GaTe(InTe) heterostructures for photovoltaic and photocatalytic application

Journal of Alloys and Compounds xxx (xxxx) xxx

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Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application Xiaolong Wang a, b, c, Yangyang Wang c, *, Ruge Quhe b, Yanan Tang e, Xianqi Dai d, e, **, Weihua Tang a, b, *** a

Laboratory of Information Functional Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing, 100876, China State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing, 100876, China Nanophotonics and Optoelectronics Research Center, Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing, 100094, PR China d College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan, 453007, China e School of Physics and Electronic Engineering, Zhengzhou Normal University, Zhengzhou, Henan, 450044, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 July 2019 Received in revised form 3 October 2019 Accepted 4 October 2019 Available online xxx

Converting solar energy into electrical energy or chemical energy is a promising strategy to produce renewable clean energy. Here, we investigate the electronic structures of the C2N/MTe (M ¼ Ga, In) heterostructures under strain based on the density functional theory. The C2N/MTe van der Waals heterostructures possess great room temperature stability and exhibit excellently optoelectronic properties that desired for photocatalysis and photovoltaic conversion. Furthermore, strain engineering is utilized to tune the electronic structure. The results show that the exciton Bohr radius is almost insensitive to the strain in C2N/GaTe heterostructure, while the compressive strain can decrease the exciton Bohr radius in C2N/InTe heterostructure. Moreover, the power conversion efficiency can reach 22.1% for C2N/GaTe heterostructure with 4% strain and 19.8% for C2N/InTe heterostructure with 6% strain. Our results show that the tensile strain is a great strategy to improve the optoelectronic performance of C2N/MTe heterostructures. © 2019 Elsevier B.V. All rights reserved.

Keywords: Strain effect Density functional theory Heterostructure Photovoltaics Photocatalysis

1. Introduction Although the large consumption of fossil fuel promotes the development of society, it also brings serious environment pollution. Converting solar energy into electricity and fuel by photocatalytic water splitting is expected to solve the conflict between energy consumption and environment problem. The solar cells based on different materials [1e6] have attracted extensive attention, and considerable progress has been made. In recent years, a large number of studies have shown that the twodimensional (2D) materials exhibit excellent optoelectronic

* Corresponding author. ** Corresponding author. College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan, 453007, China. *** Corresponding author. Laboratory of Information Functional Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing, 100876, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Dai), [email protected] (W. Tang).

properties, such as tunable band gap (Eg) [7,8], strong visible light absorption [9] and long exciton lifetime [10,11], which make the 2D materials a promising candidate for solar cells. The solar cells can be constructed by stacking different 2D materials in the vertical direction [12e14]. Due to the different band edges position of materials, the solar cells with the vertical heterostructure can possess the type-ІІ band alignment, which plays a important role in separating the photogenerated carriers and increasing the power conversion efficiency (PCE) [15e17]. On the other hand, the heterostructures constructed in the lateral direction also exhibit great potential in the application of solar cells. For example, the BX1-BX2 (X1 ¼ P, As; X2 ¼ As, Sb) lateral heterostructures have ultrahigh carrier mobilities, type-ІІ band structures and strong polarization sensitivity, and the PCE of BX1-BX2 lateral heterostructures are larger than 15% [18]. In addition, the dopants have also been used to improve the performance of solar cells, for example, the N dopant can effectively modulate the band structure of the GaX/SnS2 heterostructure and increase the PCE to 16%, which is an increase of 162% compared to the pristine case [19]. Compared with photovoltaics, photocatalytic water splitting for

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Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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H2 and O2 evolution is another effective method to utilize solar energy. Many photocatalysts and theoretical models have been designed and proposed [20e24], such as particulate photocatalysts with core-shell structure [25], Z-scheme photocatalytic system [26,27] and built-in electric field modulation [28]. The black phosphorus/red phosphorus heterostructure synthesized by wetchemistry method can effectively separate the photogenerated electron-hole pair and achieve Z-scheme photocatalytic water splitting [29]. The InSe/g-C3N4 heterostructure possesses the strong oxidation potential to drive the oxygen evolution reaction, and the photocatalytic performance of InSe/g-C3N4 heterostructure can be improved under compressive strain [30]. The 2D In2X3 (X ¼ S, Se, Te) heterostructures possess the excellent optical absorption and strong build-in electric field, which may drive photocatalytic water splitting in the infrared region [31]. In the experiment, it is difficult to eliminate the strain, and the strain can significantly influence the optoelectronic properties of the materials, such as the non-radiative recombination [32], the band structure [33], the PCE [34], and so on. Therefore, it is interesting and important to investigate the strain effect on the performance of optoelectronic devices. In this work, the monolayer C2N and MTe (M ¼ Ga, In) are selected to construct heterostructures for application in photovoltaics and photocatalytic water splitting. Based on the density functional theory (DFT), we investigate the electronic structures of the C2N/MTe heterostructures and discuss the potential methods to improve the photocatalytic performance of the C2N/MTe heterostructures. In addition, the strain effect on the excitonic Bohr radius and the optoelectronic performance are investigated. The results show that the tensile strain can be a potential method to improve the PCE of the two heterostructures.

