Light Absorption In Semiconductors Research Articles

A Photoelectrochemical and Spectroscopic Studies of Photoelectrodes with Hematite∣Catalyst∣Electrolyte Interface Toward Oxygen Evolution Reaction

One of the recent issues in the world is climate change caused by global warming. Many countries have set an ultimate goal for carbon neutrality to prevent from further increase of the global temperatures. In this regard, research on development of renewable energy sources without any carbon emissions has been intensively conducted. Among many promising chemical fuels with sustainability, Hydrogen (H2) can be produced by water (H2O) splitting.Especially, water splitting driven by photoelectrochemical reactions has great advantages in H2 generation: photovoltage generated by light absorption in semiconductors may significantly reduce the applied overpotential which should be provided by external circuits. This photoelectrochemical system, called photoelectrochemical cell (PEC) for water splitting is basically composed of photoelectrodes, catalysts, and membranes for generation of photogenerated charge carriers, reducing overpotential by improving charge transfer kinetics, and prevention of H2 and Oxygen (O2) crossover, respectively. When photons from solar light with energies greater than band gap are incident on the photoelectrodes, an electron-hole pair (EHP) is formed in the semiconductor. These EHPs are separated and transported to the surface, driven by band bending formed at the semiconductors. In photoanodes, the most of the charge carriers arrived at the electrode surface are holes (h+s), which oxidizes H2O at the photoelectrode-electrolyte interface. The transfer of holes accompanies the proton transfers, thereby producing O2. The separated electrons (e−s) should travel to the (photo)cathode along the external circuit and generate H2. However, the overall efficiency of water splitting using PEC still confronts many challenges for wide commercialization, including the low solar-to-hydrogen (STH) efficiencies. In order to improve the performance, the semiconductor efficiently utilizes the photons in the visible light region, which takes 45% of solar irradiation. In this perspective, hematite (α-Fe2O3) is one of the most promising candidates for the semiconductor as photoanodes owing to is narrow band gap of 1.9 ~ 2.2 eV, which is in the range between 550 ~ 650 nm. Meticulous cares should be also taken in catalysts localization to prevent from the parasitic photon loss, as well as the selection of catalytic materials. Because catalysts attribute to the improved charge transfer kinetics but also simultaneously become the sites that obstruct the light penetration to the semiconductor, resulting in the lower light absorption. Furthermore, physical and chemical composition of catalysts on the semiconductor surface significantly influence the charge transfer rates for the photoelectrochemical reactions by varying the electrical field gradient formed at the semiconductor∣catalyst∣electrolyte interface. As several procedures are additionally involved in the PEC, the electrochemical reaction pathways become more complicated rather than the one in electrolyzers utilizing conductive electrodes. Therefore, underlying understanding in semiconductor∣catalyst∣electrolyte interface is crucial to achieve the high performance in PECs.Herein, we demonstrate the hematite∣catalyst∣electrolyte interface as the photoanodes to investigate the photoelectrochemical reaction pathways in water oxidation. Hematite is the perfect material as the semiconductor model, not only due to the narrow band gap in the region of visible light but also the excellent stability even when exposed to the electrolytes and electrochemical environments. The hematite is fabricated by simple hydrothermal reactions, followed by step-wise heat treatment. The structure of hematite is optimized by varying hydrothermal temperatures, affecting the crystallinity and exposed area of the conductive substrate, fluorine-doped tin oxide (FTO). The recombination by back reduction is occurred at the exposed FTO surface by pinholes in hematite, which directly contact the electrolyte. Moreover, the localization of Co-based catalysts is also easily controlled by light-guided electrodeposition. In this hematite∣catalyst∣electrolyte interface, the overall water oxidation pathways are investigated by photoelectrochemical and Raman spectroscopic studies. It is expected that our study in here contributes to the in-depth understanding on the complicated photoelectrochemical reaction mechanism, might leading to the improved performance for wide commercialization of PEC.

