Monolayer HfTeSe4: A Promising Two-Dimensional Photovoltaic Material for Solar Cells with High Efficiency

Theoretical Study of the Ternary Compound Monolayer CuP2Se for Photocatalytic Water Splitting with Efficient Optical Absorption

Utilizing photocatalytic method to produce hydrogen by splitting water is an efficient strategy to solve the hotspot issues of energy crisis and environmental pollution. Herein, we systematically investigate the corresponding properties of the reported Cu-bearing ternary compound monolayer CuP2Se by using the first-principle calculations. The monolayer CuP2Se has quite small cleavage energy of 0.51 J/m2, indicating it can be easily produced by the mechanical exfoliation method experimentally. In addition, it is an indirect bandgap semiconductor material which has a moderate value of 1.91 eV. The conduction band minimum (CBM) and valence band maximum (VBM) can perfectly straddle the redox potentials of water when a biaxial strain of -4% to 4% is applied, unveiling the high photocatalytic thermodynamic stability of monolayer CuP2Se in response to the effect of solvent tension. Remarkably, the monolayer CuP2Se also demonstrates significant sunlight capturing ability in the visible region. The outstanding electronic and optical properties suggest that the monolayer CuP2Se is undoubtedly a viable material for photocatalytic water splitting.

Two-dimensional InTeClO3: an ultrawide-bandgap material with potential application in a deep ultraviolet photodetector

Ultrawide-bandgap semiconductors, possessing bandgaps distinctly larger than the 3.4 eV of GaN, have emerged as a promising class capable of achieving deep ultraviolet (UV) light detection. Based on first-principles calculations, we propose an unexplored two-dimensional (2D) InTeClO3 layered system with ultrawide bandgaps ranging from 4.34 eV of bulk to 4.54 eV of monolayer. Our calculations demonstrate that 2D InTeClO3 monolayer can be exfoliated from its bulk counterpart and maintain good thermal and dynamic stability at room temperature. The ultrawide bandgaps may be modulated by the small in-plane strains and layer thickness in a certain range. Furthermore, the 2D InTeClO3 monolayer shows promising electron transport behavior and strong optical absorption capacity in the deep UV range. A two-probe InTeClO3-based photodetection device has been constructed for evaluating the photocurrent. Remarkably, the effective photocurrent (5.7 A m-2 at photon energy of 4.2 eV) generation under polarized light has been observed in such a photodetector. Our results indicate that 2D InTeClO3 systems have strong photoresponse capacity in the deep UV region, accompanying the remarkable polarization sensitivity and high extinction ratio. These distinctive characteristics highlight the promising application prospects of InTeClO3 materials in the field of deep UV optoelectronics.

Photocatalytic water splitting for hydrogen production with high efficiency monolayer In2Te5: a theoretical study

Employing density functional theory (DFT) calculations, we explore the excellent performance of two-dimensional (2D) semiconductor In2Te5 in photocatalytic water splitting at the theoretical level. The calculated results illustrate that 2D In2Te5 is a direct band gap semiconductor with a moderate band gap value and an ultrahigh optical absorption coefficient in the visible light region. It was found that its conduction band edge is higher than the reduction potential of water (-4.44 eV), which proves that it can split water to produce hydrogen. Furthermore, its excellent hydrogen evolution activity can be tuned under an appropriate biaxial strain. In addition, 2D In2Te5 shows a remarkable photo-generated current, suggesting that electrons and holes can be separated efficiently. Our results offer a superior candidate material for realizing photocatalytic water splitting for hydrogen evolution.

