Phase Transitions of Formamidinium Lead Iodide Perovskite under

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Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure Shaojie Jiang, Yiliang Luan, Joon I. Jang, Tom Baikie, Xin Huang, Ruipeng Li, Felix O. Saouma, Zhongwu Wang, Timothy J. White, and Jiye Fang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09316 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Journal of the American Chemical Society

Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure Shaojie Jiang,1 Yiliang Luan,2 Joon I. Jang,3 Tom Baikie,*,4 Xin Huang,5 Ruipeng Li,5† Felix O. Saouma,6 Zhongwu Wang,*,5 Timothy J. White7 and Jiye Fang*,1,2 1

Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York 13902, USA 2

Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, USA

3

Department of Physics, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, South Korea

4 5

Energy Research Institute@NTU, Nanyang Technological University, Republic of Singapore

Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York 14853, USA

6

Department of Physical Sciences, Kaimosi Friends University College, P.O. Box 385-50309, Kaimosi, Kenya

7

School of Materials Science and Engineering, Nanyang Technological University, Republic of Singapore

ABSTRACT: The pressure-induced structural evolution of formamidinium-based perovskite FAPbI3 was investigated using in-situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI3 (Pm-3m) partially and irreversibly transformed to hexagonal δ-FAPbI3 (P63mc) at a pressure less than 0.1 GPa. Structural transitions of αFAPbI3 followed the sequence of Pm-3m → P4/mbm → Im-3 → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converts to an orthorhombic Cmc21 structure between 1.26 and 1.73 GPa. During decompression, FAPbI3 recovered the P63mc structure of the δ-phase as a minor component (~18 wt%) from 2.41 - 1.40 GPa and the Pm3m structure of the α-phase becomes dominant (~82 wt%) at 0.10 GPa but with an increased fraction of δ-FAPbI3. The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI3 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI3-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.

FAPbI3 polymorphism and the relationship to PV functionality is required.

INTRODUCTION Hybrid perovskite-based photovoltaic (PV) cells have shown remarkable improvements in power conversion efficiency (PCE), increasing from ~3.8 % in 20091 to more than 21 % in 2017.2 Sunlight possesses a panchromatic spectrum, requiring absorber materials of the certain bandgap. It is known that the highest PCE at 300 K for 1 sun correlates with a bandgap of 1.2-1.4 eV.3-4 MAPbX3 (MA: methylammonium; X: halide anions) is the most widely studied hybrid perovskite with the smallest bandgap5-8 of 1.57 eV which is larger than the optimal value. The bandgap can be reduced by introducing organic groups with a larger effective ionic radius (R). For example, a replacement of MA (R = 2.70 Å)9 with a slightly larger formamidinium cation (NH2CH=NH2+, FA, R = 2.79 Å)9 in the perovskite MAPbI3 can dramatically reduce the bandgap to ~1.45 eV,5 making FAPbI3 a more suitable absorber for single-junction solar cells.10 It is also reported that FAPbI3 has a high short-circuit photocurrents without sacrificing the photovoltage owing to more efficient harvesting of red photons.11 Furthermore, FAPbI3 exhibits superior charge transport parameters including carrier mobility compared to MAPbI3.12 Therefore, exploration of

FAPbI3 polymorphism as a function of temperature has been extensively studied. At room temperature, FAPbI3 crystallizes as a yellow, non-perovskite δ-phase (P63mc) which is thermally stable. However, this material is a poor solar absorbent with a large bandgap and a chain-like structure that hinders electron transport.10 At ~150 °C, the δ-phase FAPbI3 (hereafter, δ-FAPbI3) undergoes a reconstructive phase transformation to the α-phase FAPbI3 (hereafter, α-FAPbI3) with a favorable bandgap and topology for efficient PV effects.13 The α-FAPbI3 is a cubic perovskite (Pm-3m) where [PbI6]4- octahedra are vertexconnected.14 At room temperature, the α-FAPbI3 slowly transforms to the thermodynamically stable δ-FAPbI3 irrespective of the storage environment.13 Recent studies have focused on α-FAPbI3 stability15-17 and its application in PV cells,4, 18-19 whereas pressure-dependent investigations on FAPbI3 are very limited.20 The chemical analog FAPbBr3 transforms from Pm-3m to Im-3 as the pressure increased from ambient pressure to 0.53 GPa, leading to a bandgap red-shift followed by a blue-shift at a higher pressure (2.2 GPa).21 Similarly, MAPbI3 evolves

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from I4/mcm to Im-3 below 0.3 GPa with a bandgap redshift followed by a blue-shift upon further compression.22 This phenomenon has not reported in FAPbI3.20 Thus, the motivation for the present high-pressure experiments using synchrotron X-ray diffraction (XRD) and photoluminescence (PL) techniques was to clarify the relationship between the crystallographic and electronic structures of the FAPbI3 polymorphs and the implications for photovoltaic deployment of this material.