Metasurface Photoelectrodes for Enhanced Solar Fuel Generation

AbstractTailoring optical properties in photocatalysts by nanostructuring them can help increase solar light harvesting efficiencies in a wide range of materials. Whereas plasmon resonances are widely employed in metallic catalysts for this purpose, latest advances of nonradiative, dielectric nanophotonics also enable light confinement and enhanced visible light absorption in semiconductors. Here, a design procedure for large‐scale nanofabrication of semiconductor photoelectrodes using imprint lithography is developed. Anapole excitations and metasurface lattice resonances are combined to enhance the absorption of the model material, amorphous gallium phosphide (a‐GaP), over the visible spectrum. It is shown that cost‐effective, high sample throughput is achieved while retaining the precise signature of the engineered photonic states. Photoelectrochemical measurements under hydrogen evolution reaction conditions and sunlight illumination reveal the contributions of the respective resonances and demonstrate an overall photocurrent enhancement of 5.7, compared to a planar film. These results are supported by optical and numerical analysis of single nanodisks and of the upscaled metasurface.

К ТЕОРИИ ДВУХ - И ТРЕХ ФОТОННОГО ЛИНЕЙНО - ЦИРКУЛЯРНОГО ДИХРОИЗМА В ПОЛУПРОВОДНИКАХ КУБИЧЕСКОЙ СИММЕТРИИ

The linear - circular dichroism of two and three photon absorption of light in semiconductors of cubic symmetry of hole conductivity is theoretically investigated. The matrix elements of two and three - photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. In this case, transitions associated with both non - simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficient of two and three - photon absorption of polarized radiation are determined

Design Principles for Efficient and Stable Water Splitting Photoelectrocatalysts

ConspectusPhotoelectrochemical water splitting is a promising avenue for sustainable production of hydrogen used in the chemical industry and hydrogen fuel cells. The basic components of most photoelectrochemical water splitting systems are semiconductor light absorbers coupled to electrocatalysts, which perform the desired chemical reactions. A critical challenge for the design of these systems is the lack of stability for the majority of desired semiconductors under operating water splitting conditions. One strategy to address this issue is to protect the semiconductor by covering it with a stabilizing insulator layer, creating a metal-insulator-semiconductor (MIS) architecture, which has demonstrated improved stability. In addition to enhanced stability, the insulator layer may significantly affect the electron and hole transfer, which governs the recombination rates. Furthermore, the insertion of an insulator layer leads to the introduction of additional insulator/electrocatalyst and insulator/semiconductor interfaces. These interfaces can impact the system's performance significantly, and they need to be carefully engineered to optimize the efficiencies of MIS systems. In this Account, we describe our recent progress in shedding light on the critical role of the insulator and the interfaces on the performance of MIS systems. We discuss our findings by focusing on the concrete example of planar n-type Si protected by a HfO2 insulator layer and coupled to a Ni or Ir electrocatalyst that performs the oxygen evolution reaction, one of the water splitting half-reactions. To improve our fundamental understanding of the insulator layer, we precisely control the HfO2 insulator thickness using atomic layer deposition (ALD), and we perform a series of rigorous electrochemical experiments coupled with theory and modeling. We demonstrate that by tuning the insulator thickness, we can control the flux and recombination of photogenerated electrons and holes to optimize the generated photovoltage. Despite optimizing the thickness, we find that the maximum generated photovoltage in MIS systems is often significantly lower than the upper performance limit, i.e., there are additional losses in the system that could not be addressed by optimizing the insulator thickness. We identify the sources of these losses and describe strategies to minimize them by a combination of improving the semiconductor light absorption, removing nonidealities associated with interfacial defects, and finding alternative insulators with improved charge carrier selectivity. Finally, we quantify the improvements that can be obtained by implementing these specific strategies. Our collective work outlines strategies to analyze MIS systems, identify the sources of efficiency losses, and optimize the design to approach the fundamental performance limits. These general approaches are broadly applicable to photoelectrochemical materials that utilize sunlight to produce value-added chemicals.

