Electronic and optical properties of halogen (H= F, Cl, Br)-doped Cu2O by hybrid density functional simulations

Journal Pre-proof Electronic and optical properties of halogen (H= F, Cl, Br)-doped Cu2 O by hybrid density functional simulations M. Benaissa, H. Si Abdelkader, G. Merad

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https://doi.org/10.1016/j.ijleo.2020.164440

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Please cite this article as: Benaissa M, Si Abdelkader H, Merad G, Electronic and optical properties of halogen (H= F, Cl, Br)-doped Cu2 O by hybrid density functional simulations, Optik (2020), doi: https://doi.org/10.1016/j.ijleo.2020.164440

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Electronic and optical properties of halogen (H= F, Cl, Br)-doped Cu2O by hybrid density functional simulations

M. Benaissa*, H. Si Abdelkader*, G. Merad Laboratory of Materials Discovery, Unit of Research Materials and Renewable Energies, LEPM-URMER, University of Tlemcen, Algeria.

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* Corresponding Authors Email addresses: [email protected] (M. Benaissa), [email protected] (H. Si Abdelkader)

Abstract:

First principles calculations with the HSE06 functional theory have been carried out to investigate the energetic, electronic and optical properties of halogen (H= F, Cl, Br)-doped Cu2O systems. The results

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suggest that the halogen-doped Cu2O can be easily prepared under Cu-rich conditions. In addition, the inclusion of halogen atoms in Cu2O offers good optical and electronic properties with significant

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improvements for the Cl-doping compared to other dopants and pure Cu2O in wavelength ranging between [250 nm, 380 nm]. However, the absorption of H-doped Cu2O is lower than that of pure Cu2O in the UV zone. These results confirm that halogen doped-Cu2O has n-type conductivity, especially the case of

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chlorine doping.

Keywords:

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conductivity.

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Cuprous oxide, Halogen doping, First principles calculations, Optical properties, Absorbance, n-type

1) Introduction:

There is long interest in the solar cells research community in novel photovoltaic materials. This is influenced by the desire of improving the properties of the materials currently used in market, or by the necessity of discovering alternative materials to those already used, for reasons of cost or ease of fabrication. Recently, cuprous oxide (Cu2O) has been extensively investigated as one of the promising low-cost solarcell materials [1-4]. It has many advantages, such a very high absorption coefficient, direct bandgap, nontoxic and abundant source materials and easy to scale fabrication methods. Cu2O has been known for long time to be a naturally p-type semiconductor [5]. However, n-type Cu2O was also fabricated; doping, oxygen

vacancies, and copper antisites are means to achieve the n-type conductivity in Cu2O. Generally, doping is the most important method used to change the conduction type. n-type Cu2O doped with chlorine (Cl) has been deposited electrochemically and demonstrated a resistivity reduced by 5 orders of magnitude after doping [1]. Indium-doped Cu2O thin films were fabricated by direct current magnetron co-sputtering by Cai et al [6], with donor energy level estimated to be 620.2–713.8 meV below the conduction band. Some density functional calculations have been applied to study the n-type conduction of doped Cu2O. Bai et al. [7] studied the n-type doping in Cu2O synthesized in an acidic solution environment. They found that F, Cl and Br in substitution of an O atom have low formation energies and present an n-type conduction behavior with donor levels below conduction band. Their results also showed that the Cl dopant has the shallowest donor level among the three halogens. Zhao et al. [8] presented the halogen (H =F, Cl, Br, I) doping effects on electronic structure of Cu2O and observed that the halogen doping could make Cu2O have n-type

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conductivity. But in the case of iodine doping, the impurity formation energy is positive under both conditions (O poor and O rich), suggesting that it is relatively difficult to prepare I-doped Cu2O material. Cai et al. [6] also studied In-doped Cu2O theoretically using DFT method. They found that In-doping reduced the work function and can raise the Fermi energy level of Cu2O and, therefore, lead to n-type

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conduction. Despite that n-type Cu2O has been studied experimentally and theoretically, there are still some open questions such as the doping effects of halogen on the optical properties of Cu2O and the comparison

is necessary and needs more attentions.

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of effects of halogens. So, a systematic study of effects of halogen doping on the optical properties of Cu 2O

In this paper, we have used first-principles calculations to examine the effects of halogen anion (H= F, Cl,

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Br)-doping on the formation energies, electronic structure and optical properties of Cu2O and also to compare between different halogens dopants. This paper is arranged as follows: Section 2 describes the computational details used in this study. In Section 3, we present the results on the undoped and doped

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systems. Finally, we draw conclusions in Section 4.

