Potential Applications of Halide Double Perovskite Cs2AgInX6 (X = Cl

Publication Date (Web): February 25, 2019. Copyright © 2019 American Chemical Society. Cite this:J. Phys. Chem. Lett. XXXX, XXX, XXX-XXX ...
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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1120−1125

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Potential Applications of Halide Double Perovskite Cs2AgInX6 (X = Cl, Br) in Flexible Optoelectronics: Unusual Effects of Uniaxial Strains Zhao Zhang, Jie Su,* Jie Hou, Zhenhua Lin, Zhaosheng Hu, Jingjing Chang,* Jincheng Zhang, and Yue Hao China State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Shaanxi Joint Key Laboratory of Graphene, Advanced Interdisciplinary Research Center for Flexible Electronics, School of Microelectronics, Xidian University, Xi’an 710071, China J. Phys. Chem. Lett. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 02/26/19. For personal use only.

S Supporting Information *

ABSTRACT: The discovery of halide double perovskite Cs2AgInX6 (X = Cl, Br) has provided an efficient way to search promising solar cell absorbers. Here, theoretical calculations on strained Cs2AgInX6 (X = Cl, Br) not only comprehensively and firstly help understand their physical properties but also provide a guideline to extend their potential applications. Although Cs2AgInX6 possesses a similar structure, the variations of physical properties of strained Cs2AgInX6 are different. Only compressive Cs2AgInBr6 undergoes a direct-to-indirect transition, which enables it to be a good radiation detection material. Moreover, the mobility of Cs2AgInCl6 is reduced by strains, while that of Cs2AgInBr6 is enhanced (reduced) by compression (tension). That is because the contribution degrees of Ag-dz2, dx2−y2 and In-dz2, dx2−y2 on the band edges of Cs2AgInX6 (X = Cl, Br) are inconsistent. In addition, the absorption coefficients of Cs2AgInX6 (X = Cl, Br) are deteriorated negligibly by strain, making it a potential material for further applications of photovoltaics and flexible optoelectronics.

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addition, Cs2AgInCl6, which has been successfully synthesized in experiment with band gap values of 2.0−3.0 eV and good stability, has shown great potential as a useful optoelectronic material such as a photon as well as an ionizing radiation detector.13,17 Unfortunately, few optoelectronics and photovoltaics based on Cs2AgInX6 (X = Cl, Br) in experiments have been reported due to limited understandings of physical properties of Cs2AgInX6 (X = Cl, Br). Meantime, the physical properties can be further tuned by strain engineering according to previous reports.18,19 Thus, the electronic, transport, and optical properties of Cs2AgInX6 (X = Cl, Br) and effects of uniaxial strain along the [100] direction on such properties are investigated first and comprehensively using first-principles calculations. Compared to previous studies, we have presented a promising approach toward controlling physical properties of Cs2AgInX6 (X = Cl, Br) via strain engineering, thus to obtain better performance for potential applications of solar cells and flexible optoelectronics in experiment. All calculations were performed by using density functional theory on the basis of the projector augmented wave (PAW) method20 as implemented in the VASP code.21−23 The local density approximation (LDA) was adopted for the exchange− correction functional.24,25 The cutoff energy was set to be 400 eV. A 6 × 6 × 6 and a 7 × 7 × 7 Monkhorst−Pack (MP)26 k-

n the past few years, perovskites have attracted intensive attention for solar cell applications due to the continuously increasing power conversion efficiency (PCE). Until now, the reported PCE of perovskite solar cells (PSCs) was over 23.6%, which is even higher than that achieved by some Si-based thin film solar cells.1−3 The most studied perovskite is the organic− inorganic hybrid lead halide perovskite due to its suitable band gap, high optical absorption, high carrier mobility, as well as long carrier lifetime.4−6 However, hygroscopic and volatile organic cations and toxic lead elements make organic− inorganic hybrid perovskites suffer from two major issues: toxicity and instability.7 These issues significantly hamper the applications of organic−inorganic hybrid perovskites. Recently, the discovery of lead-free halide double perovskites provided a feasible way to search air-stable and environmentally benign solar cell absorbers. The novel physical properties of these halide double perovskites make them potentials for photovoltaic (PV) and optoelectronic applications.8−11 For example, the absorption coefficient of halide double perovskite Cs2AgInBr6 is estimated to be higher than that of silicon (with a band gap of 1.1 eV).11 Moreover, Cs2AgInBr6-based solar cells may reach an ideal solar efficiency of 28%, which is close to the ideal values of CH3NH3PbI3based solar cells (30%).12 Another halide double perovskite Cs2AgInCl6 exhibits a direct band gap as well as an ultralong carrier lifetime (6 μs),13,14 which are suitable for PV applications. Moreover, manganese15 and sodium16 doped Cs2AgInCl6 can extend the visible light emission property and obtain efficient and stable white light emission, respectively. In © XXXX American Chemical Society

