Supporting Information
Enhanced Photocurrent of All-Inorganic Two-Dimensional
Perovskite Cs2PbI2Cl2 via Pressure-Regulated Excitonic Features
Songhao Guo1, Kejun Bu1, Jiangwei Li2, Qingyang Hu1,Hui Luo1, Yihui He3, Yanhui Wu1,
Dongzhou Zhang4, Yongsheng Zhao1, Wenge Yang1, Mercouri G. Kanatzidis3, and Xujie Lü1*
1 Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai
201203, China
2 Key Lab of Organic Optoelectronics, Molecular Engineering of Ministry of Education,
Department of Chemistry, Tsinghua University, Beijing 100084, China
3 Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States 4 Hawaii Institute of Geophysics & Planetology, University of Hawaii Manoa, Honolulu, Hawaii
96822, United States
Experiment details
Sample preparation.
Single crystals of Cs2PbI2Cl2 were prepared from a stoichiometric melt of CsCl (3.368 g)
and PbI2 (4.610 g) by the vertical Bridgman method. The temperature was set to 500 °C for the
hot zone and 350 °C for the cold zone. Before the growth process, the ampule was held in the hot zone for 24 h to melt completely. The ampule was then translated from the hot zone to the cold
In situ high-pressure structural characterizations.
The high-pressure environment was provided by symmetrical diamond anvil cells (DACs). Type II-a ultralow fluorescence diamonds with a culet size of 300 μm were used. The high-pressure sample chamber was formed from a pre-indented Re gasket with a thickness of about 50 μm and a hole with a diameter of about 150 μm by laser-drilling the center part of it. The Cs2PbI2Cl2 crystal and a ruby ball (for pressure measurements) were loaded inside the
chamber, and the pressures were determined by the ruby fluorescence method.1 Neon was used
for single-crystal XRD and Raman measurements to obtain high quality of single-crystal diffraction data and reduce the spectroscopy background, respectively. Mineral oil was used for other experiments.
The in situ high-pressure X-ray diffraction (XRD) experiments were performed at beamline 13ID-D of GeoSoilEnviroCARS (GSECARS) at Advanced Phonon Source (APS), Argonne National Laboratory (ANL). The wavelength of the monochromatic X-ray beam is 0.4133 Å. The diffraction patterns were collected by the MarCCD detector and integrated using the Dioptas software.2 The in situ high-pressure single-crystal XRD experiments were performed at beamline
13BM-C of GSECARS, APS, ANL. The wavelength of the monochromatic X-ray beam is 0.434 Å. The structure was solved using the SHELX software package interfaced with the OLEX2 program.3-4 A direct method was used to solve the structure, and full-matrix least-squares fitting
on F2 was used for the structure refinement.
Structure refinements for powder XRD data were carried out using the Rietveld method in General Structure Analysis System (GSAS) software.5-6 The starting structural parameters of the
where i can be assigned as V (volume), A (area), and l (length) for the volume, area, and linear compressibility, respectively.8
The unit-cell volume as a function of pressure was fitted to the second-order Birch-Murnaghan equation of state;
𝑃(𝑉) =3𝐾0 2 [( 𝑉0 𝑉) 7 3 − (𝑉0 𝑉) 5 3 ]
where V0 is the initial volume, V is the deformed volume, and K0 is bulk modulus.9
In situ high-pressure optical measurements.
High-pressure Raman spectra were collected by a Raman spectrometer with a 532.1 nm excitation laser at GSECARS, APS, ANL. As for determining the bandgap values under high pressures, we conducted both absorption and diffuse reflection measurements for the single crystal and powder sample, respectively, in a home-designed spectroscopy system in the micro-region (Gora-UVN-FL, built by Ideaoptics, Shanghai, China). The bandgap measured by absorption method is determined by extrapolating the linear portion of the α2 versus the hν curve,
where α is the absorption coefficient, h is Planck constant, and ν is the frequency of the photon. The bandgaps are also determined from the diffuse reflection measurements by using the Kubelka-Munk equation: F(R) = (1-R)2/2R, where R is the reflectance.10 The bandgap values
derived from these two methods are consistent, as shown in Figure S4.
In situ high-pressure photocurrent measurements.
Cs2PbI2Cl2 single crystal with a dimension of about 100 × 100 μm2 × 10 μm was used. The
photocurrent was recorded by a two-probe resistance measurement system in a DAC up to around 10 GPa. A cubic boron nitride (cBN) layer was inserted between the stainless-steel gasket
voltages at different pressures (Figure S12).
The photoconductivity was calculated using the following equation; 𝜎 = 𝐼 × 𝑙
𝑈 × 𝑆
where U is bias voltage, I is photocurrent, and S and l are the cross-sectional area and distance of the sample between the two electrodes, respectively.
