Full-spectrum Solar Cells Research Articles

Spatially composition-graded monolayer tungsten selenium telluride

Heterogeneous materials with spatially modulated bandgaps have many unique applications, such as super-broadband nanolasers, color engineered displays, hyperspectral detectors, and full spectrum solar cells. In this work, spatially composition-graded WSe2 − 2xTe2x flakes are synthesized through an in situ chemical vapor deposition method. Furthermore, a monolayer flake topography is confirmed by atomic force microscopy. Photoluminescence and Raman line-scanning characterization indicate the bandgap changes continuously from center (1.46 eV) to edge (∼1.61 eV) within a monolayer flake. Electronic devices based on this spatially composition-graded material exhibit tunable transfer curves. First principal calculation reveals that the electron affinity increases, while the bandgap decreases based on tellurium composition. This is consistent with experimentally observed non-monotonic dependence of the hole current on tellurium composition. This work provides the experimental groundwork for synthesis of the composition-graded transition metal dichalcogenide materials and offers a route toward tailoring their electrical properties by bandgap engineering in the future.

Effects of Al doping on the structural, electrical, and optical properties of rock-salt ZnCdO thin films grown by molecular beam epitaxy

The effects of Al doping on the structural, electrical, and optical properties of rock-salt (RS)-Zn1-xCdxO (ZnCdO) thin films were investigated. The films were grown by molecular beam epitaxy under various Al cell temperatures. Reflection high-energy electron diffraction and compositional analyses indicated that single-crystal RS-ZnCdO (x ≈ 0.83) layers were obtained with up to 7.8% Al incorporation. The electron concentration in the thin films increased with increasing Al cell temperature and was 1.5 × 1021 cm−3 at ∼7.8% Al. Al-doped RS- and wurtzite (WZ)-ZnCdO thin films with various Cd concentrations were grown under the same Al cell temperatures. The ZnCdO films had an electron concentration of 1021 cm−3 for Cd content x ranging over 0.27–1.0, regardless of the crystal structure. Al-doped RS-ZnCdO films had a 3.45-eV optical band gap at x = 0.7 because of the high electron concentration that led to a Burstein-Moss shift. This was the highest-energy optical band gap reported for RS-ZnCdO. Therefore, Al doping was very effective in expanding the optical band gap, as well as increasing the RS-ZnCdO electrical conductivity, making the films suitable as transparent conductors for full-spectrum solar cells.

An integrated thermoelectric-assisted photoelectrochemical system to boost water splitting

Common solar-driven photoelectrochemical (PEC) cells for water splitting were designed by using semiconducting photoactive materials as working photoelectrodes to capture sunlight. Due to the thermodynamic requirement of 1.23 eV and kinetic energy loss of about 0.6 eV, a photo-voltage of 1.8 V produced by PEC cells is generally required for spontaneous water splitting. Therefore, the minimum bandgap of 1.8 eV is demanded for photoactive materials in single-photoelectrode PEC cells, and the bandgap of about 1 eV for back photoactive materials is appropriate in tandem PEC cells. All these PEC cells cannot effectively utilize the infrared light from 1250 to 2500 nm. In order to realize the full spectrum utilization of solar light, here, we develop a solar-driven PEC water splitting system integrated with a thermoelectric device. The key feature of this system is that the thermoelectric device produces a voltage as an additional bias for the PEC system by using the temperature difference between the incident infrared-light heated aqueous electrolyte in the PEC cell as the hot source and unirradiated external water as the cold source. Compared to a reference PEC system without the thermoelectric device, this system has a significantly improved overall water splitting activity of 1.6 times and may provide a strategy for accelerating the application of full spectrum solar light-driven PEC cells for hydrogen production.