where EC, EM and ET are the total energy of the monolayer C2N, monolayer MTe and C2N/MTe vdW heterostructures, respectively. The binding energies and interlayer distances are listed in Table 1 and show that the type-A is the most stable structure. The ab initio molecular dynamics with NVT ensemble is employed to calculate the energy fluctuation of the type-A configuration at room temperature. Here, the calculated supercells containing 136 atoms are constructed by 2  2  1 unit cell of C2N/MTe heterostructure. The results are plotted in Fig. 2. It is clear that the C2N/MTe heterostructures are not distorted after 6 ps, and the energy fluctuation is small, which indicate that the type-A structure is stable at 300 K. Therefore, only the electronic structure of type-A configuration will be calculated and discussed in detail. The projected band structure of pristine C2N/GaTe heterostructure is illustrated in Fig. 3(c). In the heterostructure, the conduction band minimum (CBM) is dominated by N-2p and C-2p orbitals, while the valance band maximum (VBM) is dominated by the Te-5p orbital, indicating the C2N/GaTe vdW heterostructure possesses type-ІІ band alignments. Therefore, the C2N/GaTe heterostructure can drive the separation of photogenerated carriers and suppress the recombination of the photoexcited carrier. In addition, the indirect Eg of C2N/GaTe vdW heterostructure is 1.43 eV, which falls into the best scope (range from 1.0 to 1.5eV) for solar cell [45e47]. In the experiment, the strain effect on the sample is inevitable, and the strain can modulate the electronic structures of 2D materials [48,49]. The in-plane strain applied to the C2N/MTe heterostructure is calculated by

2. Methods

ε ¼ ½ða  a0 Þ = a0   100%

All the calculations are performed using the Vienna ab initio simulation package [35e38]. The generalized gradient approximations of PerdeweBurkeeErnzerhof (PBE) [39] is employed to describe the electron correlation and exchange interaction. The structural optimization and self-consistent calculations are performed using the k-points of 7  7  1 and 9  9  1 generated by the MonkhorstePackscheme [40], respectively. The calculations are performed until reach the convergence criteria for energy and force, which are set to be 104 eV and 0.01 eV/Å, respectively. The interlayer vdW interaction was calculated by the DFT-D3 method [41]. The cutoff energy was set as 500eV and the vacuum space was set as 20 Å to decouple the interactions between the periodic layers. The PBE always underestimates the Eg of the semiconductor, so the hybrid functional (HSE06) [42] was employed to obtain accurate electronic structure. In all the calculated process, the dipole correction is employed. 3. Results and discussion The lattice constants of optimized C2N, GaTe and InTe layers are 8.33, 4.14 and 4.33 Å, respectively, which agree well with previous studies [43,44]. Three typical stacking heterostructures are composed by 1  1C2N unit cell and 2  2 MTe supercell, as shown in Fig. 1. The lattice mismatch is 0.60% for the C2N/GaTe heterostructure and 3.96% for the C2N/InTe heterostructure. In calculations, the average lattice constants are selected as the lattice constants of heterostructures, and then the lattice mismatch is 0.30% for the C2N/GaTe heterostructure and 1.98% for the C2N/InTe heterostructure. The binding energies of the heterostructures are calculated according to the equation,

