Graphene/Si Schottky solar cells: a review of recent advances and prospects

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nano machining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing that they are the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depth is discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Solution-Processed Heterojunction Photodiodes Based on WSe2 Nanosheet Networks

Solution-processed photodetectors incorporating liquid-phase-exfoliated transition metal dichalcogenide nanosheets are widely reported. However, previous studies mainly focus on the fabrication of photoconductors, rather than photodiodes which tend to be based on heterojunctions and are harder to fabricate. Especially, there are rare reports on introducing commonly used transport layers into heterojunctions based on nanosheet networks. In this study, a reliable solution-processing method is reported to fabricate heterojunction diodes with tungsten selenide (WSe2 ) nanosheets as the optical absorbing material and PEDOT: PSS and ZnO as injection/transport-layer materials. By varying the transport layer combinations, the obtained heterojunctions show rectification ratios of up to ≈104 at ±1 V in the dark, without relying on heavily doped silicon substrates. Upon illumination, the heterojunction can be operated in both photoconductor and photodiode modes and displays self-powered behaviors at zero bias.

Changes in the Raman Spectrum of Monolayer Graphene under Compression/Stretching Strain in Graphene/Piezoelectric Crystal Structures

Results from studying the effect of an applied electric voltage on the Raman spectrum of graphene deposited on a lithium niobate crystal substrate with a ferroelectric domain structure are presented. The use of the principal component method for data processing in combination with correlation analysis made it possible to reveal the contribution to the change in the spectra associated with the linear deformation of the substrate due to the inverse piezoelectric effect. An effect of the graphene coating peeling was found. Furthermore, bending deformations of the graphene coating associated with the presence of a relief on the substrate were found. An analysis of the change in the spectra of graphene under the application of an electric voltage made it possible to determine the height of this relief.

Hybrid thermionic-photovoltaic converter with graphene-on-semiconductor heterojunction anode for efficient electricity generation

Thermionic energy converters are solid-state heat engines to produce electricity with significant potential, whereas the output voltage is constrained by the work function difference between cathode and anode. In this work, we originally apply a graphene-on-semiconductor heterojunction anode to a thermionic-photovoltaic (TIPV) converter to output additional voltage. Thermionic electrons are injected into the graphene layer and then recombined with photogenerated holes. Photogenerated electrons are extracted from the conduction band and reinjected into the cathode through an external load. A proof-of-concept demonstration of the TIPV converter is developed with barium surface-engineered cathode and anode. Open-circuit voltage is increased from ∼0.9 to ∼1.9 V by comparing with an identical configuration without graphene layer. The TIPV converter yields a power generation density of 2.7 kW/m² with an electronic efficiency of ∼27%. This work paves the way for the development of TIPV converter toward high power density.

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A novel thermionic energy converter is originally proposed and measured

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Open-circuit voltage is increased from 0.9 to 1.9 V by the photovoltaic effect

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A power density of 2.7 kW/m² with an electronic efficiency of ∼27% is measured

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Ultralow work function is obtained by forming barium surface dipoles

The Graphene Structure’s Effects on the Current-Voltage and Photovoltaic Characteristics of Directly Synthesized Graphene/n-Si(100) Diodes

Graphene was synthesized directly on Si(100) substrates by microwave plasma-enhanced chemical vapor deposition (MW-PECVD). The effects of the graphene structure on the electrical and photovoltaic properties of graphene/n-Si(100) were studied. The samples were investigated using Raman spectroscopy, atomic force microscopy, and by measuring current–voltage (I-V) graphs. The temperature of the hydrogen plasma annealing prior to graphene synthesis was an essential parameter regarding the graphene/Si contact I-V characteristics and photovoltaic parameters. Graphene n-type self-doping was found to occur due to the native SiO₂ interlayer at the graphene/Si junction. It was the prevalent cause of the significant decrease in the reverse current and short-circuit current. No photovoltaic effect dependence on the graphene roughness and work function could be observed.

