 4, one can say that by augmenting the pressure from zero to 1.2 GPa, the emission wavelength of this structure increases from 410nm to 413nm. To further investigate the effect of pressure on the perovskites structures Diffuse Reflectance Spectroscopy was used. Based on Figure 5, it can be concluded that the energy gap for CH3NH3PbCl3, through applied pressure up to 1.2 GPa, alters from 2.33 to 2.39eV.

However, this enhancement in the energy gap is not observed in the photoluminescence spectra (Figure 4). On the other hand, DRS can give precise information regarding the energy bandgap, so the difference between the photoluminescence and the diffuse reflectance spectra can be attributed to the change from direct to the indirect bandgap, or vice versa. In this structure, since the emission wavelength is increased, it can be concluded that the direct bandgap is changed into the indirect bandgap. 8 ACS Paragon Plus Environment

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Figure 6-a shows the photoluminescence spectrum of the CH3NH3PbBr3 under various applied pressures. As can be seen from the figure, increment of the applied pressure from zero up to 1.2GPa decreases the emission wavelength of this structure from the wavelength of 536nm to 530nm. Plus, Figure 6-b presents the variation of photoluminescence spectra (eV) as a function of pressure, which shows an increment in the energy gap. Additional investigation of the effect of pressure on the energy gap of CH3NH3PbBr3, using DRS in Figure 7, clearly states that the energy gap of CH3NH3PbBr3 is decreased about 0.02eV, from 2.31 to 2.29eV, by the increase of pressure up to 1.2GPa. The difference between the photoluminescence spectrum and DRS can be attributed to the change between the direct and indirect bandgap. Following the first two perovskites, the photoluminescence spectrum of the CH3NH3PbI3 under various pressures is presented in Figure 8. It can be seen from the figure that by slightly increasing the pressure up to 0.11GPa, the emission wavelength of this structure is increased from 751nm to 784nm, which indicates a significant reduction in the energy gap of the investigated perovskite. Subsequently, by augmenting the applied pressure up to 1.2GPa no considerable change in the wavelength of emission is observed. In other words, the increment of the applied pressure results in a significant change in the energy gap in the beginning, but continuous applying of pressure bears no considerable effect and the energy gap remains constant. To confirm the obtained results of the photoluminescence of the perovskite CH3NH3PbI3, the DRS method was used (Figure 9). According to Figure 9, it is obvious that the energy gap of CH3NH3PbI3, through a pressure up to 0.11GPa, decreases from 1.77 to 1.56eV. To continue applying the pressure have no considerable effect on the energy gap of this 9 ACS Paragon Plus Environment


perovskite. These results are in agreement with the outcome of the photoluminescence spectrum and they indicate that the energy gap of this structure is in fact direct.

4. Conclusion In summary, lead halide perovskites precursors were synthesized with different halide sources (CH3NH3PbX3, X=Cl, Br and I), by the molar ratio of CH3NH2/PbX2: 6/1, using the solution chemistry method. Tuning the optical bandgap of halide perovskites were achieved by substitution of X=Cl by Br and I in CH3NH3PbX3, and changed from direct to indirect bandgap through varying the various applied pressures from 0 to 1.2 GPa. The ability to tune the emission spectra or optical bandgap, through external pressure and halogen replacement in perovskite structures are major advantages of hybrid perovskites. Therefore, this tunability opens a new area of possibilities for perovskites base optical devices with engineered band structure.

Acknowledgement The authors thank the University of Mohaghegh Ardabili for their cooperation and supports.


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[24] Szafra ń ski, M.; Katrusiak, A. Mechanism of pressure-induced phase transitions, amorphization, and absorption-edge shift in photovoltaic methylammonium lead iodide. J. Phys. Chem. Lett. 2016, 7, 3458-3466. [25] Maculan, G.; Sheikh, A. D.; Abdelhady, A. L.; Saidaminov, M. I.; Haque, M. A.; Murali, B.; Alarousu, E.; Mohammed, O. F.; Wu, T.; Bakr, O. M. CH3NH3PbCl3 single crystals: inverse temperature crystallization and visible-blind UV-photodetector. J. Phys. Chem. Lett. 2015. 6, 3781-3786. [26] Kojima, A.; Teshima, K.;Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. [27] Luo, S.; Daoud, W. A. Crystal structure formation of CH3NH3PbI3-xClx perovskite. Materials. 2016, 9, 123-136. [28] Mirershadi, S.; Ahmadi Kandjani, S. Efficient thin luminescent solar concentrator based on organometal halide perovskite. Dyes Pigm. 2015, 120, 15-21. [29] Zhang, Z.; Ren, L.; Yan, H.; Guo, Sh.; Wang, Sh.; Wang, M.; Jin, K. Bandgap narrowing in Bi-doped CH3NH3PbCl3 perovskite single crystals and thin films. J. Phys. Chem. C, 2017, 121 (32),17436-17441. [30] Mirershadi, S.; Sattari, F.; Mohamadi Saridaragh, M. Effects of halogen replacement on the efficiency of luminescent solar concentrator based on methylammonium lead halide perovskite. Sol. Energy Mater. Sol. Cells. 2018, 186, 365-372. [31] Mott, N. F.; Davis, E. A. Electronic processes in non-crystalline materials; Clarendon Press, Oxford, 1979.


