Narrow Band Gap in La0.8MnO2.8 as a New Promising Solar Cell Absorber

In recent years, great interest has been given to manganite oxides thanks to their various proprieties which that make them gaining interest in technological applications (sensor applications, magnetic storage media, solar cell applications...) [1,2]. One of the mean challenges is finding a new ecological material that can be easily synthesized, cheap and non-toxic as a light absorber in photovoltaic applications [3-5]. LaMnO3 is one of the interesting manganite family materials as an antiferromagnetic insulator with trivalent manganese ions (Mn3+). The creation of a vacancy in La sites induces the formation mixed valence which mediate a ferromagnetic interaction between Mn3+ and Mn4+ cations [6]. Which present the main origin of the physical properties and application fields diversity of this kind of materials. In another hand, DFT calculations present crucial tool alloying to predict the performance of the studied materials. In this work, we mainly focused on the study of the optical properties of La0.8MnO2.8 material, which presents a very important key to estimate the material optimal performance to the application in solar cells domains [7].

0.80.23LaMnOγ powder was elaborated using the Sol-Gel as reported in our previous work [1,8]. To create vacancy in oxygen sites with a controller manner, the0.80.23LaMnOγcompound was placed into a vacuum quartz tubes for at 900 °C for 24j using the titanium as an oxygen absorber according to the following Equation:

To a successful numerical study 0.80.22.8LaMnOγ manganite, we are used the experimental parameters obtained by the DRX refinement. After that, we have created a 2*2*1 super cell contains 19 La, 24 Mn, and 67 O atoms. The ab intio calculations based on the full-potential linearized-augmented plane-wave (FP-LAPW) method as implemented in WIEN2k [9] code was performed [10]. For the exchange-correlation potential, we employed the Generalized Gradient Approximation (GGA) PBE (Perdew-Burke-Ernzerhof) [11]. The behaviour of the localized Mn 3d electrons is taken in consideration by include the orbitaldependent, on-site Coulomb potential (Hubbard U) Ueff=U-J=4.0eV [12]. The integrations in the Brillouin zone are performed on a 5×5×2 grid and the convergence of the Self-Consistent Cycles (SCF) was a considered when the energy difference between them was less than 10-4 Ry.

The structural refinement of X-ray diffraction pattern using Rietveld method, as displayed in (Figure 1a), shows that La0.8γ 0.2MnO2.8 material presents a coexistence of a minor Pnma orthorhombic phase with and a major R3c rhombohedral one. The obtained structural parameters are repotted in our previous work [1]. Figure 1b presents the morphology the prepared powder observed by SEM technology proving nanometric particle size. Figure 2 presents the variation of the scaled magnetization as function of temperature. The second derivate of the magnetization (Figure 2) shows that La0.8γ 0.2MnO2.8 material presents a magnetic transition from a ferromagnetic (metallic) order to a paramagnetic (Insulator/Semiconductor) one at Tc=232K when the temperature increases. Figure 3 proves the total density states obtained using DFT+U calculation showing an asymmetric behavior between the spins up and down states which indicate the magnetic behavior of the studied material [8]. It is clear that the Fermi level is occupied by density levels which indicate the metallic behavior of the studied material. While the spin-down states show a semiconductor behavior. The coexistence of both characteristics proves the semimetallic behavior of the studied materials [8]. The band structure for the spin-down states (Figure 4) proves a direct energy gap about 1.5eV. Interestingly, this value is very close to thus the Shockley- Queisser band gap of 1.34eV of a single junction solar cell [13]. It is also clear from the (Figure 5) that La0.8γ 0.2MnO2.8 compound has a high light absorption. Therefore, this material could be considered as good candidate for photovoltaic applications. The inset of (Figure 5) presents the Tauc plot. We notice that the optical band gap in z direction is the same as the electronic band gap. However, we notice the existence of an optical anisotropy between xx and zz directions. In order to understand the transport mechanism in this material, we calculated the electrons and holes effective mass (me* and mh*) using a polynomial fit of the minimum and maximum of B.C and B.V, respectively using the following relation.

Figure 1a: Rietveld refinement for La0.8γ 0.2MnO2.8 sample.

Figure 1b: SEM image for La0.8γ 0.2MnO2.8 sample.

Figure 2: The temperature dependence of the magnetization, the inset presets the second derivate of the magnetization as function of temperature.

Figure 3: The total density of state for La0.8γ 0.2MnO2.8 sample.

Figure 4: Majority (up) and minority (down) spin band structures of La0.8γ 0.2MnO2.8 .

Figure 5: The variation of the absorption coefficient as a function of the wavelength, the inset shows the Tauc plot.

The obtained values are me*=2.54 m0 and mh*=0.892 m0. So, we can calculate the electron and hole thermal velocity Vth using this relation:

We obtain Vth=7.326*104m/s and 1.236*105m/s for electrons and holes, respectively.

The electron and holes density can be calculate using the following relations [14]:

The obtained parameters are summarized in Table 1. We found then: n=4.11*1011cm-3 and p=3.47*108cm-3. The holes density is negligible to the electrons one, which suggests that the material, in spin down states, is an n-type semiconductor.

Table 1: Band gap and transport properties.

To sum up, the La0.8γ 0.2MnO2.8 synthesized by Sol-Gel route shows a coexistence of a minor Pnma orthorhombic phase and a major R3c rhombohedral one. The DFT calculation exhibits that the prepared nanoparticles present a wide absorption with a narrow band gap witch make this material considered as potential candidate to the photovoltaic applications.

The authors acknowledge the support of the Tunisian Ministry of Higher Education and Scientific Research and within the framework of Tunisian-Portuguese cooperation in the field of scientific research and technology (Project of Université of Sfax- Université of Aveiro).

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