Received: January 15, 2019 Accepted: February 25, 2019 Published: February 25, 2019 1120

DOI: 10.1021/acs.jpclett.9b00134 J. Phys. Chem. Lett. 2019, 10, 1120−1125

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The Journal of Physical Chemistry Letters

Figure 1. (a) Crystal structures of the strained Cs2AgInX6 (X = Cl, Br). (b) Relationship between total energy and the applied strain. The quadratic fitting of the data gives the elastic constant of Cs2AgInX6 (X = Cl, Br). DOS of (c) Cs2AgInCl6 and (d) Cs2AgInBr6, respectively.

Figure 2. Normalized band gap, CBM, and VBM as functions of uniaxially strained (a) Cs2AgInCl6 and (b) Cs2AgInBr6, respectively. The shadowed area in (b) represents the strain range of the indirect band gap region.

and Cs2AgInBr6 (obtained by fitting the energy-strain curves, as shown in Figure 1b) are 71.02 and 67.73 N/m, respectively, which are lower than those of two-dimensional transition metal dichalcogenides.30 It suggests that these double perovskites easily undergo strain and can potentially be used for flexible nanodevices. Thus, investigating the electronic and optical properties of strained Cs2AgInX6 (X = Cl, Br) is important. To accurately understand the strain effects, the electronic structures of unstrained Cs2AgInX6 (X = Cl, Br) are comparatively investigated by LDA, GGA-PBE, GGA-91, PBE0, and HSE methods, as displayed in Figure S1. It can be found that the LDA method is sufficient to investigate the electronic properties of Cs2AgInX6 (X = Cl, Br) perovskite, except for the band gaps. Thus, Figure 1 gives the density of states (DOS) of Cs2AgInX6 (X = Cl, Br) obtained by the LDA method. For the Cs2AgInCl6 perovskite, it is a direct band gap material with the conduction band minimum (CBM) and valence band maximum (VBM) located at the Γ point. Its CBM is mainly composed of Ag-s states of the AgX6

mesh with a Gamma k-point were adopted for structural relaxation and electronic calculation, respectively. The structures were fully relaxed until the maximum residual force on each atom was smaller than 0.01 eV/Å, and the selfconsistent field energy was set to be 10−5 eV. Spin−orbit coupling was not taken into consideration here because it negligibly affects the electronic structures of Ag/In halide double perovskites.14,27 Similar methods have been employed to investigate the natural physical properties of Cs2InAgX6 (X = Cl, Br).11,28,29 The optimized lattice constants obtained by the LDA functional are about 10.22 and 11.43 Å for cubic Cs2AgInCl6 and Cs2AgInBr6, respectively, which are in good agreement with previous reports,11,12,29 as listed in Table S1. Cs2AgInX6 (X = Cl, Br) consists of InX6 and AgX6 octahedra, as displayed in Figure 1a. The lengths of Ag−X bonds of Cs2AgInX6, especially those of Cs2AgInBr6, are slightly larger than those of In−X bonds, as listed in Table S1, due to the larger radius of Ag and Br ions. In addition, the elastic constants of Cs2AgInCl6 1121

DOI: 10.1021/acs.jpclett.9b00134 J. Phys. Chem. Lett. 2019, 10, 1120−1125

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Figure 3. Projected band structures of (a) Cs2AgInCl6 and (b) Cs2AgInBr6 with −10, 0, and 10% uniaxial strains.