Eb ¼ EC þ EM  ET

where a0 and a represent the optimized lattice parameter without and with strain, respectively. Fig. 3(a), (b) plot the projected band structure of C2N/GaTe heterostructure with compressive strain. The type-ІІ band alignment is maintained in the C2N/GaTe heterostructures, while the degree of orbital hybridization is changed under compressive strain. The C-2p and N-2p orbitals split at the CBM, and only the N-2p orbital contributes to the CBM. When the strain increases to 4%, the VBM moves to the G point, thus the C2N/GaTe heterostructure transforms from an indirect semiconductor into a direct semiconductor. Owing to the movement of the band edges, the Eg of C2N/GaTe heterostructure becomes to 1.53 eV under 2% strain and 1.40 eV under 4% strain. Fig. 3(d)-(f) plot the projected band structure of C2N/GaTe heterostructure under tensile strain. Different from the compressive strain, the tensile strain has little effect on the orbital coupling near the band edges, but the band edges move gradually towards the Fermi level (EF), then the Eg of C2N/GaTe heterostructure decrease to 1.31 eV for 2% strain, 1.23 eV for 4% strain and 1.02 eV for 6% strain. The significant decrease of the Eg is beneficial to increase the light absorption, but the CBM of GaTe layer moves faster than that of C2N layer, which results in C2N/GaTe heterostructure possesses a type-І band alignment under 6% strain. The difference between the VBM and top valence band at G point in C2N/GaTe vdW heterostructures are 68, 75, 99, 150, 202 meV under 2%, 0%, 2%, 4% and 6% strain, respectively, which are larger than the thermal energy at room temperature (KT~30 meV). Zhao and his co-authors reported that this situation can effectively retard the recombination of photogenerated carriers [31]. Fig. 4 illustrates the projected band structures of C2N/InTe heterostructure under different strains. The projected band structures of pristine C2N/InTe heterostructure is illustrated in Fig. 4(c), the

Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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Fig. 1. The structure of C2N/MTe heterostructures.

Table 1 The optimized parameters of the C2N/MTe heterostructures, d represents the interlayer distance, Eb represents the binding energy.

C2N/GaTe C2N/InTe C2N/GaTe C2N/InTe

d (Å) Eb (eV)

CBM is composed of C-2p and N-2p orbitals, while the VBM is composed of Te-5p orbital, indicating that the C2N/InTe heterostructure possesses the type-ІІ band alignments. However, the VBM of InTe layer locates at G point in pristine C2N/InTe heterostructure, which is distinct from the pristine InTe monolayer [44]. As for this, one should note that 1.98% lattice mismatch exists in pristine C2N/ InTe heterostructure, in other words, the InTe layer suffers 1.98% strain with respect to the free-standing monolayer InTe. Due to the strain effect, the pristine C2N/InTe heterostructure presents both the type-ІІ band alignment and the direct Eg of 1.55 eV. Fig. 4(a), (b) plot the projected band structure of C2N/InTe heterostructure under compressive strain. The CBM of C2N/InTe heterostructure move towards the EF under the compressive strain, while the VBM barely moves relative to EF, thus the Eg decrease to 1.41 eV under 2% strain and 1.13 eV under 4% strain. The projected band structures of C2N/GaTe heterostructure under tensile strain are illustrated in Fig. 4(d)-(f). The tensile strain disturbs weakly the conduction band edge, while the tensile strain shifts the valence band towards the EF, thus the Eg of C2N/InTe heterostructure is reduced, as shown in Table 2. Moreover, the C2N/InTe heterostructure become to an indirect semiconductor due to the VBM deviates from the G point under the tensile strain. The difference between the VBM and top valence band at G point in C2N/InTe vdW heterostructures are 35, 36 and 49 meV under 2%, 4% and 6% strain, respectively, which also are larger than the thermal energy at room temperature (KT~30 meV). The work function of C2N, GaTe and InTe monolayer are 5.77, 5.10 and 5.31 eV, respectively. The difference of work function originates from the EF difference, which can result in the interfacial charge transfer. The charge transfer of the C2N/MTe heterostructures normal to the interface is calculated by

ð

ð

ð

Dr ¼ rT ðx; y; zÞdxdy  rCN ðx; y; zÞdxdy  rM ðx; y; zÞdxdy where rT ðx; y; zÞ; rCN ðx; y; zÞ and rM ðx; y; zÞ are the charge density at the point (x,y,z) in the C2N/MTe heterostructure, C2N monolayer and MTe monolayer, respectively. Fig. 5(a) show that the significant

