- Email: [email protected]
CsPbBr3 heterostructure
Accepted Manuscript First-principles investigations of electronic and optical properties in the MoS2/ CsPbBr3 heterostructure Cheng-Sheng Liao, Qian-Qi Zhao, Yu-Qing Zhao, Zhuo-Liang Yu, Hong Zhou, PengBin He, Jun-Liang Yang, Meng-Qiu Cai PII:
S0022-3697(18)33411-5
DOI:
https://doi.org/10.1016/j.jpcs.2019.06.008
Reference:
PCS 9060
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 17 December 2018 Revised Date:
10 June 2019
Accepted Date: 11 June 2019
Please cite this article as: C.-S. Liao, Q.-Q. Zhao, Y.-Q. Zhao, Z.-L. Yu, H. Zhou, P.-B. He, J.-L. Yang, M.-Q. Cai, First-principles investigations of electronic and optical properties in the MoS2/CsPbBr3 heterostructure, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/ j.jpcs.2019.06.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT First-principles investigations of electronic and optical properties in the MoS2/CsPbBr3 heterostructure Cheng-Sheng Liao1, Qian-Qi Zhao1, Yu-Qing Zhao1, Zhuo-Liang Yu1, Hong Zhou1, Peng-Bin He1*, Jun-Liang Yang2, Meng-Qiu Cai1,3, * Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education &
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1
Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha, 410082, China
Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of
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2
Physics and Electronics, Central South University, Changsha, 410083, Hunan, China Synergetic Innovation Center for Quantum Effects and Applications (SICQEA),
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3
Hunan Normal University, Changsha, 410081, China
Abstract
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Corresponding author. E-mail address: [email protected];[email protected]
There has been great interest inhow to boost the performance of perovskite solar cells
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effectively. Building heterostructures between layered two-dimensional materials and perovskites is a common way to achieve the goal. In this work, the electronic and optical properties of the MoS2/CsPbBr3 heterostructure are studied systematically by
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density functional theory calculations. The calculated results show that the heterostructure can effectively retain some of the excellent properties of individual MoS2 and CsPbBr3. Furthermore, the optical performance of the two kinds of interfaces—CsBr/MoS2 contact and PbBr2/MoS2 contact—is better than that of the individual perovskite layers. We attribute the increased light absorption to the type II electronic band alignment of the heterostructure, interfacial charge transfer, and electron-hole separation. This work may be beneficial for improving the properties of perovskite solar cells and may open a way for the theoretical exploration of inorganic perovskite–based heterostructures. 1
ACCEPTED MANUSCRIPT 1. Introduction Recently, hybrid organic-inorganic halide perovskites (ABX3, where A is CH3NH3+ or CH(NH2)2+, B is Pb2+ or Sn2+, and X is Cl−, Br−, or I−) have been widely studied as promising light-harvesting materials and light-emitting materials [1–3]. The new
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generation of hybrid organic-inorganic halide perovskites have great electrical transmission characteristics, low cost, tunable bandgaps, and outstanding light absorption performance [4–6]. In a few years, the power conversion efficiency of solar cells based on hybrid organic–inorganic halide perovskites has been rapidly
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increased to 22.1%, which is very close to the efficiency of monocrystalline silicon [7]. Despite the excellent performance, the unstable monovalent organic cations of
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hybrid organic-inorganic halide perovskites obstruct their commercial prospects [8]. In general, inorganic materials are stabler than organic materials, especially at high temperature. Therefore, all-inorganic halide perovskites (CsBX3, where B is Pb2+ or Sn2+, and X is Cl−, Br−, or I−) were proposed and have been developed rapidly in the past few years. Typical inorganic halide perovskiteshave high carrier mobilities,
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long carrier diffusion lengths, and suitable bandgaps, and most importantly, compared with hybrid organic-inorganic perovskites, all-inorganic halide perovskites exhibit better stability against moisture and heat [9]. Liang et al. [10] successfully prepared
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all-inorganic perovskite solar cells that can maintain performance for more than 3 months in humid air (90–95% relative humidity, 25 temperatures (100
and -22
) and tolerate extreme
). The optoelectronic performance of perovskites is
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another key point for achieving high-efficiency solar cells. Kulbak et al. [11] showed that the all-inorganic perovskite CsPbBr3 is able to work as effectively as an organic-inorganic perovskite in many photovoltaic aspects, especially in producing high open-circuit voltages. Zhang et al. [12] achieved an electroluminescent LED based on CsPb(Br/I)3 perovskite nanocrystals, generating great photoluminescence quantum yield and effectively extending the photoluminescence lifetime. However, the light absorption efficiencies of traditional all-inorganic perovskite solar cells are often lower than those of their organic-inorganic counterparts. To date, many efforts have been made to tune the optical properties of all-inorganic perovskites for 2
ACCEPTED MANUSCRIPT high-efficiency solar cells [13]. For instance, Kam et al. [14] fabricated an optoelectronic device based on Cs0.925K0.075PbI2Br perovskite by the doping method; it demonstrated an obvious increase in light absorption over the whole wavelength range. Moreover, the device based on Cs0.925K0.075PbI2Brhas a longer lifetime in air.