as displayed in Figure 2a. At the same time, no matter how compressive and tensile the strain, Cs2AgInCl6 retains the direct band gap character with the CBM and VBM both located at the Γ point. These could lead to wide application of Cs2AgInCl6 in flexible electronics. Different from Cs2InAgCl6, the band gap of Cs2InAgBr6 slightly enlarges at first and then decreases significantly as the compression increases, while it decreases slowly and monotonously as the tension increases. Moreover, the variation rate of the band gap of Cs2InAgBr6 is higher than that of Cs2InAgCl6. It should be noted that Cs2InAgBr6 just maintains the direct band gap character with the CBM and VBM both located at the Γ point under the tensile condition and small compressive condition. A direct-toindirect transition occurs for Cs2InAgBr6 when the compressive strain is larger than 4%, which enables the Cs2InAgBr6 to be a good radiation detection material. Meanwhile, the VBM shifts from the Γ point to the X point. As a result, the VBM of Cs2InAgBr6 decreases at first until the compressive strain reaches 2%, and after that, it increases slightly and then shifts down as the compressive strain continues to enlarge. For the CBM of Cs2InAgBr6, it increases from the compression to tension, like that of Cs2InAgCl6. The reason for these behaviors is elucidated by inspecting the character of the orbitals that form the band edge states, as demonstrated in Figure 3. The CBM of Cs2AgInX6 (X = Cl, Br) is dominated by σ* antibonding, which consists of s orbitals of Ag/In ions and p orbitals of halide ions, irrespective of compressive and tensile strain (shown in Figure 3a). Compressive and tensile strain can enhance and weaken the σ* antibonding, respectively.19 Thus, the CBM of Cs2AgInX6 (X = Cl, Br) reduces and increases with increasing compressive and tensile strain, respectively. Comparing the contribution of orbitals on the CBM, more contributions of Ag s orbitals and fewer contributions of In s orbitals are observed for

octahedron and In-s orbitals of the InX6 octahedron, and its VBM is dominated by Ag-d (dz2, dx2−y2) and Cl-p (px + py + pz) states of the AgX6 octahedron coupling with the small In-d (dz2, dx2−y2) orbitals of the InX6 octahedron. Compared to the Cs2AgInCl6 perovskite, a similar band structure is observed for the Cs2AgInBr6 perovskite. However, the CBM of the Cs2AgInBr6 perovskite is dominated by Ag-s states of the AgX6 octahedron coupling with negligible In-s orbitals of the InX6 octahedron; the VBM of the Cs2AgInBr6 perovskite is dominated by In-d (dz2, dx2−y2) and Br-p (px + py + pz) states of the AgX6 octahedron and small Ag-d (dz2, dx2−y2) orbitals of the InX6 octahedron, which is in contrast to those of the Cs2AgInCl6 perovskite. In addition, the band gaps of Cs2AgInCl6 and Cs2AgInBr6 perovskites are 0.65 and 0.58 eV, respectively, which are lower than those obtained by the HSE functional (2.43 and 1.33 eV for Cs2AgInCl6 and Cs2AgInBr6, respectively, as displayed in Figure S1) and the experimental values because LDA methods always underestimate the band gap values.11 Figure 2 displays the normalized band gaps and band edges of strained Cs2AgInX6 perovskites obtained by the HSE functional. The Cs 6s semicore state in the deep energy level is employed as a reference energy level to measure the band edge variations. For Cs2AgInCl6, its band gap reduces slightly and then approaches a constant with either increasing compressive or tensile strain. Moreover, the variation under the compressive strain condition is larger than that under the tensile strain condition. For example, the band gaps of Cs2AgInCl6 with 10% compressive and tensile strains are reduced 16 and 13%, respectively. This is because both the CBM and VBM of Cs2AgInCl6 shift down and up with increasing compressive and tensile strain, respectively. Meanwhile, the variation rate of the band edges under the compressive strain is higher than that under the tensile strain, 1122

DOI: 10.1021/acs.jpclett.9b00134 J. Phys. Chem. Lett. 2019, 10, 1120−1125

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Figure 4. Calculated effective masses and mobilities of strained Cs2AgInX6 (X = Cl, Br) for (a) electrons and (b) holes, respectively.

