Pressure-Tailored Band Gap Engineering and Structure Evolution of

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Pressure-Tailored Band Gap Engineering and Structure Evolution of Cubic Cesium Lead Iodide Perovskite Nanocrystals Ye Cao, Guangyu Qi, Chuang Liu, Lingrui Wang, Zhiwei Ma, Kai Wang, Fei Du, Guanjun Xiao, and Bo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01673 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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The Journal of Physical Chemistry

Pressure-Tailored Band Gap Engineering and Structure Evolution of Cubic Cesium Lead Iodide Perovskite Nanocrystals Ye Cao,1 Guangyu Qi,1 Chuang Liu,1 Lingrui Wang,1 Zhiwei Ma,1 Kai Wang,1 Fei Du,1,2 Guanjun Xiao,1,* and Bo Zou1 1 State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China,

2 Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China.

AUTHOR INFORMATION Corresponding Author [email protected]

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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

ABSTRACT: Metal halide perovskites (MHPs) have attracted increasing research attention given the ease of solution processability with excellent optical absorption and emission qualities. However, effective strategies for engineering the band gap of MHPs to satisfy the requirements of practical applications are difficult to develop. Cubic cesium lead iodide (α-CsPbI3), a typical MHP with an ideal band gap of 1.73 eV, is an intriguing optoelectric material owing to the approaching Shockley-Queisser limit. Here, we carried out a combination of in situ photoluminescence, absorption and angle dispersive synchrotron X-ray diffraction spectra to investigate the pressure-induced optical and structural changes of α-CsPbI3 nanocrystals (NCs). The α-CsPbI3 NCs underwent a phase transition from cubic (α) to orthorhombic phase and subsequent amorphization upon further compression. The structural changes with octahedron distortion to accommodate the Jahn−Teller effect were strongly responsible for the optical variation with the increase of pressure. First-principles calculations reveal that the band gap engineering is governed by orbital interactions within the inorganic Pb-I frame through the structural modification. Our high pressure studies not only established structure-property relationships at the atomic scale of α-CsPbI3 NCs, but also provided significant clues in optimizing photovoltaic performance, thus facilitating the design of novel MHPs with increased stimulus-resistant capability.


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Introduction

The rapid development of hybrid organic-inorganic perovskite solar cells experienced an increase in power conversion efficiency from 3.8% to more than 20% in only a few years.1,2 Multiple studies focused on improving the stability of hybrid organic-inorganic perovskites. The recent success of replacing the organic cation with inorganic cation in metal halide perovskite (MHPs) solar cells indicates that all-inorganic metal halide perovskite nanocrystals (NCs) are highly desired in a high demand owing to their excellent stability, ultrahigh photoluminescence (PL) quantum yield, and composition-dependent luminescence with a wide color gamut.3-7 The orthorhombic CsPbBr3 NCs exhibited a band gap of 2.52 eV with the green emission at 514 nm. While the cubic phase cesium lead iodide (α-CsPbI3) perovskites with a band gap of 1.73 eV is one of the most notable optoelectric materials owing to the approaching Shockley–Queisser limit.8 CsPbI3 perovskite normally exists in the orthorhombic (δ) phase at room temperature, whereas, the cubic phase of CsPbI3 is unstable. The instability of α-CsPbI3 under ambient conditions presents a major fabrication hurdle. Therefore, the stability of α-CsPbI3 NCs should be explored for further application of this excellent solar cell material. It was reported that nanocrystallinity played a decisive role for the stabilization of metastable phase owing to the unsaturated coordination of surface atoms and the enhanced surface energy.9 Accordingly, by restricting the physical dimensions of the α-CsPbI3 perovskite crystallites to a nanometer region, the cubic crystalline phase of CsPbI3 can be stabilized at ambient conditions with unique spatial confinement effects.10 This change allows large spectral tunability, thus facilitating the band gap optimization for the devices.11,12