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2) Computational methods:

First principal calculations were performed using the VASP package [9,10] based on the density functional

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theory (DFT) [11,12]. We adopted the generalized gradient approximation (GGA) for the exchange correlation functional of Perdew-Burke-Ernzerhof (PBE) [13] with the projector augmented wave (PAW) method to describe the electron-ion interactions [14]. The valence electron configurations used in this research were 3d104s1 for Cu, 2s22p4 for O, 2s22p5 for F, 3s23p5 for Cl and 4s24p5 for Br. The geometry optimization after adding the impurity in Cu2O is performed using DFT-PBE calculations. While, the total energy, the electronic structure and optical properties calculations are performed using the Heyd-ScuseriaErnzerhof (HSE) hybrid density functionals approach [15,16]; where HSE gives the most accurate electronic structure for stoichiometric Cu2O [17]. In this work, the fraction of Hartree-Fock exchange and the screening parameter are set to the default of HSE06 functional. The integration over the first Brillouin zone

were made by using a 4×4×4 Gamma-centered k-points grid generated according to the Monkhorst–Pack scheme [18] with a cut-off energy restricting the number of plane waves in the basis set to 520 eV. All the structures were allowed to relax according to the Hellman-Feynman forces using the conjugate gradient algorithm until all the forces acting on the atoms were less than 5 meV/Å3 and the convergence tolerance of the total energy was set to 0.5 meV.

3) Results and discussions 3.1. Structural properties The Cu2O crystal has a cubic structure with Pn3m space group as shown in Figure 1 (a). Each oxygen atom is surrounded by four copper atoms in a tetrahedral configuration. The doping systems were simulated using the periodic supercell with concentration of 6.25 %. A 2×2×2 supercell of unit cell of Cu2O containing 48

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atoms was constructed; where one O ion is replaced by a halogen ion (H= F, Cl, Br) as displayed in Figure 1 (b). The calculated lattice parameters and bond lengths of pure and halogen doped systems are summarized in Table 1. For pure Cu2O, the calculated parameter a=4.31 Å is in good agreement with the experimental value (a=4.27 Å [19]) and previous calculations (a=4.32 Å [20]). For halogen doped, we notice that the

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lattice parameters are remarkably changed by 0.21 %, 0.60 % and 0.30 % for F, Cl and Br doping compared to pure case, respectively. This change can be explained by the difference of atomic radius between F (0.50

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Å), Cl (1.00 Å), Br (1.15 Å) and the host atom O (0.60 Å). We can observe that the contraction increases in the order of Cu-F < Cu-Br < Cu-Cl. Regarding bond lengths, we found a difference of 8.09 %, 18.15% and

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15.37% for F, Cl and Br doping compared to Cu2O bond length, respectively. These results show that the

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3.2. Formation energies

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bond length increase with increasing atomic number.

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To explore the stability of halogen-doped Cu2O, the formation energies of the pure and doping Cu2O were calculated according to the following formulas [21,22]: 𝐶𝑢 𝑂

2 𝛥𝐸𝑓𝑜𝑟𝑚 = 𝐸𝐶𝑢2 𝑂 − (2𝜇𝐶𝑢 + 𝜇𝑂 )(1)

𝛥𝐸𝑓𝑜𝑟𝑚 = 𝐸𝐶𝑢2 𝑂:𝐻 − 𝐸𝐶𝑢2 𝑂 − (𝑛𝐻 𝜇𝐻 − 𝑛𝑂 𝜇𝑂 )(2)

Where 𝐸𝐶𝑢2𝑂 and 𝐸𝐶𝑢2 𝑂:𝐻 correspond to the total energies of pure and halogen-doped Cu2O supercell, respectively. 𝜇𝐶𝑢 ,𝜇𝑂 and 𝜇𝐻 are the chemical potentials of Cu, O, and H= F, Cl, Br elements, respectively. 𝑛𝐶𝑢 and 𝑛𝐻 represent the number of Cu atoms removed from, and halogen atoms added to the pristine Cu2O