Symmetry-breaking-induced ferroelectric HfSnX3 monolayers and their tunable Janus structures: promising candidates for photocatalysts and nanoelectronics

Designing novel two-dimensional (2D) ferroelectric materials by symmetry breaking and studying their mechanisms play important roles in the discovery of new ferroelectric photocatalysts and nanoelectronics. In this study, we have systematically investigated a series of novel ferroelectric 2D HfSnX3 (X = S, Se and Te) monolayers through first-principles calculations. We found that each HfSnX3 monolayer contains a stable ferroelectric phase (FP) and a paraelectric phase (PP). The large polarization (up to 1.64 μC cm-2) in the FP can significantly bend the oxidation reduction potential of water, making HfSnX3 monolayers become excellent ferroelectric photocatalysts. Specifically, by designing a Janus structure to break the symmetry of the PP, we have excitingly obtained a stable Hf2GeSnSe6 (referred to as HGSS) monolayer with triple polarized states. HGSS not only possesses great visible light absorption properties (about 3 × 105 cm-1) as photocatalysts but also successfully solves the dead layer problem previously reported in practical applications. In addition, by constructing a heterostructure with graphene, HGSS has great application in the design of controllable ultrathin p-n junctions. Overall, our study not only predicts a series of potential ferroelectric photocatalytic materials, but also provides valuable insights for designing tunable polarized materials and nanoelectronics.

Moderate direct band-gap energies and high carrier mobilities of Janus XWSiP2 (X = S, Se, Te) monolayers via first-principles investigation

Two-dimensional (2D) Janus materials with extraordinary properties are promising candidates for utilization in advanced technologies. In this study, new 2D Janus XWSiP2 (X = S, Se, Te) monolayers were constructed and their properties were systematically analyzed by using first-principles calculations. All three structures of SWSiP2, SeWSiP2, and TeWSiP2 exhibit high energetic stability for the experimental fabrication with negative and high Ecoh values, the elastic constants obey the criteria of Born-Huang, and no imaginary frequency exists in the phonon dispersion spectra. The calculated results from the PBE and HSE06 approaches reveal that the XWSiP2 are semiconductors with moderate direct band-gaps varying from 1.01 eV to 1.06 eV using the PBE method, and 1.39 eV to 1.44 eV using the HSE06 method. In addition, the electronic band structures of the three monolayers are significantly affected by the applied strains. Interestingly, the transitions from a direct to indirect semiconductor are observed for different biaxial strains εb. The transport parameters including the carrier mobility values along the x direction μx and y direction μy were also calculated to study the transport properties of the XWSiP2. The results indicate that the XWSiP2 monolayers not only have high carrier mobilities but also anisotropy in the transport directions for both holes and electrons. Together with the moderate and tunable energy gaps, the XWSiP2 materials are found to be potential candidates for application in the photonic, photovoltaic, optoelectronic, and electronic fields.

W4PCl11 monolayer: an unexplored 2D material with moderate direct bandgap and strong visible-light absorption for highly efficient solar cells

The discovery of novel two-dimensional (2D) materials with excellent electronic and optoelectronic properties have attracted much scientific attention. Based on the first-principles calculations, we predict an unexplored 2D W₄PCl₁₁ monolayer, which is potentially strippable from its bulk counterpart with the exfoliation energy of only 0.16 J m^-2. The dynamical, thermal, and mechanical stabilities have also been confirmed. Remarkably, W₄PCl₁₁ monolayer is direct semiconductor with a bandgap of 1.25 eV, which endows the monolayer with very strong visible-light absorption in the magnitude of 10⁵ cm^-1. Meanwhile, the calculated carrier mobilities of W₄PCl₁₁ monolayer can reach to 10³ cm² V^-1 s^-1. Considering the moderate direct bandgap and high carrier mobility, W₄PCl₁₁ monolayer should be a superior candidate for the donor material of excitonic solar cells. The estimated power conversion efficiency of the fabricated W₄PCl₁₁/Bi₂WO₆ heterojunction reaches as high as 21.64%, which much superior to those of most recently reported 2D heterojunction. All these outstanding properties accompanied with its experimental feasibility endows W₄PCl₁₁ monolayer with promising photovoltaic applications.

Thermoelectric performance of novel single-layer ZrTeSe4

Single-layer ZrTeSe4 is a novel 2D semiconductor as well as a promising candidate for 2D thermoelectric materials.