EXPERIMENTAL Sample preparation: FA- and lead-containing solutions were separately prepared by dissolving 1.9 mmol of formamidine acetate (99%, Sigma-Aldrich) in 0.5 mL of hydriodic acid (HI, 57 wt% in water, Sigma-Aldrich) and dissolving 1.8 mmol lead(II) acetate trihydrate (≥99%, Sigma-Aldrich) in 4.0 mL of hydriodic acid in a flask heated in an oil bath (100 oC). The FA-containing solution was then slowly introduced to the Pb-containing solution in a round-bottom flask, generating a yellow precipitate. The HI solution was added dropwise to this mixture at 100 oC under stirring until the yellow precipitate was completely dissolved. This solution was maintained at 100 o C for 2 h under vigorous stirring, then slowly cooled to 75 o C and held at this temperature overnight without stirring to yield yellow acicular FAPbI3 crystals. The product was harvested by filtration, washed using diethyl ether (J & T Baker), and dried in a vacuum oven at room temperature. Black α-FAPbI3 (Pm-3m) powders were obtained by heating the yellow crystals at 170 ⁰C for 30 min in argon. A portion of the black FAPbI3 product was introduced to a diamond anvil cell (DAC), whereas the unused material was sealed in a vial at ambient pressure as a “reference” specimen for future phase comparison. High-pressure setup: The DAC was prepared by preindenting the stainless steel gasket to a thickness of ~100 µm from 250 µm through which a ~200 µm diameter hole was drilled and served as the sample chamber. The perovskite was loaded with several ruby chips to calibrate the pressure by laser-excited ruby fluorescence technique during the in-situ experiments. Synchrotron XRD and PL spectra were collected successively. X-ray diffraction and photoluminescence characterizations: Synchrotron powder XRD patterns were collected at the B1 station of the Cornell High Energy Synchrotron Source (CHESS). Monochromatic X-rays (25.514 keV) was collimated using a double pinholealigned circular tube into the small X-ray beam (100 microns in diameter). X-ray scattering signals were collected using a large area Mar345 detector. The sample-todetector distance and other detector parameters were calibrated using a CeO2 powder standard. The raw twodimensional (2D) images were integrated and analyzed by the Fit2D package.23 The laser-excited PL spectra were recorded using a Princeton Acton SP-300i system from 660 nm to 940 nm. The samples loaded in the DAC were measured under compression or decompression with insitu measurements of each diffraction pattern. A 532-nm diode laser was used for excitation and the emitted light

was collected through a spectrometer with a 300g/mm grating and exposure time of 3s. In order to make a consistent comparison and correlation between the collected diffraction and PL data sets, the pressuredependent compression and decompression experiments were repeated in the same manner. A time-integrated photoluminescence (TIPL) spectrum of the δ-FAPbI3 was also measured at ambient conditions under UV radiation. The sample was flame-sealed in a quartz capillary tube, and loaded into a homemade sample holder that was mounted on a Z-scan translation stage. Coherent light (wavelength: 1064 nm) was first produced using an EKSPLA PL-2250 series diode-pumped picosecond Nd:YAG laser with a pulse width of 30 ps and a repetition rate of 50 Hz. The Nd:YAG laser pumped an EKSPLA Harmonics Unit (HU) H400 where the input beam was frequency tripled by a successive cascade of nonlinear wave mixing to obtain the UV excitation source. Fitting Software: Pawley fitting of the lattice parameters and phase component were carried out using the Topas (version 3) software package (1999-2000 Bruker AXS). Previously reported lattice parameters of α-FAPbI3 at ambient pressure as determined by neutron diffraction14 were used as the starting point for refinement. The corresponding synchrotron XRD pattern was simulated using CrystalMaker.24