Two- and Three-Photon Linear Circular Dichroism in Semiconductors of Cubic Symmetry

The linear-circular dichroism of two- and three-photon absorption of light in semiconductors of hole conductivity with cubic symmetry is theoretically investigated. The matrix elements of two- and three-photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. Transitions associated with both non-simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficients of two- and three-photon absorption of polarized radiation are determined.

Noncovalent interaction stabilizes the 2,4-Dinitrophenylhydrazone Derivatives over g-C3N4 surface to enhance optical properties: Synthesis, characterization, and DFT investigation

Immobilization of organic compounds on semiconductor surface was recently exploited to enhance the semiconductor light absorption and photocatalytic performance. Here, synthesis of 2,4-Dinitrophenylhydrazone-benzaldehyde (DPHH), 2,4-Dinitrophenylhydrazone-2-thiophenecarboxyaldehyde (DPHS), 2,4-Dinitrophenylhydrazone-4-dimethylaminobenzaldehyde (DPHN) and 2,4-Dinitrophenylhydrazone-4-aminobenzoic acid (DPHB) was introduced and characterized with different spectroscopic techniques such as FT-IR and UV–Vis spectroscopy. The UV–vis absorption spectra of the hydrazone compounds were stabilized (red shifted) in polar solvents such as N,N-dimethylformamide (DMF) and acetonitrile (ACN). Density functional theory (DFT) was used to investigate the structures conformations and stabilizing forces. Doping of g-C3N4 nanosheets with 5% of the hydrazone derivatives decreases the optical band gap from 2.71 eV to 2.62 eV with DPHN derivatives. The DFT investigation coupled with reduced density gradient (RDG) show that hydrazone derivatives was stabilized on g-C3N4 by vdW forces. These results shows that the hydrazone derivatives could be potentially used as sensitizers for photocatalysis and solar cells systems.

Двух- и трехфотонный линейно-циркулярный дихроизм в полупроводниках кубической симметрии

The linear-circular dichroism of two and three photon absorption of light in semiconductors of cubic symmetry of hole conductivity is theoretically investigated. The matrix elements of two and three-photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. In this case, transitions associated with both non-simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficient of two and three-photon absorption of polarized radiation are determined.

Двух- и трехфотонный линейно-циркулярный дихроизм в полупроводниках кубической симметрии

The linear-circular dichroism of two and three photon absorption of light in semiconductors with cubic symmetry of hole conductivity is theoretically investigated. The matrix elements of two and three-photon optical transitions occurring between the subbands of the semiconductor valence band are calculated. In this case, transitions associated with both non-simultaneous absorption of individual photons and simultaneous absorption of two photons are taken into account, and the spectral and temperature dependences of the coefficient of two and three-photon absorption of polarized light are determined.

Enhancement of sub-bandgap light absorption in perovskite semiconductor films via critical coupling.

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

Enhanced Photoelectrochemical Energy Conversion in Ultrathin Film Photoanodes with Hierarchically Tailorable Mesoscale Structure

AbstractHighly porous electrode designs are often employed for photoelectrochemical energy conversion applications. “Inverse opal” structures generate high surface area electrodes to enhance light absorption in semiconductors with short carrier collection lengths, effectively increasing the optical depth of ultrathin film photoelectrodes. Here, the fabrication of hierarchically structured, “host–guest” photoelectrodes based on selective atomic layer deposition of ZnO in composite polystyrene–SiO2 nanosphere films is described. Nanostructured scaffolds for ultrathin film photoanodes are prepared with a facile, continuously tunable solution‐phase synthesis. The characteristic length scales for absorption, carrier collection, and mass transport can be independently engineered into the electrode by choosing appropriate colloidal components for the composite scaffold. 20 nm ZnO photoanode layers based on the “host–guest” architecture exhibit roughly 500 times the photocurrent generated on an equivalent planar electrode and a 430% increase over a photoanode structured by a scaffold comprised of a close‐packed assembly of identical SiO2 nanospheres. This results from an improved balance of reactant mass transport and the locus of light absorption throughout the electrode. This approach offers a facile route for preparing strategically nanostructured photoelectrodes based on strategies developed from more complex fabrication techniques.