Type-A

Type-B

Type-C

3.43 3.36 1.99 1.68

3.59 3.50 1.96 1.66

3.57 3.48 1.97 1.67

charge redistribution occurs at the interface of the C2N/GaTe heterostructure. Based on the analysis of atoms (C, N and Te) radius, the charge depletes at the interfacial Te atoms and accumulates at the C2N layer, implying that the electron transfer from GaTe layer to C2N layer. Furthermore, the compressive strain increases the charge transfer, while the compressive strain decreases the charge transfer. In the C2N/InTe heterostructure, the distribution of the averaged difference charge density is similar to the case of C2N/GaTe heterostructure. The Bader charge analysis is performed to quantify the charge transfer from MTe to C2N layer, the trend agrees with the averaged difference charge density, as shown in Table 2. Here, one can see that the amount of charge transfer in the C2N/MTe heterostructures is very small, although the charge redistribution is significant. The previous study [50] reports that the band edge positions can influence the photocatalytic performance of the semiconductor. Therefore, it is interesting to investigate the photocatalytic property of C2N/MTe heterostructure under different strain. Fig. 6(a) plots the band edge positions of C2N/MTe heterostructure. Clearly, the VBM of C2N layer and the CBM of MTe layer shift down from 4% strain to 6% strain, while the fluctuation occurs in the CBM of C2N and the VBM of MTe. Fortunately, the redox potentials of water lie in the Eg of C2N/GaTe heterostructure under 2% strain and of C2N/InTe heterostructure under 0% strain, which implies that the hydrogen production can occur at the surface of C2N layer, while the oxygen production can occur at the surface of MTe layer. The two photocatalytic processes occurring at different surface are beneficial to improve the photocatalytic performance. To further analyze the properties of C2N/MTe heterostructures as a photocatalyst, Fig. 6(b) schematically shows the type-ІІ band alignment and charge transfer. The built-in electric field with the direction from MTe layer to C2N layer would form at the interface due to the difference in work function. It is reported that the built-in electric field can result in the Z-scheme photocatalytic system [51]. If the Zscheme photocatalysis can be realized in the C2N/MTe heterostructure, the photogenerated electrons can be extracted from C2N layer to MTe layer and recombine with the holes in MTe layer, resulting that photogenerated electrons only accumulate in CBM of

Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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Fig. 2. The results of molecular dynamics simulation, (a) C2N/GaTe heterostructure, (b) C2N/InTe heterostructure.

MTe layer and photogenerated holes only accumulate in VBM of C2N layer, which can improve the potential difference between the band edges and the redox potentials, thus improving photocatalytic capacity of the C2N/MTe heterostructures. However, this mechanism in C2N/MTe heterostructures may be ineffective due to the weak built-in electric field originated from the weak charge transfer. It is reported that the surface absorption is an effective method to increase the electric field and achieve this Z-scheme photocatalytic system [28]. In addition, inserting the electron mediator (donor/acceptor) or conductor at the interface of heterostructure also exhibit the potential to form the Z-scheme photocatalytic system [52,53]. Therefore, inserting electron mediator or surface absorption may be the efficient methods to improve the photocatalytic performance of C2N/MTe heterostructures. The optical absorption coefficient of the C2N/MTe heterostructures is calculated by

1 pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ε21 ðuÞ þ ε22 ðuÞ  ε1 ðuÞ

a ¼ 2u

where ε2 ðuÞ and ε1 ðuÞ are the imaginary and real parts of the dielectric function εðuÞ, respectively. For comparison purpose, the

optical absorption coefficients of monolayer C2N, GaTe and InTe are plotted in Fig. 7(a)-(c). The absorption coefficients aX and aY are same in monolayer C2N, GaTe and InTe, indicating that optical absorption is isotropous in X and Y directions (a//), while the anisotropy is observed between a// and aZ. Fig. 7(d), (e) and (g), (h) illustrate the absorption coefficients of C2N/GaTe and C2N/InTe heterostructures under different strains, respectively. In a//, both C2N and MTe layers contribute to the characteristic absorption peaks. In the aZ, the MTe layer dominates the characteristic absorption peaks ranged from 0 to 10 eV, while the C2N layer dominates the absorption peaks ranged from 12 to 20 eV. The absorption edges start from about 1.43 eV for pristine C2N/GaTe heterostructure and 1.55eV for pristine C2N/InTe heterostructure, which originates from the electron hopping from the VBM (Te-5p orbital) to the CBM (C-2p and N-2p orbitals). Fig. 7(f) and (i) plot the optical absorption spectra of C2N/GaTe and C2N/InTe heterostructures, respectively. Compared with the pristine case, the optical absorption of C2N/MTe heterostructures increases significantly in the region above 526 nm under 4% and 6% strain, which are beneficial for increasing the absorption of visible light. Due to the Coulomb interaction, excited electrons and holes cannot be separated quickly and form electron-hole pairs, i.e.,

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Fig. 3. Projected band structure of C2N/GaTe heterostructure under strain, (a) for 4% strain, (b) for 2% strain, (c) for 0% strain, (d) for 2% strain, (e) for 4% strain, and (f) for 6% strain.