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Pressure, as an important thermodynamic parameter, provides a powerful means to study the structural and electronic behavior of MHP materials. Pressure loading by a diamond anvil cell (DAC) can lead to a close packing and reduce the interatomic distances, which profoundly modify electronic orbitals and bonding patterns.13-20 In recent years, high-pressure studies on MHP materials have set off a new wave of upsurge.21-25 Significant progress in the pressure response of methylammonium-based lead halide perovskites was achieved by several independent groups.26,27 However, most high-pressure objects are confined to the organometallic halide perovskites, with instability and sensitivity to oxygen/water largely limiting actual photovoltaic applications.28-32 Recent achievements on pressure-sintered process and band gap modulations of CsPbBr3 NCs extended the interest to all-inorganic MHP NCs at extremes.24,33-35

Here, we performed a systematic study to explore the behavior of α-CsPbI3 NCs, all-inorganic halide structured perovskites, under high

pressure conditions.

The

pressure-driven structure and optical properties of this material were characterized by combining in situ synchrotron X-ray diffraction (XRD), optical absorption, and photoluminescence (PL) measurements. All high pressure experimental results indicated that α-CsPbI3 NCs underwent a structural transition from cubic to orthorhombic phase at approximately 0.39 GPa. High-pressure absorption and PL experiments on α-CsPbI3 NCs exhibited a red shift followed by a blue shift in wavelength. The theoretical computations speculated that the orbital interactions associated with the shrinkage of Pb-I bond lengths and the variations of Pb-I-Pb bond angles in lead-iodine octahedral network led to the pressure-driven structural modulation and band gap shift. Our findings not only provide the


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fundamental relationship between structural variations and optoelectronic properties of α-CsPbI3 NCs but also offer an in-depth insight into the microscopic physicochemical mechanism of the MHP nanosystem at extremes.



Experimental Section α-CsPbI3 NCs were synthesized according to previously reported literature.10 Cesium carbonate (0.25 g, Cs2CO3, Sigma-Aldrich, 99.9%) was loaded in a 50 mL three-neck flash along with oleic acid (1 mL, OA, Sigma-Aldrich, 90%) and octadecene (25 mL, ODE, Sigma-Aldrich, 90%). The mixture was stirred for 30 mins at 120 oC under N2 until the solution was clear. Prior to use, the temperature of Cs-oleate was reduced to 70 oC and the solution was stored in N2.

25 mL of ODE, 0.5 g of PbI2 (Alfa Aesar, 99%), 2.5 mL of OA, and 2.5 mL of oleylamine (OLA, Sigma-Aldrich, 70%) were loaded in a 50 mL three-neck flash, and dried under N2 at 120 oC for 1 h. Until PbI2 was completely dissolved, the solution was heated under N2 to 170 o

C, and the Cs-oleate (4 mL) precursor was rapidly injected into the reaction mixture. About

30 s after injection, the reaction mixture turned dark red and was quenched by the ice-water bath.

In order to improve the stability of the obtained products, the resultant mixture was isolated twice by using methyl acetate (MeOAc, anhydrous 99.5%) and hexane by centrifuging for 5 mins at 8000 rpm. The resulting samples were characterized by transmission electron microscopy (TEM) and high-resolution TEM performed on a JEM-2200FS with an emission gun operating at 200 kV.

In situ high-pressure experiments were conducted in a symmetric diamond anvil cell (DAC) apparatus with a culet size of 400 µm. The sample chamber was generated by drilling a 150 µm-diameter hole in the center of T301 stainless steel gasket, which was preindented to 45


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µm by the diamonds. The sample was loaded into the gasket cavity together with a ruby ball, which was used to determine the actual pressure by the standard ruby fluorescent technique.36 Silicon oil (Dow Corning Corporation, 10 cSt) was applied as the pressure transmitting medium to provide the hydrostaticity.

The in situ high-pressure PL and absorption spectra were recorded with an optical fiber spectrometer (Ocean Optics, QE65000). Using a semiconductor laser with an excitation wavelength of 355 nm as the excitation source, we collected the pressure-dependent PL spectra. The PL micrographs of the samples upon compression were taken with a camera (Canon Eos 5D mark II) installed on a microscope (Ecilipse TI-U, Nikon). A deuterium-halogen light source was used for the absorption measurements under high pressure.