Cs2AgInCl6 and Cs2AgInBr6, respectively, as demonstrated in band structures in Figure 3. The hole effective masses of Cs2AgInX6, especially those of Cs2AgInBr6, reduce gradually with increasing compressive strain. Moreover, the hole effective masses of Cs2AgInX6 decrease sharply when the compression is about 4% due to the direct-to-indirect transition of compressive Cs2AgInX6. Figure 4 also exhibits the room-temperature electron and hole mobilities of Cs2AgInX6 (X = Cl, Br) with different strains. Although Cs2AgInBr6 possesses larger effective masses than Cs2AgInCl6, the carrier mobilities of Cs2AgInBr6 are higher than those of Cs2AgInCl6 because Cs2AgInBr6 shows a lower deformation potential than that of Cs2AgInCl6 (as displayed in Figure S3), and the carrier transport is inversely proportional to the effective mass and deformation potential.31,32 The calculated electron and hole mobilities of unstrained Cs2AgInCl6 are 976.79 and 34.41 cm2· V−1·s−1, respectively. The calculated electron and hole mobilities of unstrained Cs2AgInBr6 are 997.06 and 27.52 cm2·V−1·s−1, respectively. When Cs2AgInX6 undergoes strain, the electron mobilities of Cs2AgInCl6 are reduced by both compression and tension, and the electron mobilities of Cs2AgInBr6 are enlarged and decreased by compressive and tensile strain, respectively. It suggests that the electron mobilities of Cs2AgInBr6 can be improved by squeezing or a high-pressure atmosphere (viz., under the compression condition) in experiment. These characters are opposite of the variation of electron effective masses, as the above analysis shows . When the strain increases to 10%, these electron mobilities are still high. The electron mobilities of Cs2AgInCl6 with 10% compressive and tensile strains are calculated to be 822.90 and 680.04 cm2·V−1·s−1, respectively. The electron mobilities of Cs2AgInBr6 with 10% compressive and tensile strains are calculated to be 1201.94 and 804.38 cm2·V−1·s−1, respectively. For the hole transport mobilities of Cs2AgInX6, their variations are also opposite of those of hole effective masses, as comparatively demonstrated in Figure 4. Interestingly, tensile strain has little influence on the hole mobilities, while compressive strain strongly enlarges the hole mobilities of Cs2AgInX6, especially those of Cs2AgInBr6. For instant, the calculated hole mobilities of Cs2AgInCl6 and Cs2AgInBr6 with 10% compression are up to 186.46 and 370.59 cm2·V−1·s−1, respectively, which are larger than those of unstrained Cs2AgInX6. Besides electron transport, the optical absorption coefficient is another significant property for the perovskite photoelectric device. Figure 5 shows the absorption spectra of Cs2AgInCl6 and Cs2AgInBr6 under different strains. The absorption

Cs2AgInBr6. As a result, the reduced rate of CBM of Cs2AgInBr6 is different than that of Cs2AgInCl6. The above analysis shows that the VBM at the Γ point of Cs2AgInCl6 mainly consists of π bonding, which is composed of the hybridization between Cl-p orbitals and In/Ag-dz2, dx2−y2 orbitals. Such π bonding can be weakened and enhanced by compressive and tensile strain, respectively. Thus, the energies of the VBM of Cs2AgInCl6 shift up and down with the increasing compressive and tensile strain, respectively. Note that, dx2−y2 is the in-plane orbital and dz2 is the out-of-plane orbital, and the former is more sensitive to the in-plane strain. Therefore, significant orbital splitting occurs at the Γ point, as displayed in Figure 3a. Compared to Cs2AgInCl6, similar π bonding is observed for the VBM of Cs2AgInBr6. However, the π bonding of Cs2AgInBr6 is dominated by the Br-p orbitals with negligible dz2 and dx2−y2 orbitals of In/Ag ions, as displayed in Figures 3b and S2. At the same time, the π bonding at the L point consists of significant dz2, dx2−y2 orbitals of In/Ag ions and small Br-p orbitals. The energies of both kinds of π bondings are reduced and enlarged by the compressive and tensile strain, respectively. It should be noted that, nevertheless, the variation rate of π bonding at the Γ point is larger than that at the L point. Thus, the energies of the VBM of Cs2AgInBr6 increase under tensile conditions, while they reduce at first and then increase and then reduce under compressive conditions, resulting in a direct-to-indirect transition under compressive conditions. Besides the band edges, transport properties are also usually tuned by strain engineering. Figure 4 illustrates the effective masses along the strain direction dependent on strain. For unstrained Cs2AgInCl6 and Cs2AgInBr6, their electron (hole) effective masses are 0.27 m0 (1.01 m0) and 0.39 m0 (2.16 m0), respectively, which are consistent with previous reports.10 All of the electron effective masses of Cs2AgInX6 are lower than hole effective masses because CBM shows a sharper parabolic curve, as demonstrated in band structures in Figure 3. The electron and hole effective masses of Cs2AgInCl6 are lower than those of Cs2AgInBr6. For tensile Cs2AgInX6, their electron effective masses enlarge monotonously, while the hole effective masses reduce slightly, especially for Cs2AgInBr6, with increasing tensile strain. It suggests that tensile strains may have obvious and weak effects on the electron and hole transports of Cs 2AgInX 6, respectively. In the case of compressive Cs2AgInX6, the electron effective mass of Cs2AgInCl6 enlarges by increasing the compressive strain, while that of Cs2AgInBr6 varies oppositely because the compressive strain broadens and sharpens the CBM of 1123