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Figure captions: Figure 1. X-ray diffraction pattern of (A) CH3NH3PbCl3, (B) CH3NH3PbBr3 and (C) CH3NH3PbI3 perovskites. Figure 2. UV-visible absorption spectra of (A) CH3NH3PbCl3, (B) CH3NH3PbBr3 and (C) CH3NH3PbI3 perovskites. Figure 3. (a) Diffuse reflectance spectra of of (A) CH3NH3PbCl3, (B) CH3NH3PbBr3 and (C) CH3NH3PbI3 perovskites. (b) Tauc’s plot of (αhν)2 as a function of photon energy (hν) for the (A) CH3NH3PbCl3, (B) CH3NH3PbBr3 and (C) CH3NH3PbI3 perovskites. Figure 4. Photoluminescence spectra of the CH3NH3PbCl3 perovskites under external applied pressure (A) 0, (B) 0.66 and (C) 1.20 GPa. Figure 5. Tauc’s plot of (αhν)2 as a function of photon energy (hν) for the CH3NH3PbCl3 perovskites under external applied pressure (A) 0, (B) 0.11 and (C) 1.20 GPa. Figure 6. (a) Photoluminescence spectra of the CH3NH3PbBr3 perovskites under external applied pressure (0-1.20 GPa). (b) Pressure dependence of the energy to main peak as extracted from PL data. Figure 7. Tauc’s plot of (αhν)2 as a function of photon energy (hν) for the CH3NH3PbBr3 perovskites under external applied pressure (A) 0, (B) 0.66 and (C) 1.20 GPa. Figure 8. Photoluminescence spectra of the CH3NH3PbI3 perovskites under external applied pressure (A) 0, (B) 0.11, (C) 0.66 and (D) 1.20 GPa. Figure 9. Tauc’s plot of (αhν)2 as a function of photon energy (hν) for the CH3NH3PbI3 perovskites under external applied pressure (A) 0, (B) 0.11 and (C) 1.20 GPa.



Fig. 1.

A

CH3NH3PbCl3 B CH NH PbBr 3 3 3 C CH NH PbI 3 3 3

4

(112) (211) (220) (202)

(100)

(224) (314)

(310)(312)

Intensity (a.u.)

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C

(100) (200) (210)

(110)

(220) (300)

B

(100) (200) (110)(111) (210)(211)

(221)

A

0

15

20

25

30

35

40

45

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2θ (degree)

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65

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Fig. 2.

A CH NH PbCl 3 3 3 B CH NH PbBr 3 3 3 C CH NH PbI 3 3 3 1

Intensity (a.u.)

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


405

656

522

A

C

B

0

400

450

500

550

600

650

Wavelength (nm)


700

750


Fig. 3.

100

A B C

a 80

580

CH3NH3PbCl3 CH3NH3PbBr3 CH3NH3PbI3

B

60

593

A

795

40

20

C

515 0 450

500

550

600

650

700

750

800

Wavelength (nm)

b

A CH NH PbCl 3 3 3 B CH NH PbBr 3 3 3 C CH NH PbI 3 3 3

(αhν)2 (cm-1 eV)2

Reflectance (%)

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 18 of 25

A

B

C 0

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

hν (eV)


2.5

2.6

2.7

2.8

Page 19 of 25

Fig. 4.

CH3NH3PbCl3 A

Normalized PL Intensity (a.u.)

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


B C

0.00 GPa 0.66 GPa 1.20 GPa

1.0

A B C

400

404

408

412

416

Wavelength (nm)


420

424


Fig. 5.

CH3NH3PbCl3

A

1

A

(αhν)2 (cm-1 eV)2

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 20 of 25

B

B C

C

0.00 GPa 0.11 GPa 1.20 GPa

0

2.040

2.091

2.142

2.193

2.244

2.295

2.346

hν (eV)


2.397

2.448

2.499

Page 21 of 25

Fig. 6.

a

1

0.00 GPa 0.11 GPa 0.22 GPa 0.66 GPa 0.88 GPa 1.20 GPa


CH3NH3PbBr3

0

500

520

540

560

580

600

Wavelength (nm)

2.345

b 0.98

2.340

PL Peak Position (eV)

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


2.335

0.91

526

528

530

532

534

536

538

540

542

2.330 2.325 2.320 2.315 0.0

0.2

0.4

0.6 0.8 Pressure (GPa)


1.0

1.2


Fig. 7.

1

CH3NH3PbBr3

A B C

(αhν)2 (cm-1 eV)2

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 22 of 25

A

0.00 GPa 0.66 GPa 1.20 GPa

B C

0

2.163 2.184 2.205 2.226 2.247 2.268 2.289 2.310 2.331

hν (eV)


Page 23 of 25

Fig. 8.

1.8

A

CH3NH3PbI3

B


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


C D

0.00 GPa 0.11 GPa 0.66 GPa 1.20 GPa

A B C D

560

600

640

680

720

760

800

Wavelength (nm)


840

880

920

960


Fig. 9.

1.4

CH3NH3PbI3 A B C

A

(αhν)2 (cm-1 eV)2

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 24 of 25

0.00 GPa 0.11 GPa 1.20 GPa

B

C

0.0

1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

hν (eV)


Page 25 of 25

TOC Graphic

0.00 GPa 0.11 GPa 0.22 GPa 0.66 GPa 0.88 GPa 1.20 GPa

2.345 1.0

2.340 0.9

PL Peak Position (eV)

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


2.335 0.8

520

525

530

535

540

545

550

2.330 2.325

Under UV

2.320 2.315 0.0

0.2

0.4

0.6 Pressure (GPa)


0.8

1.0

1.2

Pressure-Induced Optical Bandgap Transition in Methylammonium

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