Theoretical investigations of novel Janus Pb2SSe monolayer as a potential multifunctional material for piezoelectric, photovoltaic, and thermoelectric applications

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb₂SSe, with a black phosphorus-like structure and an indirect band gap of 1.59 eV as well as high carrier mobility (526-2105 cm² V^-1 s^-1), displays outstanding potentials in the energy conversion between nanomechanical energy, solar energy, waste heat, and electricity, which has been comprehensively studied utilizing DFT-based simulations. The research results reveal that monolayer Pb₂SSe not only possesses giant in-plane piezoelectricity of d₁₁ = 75.1 pm V^-1 but also superhigh out-of-plane piezoelectric coefficients (d₃₁ = 0.5 pm V^-1 and d₃₃ = 15.7 pm V^-1). Meanwhile, by constructing Pb₂SSe bilayers, the out-of-plane piezoelectric coefficients can be significantly enhanced (d₃₁ = 19.2 pm V^-1 and d₃₃ = 194.7 pm V^-1). In addition, owing to the small conduction band offset, suitable donor band gap and excellent light absorption capability in the Pb₂SSe/SnSe (Pb₂SSe/GeSe) heterostructure, the power conversion efficiencies were calculated to be up to 20.02% (Pb₂SSe/SnSe) and 19.28% (Pb₂SSe/GeSe), making it a promising candidate for solar energy collection. Furthermore, from the thermoelectric electron and phonon transport calculations, it can be found that the Pb₂SSe monolayer is an n-type thermoelectric material with ultrahigh ZT = 2.19 (1.52) at room temperature, which can be traced back to its ultralow κ_L = 0.78 (0.99) W m^-1 K^-1, and superhigh PF = 10.18 (8.25) mW m^-1 K^-2 along the x(y) direction at the optimal doping concentration at 300 K. The abovementioned versatile characteristics in the Janus Pb₂SSe monolayer, along with its comprehensive stabilities (energy, dynamic, thermal, and mechanical stabilities), highlight its potential in clean energy harvesting.

Sb2TeSe2 Monolayers: Promising 2D Semiconductors for Highly Efficient Excitonic Solar Cells

On the basis of density functional theory computations, we demonstrated that two-dimensional (2D) α- and β-Sb2TeSe2 monolayers are promising candidates for constructing high-efficiency heterojunction excitonic solar cells. These two 2D materials possess moderate band gaps (∼1.1 eV), which can be flexibly tuned by applying external strains. They possess high carrier mobility (∼3000 cm² V^-1 s^-1) and can absorb sunlight over the whole range of the solar spectrum. Remarkably, the α- and β-Sb2TeSe2 monolayers can form desirable type II heterostructures with HfSe2 and BiOI monolayers, respectively. The power conversion efficiencies of α-Sb₂TeSe₂/HfSe₂ and β-Sb₂TeSe₂/BiOI heterojunction excitonic solar cells can reach 22.5 and 20.3%, respectively. Since α-Sb2TeSe2 and β-Sb2TeSe2 monolayers have good stability and high synthesis feasibility, they have important applications in photovoltaic solar cell devices.