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Qiu et al. [15] reported the semiconducting perovskite Cs2SnI6 oxidized from CsSnI3; it exhibits a high absorption coefficient of more than 105 cm-1, and was successfully used as a light-absorbing layer for solar cells. In addition to these examples, it is well known that building a heterostructure is an effective method to achieve optical
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performance enhancement. On the one hand, constructing heterostructures helps to broaden the sunlight absorption spectrum, thereby increasing the efficiency of
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materials. On the other hand, the interface engineering of heterostructures is beneficial to increase the inbuilt electric field, which can effectively increase the injection efficiency. Finally, it is possible to reduce the consumption of raw materials and thus reduce costs by building heterostructures by growing cells on low-cost glass, ceramic, or even flexible substrates. With the rise of layered two-dimensional (2D)
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materials such as graphene, hexagonal BN, and especially transition metal dichalcogenides, the study of heterostructures has experienced remarkable progress [16–18]. Monolayer MoS2 has huge potential for next-generation electronics.
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Field-effect transistors based on MoS2 exhibit great thermal stability and a large on-off current ratio [19]. Therefore, use of MoS2 as the substrate to build a heterostructure can be a promising method to achieve positive regulation of the
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properties of materials. Experimentally, Ceballos et al. [20] reported ultrafast transfer of electrons and holes in a van der Waals heterostructure formed by MoS2 and MoSe2 monolayers, and the lifetime of indirect excitons in the heterostructure was found to be longer than that of direct excitons in individual monolayers. The theoretical work of Liu et al. [21] indicates that in the heterostructure composed of MoS2 and graphene, the Schottky barrier height can be modulated by adjustment of the interlayer distance between the two kinds of materials. Meanwhile, the combination of 2D materials and halide perovskites is become a promising approach in the field of optoelectronics. Luet al. [22] prepared a high-performance 2D hybrid photodetector by depositing the 3
ACCEPTED MANUSCRIPT layered halide perovskite CH3NH3PbI3 onto monolayer WSe2; it is sensitive to a wide spectral range and has better output characteristics than devices based on original materials. Peng et al. [23] investigated the defect engineering of the MoS2 and CH3NH3PbI3 interface, successfully overcoming the drawbacks of short optical
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absorption paths. Besides, there have been gradual developments in the design of heterostructures formed between all-inorganic halide perovskites and 2D materials. For example, Liu et al. [24]studied theoretically the electronic and optical properties of CsPbI3 and black phosphorus monolayer heterostructures. Obviously, contacts
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between layered 2D materials and halide perovskites have received increasing attention, but reports on heterostructures based on all-inorganic perovskites are
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comparatively fewer.
In this work, to improve the light absorption performance of the all-inorganic perovskite CsPbBr3, the electronic and the optical properties of heterostructures based on CsPbBr3 and monolayer MoS2 were investigated by density functional theory (DFT) calculations. The changes that occur when both terminated surfaces of CsPbBr3 (CsBr
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surface and PbBr2 surface) are brought into contact with MoS2 were studied. This work may be beneficial for improving the properties of perovskite solar cells and may open a way for the theoretical exploration of inorganic perovskite–based
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heterostructures.
2. Model and details of calculations
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Our calculations were based on DFT with the projected augmented wave method, as implemented
in
Vienna
Ab
Initio
Simulation
Package
[25,
26].
The
Perdew-Burke-Ernzerhof (PBE)-type of generalized gradient approximation (GGA) was used to describe the exchange-correlation energy [27, 28]. Underestimating the value of the bandgap is one of the drawbacks of the conventional DFT method [29]. Therefore, the screened hybrid functional HSE06 was used to correct the underestimated bandgaps of the heterostructures. The initial structures of the heterostructures consist of monolayer 2H-MoS2 and cubic CsPbBr3. Because of the octahedral
symmetry,
two
kinds
of 4
different
terminated
surfaces
of
ACCEPTED MANUSCRIPT CsPbBr3—CsBr-terminated surface and PbBr2-terminated surface—can match with MoS2. The initial geometry structures of CsBr/MoS2 contact and PbBr2/MoS2 contact are shown in Fig. 1. For the simulation of the heterostructures, an energy cutoff of 450 eV and a 2×4×1 Monkhorst-Pack k-point mesh were used to achieve geometrical
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optimizations and self-consistence. A vacuum space thickness of 15 Å in the z direction was used to avoid interactions between neighboring layers[30, 31]. All structures were fully relaxed until the final Hellmann-Feynman forces on each atom were smaller than 0.03 eV/Å, and the convergence threshold for energy was set to
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10−4 eV [32]. We used the DFT-D3 van der Waals correction function to describe the van der Waals interaction [33]. For the convenience of analysis regarding work
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functions and charge transfer, a dipole correction along the z direction (IDIPOL=3) was used to calculate the electrostatic potential of CsPbBr3 and MoS2. The calculated lattice parameters of isolated CsPbBr3 and isolated MoS2 are a=b=c=5.988 Å and a=b=3.182 Å, respectively, which are in very good agreement with the experimental results. We built the heterostructures using the Atomistix ToolKit software package
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[34] to obtain a good compromise between a small supercell and low strain; the mean absolute strain calculated by Atomistix ToolKit is about 2.3%, which is reasonably small. The detailed matching method can be seen in Fig. S1. We defined the
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equilibrium interlayer distance d1 as the distance between the S atom at the bottom of the MoS2 layer and the Cs atom at the top octahedron of CsPbBr3 in the CsBr/MoS2 contact, and d2 was defined as the distance between the S atom at the bottom of the
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MoS2 layer and the Pb atom at the top octahedron of CsPbBr3 in the PbBr2/MoS2 contact. Before calculating the electronic structure and optical properties, we first investigated the selection of the interlayer distance, as shown in Fig. 2. The binding energy (Eb) can be calculated as [35–40] ,
where
,
, and
represent the total energies
of the hybrid heterostructure, isolated CsPbBr3, and isolated MoS2, respectively, and n 5
ACCEPTED MANUSCRIPT represents the total number of CsBr/MoS2 contacts or PbBr2/MoS2 contacts. We found that the stablest interlayer distance is about 3.6 Å for CsBr/MoS2 contact and 3.4 Å for PbBr2/MoS2 contact. Therefore, we chose initial d1= 3.6 Å and d2= 3.4 Å for the following calculations. Eb was calculated to be -176.4 meV for optimized CsBr/MoS2
CsBr/MoS2 contact and PbBr2/MoS2 contact are stable.