RESULTS AND DISCUSSION Freshly annealed black FAPbI3 at ambient pressure was found by synchrotron powder diffraction to be phase pure α-FAPbI3 (Fig. 1(a,b)). Pawley fitting (Fig. S1a) verified Pm-3m symmetry14 with a cell edge of a = 6.3651(1) Å. For compression < 0.1 GPa, weak δ-FAPbI3 reflections (Fig. S1) were detected (Fig. 1c). Rietveld fitting suggested a ~3.5 wt% (Rwp = 6.77%) fraction of δ-FAPbI3 at ~0.1 GPa as shown in Supporting Information. α-FAPbI3 is reported to spontaneously and slowly transform to δ-FAPbI3 at ambient conditions, with complete conversion of a single crystal sample requiring up to 10 days.13 For powders, the transition time ranges from a few hours to several days, depending on crystallinity and particle size.4, 12 The present experiments found the α- and δ-FAPbI3 transition accelerated by pressure (Fig. 1c), while the reference αFAPbI3 did not contain detectable δ-FAPbI3 at the same time. The large thermal hysteresis during α- and δ-FAPbI3 transition confirms a potential energy barrier,25 as does the kinetic trapping of α-FAPbI3 by rapid quenching. While pressure lowers the energy barrier, the majority of α-FAPbI3 is retained even to 6.59 GPa at the onset of amorphization (vide infra). Moreover, α-FAPbI3 metastability is evidenced by (111)cubic anisotropic strain which is released by transformation to δ-FAPbI3,26 a characteristic not replicated in the FAPbBr321 and MAPbI322 analogs. Before a discussion of the α-FAPbI3 structural transitions during decompression (Fig. S2), the pressure-induced δ-FAPbI3 needs to be addressed.


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Journal of the American Chemical Society out for α-FAPbI3. Increasing the pressure to 0.46 GPa transformed cubic Pm-3m to tetragonal P4/mbm symmetry (Fig. 2) that is a maximal isomorphic subgroup of Pm-3m with topological in-phase a0a0c+ rotation of the [PbI6]4- octahedra along [001].27,28 For halide perovskites, density functional theory (DFT) calculations show that P4/mbm is the preferable symmetry with a lower electrostatic energy and higher bond valence for the A-site,29 As expected, δ-FAPbI3 transformed into Cmc21 at 1.51 - 1.97 GPa (Fig. 2a and Fig. S1l) and co-existed with P4/mbm perovskite at 3.13 GPa (Fig. 2b). Pawley fitting (Fig. S1) suggests that the high-pressure phase adopts the Im-3 supercell with a doubled cell-edge, which is realized by a+a+a+ tilting (Fig. 3). The perovskite Im-3 structure and non-perovskite Cmc21 structures coexist to 6.59 GPa and partially amorphizes (Fig. 2b). Aperiodicity is initiated from the onset of the P4/mbm to Im-3 transformation at 3.13 GPa (Fig. 2b, Fig. S1p) consistent with observations from single crystal MAPbI3 under 2.5 GPa hydrostatic compression.30 This contrasts with the present study, where the powder was subject to deviatoric stress.

Figure 1. XRD patterns of α-FAPbI3 powder. (a) Experimental pattern at ambient pressure; (b) Calculated pattern at ambient pressure; (c) Experimental pattern at 0.10 GPa, showing the minor peaks (with stars) of δ-FAPbI3. The inset in (c) shows the structure of δ-FAPbI3.

Since co-existing δ-FAPbI3 may compromise the structural analysis of α-FAPbI3, high-pressure measurements of phase pure δ-FAPbI3 were conducted to 6.81 GPa (Fig. S3). The lattice parameters of δ-FAPbI3 (P63mc) at ambient pressure were determined by Pawley fitting to be a = 8.6841(1) Å and b = 7.9356(1) Å (Fig. S4a). A transition to orthorhombic Cmc21 occurred at 1.26 - 1.73 GPa during compression, and the P63mc phase was restored at ~0.57 GPa during decompression (Figs. S3-S5). PL spectra were recorded from pure δ-FAPbI3 through the compression/decompression cycle (from 0.57 to 2.3 GPa during compression and from ~1.97 to 0.24 GPa during decompression) as shown in Figs. S6-S8 and Tables S1-S2. Details are provided in the Supporting Information (Pages S6-S7). Having monitored the structural behavior of pure δFAPbI3 under pressure, similar experiments were carried