Fig. 4. Projected band structure of C2N/InTe heterostructure under strain, (a) for 4% strain, (b) for 2% strain, (c) for 0% strain, (d) for 2% strain, (e) for 4% strain, and (f) for 6% strain.

excitons. The degree of separation of excitons has a great influence on the utilization of photons. Factors affecting exciton separation are exciton lifetime, exciton radius, diffusion coefficient, carrier mobility, and so on. Due to the computational cost limitation, here we calculate the exciton Bohr radius based on the PBE functional.

The excitonic Bohr radius is calculated by Refs. [54,55].

a’ ¼ aH ε’

m0 u

where the aH , ε’ , m0 and u are the Bohr radius of the hydrogen atom,

Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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Table 2 The Eg, charge transfer (DQ), PCE (h) and relative error rate (Dh/h) of C2N/MTe heterostructures.

C2N/GaTe

Eg (eV) DQ (e)

h Dh/h

C2N/InTe

Eg (eV) DQ (e)

h Dh/h

4%

2%

0%

2%

4%

6%

1.40 0.065 9.15% 0.138 1.13 0.067 9.70% 0.156

1.53 0.062 9.21% 0.129 1.41 0.058 9.01% 0.135

1.43 0.060 10.74% 0.133 1.55 0.056 7.72% 0.128

1.31 0.057 15.35% 0.141 1.46 0.053 8.32% 0.132

1.23 0.056 22.10% 0.146 1.32 0.051 13.35% 0.140

1.02 0.053

1.26 0.050 19.51% 0.144

radius are listed in Table 3. In C2N/GaTe heterostructure, the exciton Bohr radius is approximately twice the lattice constant and is almost unaffected by the strain. In C2N/InTe heterostructure, the exciton Bohr radius is affected significantly by the strain and shows a decreasing trend from compressive strain to tensile strain. The decreased exciton radius is not conducive to split the exciton. The electronic structures show that the C2N/MTe heterostructures may be potential candidates for solar cell. From an application perspective, the PCE is the key point to evaluate the performance of the solar cell. The PCE of the heterostructure is calculated by Ref. [16].

FF

h¼



Eg  DEc



ð  ∞ Jph ðZuÞEQEðZuÞ dðZuÞ e  0:3V e Zu Eg ð∞ Jph ðZuÞdðZuÞ Eg

Fig. 5. Averaged electron density difference of (a) C2N/GaTe heterostructure and (b) C2N/InTe heterostructure.

macroscopic dielectric constant, electron mass and reduced mass of the carriers, respectively. This method cannot accurately calculate the exciton Bohr radius, but it should be effective for analyzing the changing trend of the exciton Bohr radius in the strained C2N/MTe heterostructures. To calculate accurately the macroscopic dielectric constant, the contributions from electrons (εe ) and ions (εi ) are considered. The macroscopic dielectric constant and exciton Bohr

where FF is the band-fill factor, which relates to the carrier resistance and recombination process [56]. Here, the FF equal to 0.65. ððEg DEc Þ =e 0:3VÞ is the estimated maximum open-circuit voltage (Voc ). e is the electron charge. The Eg is the band gap of MTe donor. Jph ðZuÞ is the solar energy flux at AM1.5. EQEðZuÞ is the external quantum efficiency, here, the EQEðZuÞ is estimated to 100%. The calculated results are listed in Table 2. The maximum PCE is 22.10% for C2N/GaTe heterostructure with 4% strain and 19.51% for C2N/InTe heterostructure with 6% strain, which are larger than the phosphorene heterojunction [16] CBN-PCBM [57] and polymer [58]. The PCE increases monotonously from 4% strain to 4% strain in C2N/GaTe heterostructure, while the PCE is increased both under compressive strain and tensile strain in C2N/InTe heterostructure. To reveal the factors of strain-regulated PCE, Fig. 8 plots the PCE of C2N/MTe heterostructures as a function of conduction band offset and Eg of the donor. Clearly, the strain effectively modulates the conduction band offset and the Eg of the donor, which can influence the Voc and the light absorption. In C2N/GaTe heterostructure, the

Fig. 6. (a) The band edge alignment of C2N/MTe heterostructures with different strain. (b) Schematic representation of the band alignment of C2N/MTe heterostructures.

Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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Fig. 7. Optical absorption coefficient of C2N/MTe heterostructures.

Table 3 The calculated macroscopic dielectric constant and excitonic Bohr radius.