In situ high-pressure angle-dispersive X-ray diffraction (ADXRD) patterns were measured with a monochromatic wavelength of 0.6199 Å at beamline 15U1, Shanghai Synchrotron Radiation Facility (SSRF). CeO2 was utilized as the standard sample for the calibration. The pattern of intensity versus diffraction angle 2θ was plotted based on the FIT2D program, which integrated and analyzed the 2D images collected. All the high-pressure experiments were conducted at room temperature.

The CASTEP module in Materials Studio was carried out for the simulation of the band structure and the partial density of states (PDOS). First-principles density functional theory (DFT) computations were performed using the plane-wave pseudopotential technique with the generalized gradient approximation (GGA).37 A plane-wave basis set with an energy



cutoff of 910 eV was applied. The PDOS of isolated atoms were collected by adopting the identical pseudopotential of the GGA.


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Results and discussion The TEM and HRTEM images of the synthesized α-CsPbI3 NCs are shown in Figure 1. The images exhibit highly monodispersed nanocube morphology and good crystallinity. The inset in Figure 1a presents a bright red color of α-CsPbI3 NCs colloid under UV light irradiation. The lattice spacing of α-CsPbI3 NCs was 6.2 Å, which matched the (100) plane of cubic-structured α-CsPbI3 (Figure 1b). The relative particle size distribution of the obtained α-CsPbI3 NCs indicated that the average diameter of the sample was 10.0 nm with the standard deviation of 1.2 nm (Figure S1).

Figure 1. (a) TEM image and (b) HRTEM image of α-CsPbI3 NCs upon compression. The inset presents the optical photograph under UV light. (c) Changes in PL spectra of α-CsPbI3 NCs under pressure. The right panel represents the microphotographs in the sample chamber at selected pressures of 1 atm, 0.38 GPa, 1.07 GPa, and 3.43 GPa with the assistance of a microscope.



We should characterize the corresponding optical properties of these materials, which are utilized as promising optoelectronic materials for α-CsPbI3 NCs. Thus, we carried out in situ high-pressure PL measurement on α-CsPbI3 NCs. As depicted in Figure 1c, the PL peak exhibited a tiny red shift below 0.38 GPa at the beginning of the whole pressurization, followed by a successive blue shift until further compression to 3.43 GPa. The turning point of the PL location at 0.38 GPa is related to the crystal structure change of α-CsPbI3 NCs under high pressure. The PL location of α-CsPbI3 NCs evolved linearly from 689 nm to 691 nm and then to 664 nm before and after the turning point of 0.38 GPa (Figure S2). The pressure coefficients were approximately -16.67 and +24.24 meV/GPa, respectively.

In addition, PL intensity exhibited persistent reduction with increasing pressure, until fluorescence was abruptly quenched at 3.43 GPa. The reduction of PL intensity was caused by the distortion of crystal structure. The optical micrographs of α-CsPbI3 NCs as a function of pressure in the DAC chamber clearly revealed the darkening trend of PL brightness (right panel in Figure 1c). The PL color changed from bright red to dark red and then disappeared with increasing pressure. Upon decompression, the PL intensity and location recovered, suggesting the reversibility of the pressure-induced optical variation of α-CsPbI3 NCs (Figure S3).

The high-pressure optical absorption spectra of α-CsPbI3 NCs are shown in Figure S4. Under ambient conditions, the sample exhibited notable absorbance at the band edge of 715 nm. With the increase in pressure, the absorption presented a gradual red shift below 0.38 GPa, followed by a sustaining blue shift. The sudden change in absorption spectra at 0.38


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GPa agreed with the discontinuity in the PL spectra. Upon further compression to 3.43 GPa, the original absorption edge disappeared, as was consistent with the quenching in the PL spectra. After total pressure release, the absorption recovered to the initial position.