Well-organized migration of electrons for enhanced hydrogen evolution: Integration of 2D MoS2 nanosheets with plasmonic photocatalyst by a facile ultrasonic chemical method

The construction of a plasmonic photocatalyst is an efficient way to suppress detrimental electrons-holes recombination and extend the spectral range of light absorption in semiconductors. However, the facilitation effect in the aspect of electrons-holes separation is great limited as the lack of a driving force compelled the electrons or holes migration to surface catalytic sites makes them flow randomly in semiconductors. In this work, we confirm that the integration of MoS2 nanosheets formed two dimensional (2D) layered heterojunction with C3N4 with Au-C3N4 plasmonic photocatalyst can further enhance electrons-holes separation through the formation of Au-C3N4-MoS2 nanostructure by a facile ultrasonic chemical method. The integrated MoS2 nanosheets extract the electrons not only from C3N4 due to a building up of 2D layered heterojunction but also from plasmonic Au via a “pipe” played by C3N4. The electrons “pump” role of the 2D MoS2 nanosheets makes electrons flow randomly turn into the well-organized migration direction, promoting the electrons-holes more efficient separation and lifetime prolongation. Meanwhile, MoS2 nanosheets also increase the light absorption of the photocatalyst owing to its inherent strength of the narrower band gap. Enabled by integration of 2D MoS2 nanosheets, the hydrogen production rate is 2.08 times higher than that of its counterpart Au-C3N4. This work highlights a new window to employ 2D layered heterojunction for enhanced photocatalytic hydrogen evolution performance.

Linearly and circular dichroism in a semiconductor with a complex valence band with allowance for four-photon absorption of light

This paper presents a theoretical study of the linear and circular dichroism of multiphoton absorption of light in semiconductors with a complex valence band. Matrix elements of optical transitions between subbands of the valence bands of a p-GaAs semiconductor are calculated. Transitions connected with both nonsimultaneous absorption of single photons and simultaneous absorption of two photons are taken into account. An expression for the temperature dependence of the coefficient of multiphoton absorption of polarized radiation with allowance for transitions between subbands of heavy and light holes is obtained.

Solar-Powered Plasmon-Enhanced Heterogeneous Catalysis

AbstractPhotocatalysis uses semiconductors to convert sunlight into chemical energy. Recent reports have shown that plasmonic nanostructures can be used to extend semiconductor light absorption or to drive direct photocatalysis with visible light at their surface. In this review, we discuss the fundamental decay pathway of localized surface plasmons in the context of driving solar-powered chemical reactions. We also review different nanophotonic approaches demonstrated for increasing solar-to-hydrogen conversion in photoelectrochemical water splitting, including experimental observations of enhanced reaction selectivity for reactions occurring at the metalsemiconductor interface. The enhanced reaction selectivity is highly dependent on the morphology, electronic properties, and spatial arrangement of composite nanostructures and their elements. In addition, we report on the particular features of photocatalytic reactions evolving at plasmonic metal surfaces and discuss the possibility of manipulating the reaction selectivity through the activation of targeted molecular bonds. Finally, using solar-to-hydrogen conversion techniques as an example, we quantify the efficacy metrics achievable in plasmon-driven photoelectrochemical systems and highlight some of the new directions that could lead to the practical implementation of solar-powered plasmon-based catalytic devices.

Linear-Circular Dichroism of Four-Photon Absorption of Light in Semiconductors with a Complex Valence Band

Matrix elements of optical transitions occuring between the subbands of the valence band of a p-GaAs type semiconductor are calculated. Transitions associated with the non-simultaneous absorption of single photons and simultaneous absorption of two photons are taken into account. The expressions are obtained for the average values of the square modulus of matrix elements calculated with respect to the solid angle of the wave vector of holes. Linear-circular dichroism of four-photon absorption of light in semiconductors with a complex valence band is theoretically studied.