C2N/GaTe

C2N/InTe

εe εi a’ (Å) εe εi a’ (Å)

4%

2%

0%

2%

4%

5.06 1.57 18.16

5.06 1.41 16.81

5.10 1.33 19.62

5.12 1.16 17.73

5.30 0.91 18.59

5.29 1.06 18.56

5.06 1.06 16.92

4.89 1.08 17.39

4.83 1.14 10.64

4.69 1.26 11.28

Eg of the donor and the band offset are reduced under tensile strain, while the case is contrary to compressive strain, thus the PCE is increased monotonously ranged from 4% strain to 4% strain. In C2N/InTe heterostructure, the tensile strain effectively reduces the conduction band offset and Eg, then the PCE is increased significantly. Therefore, the tensile strain is a potential method to tailor optical properties and increase the PCE of the C2N/MTe heterostructures. Given the estimated values, we calculated the relative error on the efficiency Dh=h according to the following equation

Dh 1 ðDVoc ; DEQEÞ ¼ h h

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi    2 2  vh vh DEQE  DVoc vEQE vVoc

Here, the error DEQEðZuÞ is assumed to 0.1 and the DVoc is assumed to 0.1. The calculated results are listed in Table 2. The PCE range is ð9:15 ±1:26  22:10 ±3:23Þ% for C2N/GaTe

6%

4.63 1.32 10.47

heterostructures and ð7:72 ±0:99  19:51 ±2:81Þ% for C2N/InTe heterostructures. In experiment, many materials and methods have been used to prepare photocatalytic and photovoltaic devices. These experimental results [59e61] show that the type-ІІ band alignment or tunable the band edge positions are the important factors to improve the performance of the devices. Although the C2N/MTe heterostructures have not been investigated widely in experiment, the DFT calculation shows that the C2N/MTe heterostructures possess tunable band alignment, which indicates that the C2N/MTe heterostructures are the promising candidates to synthetic photoelectronic devices.

4. Conclusions Based on the density functional theory, we have investigated the

Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559

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Fig. 8. Power conversion efficiency contour.

electronic structure and optical properties of the strained C2N/MTe vdW heterostructures. The C2N/MTe heterostructures possess the type-ІІ band alignment, which can drive the separation of photogenerated carriers. The C2N/GaTe heterostructure with 2% strain and the pristine C2N/InTe heterostructure reach the primary requirements for photocatalytic water splitting. The excitonic Bohr radius is almost not influenced in C2N/GaTe heterostructure by the strain, while it is decreased by the compressive strain in C2N/InTe heterostructure. The tensile strain can efficiently improve the light absorption and open circuit voltage of the C2N/MTe heterostructures, thus improving the PCE of the C2N/MTe heterostructures. In particular, the PCE of C2N/GaTe heterostructure with 4% strain is larger than 22%. The results provide valuable information for the experimental design of high-performance C2N/MTe heterostructures based devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51572033, 11704406, 11604019 and 61674053) and the Natural Science Foundation of Henan Province (Grant No.162300410325) and the High Performance Computing Center of Henan Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152559. References [1] W. Zheng, R. Lin, L. Jia, F. Huang, Vacuum-ultraviolet-oriented van der Waals photovoltaics, ACS Photonics 6 (2019) 1869e1875. [2] X. Liu, Y. Yan, A. Honarfar, Y. Yao, K. Zheng, Z. Liang, Unveiling excitonic dynamics in high-efficiency nonfullerene organic solar cells to direct morphological optimization for suppressing charge recombination, Adv. Sci. 6 (2019) 1802103. [3] M. Kumari, V. Singh Kundu, S. Kumar, N. Chauhan, S. Siwatch, Synthesis, characterization and dye-sensitized solar cell application of Zinc oxide based coaxial core-shell heterostructure, Mater. Res. Express 6 (2019), 085050. [4] M.H. Ali, M.M.A. Moon, M.F. Rahman, Study of ultra-thin CdTe/CdS heterostructure solar cell purveying open-circuit voltage ~1.2 V, Mater. Res. Express 6 (2019), 095515. [5] P. Zhang, X. Li, K. Zheng, Y. Zou, X. Feng, H. Cui, L. Tao, X. Li, X. Chen, SiAs2/GeP2 heterostructure for solar cell: a first-principles calculation, Chem. Phys. Lett.

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Please cite this article as: X. Wang et al., Designing strained C2N/GaTe(InTe) heterostructures for photovoltaic and photocatalytic application, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152559