First-principles calculations
First-principles calculations were performed in the framework of density functional theory (DFT) using the Vienna Ab Initio Simulation Package (VASP).11-12 The Perdew-Burke-Ernzehof
(PBE) version of the generalized gradient approximation (GGA) was used to describe the exchange correlation functional, and the projector augmented wave (PAW) method was used in the present work.13 Here, the cutoff energy of the plane wave was chosen at 400 eV for both
phases. For the structure optimizations, 6×6×2 and 6×6×6 k-points were used for the conventional cell of phase I and II, respectively. The convergence criteria are that the changes in total energies between two successive electronic steps are less than 10-5 eV, and all the
Hellmann-Feynman force acting on each atom is less than 0.01 eV /Å. The long-range van der Waals interactions should be taken into account for layered structures.14 In light of this, we
employed the DFT-D3 scheme to calculate the long-range dispersion correction up to 60 Å.15
High-symmetry points in the Brillouin zones (X, Γ, M, Y, and A represent (0, 0.5, 0), (0, 0, 0), (0.5, 0.5, 0), (0.5, 0, 0), and (0.5, 0.5, 0.5) for phase I, while A, Γ, Y, M, L and V represent (0, 0, 0.5), (0, 0, 0), (0.5, 0.5, 0), (0.5, 0.5, 0.5), (0.5, 0, 0.5), and (0.5, 0, 0) for phase II) were
sampled along the high-symmetry sites according to the work by Wahyu and Stefeno.18 The
off-resonance Raman activity of each normal mode was then calculated by the Raman_sc code.19
The effective in-plane masses of the electron and hole were derived from the following expression;
𝑚∗= ℏ2[𝜕
2𝜀(𝑘)
𝜕𝑘2 ] −1
where the k is the wave vector along the transport direction, ε(k) represents the energy band eigenvalues, and the ℏ is the reduced Planck constant.
According to the Wannier-Mott exciton model, the exciton binding energy (Eb) is given as
the following forms:20
𝐸b = 𝑒
4
2(4𝜋𝜀0)2ℏ2×
𝑚𝑟∗
𝜀∞2 where mr* is the reduced exciton mass based on the equation,
1 𝑚𝑟∗ = 1 𝑚𝑒+ 1 𝑚ℎ
Supplementary Tables
Table S1. Crystallographic data for the phase I and phase II of Cs2PbI2Cl2.
Phase I Phase II
Pressure Ambient 2.8 GPa
Crystal system Tetragonal Monoclinic
Space group I 4/mmm C 2/m
a = 5.6382(8) Å, α = 90° a = 7.749(2) Å, α = 90°
Unit cell dimensions b = 5.6385(8) Å, β = 90° b = 7.800(2) Å, β = 112.29(4)° c = 18.879(4) Å, γ = 90° c = 9.863(14) Å , γ = 90°
Volume 600.2(2) Å3 551.6(8) Å3
Density 4.414 g/cm3 4.803 g/cm3
Crystal color Transparent yellow Yellow
Rint 0.0626 0.0309
R1 [I > 2σ(I)] 0.0306 0.0871
wR2 0.0824 0.2823
GOF 1.242 1.166
* Crystallographic data for the phase I come from literature.7 aR
1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 – Fc2)2]/ ∑[w(Fo2)2]}1/2, w = 1/[s2(Fo2) + (0.1705P)2 +
Table S2. Atomic coordinates for the phase II of Cs2PbI2Cl2 at 2.8 GPa with estimated standard deviations in parentheses. Label x y z Occupancy U Pb 0.5 0.5 0 1 0.081 Cs 0.8774(9) 0.5 0.7486(5) 1 0.075 I 0.6650(18) 0.5 0.341(3) 1 0.201 Cl 0.25 0.25 0 1 0.032
Table S3. Refined lattice parameters and the unit-cell volumes of Cs2PbI2Cl2 at different pressures. Pressure (GPa) a (Å) b (Å) c (Å) β (°) Volume (Å3) 0.0 5.630 5.562 5.527 5.493 5.470 18.88 90 598.4 0.7 18.52 90 572.9 1.2 18.35 90 560.5 1.6 18.20 90 549.1 2.1 18.07 90 540.7 2.6 7.707 7.719 9.702 112.0 534.5 3.1 7.652 7.760 9.600 111.8 529.3 3.6 7.603 7.818 9.502 111.9 524.0 4.1 7.571 7.874 9.441 112.3 520.6
Supplementary Figures
Figure S1. In situ synchrotron powder XRD patterns of Cs2PbI2Cl2 under different pressures.
Figure S3. Rietveld refinements of Cs2PbI2Cl2 at different pressures. The tetragonal structure
with space group I4/mmm was used to fit the pattern up to 2.1 GPa, while the monoclinic structure with space group C2/m was used to fit the pattern at 2.6 GPa and those at higher pressures. The blue lines denote the difference between the observed (black) and the simulated (red) profiles, and the purple verticals stand for the simulated peak positions. The green line represents the amorphous background in the profile.
Figure S4. (a) UV-Vis diffuse reflectance spectra of Cs2PbI2Cl2 at different pressures. (b) The
bandgap of Cs2PbI2Cl2 obtained by the absorption and diffuse reflection measurements for the
Figure S6. Optical images of Cs2PbI2Cl2 at different pressures. Part of these results at selected
Figure S7. Calculated band structures and the partial density of states from each element at ambient conditions (phase I) and 2.8 GPa (phase II).
Figure S8. The calculated effective masses of the electron and hole in Cs2PbI2Cl2 as a function
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