Interband absorption of photons by extended states in intermediate band solar cells

This paper considers sub-bandgap photon absorption in an InAs/GaAs quantum dot matrix. Absorption coefficients are calculated for transitions from the extended states in the valence band to confined states in the conduction band. This completes a previous body of work in which transitions between bound states were calculated. The calculations are based on the empirical k·p Hamiltonian considering the quantum dots as parallelepipeds. The extended states may be only partially extended—in one or two dimensions—or extended in all three dimensions. It is found that extended-to-bound transitions are, in general, weaker than bound-to-bound transitions, and that the former are weaker when the initial state is extended in more coordinates. This study is of direct application to the research of intermediate band solar cells and other semiconductor devices based on light absorption in semiconductors nanostructured with quantum dots.

Self-consistent T-matrix theory of semiconductor light-absorption and luminescence

Self-consistent T-matrix theory of semiconductor light-absorption and luminescence

The Transverse Field Detector (TFD): A Novel Color-Sensitive CMOS Device

A novel color-sensitive semiconductor detector is proposed. The device [named transverse field detector (TFD)] can be used to measure the color of the incident light without any color filter. The working principle is based on the capability of this device to collect the carriers, generated at different depths, at different superficial junctions by means of suitable transverse electric fields. Due to the differences in the semiconductor light absorption coefficient at different wavelengths, this device can perform color separation. The transverse components of the electric fields are generated inside the semiconductor depleted region by a suitable bias of the superficial collecting junctions. Devices with three or more collecting junctions can be implemented. This new type of color detector takes advantages from CMOS compatibility and technology scaling. The TFD is suitable to be integrated in a pixel array for imaging purposes. First results on a test device built in a completely standard CMOS 90-nm technology are shown.

Nonlinear-optical parameters of various media

A review of investigations of nonlinear refraction and nonlinear absorption of light in semiconductors, fullerenes, dyes, metals, and crystals is given. The results of measurements of the nonlinear refraction coefficients, nonlinear absorption coefficients, saturation intensities, and nonlinear susceptibilities of these media by using laser radiation at different wavelengths and the z-scan method are presented. Methods of preparation of nanoparticles based on these media and studies of their nonlinear-optical parameters are discussed. Some media are analysed from the point of view of their application as optical limiters of laser radiation. Investigations of higher nonlinearities of some media are also considered.

Thermal characterization and determination of recombination parameters in CdTe films on glass substrates by using open photoacoustic cell technique

CdTe is a semiconductor with a wide variety of applications and perspectives for electronic industry (high-efficiency photoelectric cells, infrared radiation detectors, etc.). In the present work, we used photoacoustic (PA) technique to study the thermal properties and the surface recombination velocity in CdTe/glass samples. Experimental PA phase signal as a function of modulation frequency in a heat transmission configuration was fitted to the theoretical expression for PA signal, which takes into account the heat sources resulting from the absorption of light in semiconductors and the nonradiative processes involved, which depend on their thermal, optical and electronic transport properties. By this procedure, it was possible to determine the thermal diffusivity and the surface recombination velocity in these samples. The studied samples were thin polycrystalline CdTe film deposited on glass slides. CdTe layers were deposited by a hot-wall closed-spaced vapor transport method, known as gradient recrystallization and growth. The values for the deposition parameters used in this study were the following: 700°C for the source temperature and 450 and 550°C for the substrate temperatures with three different deposition times from 5 to 20 min (then three different film thicknesses were obtained). A clear increment in the surface velocity and surface roughness is observed as the film thickness is increased.

Solar cells

Abstract Solar cells are based on the photovoltaic effect of converting solar energy into electric energy. The mechanism for solar cells is divided into steps, that is, electron-hole pair generation by absorption of light in semiconductors, separation of electron-hole pairs by built-in potential, electron-hole recombination, collection of charge carriers by metal electrodes, etc. In this article, the principle and the theories of these basic steps are presented. On the basis of these steps, methods to improve the efficiency for solar cells are discussed. The fabrication process and the situation of currently produced solar cells are also presented. Solar cells having no p-n junction, that is, photoelectrochemical solar cells and MIS solar cells, are discussed from the perspective of low-cost solar cells.