Figure 2. (a) Band gaps of α-CsPbI3 NCs at selected pressures of 1 atm, 0.38 GPa and 3.43 GPa estimated by Tauc plot. (b) Shift of band gap energy with the increase of compressive stress.

Figure 2a displays the band gaps at selected pressures of 1 atm, 0.38 GPa and 3.43 GPa, as determined via the extrapolation of the linear region to the energy axis intercept by Tauc



plot.38 We investigated the band gap Eg on α-CsPbI3 NCs in terms of Kubelka-Munk transformations: α (1)

where A and hν are the edge-width parameter and the incident photon energy, respectively. The evolution of pressure-dependent band gap energy for α-CsPbI3 NCs is shown in Figure 2b. The synthesized α-CsPbI3 NCs possessed a band gap of 1.72 eV under ambient conditions, in accordance with the reported results by Luther et al.9 With the pressure increase to 0.38 GPa, the band gap showed a slight narrowing by 0.03 eV. Upon further compression, the band gap increased from 1.69 eV to 1.76 eV, which was ascribed to occurrence of the phase transition.


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Figure 3. (a) Representative in situ ADXRD patterns of CsPbI3 NCs. (b) Refinements of the experimental (red fork), simulated (black profile), and difference (black line) ADXRD patterns at 1 atm and 2.03 GPa and decompression to ambient conditions. The blue vertical markers indicate the corresponding Bragg reflections.

To further identified the correlation between the optical properties and structure of α-CsPbI3 NCs, we carried out in situ high pressure ADXRD experiments. Figure 3 (a) shows the representative ADXRD patterns of α-CsPbI3 NCs up to 8.25 GPa and the quenched pattern. Upon compression, all the diffraction peaks shifted to increased angles, demonstrating the reduction in unit cell volume. When the pressure reached 0.39 GPa, a new notable peak at 8.3o and split peaks at 5.7o and 11.5o appeared. The changes in ADXRD patterns manifested the occurrence of a phase transition. In Figure 3 (b), the refinements of the simulated XRD patterns matched the experimental data. The blue vertical markers indicate the corresponding Bragg reflections. The initial structure at ambient pressure was a cubic phase with the lattice constant of a = 6.230(3) Å, which was highly consistent with the HTEM results. The ADXRD pattern at 2.03 GPa matches the orthorhombic phase with the space group of Pbnm. The R factors obtained from the refinement at 2.03 GPa were Rp = 0.38% and Rwp = 0.5%. The results indicated that the α-CsPbI3 NCs underwent a phase transition from cubic phase to orthorhombic phase under high pressure. We further performed the first-principles calculations to assess the enthalpies of the two phases with the increase of pressure, as shown in Figure S5. It was found that the cubic phase processed a lower enthalpy than orthorhombic phase in the initial pressure range of 1 atm to 0.3 GPa. Since the driving force of a structural transformation is the reduction in Gibbs free energy, the apparent decrease in the enthalpy of



the orthorhombic phase after roughly 0.3 GPa indicates that orthorhombic CsPbI3 was the theoretical computations strongly corroborated that α-CsPbI3 NCs underwent a phase transition from cubic structure to orthorhombic structure, which is highly consistent with our experimental results. Note that the phase transition sequence for α-CsPbI3 NCs and orthorhombic CsPbBr3 NCs was different. Upon compression, α-CsPbI3 NCs experienced a structural phase transition from cubic phase to orthorhombic phase. Whereas, when the CsPbBr3 NCs were subjected to the high pressure, an isostructural phase transition, an electronic phase transition from orthorhombic phase to orthorhombic phase occurred. Upon further compression exceeding 3.44 GPa, the diffraction peaks of CsPbI3 NCs started to broaden, and only few peaks could detected. The broad peaks and the decreased intensity of the ADXRD patterns illustrated that the crystallinity of α-CsPbI3 NCs was deteriorated, and the sample tended to be amorphous under increased pressures. After complete pressure release, the refinements of the ADXRD patterns suggested that the crystal structure transformed back to the original structure. Additionally, the morphology and crystallinity of α-CsPbI3 NCs were retained upon decompression, which could be readily deduced from the TEM and high-resolution TEM images, as shown in Figure S6. Furthermore, we summarized the detailed lattice constants of α-CsPbI3 NCs under various pressure levels in Table S1. Therein, r0 represents the pressure upon complete release of pressure to the ambient conditions.

