Center for Materials Engineering

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Computational Modeling of ZnO/WSSe van Der Waals Heterostructures for Solar Cell Applications
(Addis Ababa University, 2020-09) Girma, Mekonnen; Georgies, Alene (PhD)
Stacking of two-dimensional materials into layered Van der Waals heterostructures are recently considered as a promising candidate for applications on photovoltaic devices because they can combine advantages of individual‟s 2D materials. Janus transition metals dichalcogenides (WSSe) have emerged because of favorable electronic properties as an attractive absorbing material. We therefore systematically examine the geometric features, electronic properties, work function, and density of states, band alignment of monolayer ZnO and WSSe and their heterostructures in this work using density functional theory methods (DFT) with PBE calculations as implemented in the Quantum ESPRESSO and VASP codes. It was found that, the negative binding energies indicate all the four configurations of ZnO-WSSe heterostructures are stable and feasible. Moreover, three configurations B, C and D exhibit indirect band gaps of 1.6248 eV, 1.6319 eV and 1.3126 eV, respectively. But the other configuration A has direct band gap of 1.7106 eV. In addition, it is found that all four configurations show band alignment type-II. In type-II alignment, donor-acceptor interface band heterojunctions can easily promote electron and hole carrier transfer and separation at interface, which can significantly enhance the efficiency of photovoltaic solar cells. With direct band gap, configuration A is the preferred heterostructure for photovoltaic devices applications.
Designing A Solar PV Powered Reverse Osmosis Desalination System Using Rosa to Clean Lake Basaka Water
(Addis Ababa University, 2023-07) Bethlehem, Terzu; Georgies, Alene (PhD)
The freshwater shortage is an issue concerning safe water supply. The availability and usage of fresh water are critical for human health as well as economic and ecological stability. The continual increase in salinity and alkalinity hindered the utilization of Lake Basaka for drinking, irrigation, and residential purposes. In this study, ROSA software is used to design an appropriate membrane to filter Lake Basaka water at higher rejection and specified recovery, flux, permeate, and feed flow. The ROSA software-designed TW 30-2540 membrane for the RO System provided the required permeate flow rate of 0.21 gpm with an applied pressure of 100.94 psig, recovery of 14.89%, rejection of 99.56 %, flux of 10.80 gfd, salt passage of 0.44% at an adjusted feed pH of 8, and system temperature of 25 °C. As a consequence, ROSA Software designed the TW 30-2540 membrane for solar-powered RO to filter Lake Basaka water without displaying a design warning. In this regard, a solar-driven reverse osmosis system with five filter stages, including three pretreatment filter stages, one membrane stage, and one post-treatment filter stage, was used to filter Lake Basaka water by removing organic and inorganic components such as suspended particles, dissolved solids, and water pollutants from feed water driven by solar energy as an energy source. The samples collected for water quality examination were 4 liters of water in a clean plastic bottle as part of the sampling procedures from Lake Basaka and RO-filtered Lake Basaka water is evaluated by using AAS and MP-AES, respectively. As comparing Lake Basaka water analyzed before and after RO filtration Na, Mg, Ca, K, Co3 2-, HCo3 -, Cl-, Barium, Nitrate, Fluoride, Sulfate, Silicon, and Boron all showed a reduction in mg/l after RO filtration.
A DFT study of 2D Van der Waals Heterostructures of Janus Transition Metal Dichalcogenides with WSe2 Monolayer for Energy Applications
(Addis Ababa University, 2021-09) Samuel, Tilahun; Georgies, Alene (PhD)
Atomically thin two-dimensional layered semiconductor materials such as Transition Metal Dichalcogenides (TMDs) have a great potential for solar cells applications due to their favorable photon absorption and electronic transport properties. The combination of 2D materials in the form of van der Waals heterostructures has been proved to be an effective approach for improving the electronic properties of the material. In this work, the electronic properties, such as band structure, bandgap, and band alignment of MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 vdW heterostructures were obtained from Density Functional Theory (DFT). The potential of the exchange and correlation was calculated using the Generalized Gradient-Perdew Berk Ernzed (GGA-PBE) approximation. MoSSe/WSe2, WSeTe/WSe2, and WSSe/WSe2 heterostructures have type-II band alignments, which is advantageous for electron-hole pair separation. Photons can be absorbed directly in MoSSe/WSe2 and WSeTe/WSe2 semiconductors because they have direct bandgaps. WSSe/WSe2 semiconductor has an indirect bandgap, which means that a phonon must also be absorbed or emitted for a photon to be absorbed. The most stable stacking order in heterostructures comprising Janus monolayers of TMDs and WSe2 has been ascertained based on interlayer binding energies. The binding energies in MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 heterostructures were found to be -18216.75 eV, -38995.69 eV, and -0.3296 eV at an equilibrium interlayer space of 5.75 Å, 4.02 Å, and 4.72 Å respectively, which are more stable than the other tested configurations. The solar cell and photocatalytic applications of the heterostructures were investigated. The structures were not promising for photocatalytic applications because the band levels of semiconductors were insufficient to support full water splitting. When MoSSe/WSe2, WSSe/WSe2, and WSeTe/WSe2 were used as PV materials, the Power Conversion Efficiency (PCE) was found to be 20.16%, 20.48%, and 19.05%, respectively. The results show that it can serve as a suitable photovoltaic material with high efficiency and opening possibilities to develop solar cells based on 2D TMDs materials.