The amorphous content is preserved during decompression to 2.41 GPa (Fig. 2c, Fig. S2e) until crystallinity is completely restored at ~0.46 GPa (Fig. 2c, Fig. S2h), which is lower than the onset pressure for the amorphization (3.13 GPa) (Fig. 2b) during compression. This difference in behavior is an artifact of the experiment. The DAC gasket deforms leading to shrinkage of the sample cavity and the uniform application of pressure under compression, while the pressure was released only along the diamond anvil axis during decompression. The strain preserved in the gasket plane partially preserves the amorphous component to low pressures. Both perovskite Im-3 and nonperovskite Cmc21 structures are restored to the P4/mbm α- FAPbI3 and P63mc δ-FAPbI3 polymorphs between 2.41 GPa and 1.40 GPa, respectively (Fig. 2c, Fig. S2f). Below 0.10 GPa, the Pm-3m perovskite polymorph (Fig. 2c, Fig. S2i) is restored, confirming that the pressure-induced perovskite phase transition is reversible. However, the persistent fraction of δ-FAPbI3 (~18 wt% according to Rietveld fitting with Rwp = 7.41 %) after decompression (Fig. S9) implies that the α- to δ-FAPbI3 transition is irreversible. Finally, at the conclusion of the pressure experiments, it was confirmed that the reference sample remained stable with no δ-FAPbI3 formed. The present observations contrast with another study of FAPbI3 polymorphism where Pm-3m → Imm2 → Immm → partially amorphous transitions were reported during compression without a trace of δ-FAPbI3, and preservation of the Imm2 phase after pressure release.20



The in-situ PL spectrum contains a feature at ~795 nm at 0.10 GPa (Fig. 4 and Table 1, Section I) indicative of mixed α- and δ-FAPbI3, and cannot be used to precisely determine the bandgap of the α-FAPbI3 even though the impurity fraction is minimal. The PL peak red-shifted to 807 nm as the pressure was increased to 1.51 GPa (Fig. 4), beyond which it became vanishingly weak until it reappeared at 2.88 GPa (at 803 nm). At pressures >2.88 GPa, the feature was no longer detected (Fig. 4 and Table 1, Section II). Meanwhile, a new PL peak emerges at 854 nm attributable to the P4/mbm polymorph from 0.46 GPa that red-shifts to ~868 nm at 2.88 GPa (Fig. 4 and Table 1, Section III). After transformation to the Im-3 polymorph at 3.13 GPa and at higher pressures the PL peaks became too weak for analysis (Fig. 4). These observations suggest that the bandgap is strongly affected by the pressure-


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Journal of the American Chemical Society

Figure 2. XRD patterns of α-FAPbI3 under pressure (a, b), compression; and (c), decompression.

Figure 3. Characteristic PbI6 octahedral tilt in α- and δ-FAPbI3. Blue figures, rotations of three pressure-related polymorphs of α-FAPbI3 with Glazer symbols; brown figures, P63mc polymorph of δ-FAPbI3.

induced re-orientation of the [PbI6]4- octahedra and possibly other structural factors. While the red-shift of these peaks may be indicative of a narrower bandgap, this could not be confirmed. A plausible explanation is that all of them originate from the defect levels common to the material, regardless of a specific phase. Consequently, they

are not directly associated with the bandgap and ineffective for light harvesting.



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Figure 5. PL spectra of α-FAPbI3 upon release of pressure. Figure 4. PL spectra of α-FAPbI3 upon increase of pressure.

Table 1. PL peaks of α-FAPbI3 as a function of pressure applied during compression Pressure (GPa)

Section I

Section II

Section III

Table 2. PL peaks of α-FAPbI3 as a function of pressure applied during decompression Pressure (GPa)

Section I

Section II

Section III

λ1 (nm)

E1 (eV)

λ2 (nm)

E2 (eV)

λ3 (nm)

E3 (eV)

λ1 (nm)

E1 (eV)

λ2 (nm)

E2 (eV)

λ3 (nm)

E3 (eV)