To further analyze the structural evolution, we fitted the experimental pressure-volume (P-V) date (Figure 4) ranging from ambience to 3.44 GPa by utilizing the third-order Birch-Murnaghan equation of state as follows:


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PV

1

B

!

#

4

1$

(2)

where V0 is the zero-pressure volume, B0 is the bulk modulus at ambient pressure, and B0’ is a parameter for the pressure derivative. With B0’ fitted at 4, the unit cell volume per atom of the cubic phase (V0) was 48.8(3) Å3 and the bulk modulus (B0) was 7.5(1) GPa. During phase transition, the α-CsPbI3 NCs experienced a volume collapse of 2.2% per atom. After the phase transition, As compared with the original cubic phase, the higher value (18.0(3) GPa) of B0 (II) relative to that in the original cubic phase indicates that high-pressure phase of α-CsPbI3 NCs cannot be easily compressed after phase transition. The enhancement of the bulk modulus may be attributed to surface effect or structural disorder.39,40

Figure 4. Pressure dependence of the unit cell volume of CsPbI3 NCs fitted by the third-order Birch−Murnaghan equation of state.

The pressure-induced electronic structure can be illustrated by the first-principles calculations of α-CsPbI3. The calculated results in band gap energy tended to decrease in the low-pressure phase (cubic phase), followed by an increase in the high-pressure phase



(orthorhombic phase). The results matched well with the experiments. The typical energy band structures and density of states of α-CsPbI3 at selected pressures of 1 atm, 0.4 and 3.0 GPa are shown in Figure 5. The initial direct band gap energy was 1.84 eV under ambient conditions with space group Pm 3 m as structural model. As the pressure increased to 0.4 GPa, the direct band gap energy was decreased to approximately 1.77 eV. Upon further compression to 3.0 GPa, the direct band gap energy increased to 2.17 eV by means of the orthorhombic structure model. The partial density of states shows that the valence band maximum (VBM) is virtually attributed to the antibonding hybridization between the Pb s and I p orbitals in the [PbI6]4- octahedral network, whereas the conduction band minimum (CBM) was determined by the strong nonbonding Pb p orbitals.41


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Figure 5. Calculated band structures and the partial density of states (PDOS) of α-CsPbI3 NCs at selected pressures of 1 atm, 0.4 GPa and 3.0 GPa.



Figure 6. (a) Schematic models of polyhedral views of cubic and orthorhombic CsPbI3 perovskites under high pressure. (b and c) Schematic illustrations of the band gap changes governed by the Pb-I-Pb bond angle within PbI6 octahedral framework before and after structural phase transition.

The changes in the optical properties can be elucidated by considering the structural phase transition of α-CsPbI3 NCs under high pressure given that band gap of α-CsPbI3 perovskites was mainly determined by the changes in the VBM and CBM. The CBM is less sensitive to the contraction of the Pb-I bond length and slightly changed upon compression owing to the nonbonding characteristic of Pb 6p orbitals. Therefore, the band gap energy is mainly dominant by the VBM change of α-CsPbI3 NCs in the pressure processing. As depicted in Figure 6a, the [PbI6]4- groups include regular octahedral networks for cubic α-CsPbI3. The