3. Results and discussion
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3.1. Electronic properties
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contact and -179.8 meV for optimized PbBr2/MoS2 contact, which proves that both
In our modeling, we first calculated the electronic energy bands of isolated MoS2,
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isolated CsPbBr3, CsBr/MoS2contact, and PbBr2/MoS2contact, as shown in Fig. 3. Blue and red lines represent the contributions of the band from CsPbBr3 and monolayer MoS2, respectively. The k-point paths in the first Brillouin zone for the heterostructure and isolated components are Γ (0 0 0), M (0.5 0.5 0), and K (-1/3 2/3 0). From the band structure diagrams, we found that both CsBr/MoS2 contact and
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PbBr2/MoS2 contact are type II heterostructures. Generally, type II band alignment is more conducive to the separation of photogenerated carriers and thus enhances the light absorption properties even though the band structure exhibits an indirect
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bandgap. To correct the underestimated bandgaps of the heterostructures caused by the PBE functional, we described the electronic structure of CsBr/MoS2 contact by the GGA+HSE06 hybrid functional; the contrast pictures are plotted in Fig. S2. The value
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of the bandgap for CsBr/MoS2 contact increased from 0.83 eV to 1.44 eV. However, the band dispersion of the band edge, including the conduction band minimum and the valence band maximum, was nearly unchanged except for the different bandgap. Therefore, we chose the PBE functional to explore the properties of CsPbBr3/MoS2 heterostructures in the rest of our study. It can be observed that the band structures of MoS2 and CsPbBr3 in the heterostructure have nearly the same shape as those of their isolated counterparts. Therefore, the band structure of the MoS2/CsPbBr3 heterostructure appears to be a simple superposition of the band structures of the individual components, indicating 6
ACCEPTED MANUSCRIPT that the interaction at the interface is too weak to destroy the structural symmetry of MoS2 and CsPbBr3. Furthermore, the interlayer distances d1 and d2 for the heterostructures are both larger than the bond lengths of isolated CsPbBr3 and MoS2, so we can conclude that there is a weak van der Waals interaction between MoS2 and
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CsPbBr3. In addition, since the energy band structures of the individual components were not significantly changed in the heterostructures, it is very likely that the MoS2/CsPbBr3 heterostructure will have some of the superior electronic properties of monolayer MoS2 and CsPbBr3. In previous work by Liuet al.[24], the band structure
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of the graphene–hexagonal boron nitride–molybdenum disulfide van der Waals heterostructure nearly conformed to the sum of its constituents. Consequently, the
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heterostructure has the intrinsic properties of graphene and MoS2, which corroborates our conclusion above.
3.2. Charge transfer
Next, to explore the interface interaction and charge transfer more clearly, the charge
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density difference diagrams of the heterostructures are plotted in Fig. 4. Here, the charge density difference can be written as ∆ρ ( z ) = ρ Het − ρ CsBr/PbBr2 − ρ MoS2 , where ρHet represents the charge density of CsBr/MoS2 contact or PbBr2/MoS2
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contact, and ρ MoS2 and ρCsBr/ PbBr2 are the charge densities of isolated MoS2 and isolated CsBr or the PbBr2-terminated surface for CsPbBr3, respectively. Green filled
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isosurfaces represent the charge depletion and red filled isosurfaces represent the charge accumulation. Figure 4(a) and 4(c) shows lateral views of the three-dimensional charge density difference for CsBr/MoS2 contact and PbBr2/MoS2 contact. Figure4(b) and 4(d)shows the plane-averaged charge difference ∆ρ of optimized CsBr/MoS2 contact and optimized PbBr2/MoS2 contact. We can see that the charge depletion and accumulation are mainly concentrated near the interface area. For the charge distribution at the interface, we find ∆ρ > 0 near the MoS2 surface and ∆ρ < 0 near the CsBr surface or PbBr2 surface, which indicates that charges 7
ACCEPTED MANUSCRIPT reduce at the terminated surface of CsPbBr3 and then accumulate at the monolayer MoS2; in other words, the charges can be transferred from CsPbBr3 to MoS2 in heterostructures, consequently causing the separation of electron-hole pairs. As a result, an internal polarized field can be generated, which effectively keeps
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photoinduced electrons and holes from recombining. One can therefore expect that the light absorption properties of perovskites should be effectively improved.
To better confirm our conclusions, we also investigated the work function of different terminated surfaces for CsPbBr3 and of isolated MoS2. The work function is
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given by the difference between the vacuum energy level and the Fermi energy level of CsPbBr3 and MoS2, which can be obtained from electrostatic potential diagrams, as
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shown in Fig. S3. The calculated work functions are 5.79 eV for monolayer MoS2, 4.78 eV for CsPbBr3 with a CsBr-terminated surface, and 5.62 eV for CsPbBr3 with a PbBr2-terminated surface. The work function of MoS2 is larger than that of CsPbBr3 on either CsBr/MoS2 contact or PbBr2/MoS2 contact, demonstrating that MoS2 can
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acquire charges from CsPbBr3 in heterostructures.