1.40

718.1

1.727

784.4

1.581

-

-

0.10

724.9

1.711

795.8

1.558

853.1

1.454

1.15

717.8

1.728

784.3

1.581

-

-

0.24

725.4

1.709

796.7

1.556

853.5

1.453

0.46

724.7

1.711

799.7

1.551

856.5

1.448

0.46

726.9

1.706

800.3

1.549

854.8

1.451

0.10

726.4

1.707

776.7

1.597

-

-

0.93

-

-

804.8

1.541

855.7

1.449

1.26

-

-

805.8

1.539

857.1

1.447

1.51

-

-

807.3

1.536

859.6

1.443

1.97

-

-

809.3

1.532

864.3

1.435

2.30

-

-

809.8

1.531

867.5

1.429

2.55

-

-

808.3

1.534

867.7

1.429

2.88

732.1

1.694

803.6

1.543

867.9

1.429

3.13

-

-

799.3

1.551

868.1

1.428

During decompression, both the PL peaks are restored from 2.41 and 1.4 GPa when the Im-3 and Cmc21 crystallizes (Fig. 5 and Table 2, Sections I & II). The PL peak at ~718 nm corresponds to the transition from the Cmc21 to the P63mc structure in δ-FAPbI3 that comprises a larger fraction at this pressure, whereas the peak at ~784 nm is assigned to the transition from the Im-3 to the P4/mbm polymorph in α-FAPbI3. The PL peak at ~860 nm was not detected during decompression, suggesting it is not associated with the bandgap but reflects a decrease in defect concentration during recrystallization. The peak position and intensity remain steady as the pressure was further decreased to 1.15 GPa (Fig. 5), but at 0.46 GPa, the 718 nm feature weakens and red-shifts to 724 nm, whereas the second peak at 784 nm intensifies and red-shifts to 800 nm. An additional weak peak appears at 856 nm at this pressure (Fig. 5 and Table 2, Section III) but is lost at 0.10 GPa when two additional peaks become evident at 726


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Journal of the American Chemical Society nm and 776 nm. The differences in PL features during compression/decompression can be attributed to varying fractions of δ-FAPbI3. Concerning the lattice cell change, Fig. S10 and Table S3 show a variation of the unit cell volume as a function of applied pressure during compression. This comparison indicates that the cell volume contractions at pressures of 0.24 GPa and 2.88 GPa, which are associated with the αFAPbI3 phase transitions of Pm-3m → P4/mbm and P4/mbm → Im-3, are ~2.2 % and ~1.4 %, respectively.

positions of δ-FAPbI3 at ambient and high pressures, selected profiles of Rietveld fitting, change of lattice cell volume as a function of applied pressure. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Authors * T. Baikie: [email protected]; Z. Wang: [email protected]; J. Fang: [email protected]

Present Addresses †

Present address: National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA Author Contributions All authors have approved the manuscript.

Funding Sources CEI, NSF and PRF.

Notes Figure 6. Illustrations of the phase transition of α-FAPbI3 during compression and decompression.

CONCLUSIONS An in-situ XRD and PL investigation found α-FAPbI3 Pm-3m perovskite to partially and irreversibly convert to non-perovskite δ-FAPbI3 P63mc by compression to 0.1 GPa. The remaining sample fraction undergoes displacive transformation to three FAPbI3 polymorphs in the sequence of Pm-3m → P4/mbm → Im-3 → partially amorphous from ambient pressure to 6.59 GPa (Fig. 6, left). Concomitantly, δ-FAPbI3 adopts a Cmc21 structure from 1.26 to 1.73 GPa. Upon decompression, the α-FAPbI3 and δ-FAPbI3 dimorphs are restored (Fig. 6, right) in the proportions ~82 wt% and ~18 wt%, respectively. The PL behavior for α- and δ-FAPbI3 are most likely associated with transitions involving the defect levels rather than the bandgap variations only. Upon compression, a slight reduction of the α-FAPbI3 bandgap before 1.51 GPa is indicated but further confirmation using techniques such as absorption spectroscopy31-32 is necessary. It is concluded that pressure induces both displacive and reconstructive transitions in FAPbI3 polymorphs including irreversible conversion to δ-FAPbI3. In order to promote the photon harvest efficiency of α-FAPbI3 through modification of its structure and bandgap by pressurization, the accelerated phase transition to the hexagonal non-perovskite structure (δ-FAPbI3) should be considered.

ASSOCIATED CONTENT Supporting Information. Pawley fitting of α- and δ-FAPbI3 during compression and decompression, discussion on diffraction and PL spectra of δ-FAPbI3 under pressure, XRD patterns of δ-FAPbI3 under pressure, PL spectra and peak

ORCID ID: S. Jiang, 0000-0001-8692-0673; Y. Luan, 0000-0002-3632-8396; J. Jang, 0000-0002-1608-8321; T. Baikie, 0000-0001-8853-9278; F. Saouma, 0000-0002-9413-1866; Z. Wang, 0000-0001-9742-5213; T. White, 0000-0002-4380-9403; J. Fang, 0000-0003-3703-3204

ACKNOWLEDGMENTS We would like to acknowledge Custom Electronics Inc. for the financial support. We also thank Kevin K. Fang for his contribution to the experimental determination. This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208. S. J. and Y. L. acknowledge the partial support from ACS PRF (PRF# 58196-ND10).

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Phase Transitions of Formamidinium Lead Iodide Perovskite under

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