DOI: 10.1021/acs.jpclett.9b00134 J. Phys. Chem. Lett. 2019, 10, 1120−1125

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Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00134. Additional tables and figures showing lattice constants and band structures of cubic Cs2AgInX6 (X = Cl, Br) obtained by different methods, orbital-projected band structure of cubic Cs2AgInX6 (X = Cl, Br), and deformation potentials of the VBM and CBM for cubic Cs2AgInX6 (X = Cl, Br) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 5. Calculated absorption coefficients of Cs2AgInX6 (X = Cl, Br) under strain of −10, 0, and 10%, in comparison to CH3NH3PbI3.

*E-mail: [email protected] (J.S.). *E-mail: [email protected] (J.C.). ORCID

Jie Su: 0000-0003-4431-7184 Zhenhua Lin: 0000-0002-2965-1769 Jingjing Chang: 0000-0003-3773-182X Jincheng Zhang: 0000-0001-7332-6704

coefficients of unstrained Cs2AgInBr6 are higher than those of unstrained Cs2AgInCl6 in the infrared and visible light regions. Moreover, in the infrared and long-wave visible light regions, the absorption coefficients of Cs2AgInCl6 are close to those of CH3NH3PbI3, and the absorption coefficients of Cs2AgInBr6 are even larger than those of CH3NH3PbI3, as shown in Figure 5. In the ultraviolet region, both of the absorption coefficients of Cs2AgInX6 (X = Cl, Br) are lower than those of CH3NH3PbI3. For the strained Cs2AgInBr6, the absorption coefficients are not deteriorated by both the compression and tension, even though the strain is as large as 10%. In the case of strained Cs2AgInCl6, both compressive and tensile strains just slightly reduce the adsorption coefficients in the long-wave visible region. However, the order of adsorption coefficient of strain Cs2AgInCl6 is still in agreement with that of unstrained Cs2AgInCl6, as shown by the examples of strained Cs2AgInCl6 with 10% tension and compression in Figure 5. Such characteristics suggest that Cs2AgInX6, especially Cs2AgInBr6, shows great potential for application in flexible photoelectronic devices. In summary, with first-principles calculations, the electronic, transport, and optical properties of strained Cs2AgInX6 (X = Cl, Br) are investigated systematically and first. Strains occur easily in both Cs2AgInCl6 and Cs2AgInBr6 due to their low elastic constants. Although Cs2AgInX6 (X = Cl, Br) possess similar structures, the orbital contributions of Ag, In, and X elements on band edges are different, resulting in a direct-toindirect transition for compressive Cs2AgInBr6. Moreover, the band gaps of Cs2AgInCl6 are reduced negligibly by strain, different from that of strained Cs2AgInBr6. Both the effective electron mass and electron mobility of Cs2AgInCl6 (0.27 m0, 976.79 cm 2·V−1·s−1) are slightly lower than those of Cs2AgInBr6 (0.39 m0, 997.06 cm2·V−1·s−1). Interestingly, the electron mobility of Cs2AgInCl6 is reduced by strains, while that of Cs2AgInBr6 is enhanced and reduced by compression and tension, respectively. The absorption coefficients of Cs2AgInX6, especially that of Cs2AgInBr6, are deteriorated negligibly by strain, making it a potential material for further applications in photovoltaics and flexible optoelectronics. This work not only understands the physical properties of strained Cs2AgInX6 but also provides a guideline to extend their potential applications in experiment.

Author Contributions

Z.Z. and J.S. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 61604119, 61704131, and 61804111); Natural Science Foundation of Shaanxi Province (Grants 2017JQ6002 and 2017JQ6031); Initiative Postdocs Supporting Program (Grant BX20180234); Project funded by China Postdoctoral Science Foundation (Grant 2018M643578); and Fund of the State Key Laboratory of Solidification Processing in NWPU (Grant SKLSP201857). The numerical calculations in this paper were done on the HPC system of Xidian University.

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