3.3. Optical properties
Finally, we further analyzed the optical properties of isolated MoS2, isolated CsPbBr3,
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CsBr/MoS2 contact, and PbBr2/MoS2 contact. Figure 5(a) and 5(b) shows the absorption spectrum of the MoS2/CsPbBr3 heterostructure and its individual components. The absorption coefficient α is derived from the following equation
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[41–46]:
[
]
1
α = 2ω ε12 (ω ) + ε 22 (ω ) − ε 1 (ω ) , 2
where ω stands for the optical frequency, and ε 1 (ω ) and ε 2 (ω ) represent the real and imaginary parts of the dielectric function. It was found that the light absorption abilities of perovskites are effectively enhanced for both CsBr/MoS2 contact and PbBr2/MoS2contact, especially in the visible and ultraviolet regions. The imaginary part ɛ2(ɷ) of the dielectric function is closely related to the optical properties of materials. Therefore, to observe the 8
ACCEPTED MANUSCRIPT enhancement of light absorption performance more intuitively, the calculated imaginary parts ɛ2(ɷ) of the dielectric function of isolated MoS2, the CsPbBr3 surface, and the MoS2/CsPbBr3 heterostructure are illustrated in Fig. 5(c) and 5(d). The conclusions obtained are consistent with our previous discussion: both CsBr/MoS2
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contact and PbBr2/MoS2 contact exhibit increased absorption intensities. Electron and holes are generated at the interface of heterostructures due to charge transfer, forming an internal polarized electronic field, which can effectively increase the separation of electron-hole pairs [47–56]. The heterostructures exhibit an indirect band gap.
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Generally, from the perspective of energy utilization and electronic transition, semiconductors with a direct bandgap have better light utilization due to smaller
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energy consumption. However, the increase in optical absorption of the CsPbBr3/MoS2 heterostructure is mainly due to the reduction of semiconductor bandgaps and the inbuilt electric field generated by the charge transfer. In this case, the difference in the absorption mechanism between direct bandgaps and indirect bandgaps is not the main factor affecting the light absorption performance. Ninget al.
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[57]successfully explained the corresponding physical mechanism through the graphene/InAs heterostructure and the MoS2/InAs heterostructure, which is basically
4. Conclusion
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consistent with our work.
In summary, we comprehensively and systematically investigated the electronic and
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optical properties of the CsPbBr3/MoS2 heterostructure by means of first-principles calculations. The electronic band structures of both CsPbBr3 and monolayer MoS2 are nearly preserved in the combined CsPbBr3/MoS2 heterostructure. The CsBr/MoS2 contact and PbBr2/MoS2 contact both demonstrate type II band alignment. Meanwhile, building heterostructures with MoS2 can enhance the light absorption properties of CsPbBr3 effectively by interface engineering. Our work may be beneficial for improving properties of perovskite solar cells and may provide a theoretical approach for the exploration of inorganic perovskite–based heterostructures.
9
ACCEPTED MANUSCRIPT Acknowledgments The authors express their thanks to the Changsha Supercomputer Center for Computation. This work was supported by the National Natural Science Foundation of China through grant no. 51172067, the Specialized Research Fund for the Doctoral
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Program of Higher Education through grant no. 20130161110036, the Key Projects of Hunan Provincial Science and Technology Plan through grant no. 2017GK2231, and the Hunan Provincial Innovation Foundation for Postgraduates through grant no.
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CX2018B161.
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Fig. 1 (a) Side view and (c) front view of CsBr/MoS2 contact. (b) Side view and (d) front view of PbBr2/MoS2 contact. The Cs, Pb, Mo, Br, and S atoms are shown as green, gray, pink, brown and yellow balls, respectively.
Fig. 2 The binding energies (Eb) as a function of interlayer distance d1 and d2for (a) CsBr/MoS2 contact and (b) PbBr2/MoS2 contact, respectively.
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projected bands of CsPbBr3 and MoS2, respectively.
Fig. 4 Charge transfer of CsBr/MoS2 contact and PbBr2/MoS2 contact. Lateral views of the three-dimensional charge density difference diagrams for (a) CsBr/MoS2 contact and (c) PbBr2/MoS2 contact. Plane-averaged difference charge density ∆ρ (z )
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along the z direction of (b) CsBr/MoS2 contact and (d) PbBr2/MoS2 contact. The green
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regions represent charge depletion and the red regions represent charge accumulation.
Fig. 5 (a) Calculated absorption spectra of monolayer MoS2, CsPbBr3 with CsBr-terminated surface, and CsBr/MoS2 contact. (b) Calculated absorption spectra of monolayer MoS2, CsPbBr3 with PbBr2-terminated surface, and PbBr2/MoS2 contact (c) Calculated imaginary parts ɛ2(ɷ) of the dielectric function of monolayer MoS2,
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CsPbBr3 with CsBr-terminated surface, and CsBr/MoS2 contact. (d) Calculated imaginary parts ɛ2(ɷ) of the dielectric function of monolayer MoS2, CsPbBr3 with
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PbBr2-terminated surface, and PbBr2/MoS2 contact.
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Highlights •
The MoS2/CsPbBr3 heterostructure can effectively retain the excellent properties of individual MoS2 and CsPbBr3. The optical performance of the MoS2/CsPbBr3 heterostructure is better than that of the individual perovskite layers.
