The low conversion efficiency of thin-film silicon solar cells currently prevents them from competing, commercially, with the dominant crystalline silicon technology. The small thickness of the photo-active layer in thin-film silicon solar cells is an advantage for reducing raw material consumption and increasing industrial throughput, but results in poor light absorption at long wavelengths. A textured back reflector is used to increase the absorption of light that would otherwise escape the solar cell. The aim of this project is to fabricate a high-performing back reflector by analysing the influence of its surface texture on the light scattered by it. A structure, called the optical stack, was fabricated on a wide range of random textures to compare the light absorption in a hydrogenated nanocrystalline silicon (nc-Si:H) absorber. The plasmonic absorption at the silver back reflector in the optical stack was observed by 3-D optical modelling, for different surface textures. A qualitative analysis of the absorption in the optical stack and the plasmonic absorption in the back reflector, identified textures that are promising for increasing the light absorption in a nc-Si:H solar cell. Better light scattering and increased short-circuit current density (Jsc), compared to the reference back reflector, was demonstrated using the selected back reflectors.
In article number 1904302, Nicholas R. Glavin, Pulickel M. Ajayan, and co-workers review recent progress in applications for an exciting class of nanomaterials referred to as elemental 2D materials. The cover image depicts the main group elements of interest and lattice structures that drive unique properties for applications including electronics, optoelectronics, and energy. Image by Dr. Jo Richers (www.jorichers.com).
Abstract Two-dimensional transition metal dichalcogenides (TMDs) have been proposed for a wide variety of applications, such as neuromorphic computing, flexible field effect transistors, photonics, and solar cells, among others. However, for most of these applications to be feasible, it is necessary to integrate these materials with the current existing silicon technology. Although chemical vapor deposition is a promising method for the growth of high-quality and large-area TMD crystals, the high temperatures necessary for the growth make this technique incompatible with the processes used in the semiconductor industry. Herein, we demonstrate the possibility of low-temperature growth of TMDs, using tungsten selenide (WSe 2 ) as a model, by simply using moisture-assisted defective tungsten oxide (WO 3 ) precursor powders during the growth of these materials. Density functional theory calculations reveal the mechanism by which moisture promotes the defect formation on the precursor crystal structure and how it dictates the reduction of the temperature of the growth. The results were compared with the standard growth at high temperatures and with a precursor mixture with alkali salts to show the high quality of the WSe 2 grown at temperatures as low as 550 °C. To conclude, the work improves the understanding of nucleation and growth mechanisms of WSe 2 at low temperatures and provides a useful strategy for the growth of TMDs at temperatures required for the back-end-of-line compatibility with current silicon technology.
Two-dimensional (2D) materials from naturally occurring minerals are promising and possess interesting physical properties. A new 2D material "Ilmenene" has been exfoliated from the naturally occurring titanate ore ilmenite (FeTiO3) by employing liquid phase exfoliation in a dimethylformamide solvent by ultrasonic bath sonication. Ilmenene displays a [001] orientation that is confirmed by transmission electron microscopy. Probable charge transfer excitation from Fe2+Ti4+ to Fe3+Ti3+ results in ferromagnetic ordering along with the antiferromagnetic phase accompanied by enhanced anisotropy due to surface spins. The 2D nature and band gap states help ilmenene form a heterojunction photocatalyst with titania nanotube arrays, capable of broad spectrum light harvesting and separating/transferring the photogenerated charges effectively for solar photoelectrochemical water splitting.
Alloying in two-dimensional (2D) transition metal dichalcogenides (TMDCs) has allowed band gap engineering and phase transformation, as well as modulation of electronic properties. However, most of the efforts have been focused on alloying between transition metal cations. Among those that emphasize alloying between chalcogenide anions, the sulfide–selenide combinations are popular with a few reports on selenide–telluride combinations. In this work, we show a facile chemical vapor deposition method to obtain stable alloying between selenide and telluride anions in monolayer MoSe2(1–x)Te2x alloy. These alloys retain the monolayer 2H symmetry and show good photoluminescence and band gap tunability in the near-infrared region. The nature and percentage of alloying is further confirmed and quantified via AFM, XPS, and HAADF-STEM imaging and polarized Raman spectroscopy. The stability of the two chalcogens in the monolayer 2H lattice is also consistent with thermodynamic phase mixing via DFT simulations. The work demonstrates a straightforward method of synthesizing telluride-based 2D TMDC alloys for further studies and emerging applications.
Alloying/doping in two-dimensional material has been important due to wide range band gap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the band gap and also reduced the growth temperature. Here, we report synthesis of quaternary alloys MoxW1-xS2ySe2(1-y) using chemical vapour deposition. The composition of alloys has been tuned by changing the growth temperatures. As a result, we can tune the bandgap which varies from 1.73 eV to 1.84 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.
In article number 1703754, Aravind Krishnamoorthy, Chandra Sekhar Tiwary, Pulickel M. Ajayan, and co-workers show that Re-doping of transition-metal-dichalcogenide monolayers during chemical vapor deposition (CVD) offers a simple way to controllably tune crystal structure and phase composition in two-dimensional alloys. The CVD-synthesized MoReSe2 alloys show composition-dependent electronic structures and magnetic properties.
The discovery of ferromagnetism in atomically thin layers at room temperature widens the prospects of 2D materials for device applications. Recently, two independent experiments demonstrated magnetic ordering in two dissimilar 2D systems, CrI3 and Cr2 Ge2 Te6 , at low temperatures and in VSe2 at room temperature, but observation of intrinsic room-temperature magnetism in 2D materials is still a challenge. Here a transition at room temperature that increases the magnetization in magnetite while thinning down the bulk material to a few atom-thick sheets is reported. DC magnetization measurements prove ferrimagnetic ordering with increased magnetization and density functional theory calculations ascribe their origin to the low dimensionality of the magnetite layers. In addition, surface energy calculations for different cleavage planes in passivated magnetite crystal agree with the experimental observations of obtaining 2D sheets from non-van der Waals crystals.
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)