Multielement 2D layered material photodetectors

The pronounced quantum confinement effects, outstanding mechanical strength, strong light-matter interactions and reasonably high electric transport properties under atomically thin limit have conjointly established 2D layered materials (2DLMs) as compelling building blocks towards the next generation optoelectronic devices. By virtue of the diverse compositions and crystal structures which bring about abundant physical properties, multielement 2DLMs (ME2DLMs) have become a bran-new research focus of tremendous scientific enthusiasm. Herein, for the first time, this review provides a comprehensive overview on the latest evolution of ME2DLM photodetectors. The crystal structures, synthesis, and physical properties of various experimentally realized ME2DLMs as well as the development in metal-semiconductor-metal photodetectors are comprehensively summarized by dividing them into narrow-bandgap ME2DLMs (including Bi₂O₂X (X = S, Se, Te), EuMTe₃(M = Bi, Sb), Nb₂XTe₄(X = Si, Ge), Ta₂NiX₅(X = S, Se), M₂PdX₆(M = Ta, Nb; X = S, Se), PbSnS₂), moderate-bandgap ME2DLMs (including CuIn₇Se₁₁, CuTaS₃, GaGeTe, TlMX₂(M = Ga, In; X = S, Se)), wide-bandgap ME2DLMs (including BiOX (X = F, Cl, Br, I), MPX₃(M = Fe, Ni, Mn, Cd, Zn; X = S, Se), ABP₂X₆(A = Cu, Ag; B = In, Bi; X = S, Se), Ga₂In₄S₉), as well as topological ME2DLMs (MIrTe₄(M = Ta, Nb)). In the last section, the ongoing challenges standing in the way of further development are underscored and the potential strategies settling them are proposed, which is aimed at navigating the future advancement of this fascinating domain.

Type-II lateral SnSe/GeTe heterostructures for solar photovoltaic applications with high efficiency

Recently, lateral heterostructures based on two-dimensional (2D) materials have provided new opportunities for the development of photovoltaic nanodevices. In this work, we propose a novel lateral SnSe/GeTe heterostructure (LHS) with high photovoltaic performance and systematically investigate the structural, electronic and optical properties of the lateral heterostructure by using first-principles calculations. Our results show that this type of heterostructure processes excellent stability due to the small lattice mismatch and formation energy and also covalent bonding at the interface, which is greatly beneficial for the epitaxial growth of heterostructures. These heterostructures are semiconductors with type-II band alignment and their electronic properties can be effectively tuned by the size and composition ratio of the heterostructures. More importantly, it is found that these heterostructures possess high absorption over a wide range of visible light and high power conversion efficiency (up to 22.3%). These extraordinary properties make the SnSe/GeTe lateral heterostructures ideal candidates for photovoltaic applications.

Two-Dimensional Direct Semiconductor Boron Monochalcogenide γ-BTe: Room-Temperature Single-Bound Exciton and Novel Donor Material in Excitonic Solar Cells

Recently, excitonic solar cells (XSCs) with high photovoltaic performance have raised research interests because of their high power conversion efficiencies (PCEs). Herein, by using first-principles calculations, we predict that γ-BX (X = S, Se, Te) monolayers are direct semiconductors with the band gaps of 2.94, 2.71, and 1.32 eV, respectively, and maintain semiconduction in the broad strain range of 0% ≤ δ ≤ 5%. The moderate direct band gap, high transport property, dramatically high absorption from visible to the ultraviolet region, and extraordinary excitonic behavior of monolayer γ-BTe, render it promising for next-generation optoelectronic and photovoltaic devices. By choosing monolayer GeP₂ as a proper acceptor material, the practical upper limit of PCE for the heterobilayers of γ-BTe/GeP₂ reaches up to 21.76% (22.95% under strain), comparable to typical heterobilayer solar cells, making it a competitive donor material for photovoltaic device applications.

Two-dimensional ultrathin van der Waals heterostructures of indium selenide and boron monophosphide for superfast nanoelectronics, excitonic solar cells, and digital data storage devices