First Principles Investigation of Van Der Waals Heterostructures of Mos2 and Janus Transition Metal Dichalcogenides for Energy Applications
(Addis Ababa University, 2021-09) Birhan, Tesfaye; Georgies, Alene (PhD)
Recent research on the Janus transition metal dichalcogenide (JTMD) with an asymmetric structure has revealed that this material possesses interesting unique properties, notably in solar cells. This work is based on cutting-edge density functional theory (DFT) computations utilizing Generalized Gradient Approximation- Perdew–Burke–Ernzerhof functional (GGA-PBE) as implemented in the Quantum ESPRESSO and VASP codes. To find the most stable optimized heterostructures, eight basic stacking patterns were designed. Then, for MoSSe/MoS2, WSSe/MoS2, and MoSTe/MoS2 heterobilayer, the AAII-S stacking mode was more stable than the other stacking types. According to the findings, the band alignment was type-I for MoSSe/MoS2, MoSTe/MoS2, and type-II for WSSe/MoS2, within, 1.03, 0.30 and 0.84 eV are estimated bandgap, respectively. The electrical band structure, as well as band edge placements, was investigated. When the water redox and oxidation potentials of heterostructures were compared, it was discovered that MoSSe/MoS2, MoSTe/MoS2, and WSSe/MoS2 were not applicable for photocatalytic materials for full water splitting. On the other hand, MoSSe/MoS2 and MoSTe/MoS2 heterostructures were placed lower than the oxidation potential of O2/H2O, making them applicable for oxygen evolution reaction (OER). This work reveals that JTMDs/MoS2 heterostructures are often subsequent material that promotes the development of photovoltaic devices, specially MoSSe/MoS2, and WSSe/MoS2 vdWH. The power conversion efficiency (PCE) of the heterostructures is calculated, and the results show that MoSSe/MoS2 and WSSe/MoS2 show very good efficiency with values of 19.41% and 16.25%, respectively. The result is good when compared to other similar studies: GaTe-InSe (9.1%), MoS2/p-Si (5.23%), organic solar cells (11.7%), and PN-WSe2 (13.8 % ). Since the results are encouraging, we believe it is a good idea to do additional experiments on the heterostructures and adapt them to solar cell applications.
First-Principles Study Of Van Der Waals Heterostructures of MoSeTe/ZnO for Investigating Photocatalytic Water-Splitting and Photovoltaic Applications
(Addis Ababa University, 2023-06) Derese, Abraham; Georgies, Alene (PhD)
Two-dimensional (2D) heterostructures have allowed for the development of novel properties with interesting applications in photocatalytic water splitting and optoelectronic devices. Electronic properties of ZnO and Janus MoSeTe monolayers were investigated using density functional theory (DFT)-based first-principles calculations, and depending on the lattice mismatch, layered 2D MoSeTe/ZnO heterostructures were produced. In this study, the first-principles van der Waals corrected density functional theory calculations were also performed on ABI_Se, ABI_Te, and ABII_Te heterostructure.Out of eight basic stacking patterns of the ZnO/MoSeTe hetero-bilayer designed, the ABII-Te stacking mode was a more stable stacking type due to the small lattice mismatch and the binding energy. The result showed that the band alignment for ABI_Se, ABI_Te, and ABII_Te was done on the electrical band structure and band edge positions, and confirmed type two band alignment. In addition, the ABI_Se, ABI_Te, and ABII_Te configurations of ZnO/MoSeTe vdW heterostructures are indirect band gap semiconductors. The investigated 2D ZnO/MoSeTe heterostructures have an acceptable band gap for solar applications, according to a first-principles study. The power conversion efficiency of ZnO/MoSeTe heterostructure is computed, and the results exhibit ABI_Se, ABI_Te, and ABII_Te stacking orientations have high efficiency with values of 22.26%, 22.31%and22.17%, respectively. Therefore, our findings show the heterostructures have reasonable band gaps and high PCE, and exhibit type-II band alignment, which are suitable candidates for solar cell application. Furthermore, for full water splitting heterostructures cannot satisfy the band edge requirements; however, the heterostructures are a good photocatalyst for the hydrogen evolution reaction. The heterostructure's ability to split water more effectively can be improved by moving the band edges position using strain and doping.
Selective Electrochemical Reduction of Carbon Dioxide on CuS (001) Surface using Density Functional Theory
(Addis Ababa University, 2020-10) Nigussie, Birhanu; Georgies, Alene (PhD)
Electrochemical carbon dioxide reduction reaction (ECR) on surface CuS (001) is one of the most capable approaches to convert CO2 gases to formic acid and carbon monoxide products. Geometry optimization and Gibbs free energy calculation were carried out using density functional theory with general gradient approximation of Perdew–Burke–Ernzerhof (PBE) as implemented in quantum ESPRESSO and VASP software package with computational hydrogen electrode (CHE) approaches. In this work the bulk CuS, CuS (001) surface and adsorbate configuration structures of intermediate species (*H, *OCHO, *COOH, and *CO) and reduction mechanisms were optimized geometry structure and calculated Gibbs free energy. Herein, the two elementary reaction paths are proposed * + CO2  *OCHO  HCOOH, and * + CO2  *COOH  *CO + H2O CO for the formation of formic acid and carbon monoxide and their activation barrier energy for intermediate *OCHO, *COOH_Cu, *COOH_S, *CO_Cu, and *CO_S are -0.03eV, +0.74 eV, +1.47 eV, -0.32 eV, and -1.05 eV respectively at 0V vs RHE. Intermediate *OCHO and *COOH_Cu have smallest activation barrier energy than the others. This study revealed that easier reaction would be occurred on *OCHO and *COOH_Cu intermediate. So that CuS (001) surface has highly selective catalyst and favourable to the formation of formic acid and good selective to carbon monoxide through *OCHO and *COOH-Cu intermediate, respectively.

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