•
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We attribute the increasedlight absorption to the type II electronic band alignment of the heterostructure, interfacial charge transfer, and electron-hole
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1
S0022-3697(18)33411-5
DOI:
https://doi.org/10.1016/j.jpcs.2019.06.008
Reference:
PCS 9060
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 17 December 2018 Revised Date:
10 June 2019
Accepted Date: 11 June 2019
Please cite this article as: C.-S. Liao, Q.-Q. Zhao, Y.-Q. Zhao, Z.-L. Yu, H. Zhou, P.-B. He, J.-L. Yang, M.-Q. Cai, First-principles investigations of electronic and optical properties in the MoS2/CsPbBr3 heterostructure, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/ j.jpcs.2019.06.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT First-principles investigations of electronic and optical properties in the MoS2/CsPbBr3 heterostructure Cheng-Sheng Liao1, Qian-Qi Zhao1, Yu-Qing Zhao1, Zhuo-Liang Yu1, Hong Zhou1, Peng-Bin He1*, Jun-Liang Yang2, Meng-Qiu Cai1,3, * Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education &
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1
Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha, 410082, China
Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of
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2
Physics and Electronics, Central South University, Changsha, 410083, Hunan, China Synergetic Innovation Center for Quantum Effects and Applications (SICQEA),
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Hunan Normal University, Changsha, 410081, China
Abstract
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Corresponding author. E-mail address: [email protected];[email protected]
There has been great interest inhow to boost the performance of perovskite solar cells
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effectively. Building heterostructures between layered two-dimensional materials and perovskites is a common way to achieve the goal. In this work, the electronic and optical properties of the MoS2/CsPbBr3 heterostructure are studied systematically by
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density functional theory calculations. The calculated results show that the heterostructure can effectively retain some of the excellent properties of individual MoS2 and CsPbBr3. Furthermore, the optical performance of the two kinds of interfaces—CsBr/MoS2 contact and PbBr2/MoS2 contact—is better than that of the individual perovskite layers. We attribute the increased light absorption to the type II electronic band alignment of the heterostructure, interfacial charge transfer, and electron-hole separation. This work may be beneficial for improving the properties of perovskite solar cells and may open a way for the theoretical exploration of inorganic perovskite–based heterostructures. 1
ACCEPTED MANUSCRIPT 1. Introduction Recently, hybrid organic-inorganic halide perovskites (ABX3, where A is CH3NH3+ or CH(NH2)2+, B is Pb2+ or Sn2+, and X is Cl−, Br−, or I−) have been widely studied as promising light-harvesting materials and light-emitting materials [1–3]. The new
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generation of hybrid organic-inorganic halide perovskites have great electrical transmission characteristics, low cost, tunable bandgaps, and outstanding light absorption performance [4–6]. In a few years, the power conversion efficiency of solar cells based on hybrid organic–inorganic halide perovskites has been rapidly
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increased to 22.1%, which is very close to the efficiency of monocrystalline silicon [7]. Despite the excellent performance, the unstable monovalent organic cations of
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hybrid organic-inorganic halide perovskites obstruct their commercial prospects [8]. In general, inorganic materials are stabler than organic materials, especially at high temperature. Therefore, all-inorganic halide perovskites (CsBX3, where B is Pb2+ or Sn2+, and X is Cl−, Br−, or I−) were proposed and have been developed rapidly in the past few years. Typical inorganic halide perovskiteshave high carrier mobilities,
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long carrier diffusion lengths, and suitable bandgaps, and most importantly, compared with hybrid organic-inorganic perovskites, all-inorganic halide perovskites exhibit better stability against moisture and heat [9]. Liang et al. [10] successfully prepared
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all-inorganic perovskite solar cells that can maintain performance for more than 3 months in humid air (90–95% relative humidity, 25 temperatures (100
and -22
) and tolerate extreme
). The optoelectronic performance of perovskites is
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another key point for achieving high-efficiency solar cells. Kulbak et al. [11] showed that the all-inorganic perovskite CsPbBr3 is able to work as effectively as an organic-inorganic perovskite in many photovoltaic aspects, especially in producing high open-circuit voltages. Zhang et al. [12] achieved an electroluminescent LED based on CsPb(Br/I)3 perovskite nanocrystals, generating great photoluminescence quantum yield and effectively extending the photoluminescence lifetime. However, the light absorption efficiencies of traditional all-inorganic perovskite solar cells are often lower than those of their organic-inorganic counterparts. To date, many efforts have been made to tune the optical properties of all-inorganic perovskites for 2
ACCEPTED MANUSCRIPT high-efficiency solar cells [13]. For instance, Kam et al. [14] fabricated an optoelectronic device based on Cs0.925K0.075PbI2Br perovskite by the doping method; it demonstrated an obvious increase in light absorption over the whole wavelength range. Moreover, the device based on Cs0.925K0.075PbI2Brhas a longer lifetime in air.
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Qiu et al. [15] reported the semiconducting perovskite Cs2SnI6 oxidized from CsSnI3; it exhibits a high absorption coefficient of more than 105 cm-1, and was successfully used as a light-absorbing layer for solar cells. In addition to these examples, it is well known that building a heterostructure is an effective method to achieve optical
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performance enhancement. On the one hand, constructing heterostructures helps to broaden the sunlight absorption spectrum, thereby increasing the efficiency of
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materials. On the other hand, the interface engineering of heterostructures is beneficial to increase the inbuilt electric field, which can effectively increase the injection efficiency. Finally, it is possible to reduce the consumption of raw materials and thus reduce costs by building heterostructures by growing cells on low-cost glass, ceramic, or even flexible substrates. With the rise of layered two-dimensional (2D)
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materials such as graphene, hexagonal BN, and especially transition metal dichalcogenides, the study of heterostructures has experienced remarkable progress [16–18]. Monolayer MoS2 has huge potential for next-generation electronics.