Semiconducting indium selenide (InSe) monolayers have drawn a great deal of attention among all the chalcogenide two-dimensional materials on account of their high electron mobility; however, they suffer from low hole mobility. This inherent limitation of an InSe monolayer can be overcome by stacking it on top of a boron phosphide (BP) monolayer, where the complementary properties of BP can bring additional benefits. The electronic, optical, and external perturbation-dependent electronic properties of InSe/BP hetero-bilayers have been systematically investigated within density functional theory in anticipation of its cutting-edge applications. The InSe/BP heterostructure has been found to be an indirect semiconductor with an intrinsic type-II band alignment where the conduction band minimum (CBM) and valence band maximum (VBM) are contributed by the InSe and BP monolayers, respectively. Thus, the charge carrier mobility in the heterostructure, which is mainly derived from the BP monolayer, reaches as high as 12 × 10³ cm² V^-1 s^-1, which is very much desired in superfast nanoelectronics. The suitable bandgap accompanied by a very low conduction band offset between the donor and acceptor along with robust charge carrier mobility, and the mechanical and dynamical stability of the heterostructure attests its high potential for applications in solar energy harvesting and nanoelectronics. The solar to electrical power conversion efficiency (20.6%) predicted in this work surpasses the efficiencies reported for InSe based heterostructures, thereby demonstrating its superiority in solar energy harvesting. Moreover, the heterostructure transits from the semiconducting state (the OFF state) to the metallic state (the ON state) by the application of a small electric field (∼0.15 V Å^-1) which is brought about by the actual movement of the bands rather than via the nearly empty free electron gas (NFEG) feature. This thereby testifies to its potential for applications in digital data storage. Moreover, the heterostructure shows strong absorbance over a wide spectrum ranging from UV to the visible light of solar radiation, which will be of great utility in UV-visible light photodetectors.

Emerging Group-VI Elemental 2D Materials: Preparations, Properties, and Device Applications

Due to the ultrathin thickness and dangling-bond-free surface, 2D materials have been regarded as promising candidates for future nanoelectronics. In recent years, group-VI elemental 2D materials have been rediscovered and found superior in electrical properties (e.g., high carrier mobility, high photoconductivity, and thermoelectric response). The outstanding semiconducting properties of group-VI elemental 2D materials enable device applications including high-performance field-effect transistors and optoelectronic devices. The excellent environmental stability also facilitates fundamental studies and practical applications of group-VI elemental 2D materials. This Review first focuses on the crystal structures of group-VI elemental 2D materials. Afterward, preparation methods for nanostructures of group-VI materials are introduced with comprehensive studies. A brief Review of the electronic structures is then presented with an understanding of the electrical properties. This Review also contains the device applications of group-VI elemental 2D materials, emphasizing transistors, photodetectors, and other appealing applications. Finally, this Review provides an outlook for the development of group-VI elemental 2D materials, highlighting the challenges and opportunities in fundamental studies and technological applications.

Promising photovoltaic efficiency of a layered silicon oxide crystal Si3O

Computational searching and screening of new functional materials exploiting Earth abundant elements can accelerate the development of their energy applications. Based on the state-of-the-art material search algorithm and ab initio calculations, we demonstrate a recently suggested stable silicon oxide with a layered structure (Si₃O) as an ideal photovoltaic material. With many-body first-principles approaches, the monolayer and layered bulk of Si₃O show direct quasiparticle gaps of 1.85 eV and 1.25 eV, respectively, while an optical gap of about 1.2 eV is nearly independent of the number of layers. Spectroscopic limited maximum efficiency (SLME) is estimated to be 27% for a thickness of 0.5 μm, making it a promising candidate for solar energy applications.

Two-Dimensional Tellurium: Progress, Challenges, and Prospects

Since the successful fabrication of two-dimensional (2D) tellurium (Te) in 2017, its fascinating properties including a thickness dependence bandgap, environmental stability, piezoelectric effect, high carrier mobility, and photoresponse among others show great potential for various applications. These include photodetectors, field-effect transistors, piezoelectric devices, modulators, and energy harvesting devices. However, as a new member of the 2D material family, much less known is about 2D Te compared to other 2D materials. Motivated by this lack of knowledge, we review the recent progress of research into 2D Te nanoflakes. Firstly, we introduce the background and motivation of this review. Then, the crystal structures and synthesis methods are presented, followed by an introduction to their physical properties and applications. Finally, the challenges and further development directions are summarized. We believe that milestone investigations of 2D Te nanoflakes will emerge soon, which will bring about great industrial revelations in 2D materials-based nanodevice commercialization.