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[PbI6]4- octahedral in the cubic α-CsPbI3 underwent a contraction with the increase in pressure. The major Pb-I-Pb bond angle remained at nearly 180o (Figure S7), but the Pb-I band length decreased (Table S2). Consequently, the Pb 6s orbital (represented by light blue sphere) and I 5p orbital (represented by brown spindle) became closer with increasing pressure (Figure 6b). This change leads to an increased degree of Pb-I electron cloud overlap. Hence, the VBM energy increases as a result of the enhanced coupling between the Pb 6s and the I 5p orbitals, thereby ultimately causing the red shift of the band gap with pressure.42,43 Meanwhile, the blue shift in band gap is caused by lattice distortions. When the phase transition from cubic to orthorhombic structure occurred, the [PbI6]4- octahedra underwent a stark distortion to accommodate the Jahn−Teller effect, and the averaged Pb-I-Pb bond angle markedly decreased in orthorhombic phase with increasing pressure (Figure S7 and Table S3), resulting in the reduced overlap between electron clouds of the Pb 6s and I 5p. Although the Pb-I bond length decreased (Table S4), the reduction of Pb-I-Pb bond angle significantly influenced the coupling of the Pb 6s and I 5p electron clouds. Therefore, the coupling of Pb 6s and I 5p orbitals weakened, resulting in the decrease energy of the VBM (Figure 6c). As compared with the studies of orthorhombic CsPbBr3 NCs,33 the band gap red shift of CsPbI3 NCs and CsPbBr3 NCs was caused by the energy lift of the VBM. However, the mechanism of the band gap blue shift for cubic CsPbI3 NCs under pressures was substantially distinct. In the case of orthorhombic CsPbBr3 NCs at ambient conditions, the CBM was almost contributed from the Pb 6p state, because the Pb 6p orbital possessed a much greater energy level than Br 4s state. When much higher pressure (beyond 2 GPa) was applied to orthorhombic CsPbBr3 NCs, the CBM state happened to be dominated by the strong coupling



of Pb 6p and Br 4p orbitals. Accordingly, the blue jump can be interpreted from the significant shift to higher energy level of the CBM by the exertion of pressure, thereby leading to the widened band gap. Overall, the pressure-driven optical change of CsPbI3 NCs can be determined by just considering the VBM shift through the orbital interactions within Pb-I octahedral framework upon compression. Anyway, the current high-pressure study of α-CsPbI3 NCs provides new insights into the band gap tuning based on the relationship between structural changes and optical properties in metal halide perovskites.

Conclusions

In summary, the pressure-engineered structure and optical properties of α-CsPbI3 NCs were systematically investigated by means of a symmetric DAC apparatus. The combined results of the ADXRD patterns with in situ PL and absorption spectra, the α-CsPbI3 NCs experienced a phase transition from cubic to orthorhombic phase under high pressure. The band gap of α-CsPbI3 halide perovskite NCs exhibited an initial narrowing, followed by a successive blue shift with pressure. The first-principles calculations demonstrated that the variations in band gaps were induced by orbital interactions associated with the synergistic effects of the pressure-induced variations of Pb-I bond length and Pb-I-Pb bond angle within the PbI6 octahedral network. These findings provide insight into the microscopic physicochemical mechanism on the band gap engineering of compressed MHP nanosystems through experiments and theoretical levels and offer a new route to produce improved materials-by-design for different applications.


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ASSOCIATED CONTENT

Supporting Information Relative particle distribution histogram, Pressure-dependent PL location, Pressure-dependent PL spectra upon decompression, Changes in absorption spectra under selected pressures, Calculated enthalpies of cubic and orthorhombic CsPbI3, TEM image and high-resolution image of α-CsPbI3 NCs after compression, Lattice constants under pressure, Ball and stick model of the adjacent [PbI6]4- groups in CsPbI3 NCs, Bond length of Pb-I bond for cubic phase.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work is supported by the National Science Foundation of China (Nos. 21725304, 11774125, 21673100, 91227202 and 11774120), the Chang Jiang Scholars Program of China (No. T2016051), Changbai Mountain Scholars Program (No. 2013007), National Defense Science and Technology Key Laboratory Fund (6142A0306010917), and Program for Innovative Research Team (in Science and Technology) in University of Jilin Province.



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X.; Yang, W. G.; Zhu, K.; et al. Simultaneous Band-Gap Narrowing and Carrier-Lifetime Prolongation of Organic-Inorganic Trihalide Perovskites. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 8910.



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Pressure-Tailored Band Gap Engineering and Structure Evolution of

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