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Field-effect transistors based on MoS2 exhibit great thermal stability and a large on-off current ratio [19]. Therefore, use of MoS2 as the substrate to build a heterostructure can be a promising method to achieve positive regulation of the
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properties of materials. Experimentally, Ceballos et al. [20] reported ultrafast transfer of electrons and holes in a van der Waals heterostructure formed by MoS2 and MoSe2 monolayers, and the lifetime of indirect excitons in the heterostructure was found to be longer than that of direct excitons in individual monolayers. The theoretical work of Liu et al. [21] indicates that in the heterostructure composed of MoS2 and graphene, the Schottky barrier height can be modulated by adjustment of the interlayer distance between the two kinds of materials. Meanwhile, the combination of 2D materials and halide perovskites is become a promising approach in the field of optoelectronics. Luet al. [22] prepared a high-performance 2D hybrid photodetector by depositing the 3
ACCEPTED MANUSCRIPT layered halide perovskite CH3NH3PbI3 onto monolayer WSe2; it is sensitive to a wide spectral range and has better output characteristics than devices based on original materials. Peng et al. [23] investigated the defect engineering of the MoS2 and CH3NH3PbI3 interface, successfully overcoming the drawbacks of short optical
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absorption paths. Besides, there have been gradual developments in the design of heterostructures formed between all-inorganic halide perovskites and 2D materials. For example, Liu et al. [24]studied theoretically the electronic and optical properties of CsPbI3 and black phosphorus monolayer heterostructures. Obviously, contacts
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between layered 2D materials and halide perovskites have received increasing attention, but reports on heterostructures based on all-inorganic perovskites are
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comparatively fewer.
In this work, to improve the light absorption performance of the all-inorganic perovskite CsPbBr3, the electronic and the optical properties of heterostructures based on CsPbBr3 and monolayer MoS2 were investigated by density functional theory (DFT) calculations. The changes that occur when both terminated surfaces of CsPbBr3 (CsBr
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surface and PbBr2 surface) are brought into contact with MoS2 were studied. This work may be beneficial for improving the properties of perovskite solar cells and may open a way for the theoretical exploration of inorganic perovskite–based
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heterostructures.
2. Model and details of calculations
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Our calculations were based on DFT with the projected augmented wave method, as implemented
in
Vienna
Ab
Initio
Simulation
Package
[25,
26].
The
Perdew-Burke-Ernzerhof (PBE)-type of generalized gradient approximation (GGA) was used to describe the exchange-correlation energy [27, 28]. Underestimating the value of the bandgap is one of the drawbacks of the conventional DFT method [29]. Therefore, the screened hybrid functional HSE06 was used to correct the underestimated bandgaps of the heterostructures. The initial structures of the heterostructures consist of monolayer 2H-MoS2 and cubic CsPbBr3. Because of the octahedral
symmetry,
two
kinds
of 4
different
terminated
surfaces
of
ACCEPTED MANUSCRIPT CsPbBr3—CsBr-terminated surface and PbBr2-terminated surface—can match with MoS2. The initial geometry structures of CsBr/MoS2 contact and PbBr2/MoS2 contact are shown in Fig. 1. For the simulation of the heterostructures, an energy cutoff of 450 eV and a 2×4×1 Monkhorst-Pack k-point mesh were used to achieve geometrical
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optimizations and self-consistence. A vacuum space thickness of 15 Å in the z direction was used to avoid interactions between neighboring layers[30, 31]. All structures were fully relaxed until the final Hellmann-Feynman forces on each atom were smaller than 0.03 eV/Å, and the convergence threshold for energy was set to
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10−4 eV [32]. We used the DFT-D3 van der Waals correction function to describe the van der Waals interaction [33]. For the convenience of analysis regarding work
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functions and charge transfer, a dipole correction along the z direction (IDIPOL=3) was used to calculate the electrostatic potential of CsPbBr3 and MoS2. The calculated lattice parameters of isolated CsPbBr3 and isolated MoS2 are a=b=c=5.988 Å and a=b=3.182 Å, respectively, which are in very good agreement with the experimental results. We built the heterostructures using the Atomistix ToolKit software package
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[34] to obtain a good compromise between a small supercell and low strain; the mean absolute strain calculated by Atomistix ToolKit is about 2.3%, which is reasonably small. The detailed matching method can be seen in Fig. S1. We defined the
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equilibrium interlayer distance d1 as the distance between the S atom at the bottom of the MoS2 layer and the Cs atom at the top octahedron of CsPbBr3 in the CsBr/MoS2 contact, and d2 was defined as the distance between the S atom at the bottom of the
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MoS2 layer and the Pb atom at the top octahedron of CsPbBr3 in the PbBr2/MoS2 contact. Before calculating the electronic structure and optical properties, we first investigated the selection of the interlayer distance, as shown in Fig. 2. The binding energy (Eb) can be calculated as [35–40] ,
where
,
, and
represent the total energies
of the hybrid heterostructure, isolated CsPbBr3, and isolated MoS2, respectively, and n 5
ACCEPTED MANUSCRIPT represents the total number of CsBr/MoS2 contacts or PbBr2/MoS2 contacts. We found that the stablest interlayer distance is about 3.6 Å for CsBr/MoS2 contact and 3.4 Å for PbBr2/MoS2 contact. Therefore, we chose initial d1= 3.6 Å and d2= 3.4 Å for the following calculations. Eb was calculated to be -176.4 meV for optimized CsBr/MoS2
CsBr/MoS2 contact and PbBr2/MoS2 contact are stable.