Engineering sensitivity and spectral range of photodetection in van der Waals materials and hybrids

Exploration of van der Waals heterostructures in the field of optoelectronics has produced photodetectors with very high bandwidth as well as ultra-high sensitivity. Appropriate engineering of these heterostructures allows us to exploit multiple light-to-electricity conversion mechanisms, ranging from photovoltaic, photoconductive to photogating processes. These mechanisms manifest in different sensitivity and speed of photoresponse. In addition, integrating graphene-based hybrid structures with photonic platforms provides a high gain-bandwidth product, with bandwidths ≫1 GHz. In this review, we discuss the progression in the field of photodetection in 2D hybrids. We emphasize the physical mechanisms at play in diverse architectures and discuss the origin of enhanced photoresponse in hybrids. Recent developments in 2D photodetectors based on room temperature detection, photon-counting ability, integration with Si and other pressing issues, that need to be addressed for these materials to be integrated with industrial standards have been discussed.

Graphene Oxide Layer-by-Layer Films for Sensors and Devices

Layer-by-layer films of poly (allylamine hydrochloride) (PAH) and graphene oxide (GO) were characterized, looking at growth with the number of bilayers, morphology, and electrical properties. The PAH/GO films revealed a linear increase in absorbance with the increase in the number of deposited bilayers, allowing the determination that 10.7 ± 0.1 mg m-2 of GO is adsorbed per unit of area of each bilayer. GO absorption bands at 146, 210, 247 and 299 nm, assigned to π-π* and n-π* transitions in the aromatic ring (phenol) and of the carboxylic group, respectively, were characterized by vacuum ultraviolet spectroscopy. The morphological characterization of these films demonstrated that they are not completely uniform, with a bilayer thickness of 10.5 ± 0.7 nm. This study also revealed that the films are composed of GO and/or PAH/GO fibers and that GO is completely adsorbed on top of PAH. The electrical properties of the films reveal that PAH/GO films present a semiconductor behavior. In addition, a slight decrease in conduction was observed when films were prepared in the presence of visible light, likely due to the presence of oxygen and moisture that contributes to the damage of GO molecules.

Inverted perovskite solar cells with enhanced lifetime and thermal stability enabled by a metallic tantalum disulfide buffer layer

Perovskite solar cells (PSCs) have proved their potential for delivering high power conversion efficiencies (PCE) alongside low fabrication cost and high versatility. The stability and the PCE of PSCs can readily be improved by implementing engineering approaches that entail the incorporation of two-dimensional (2D) materials across the device's layered configuration. In this work, two-dimensional (2D) 6R-TaS₂ flakes were exfoliated and incorporated as a buffer layer in inverted PSCs, enhancing the device's PCE, lifetime and thermal stability. A thin buffer layer of 6R-TaS₂ flakes was formed on top of the electron transport layer to facilitate electron extraction, thus improving the overall device performance. The optimized devices reach a PCE of 18.45%, representing a 12% improvement compared to the reference cell. The lifetime stability measurements of the devices under ISOS-L2, ISOS-D1, ISOS-D1I and ISOS-D2I protocols revealed that the TaS₂ buffer layer retards the intrinsic, thermally activated degradation processes of the PSCs. Notably, the devices retain more than the 80% of their initial PCE over 330 h under continuous 1 Sun illumination at 65 °C.

Catalyst-Less and Transfer-Less Synthesis of Graphene on Si(100) Using Direct Microwave Plasma Enhanced Chemical Vapor Deposition and Protective Enclosures

In this study, graphene was synthesized on the Si(100) substrates via the use of direct microwave plasma-enhanced chemical vapor deposition (PECVD). Protective enclosures were applied to prevent excessive plasma etching of the growing graphene. The properties of synthesized graphene were investigated using Raman scattering spectroscopy and atomic force microscopy. Synthesis time, methane and hydrogen gas flow ratio, temperature, and plasma power effects were considered. The synthesized graphene exhibited n-type self-doping due to the charge transfer from Si(100). The presence of compressive stress was revealed in the synthesized graphene. It was presumed that induction of thermal stress took place during the synthesis process due to the large lattice mismatch between the growing graphene and the substrate. Importantly, it was demonstrated that continuous horizontal graphene layers can be directly grown on the Si(100) substrates if appropriate configuration of the protective enclosure is used in the microwave PECVD process.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