supercell, respectively. Both 𝜇𝐶𝑢 and 𝜇𝑂 are constrained by the condition of 𝜇𝐶𝑢2𝑂 = 2𝜇𝐶𝑢 + 𝜇𝑂 . Upper limits of 𝜇𝐶𝑢 and 𝜇𝑂 are determined by metallic Cu and molecular oxygen following the formula: 0 𝜇𝐶𝑢 ≤ 𝜇𝐶𝑢 , 𝜇𝑂 ≤ 𝜇𝑂0 (3) 0 Where 𝜇𝐶𝑢 and 𝜇𝑂0 are total energies per atom of metal phase Cu and molecular oxygen, respectively. So, the

rang of 𝜇𝑂 is defined as: 𝐶𝑢 𝑂

2 𝛥𝐸𝑓𝑜𝑟𝑚 + 𝜇𝑂0 ≤ 𝜇𝑂 ≤ 𝜇𝑂0 (4)

The chemical potential of the halogen dopants is determined by the energy per atom of molecular halogen. The results of formation energies calculations of pure and halogen-doped Cu2O using HSE are given in Table 2. The calculated formation energy of pure Cu2O is in excellent agreement with the available

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experimental data [23]. The obtained formation energies of halogen-doped Cu2O reveal that the systems in Cu-rich conditions are more energetically favorable than O-rich, owing to the lower values of the ∆Eform. The results indicate that the halogen-doped Cu2O can be easily prepared under Cu-rich conditions. We further note that the ∆Eform of F-doping in Cu2O is the lowest than other studied halogen dopants under both

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conditions.

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3.3. Electronic properties

The total and partial densities of states (DOS) of pure and halogen-doped Cu2O using HSE functional are

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represented in Figure 2.

For the pure Cu2O (Figure 2 (a)), DOS consists of two zones in the valence band. The first is between -4 eV and 0 and is comprised of Cu 3d stats with a small contribution of O 2p states. The second zone between -8

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eV and -5 eV consists primarily of O 2p and Cu 3d states and leads to a strong hybridization of these states. With regard to the conductive band, Cu 3d and O 2p states present a low contribution. For halogen-doped Cu2O, the F, Cl, Br-doped Cu2O systems are illustrated in Figure 2 (b, c and d),

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respectively. The DOS show similar states to the pure Cu2O. However, there are new peaks of F 2p, Cl 3p and Br 4p around -10.2 eV, - 10 eV and -10.8 eV, respectively.

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The major effect observed in the case of doping is on the conduction band states. Halogen anion p states leads to the splitting of Cu d states and shifting it at lower energy which leads to raise the Fermi level of Cu2O. This confirms that halogen doped-Cu2O can have n-type conductivity. We also calculated the gap energy of a pure and halogen doped-Cu2O by HSE approximations. The results are listed in Table 3. The calculated band gap energy of Cu2O using GGA is 0.49 eV, only 23% of the empirical data (2.17 eV [24]). HSE functional gives band gap of 1.89 eV in good agreement with the experiment. In addition, the band gap of H-doped decreases with respect to bulk Cu2O as can be seen in the Table 3 in the order ( Eg Cu2O: F < Eg Cu2O: Br < Eg Cu2O: Cl < Eg Cu2O), which is in good agreement with the previous results [8]. 3.4. Optical properties

We can determine optical properties using the complex dielectric function 𝜀(𝜔) = 𝜀1 (𝜔) + 𝑖𝜀2 (𝜔) [25]. The imaginary part of the dielectric function 𝜀2 (𝜔) was calculated from the momentum matrix elements between the occupied and unoccupied wave functions. 𝜀2 (𝜔) =

2𝑒 2 𝜋 2 ∑ |⟨𝛹𝑘𝐶 |𝑢 ^ ⋅ 𝑟|𝛹𝑘𝑉 ⟩| 𝛿(𝐸𝑘𝐶 − 𝐸𝑘𝑉 − ℏ𝜔)(5) 𝛺𝜀0 𝑘,𝑉,𝐶

𝜀1 (𝜔) = 1 +

∞ 2 𝜔′𝜀2 (𝜔′)𝑑𝜔′ (6) 𝑝∫ 𝜋 0 (𝜔′2 − 𝜔 2 )

Different optical properties of the material such as the absorption coefficient 𝛼(𝜔), reflectivity 𝑅(𝜔) and the refractive index 𝑛(𝜔) can be derived using, 𝜀2 and 𝜀1 , where [26]: 1

𝑅(𝜔) = |

√𝜀(𝜔) + 1

2

| (8)

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𝜀2 (𝜔) (9) 2 (𝜔) 𝜀1 + 𝜀22 (𝜔)