3. Results and discussion
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3.1. Electronic properties
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contact and -179.8 meV for optimized PbBr2/MoS2 contact, which proves that both
In our modeling, we first calculated the electronic energy bands of isolated MoS2,
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isolated CsPbBr3, CsBr/MoS2contact, and PbBr2/MoS2contact, as shown in Fig. 3. Blue and red lines represent the contributions of the band from CsPbBr3 and monolayer MoS2, respectively. The k-point paths in the first Brillouin zone for the heterostructure and isolated components are Γ (0 0 0), M (0.5 0.5 0), and K (-1/3 2/3 0). From the band structure diagrams, we found that both CsBr/MoS2 contact and
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PbBr2/MoS2 contact are type II heterostructures. Generally, type II band alignment is more conducive to the separation of photogenerated carriers and thus enhances the light absorption properties even though the band structure exhibits an indirect
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bandgap. To correct the underestimated bandgaps of the heterostructures caused by the PBE functional, we described the electronic structure of CsBr/MoS2 contact by the GGA+HSE06 hybrid functional; the contrast pictures are plotted in Fig. S2. The value
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of the bandgap for CsBr/MoS2 contact increased from 0.83 eV to 1.44 eV. However, the band dispersion of the band edge, including the conduction band minimum and the valence band maximum, was nearly unchanged except for the different bandgap. Therefore, we chose the PBE functional to explore the properties of CsPbBr3/MoS2 heterostructures in the rest of our study. It can be observed that the band structures of MoS2 and CsPbBr3 in the heterostructure have nearly the same shape as those of their isolated counterparts. Therefore, the band structure of the MoS2/CsPbBr3 heterostructure appears to be a simple superposition of the band structures of the individual components, indicating 6
ACCEPTED MANUSCRIPT that the interaction at the interface is too weak to destroy the structural symmetry of MoS2 and CsPbBr3. Furthermore, the interlayer distances d1 and d2 for the heterostructures are both larger than the bond lengths of isolated CsPbBr3 and MoS2, so we can conclude that there is a weak van der Waals interaction between MoS2 and
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CsPbBr3. In addition, since the energy band structures of the individual components were not significantly changed in the heterostructures, it is very likely that the MoS2/CsPbBr3 heterostructure will have some of the superior electronic properties of monolayer MoS2 and CsPbBr3. In previous work by Liuet al.[24], the band structure
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of the graphene–hexagonal boron nitride–molybdenum disulfide van der Waals heterostructure nearly conformed to the sum of its constituents. Consequently, the
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heterostructure has the intrinsic properties of graphene and MoS2, which corroborates our conclusion above.
3.2. Charge transfer
Next, to explore the interface interaction and charge transfer more clearly, the charge
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density difference diagrams of the heterostructures are plotted in Fig. 4. Here, the charge density difference can be written as ∆ρ ( z ) = ρ Het − ρ CsBr/PbBr2 − ρ MoS2 , where ρHet represents the charge density of CsBr/MoS2 contact or PbBr2/MoS2
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contact, and ρ MoS2 and ρCsBr/ PbBr2 are the charge densities of isolated MoS2 and isolated CsBr or the PbBr2-terminated surface for CsPbBr3, respectively. Green filled
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isosurfaces represent the charge depletion and red filled isosurfaces represent the charge accumulation. Figure 4(a) and 4(c) shows lateral views of the three-dimensional charge density difference for CsBr/MoS2 contact and PbBr2/MoS2 contact. Figure4(b) and 4(d)shows the plane-averaged charge difference ∆ρ of optimized CsBr/MoS2 contact and optimized PbBr2/MoS2 contact. We can see that the charge depletion and accumulation are mainly concentrated near the interface area. For the charge distribution at the interface, we find ∆ρ > 0 near the MoS2 surface and ∆ρ < 0 near the CsBr surface or PbBr2 surface, which indicates that charges 7
ACCEPTED MANUSCRIPT reduce at the terminated surface of CsPbBr3 and then accumulate at the monolayer MoS2; in other words, the charges can be transferred from CsPbBr3 to MoS2 in heterostructures, consequently causing the separation of electron-hole pairs. As a result, an internal polarized field can be generated, which effectively keeps
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photoinduced electrons and holes from recombining. One can therefore expect that the light absorption properties of perovskites should be effectively improved.
To better confirm our conclusions, we also investigated the work function of different terminated surfaces for CsPbBr3 and of isolated MoS2. The work function is
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given by the difference between the vacuum energy level and the Fermi energy level of CsPbBr3 and MoS2, which can be obtained from electrostatic potential diagrams, as
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shown in Fig. S3. The calculated work functions are 5.79 eV for monolayer MoS2, 4.78 eV for CsPbBr3 with a CsBr-terminated surface, and 5.62 eV for CsPbBr3 with a PbBr2-terminated surface. The work function of MoS2 is larger than that of CsPbBr3 on either CsBr/MoS2 contact or PbBr2/MoS2 contact, demonstrating that MoS2 can
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acquire charges from CsPbBr3 in heterostructures.