On-site growth method of 3D structured multi-layered graphene on silicon nanowires

An experimental method is described in which a orderly 3D array of graphene sheets is grown to conform to the shape of an underlying nanowire (NW) substrate that remains on-site. The procedure uses a sacrificial nickel catalyst-based CVD growth process that is capable of producing graphene onto an insulating SiO₂ substrate. Nano-imprint silicon NWs serve both as the scaffolding for the catalyst and as the final underlying substrate. The graphene is polycrystalline and multi-layered as expected from this nickel catalyzed growth method. This presents a novel and quick method that can be used to produce conductive graphene sheets in precise shapes and configurations seen in complex device applications but which are difficult to produce with current transfer methods. The geometry of the nanostructured substrate itself contributes to the on-site growth method by making it difficult for the graphene to wash off during wet etching. The SiNWs used in this research have increased surface area and a light trapping effect that, in combination with the graphene, can be used in future sensor and photovoltaic device applications.

Graphene/Semiconductor Heterostructure Wireless Energy Harvester through Hot Electron Excitation

Recharging the batteries by wireless energy facilitates the long-term running of the batteries, which will save numerous works of battery maintenance and replacement. Thus, harvesting energy form radio frequency (RF) waves has become the most promising solution for providing the micropower needed for wireless sensor applications, especially in a widely distributed 4G/5G wireless network. However, the current research on rectenna is mainly focused on the integrated antenna coupled with metal-insulator-metal tunneling diodes. Herein, by adopting the plasmon excitation of graphene and quantum tunneling process between graphene and GaAs or GaN, we demonstrated the feasibility of harvesting energy from the 915 MHz wireless source belonging to 5G in the FR1 range (450 MHz-6 GHz) which is also known as sub-6G. The generated current and voltage can be observed continuously, with the direction defined by the built-in field between graphene and GaAs and the incident electromagnetic waves treated as the quantum energy source. Under the RF illumination, the generated current increases rapidly and the value can reach in the order of 10^-8-10^-7 A. The harvester can work under the multiple channel mode, harvesting energy simultaneously from different flows of wireless energy in the air. This research will open a new avenue for wireless harvesting by using the ultrafast process of quantum tunneling and unique physical properties of graphene.

Materials for Photovoltaics: State of Art and Recent Developments

In recent years, photovoltaic cell technology has grown extraordinarily as a sustainable source of energy, as a consequence of the increasing concern over the impact of fossil fuel-based energy on global warming and climate change. The different photovoltaic cells developed up to date can be classified into four main categories called generations (GEN), and the current market is mainly covered by the first two GEN. The 1GEN (mono or polycrystalline silicon cells and gallium arsenide) comprises well-known medium/low cost technologies that lead to moderate yields. The 2GEN (thin-film technologies) includes devices that have lower efficiency albeit are cheaper to manufacture. The 3GEN presents the use of novel materials, as well as a great variability of designs, and comprises expensive but very efficient cells. The 4GEN, also known as "inorganics-in-organics", combines the low cost/flexibility of polymer thin films with the stability of novel inorganic nanostructures (i.e., metal nanoparticles and metal oxides) with organic-based nanomaterials (i.e., carbon nanotubes, graphene and its derivatives), and are currently under investigation. The main goal of this review is to show the current state of art on photovoltaic cell technology in terms of the materials used for the manufacture, efficiency and production costs. A comprehensive comparative analysis of the four generations is performed, including the device architectures, their advantages and limitations. Special emphasis is placed on the 4GEN, where the diverse roles of the organic and nano-components are discussed. Finally, conclusions and future perspectives are summarized.