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𝑛(𝜔) =

√𝜀(𝜔) − 1

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2 𝜔 𝛼(𝜔) = √2 ( ) [√𝜀12 (𝜔) − 𝜀22 (𝜔) − 𝜀1 (𝜔)] (7) 𝐶

Figure 3 represents the calculated absorption coefficient of H-doped Cu2O compared with the pure case

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using the HSE06 approach. The obtained results can be used to observe trends in the effect of dopants on band gaps and absorption coefficient. A preliminary analysis indicates that the absorption coefficient of Cl:

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Cu2O and F: Cu2O are similar to the pure one in the region [380,550] nm. While that the Br: Cu2O shows the extension of the absorption edge towards lower wavelengths. We further note that all three dopants present remarkable absorption enhancement in the region [230,380] nm compared to the pure Cu2O. It’s clear that

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the incorporation of Cl into Cu2O lattice shows the best absorption enhancement. Figure 4 represents the calculated reflectivity of H-doped Cu2O compared with the pure case. We observe

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that F: Cu2O shows low reflectivity compared to other compounds in most of the solar spectrum. In the case of Br: Cu2O, we can see that peaks of reflectivity are shifted to lower wavelength with a 50 nm shift compared to the bulk; which is in correlation with the results of band gap calculations. While, Cl: Cu2O shows great reflectivity between [310,450] nm. Figure 5 represents the variation of refractive index of H-doped Cu2O compared with the pure case. All the doped compounds have lower refractive index with respect to Cu2O in the region [150,320] nm. While in the region [320,450] nm, the refractive indexes are distinct with Cl: Cu2O being the highest and F: Cu2O being the lowest. In the case of Br: Cu2O the main peaks are shifted to higher wavelengths.

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CONCLUSION

In conclusion, we performed hybrid DFT calculations to analyze the impact of halogen doping (H= F, Cl, Br) on the energetic, electronic and optical properties of Cu2O with a concentration of x= 6.25%. The computed formation energy reveals that the halogen-doped Cu2O can be easily prepared under Cu-rich conditions. The electronic structure shows that the halogen atoms are possible donors for n-type conduction of Cu2O. Moreover, the incorporation of halogen atoms in Cu2O offers good optical properties with significant improvements for the Cl-doping compared to other dopants and pure Cu2O in wavelength ranging between [250 nm, 380 nm]. However, the optical gaps values and the absorption of H-doped Cu2O are lower than those of pure Cu2O. These results confirm that halogen doped-Cu2O has n-type conductivity, especially the case of chlorine doping.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure captions: Figure 1: (a) Unit cell of Cu2O, (b) 2 x 2 x 2 supercell of halogen-doped Cu2O, blue, red and green spheres represent Cu, O, and halogen (F, Cl, Br) atoms, respectively.

Figure 2: Density of states of (a) pure Cu2O, (b) F-doped Cu2O, (c) Cl-doped Cu2O and (d) Br-doped Cu2O.

Figure 3: The calculated absorption coefficient of H-doped Cu2O compared with the pure Cu2O case.

Figure 4: The calculated reflectivity of H-doped Cu2O compared with the pure case.

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Figure 5: The variation of refractive index of H-doped Cu2O compared with the pure case.

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Figure 2

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Figure 5

Table captions:

Table 1: Crystal structures parameters and the average bond lengths of pure and halogen doped Cu2O systems. Table 1 : a(Å)

Bond length (Å) Cu-O

Cu-H

Pure Cu2O

4.309

1.867

----

Theo. a

4.320

----

----

Exp. b

4.270

----

----

Cu2O: F

4.300

1.852

2.018

Cu2O: Cl

4.335

1.854

2.206

Cu2O: Br

4.322

1.836

2.154

(a). [20] (b). [19]

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Table 2: Formation Energies ∆Eform (in eV) of pure and halogen doped Cu2O systems. ∆Eform (eV)

Cu2O

-1.970

Exp. a

-1.750

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O-rich

-p

System

Cu-rich

-1.934

-3.900

Cu2O: Cl

1.029

-0.937

0.960

-1.006

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Cu2O: F

Cu2O: Br

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(a). [22]

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Table 3: Calculated optical band gap of pure and halogen doped Cu2O systems.

Band gap (eV)

Cu2O

Cu2O: F

Cu2O: Cl

Cu2O: Br

1.89

1.29

1.64

1.54