3.3. Optical properties
Finally, we further analyzed the optical properties of isolated MoS2, isolated CsPbBr3,
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CsBr/MoS2 contact, and PbBr2/MoS2 contact. Figure 5(a) and 5(b) shows the absorption spectrum of the MoS2/CsPbBr3 heterostructure and its individual components. The absorption coefficient α is derived from the following equation
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[41–46]:
[
]
1
α = 2ω ε12 (ω ) + ε 22 (ω ) − ε 1 (ω ) , 2
where ω stands for the optical frequency, and ε 1 (ω ) and ε 2 (ω ) represent the real and imaginary parts of the dielectric function. It was found that the light absorption abilities of perovskites are effectively enhanced for both CsBr/MoS2 contact and PbBr2/MoS2contact, especially in the visible and ultraviolet regions. The imaginary part ɛ2(ɷ) of the dielectric function is closely related to the optical properties of materials. Therefore, to observe the 8
ACCEPTED MANUSCRIPT enhancement of light absorption performance more intuitively, the calculated imaginary parts ɛ2(ɷ) of the dielectric function of isolated MoS2, the CsPbBr3 surface, and the MoS2/CsPbBr3 heterostructure are illustrated in Fig. 5(c) and 5(d). The conclusions obtained are consistent with our previous discussion: both CsBr/MoS2
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contact and PbBr2/MoS2 contact exhibit increased absorption intensities. Electron and holes are generated at the interface of heterostructures due to charge transfer, forming an internal polarized electronic field, which can effectively increase the separation of electron-hole pairs [47–56]. The heterostructures exhibit an indirect band gap.
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Generally, from the perspective of energy utilization and electronic transition, semiconductors with a direct bandgap have better light utilization due to smaller
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energy consumption. However, the increase in optical absorption of the CsPbBr3/MoS2 heterostructure is mainly due to the reduction of semiconductor bandgaps and the inbuilt electric field generated by the charge transfer. In this case, the difference in the absorption mechanism between direct bandgaps and indirect bandgaps is not the main factor affecting the light absorption performance. Ninget al.
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[57]successfully explained the corresponding physical mechanism through the graphene/InAs heterostructure and the MoS2/InAs heterostructure, which is basically
4. Conclusion
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consistent with our work.
In summary, we comprehensively and systematically investigated the electronic and
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optical properties of the CsPbBr3/MoS2 heterostructure by means of first-principles calculations. The electronic band structures of both CsPbBr3 and monolayer MoS2 are nearly preserved in the combined CsPbBr3/MoS2 heterostructure. The CsBr/MoS2 contact and PbBr2/MoS2 contact both demonstrate type II band alignment. Meanwhile, building heterostructures with MoS2 can enhance the light absorption properties of CsPbBr3 effectively by interface engineering. Our work may be beneficial for improving properties of perovskite solar cells and may provide a theoretical approach for the exploration of inorganic perovskite–based heterostructures.
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ACCEPTED MANUSCRIPT Acknowledgments The authors express their thanks to the Changsha Supercomputer Center for Computation. This work was supported by the National Natural Science Foundation of China through grant no. 51172067, the Specialized Research Fund for the Doctoral
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Program of Higher Education through grant no. 20130161110036, the Key Projects of Hunan Provincial Science and Technology Plan through grant no. 2017GK2231, and the Hunan Provincial Innovation Foundation for Postgraduates through grant no.
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CX2018B161.
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Fig. 1 (a) Side view and (c) front view of CsBr/MoS2 contact. (b) Side view and (d) front view of PbBr2/MoS2 contact. The Cs, Pb, Mo, Br, and S atoms are shown as green, gray, pink, brown and yellow balls, respectively.
Fig. 2 The binding energies (Eb) as a function of interlayer distance d1 and d2for (a) CsBr/MoS2 contact and (b) PbBr2/MoS2 contact, respectively.
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projected bands of CsPbBr3 and MoS2, respectively.
Fig. 4 Charge transfer of CsBr/MoS2 contact and PbBr2/MoS2 contact. Lateral views of the three-dimensional charge density difference diagrams for (a) CsBr/MoS2 contact and (c) PbBr2/MoS2 contact. Plane-averaged difference charge density ∆ρ (z )
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along the z direction of (b) CsBr/MoS2 contact and (d) PbBr2/MoS2 contact. The green
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regions represent charge depletion and the red regions represent charge accumulation.
Fig. 5 (a) Calculated absorption spectra of monolayer MoS2, CsPbBr3 with CsBr-terminated surface, and CsBr/MoS2 contact. (b) Calculated absorption spectra of monolayer MoS2, CsPbBr3 with PbBr2-terminated surface, and PbBr2/MoS2 contact (c) Calculated imaginary parts ɛ2(ɷ) of the dielectric function of monolayer MoS2,
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CsPbBr3 with CsBr-terminated surface, and CsBr/MoS2 contact. (d) Calculated imaginary parts ɛ2(ɷ) of the dielectric function of monolayer MoS2, CsPbBr3 with
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PbBr2-terminated surface, and PbBr2/MoS2 contact.
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The MoS2/CsPbBr3 heterostructure can effectively retain the excellent properties of individual MoS2 and CsPbBr3. The optical performance of the MoS2/CsPbBr3 heterostructure is better than that of the individual perovskite layers.
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We attribute the increasedlight absorption to the type II electronic band alignment of the heterostructure, interfacial charge transfer, and electron-hole
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