Laser-Driven Phase Segregation and Tailoring of Compositionally Graded Microstructures in Si–Ge Nanoscale Thin Films

The ability to manipulate the composition of semiconductor alloys on demand and at nanometer-scale resolutions is a powerful tool that could be exploited to tune key properties such as the electronic band gap, mobility, and refractive index. However, existing methods to modify the composition involve altering the stoichiometry by temporal or spatial modulation of the process parameters during material growth, limiting the scalability and flexibility for device fabrication. Here, we report a laser processing method for localized tailoring of the composition in amorphous silicon-germanium (a-SiGe) nanoscale thin films on silicon substrates, postdeposition, by controlling phase segregation through the scan speed of the laser-induced molten zone. Laser-driven phase segregation at speeds adjustable from 0.1 to 100 mm s-1 allows access to previously unexplored solidification dynamics. The steady-state spatial distribution of the alloy constituents can be tuned directly by setting the laser scan speed constant to achieve indefinitely long Si1-xGex microstructures, exhibiting the full range of compositions (0 < x < 1). To illustrate the potential, we demonstrate a photodetection application by exploiting the laser-written polycrystalline SiGe microstripes, showing tunability of the optical absorption edge over a wavelength range of 200 nm. Our method can be applied to pseudobinary alloys of ternary semiconductors, metals, ceramics, and organic crystals, which have phase diagrams similar to those of SiGe alloys. This study opens a route for direct laser writing of novel devices made of alloy microstructures with tunable composition profiles, including graded-index waveguides and metasurfaces, multispectral photodetectors, full-spectrum solar cells, and lateral heterostructures.

Light trapping structures and plasmons synergistically enhance the photovoltaic performance of full-spectrum solar cells.

A full-spectrum solar cell exhibits potential as an effective strategy to enhance the absorption of incident solar light. To ensure the absorption capability of solar cells, trapping structures or plasmons have emerged as two main ways of utilizing the full spectrum of solar energy. First, recent progress in the full-spectrum solar cells based on NCs was reviewed from the aspects of trapping structures and plasmon design. Moreover, the effects of light trapping and surface plasmon resonance on light absorption and photoelectronic conversion were emphasized and discussed. Finally, the application prospect of their combination in the field of full-spectrum solar cells was examined. It was pointed out that the deep exploration of the physical mechanism of photoelectric conversion, controllable preparation of the interface and stability of composite structures will become the main directions of future research.

Surface and Quantum Effects in Nanosized Semiconductor

Novel properties of nano-scale semiconductors based on the surface and quantum effects have been studied and applications identified. Spherical potential well model is used to study quantum effect whereas basic geometrical models are used for the surface effect. We have shown such effects to be the fundamental factors responsible for the novel nanosized semiconductor characteristics different from the bulk of same material. It is found that the surface area to volume ratio follows inverse power law. Thus at nanoscale, the surface to volume ratio increases significantly to enhance chemical reactivity. In addition, the increased surface area makes most nananocrystals highly soluble in liquid and dramatically lowers their melting temperature. The result strongly suggests also that the shape of the nanoparticles influences the surface area which has huge impact on their properties and performance. Our results of quantum size effect reveal that spatial confinement of charge carriers within semiconductor nanocrystals significantly modulates their properties such as size dependent absorption and emission spectra with non-zero discrete electronic transition energies as well as their blue shift band gaps. Thus by changing the size of the particle, we can literally fine-tune a material property of interest such as optical, electrical, and surface area. Specifically we found that InAs and InSb nano semiconductor optical absorption spectrum, in contrast to their bulk, can be tuned in broad range of UV to IR regions which are favorable operating wavelengths for nano photonic technology such as IR photo detectors and full spectrum solar cells applications.

First-principles study on electronic structure and photoelectric properties of zinc blende InxGa1-xN with different in doping concentrations

The first-principles ultra-soft pseudopotential plane wave based on density functional theory is used to calculate the electronic structure and photoelectric properties of zinc blende GaN and zinc blende InxGa1-xN with different In doping concentrations (x=0, 0.125, 0.25, 0.375, 0.5). It is shown that zinc blende GaN and its In-doped system are all direct bandgap semiconductor materials. With the increase of In doping concentration, the lattice constant of InxGa1-xN increases and the band gap decreases, the absorption spectrum shift to the red region. Therefore, it can be inferred that by adjusting the doping concentration of In, the light absorption range of InxGa1-xN can cover the entire solar spectrum, which means that InxGa1-xN can be worthy to fabricate photovoltaic devices such as full-spectrum solar cells.

(Invited) Semiconductor Nanowires and Nanosheets for Extremely Widely Tunable Lasing

One of the greatest advantages of semiconductor nanomaterials is the ability to grow very dissimilar materials together or on a given substrate, due to the larger tolerance to lattice mismatching than thin film growth. This advantage can be explored for a wide array of applications such as full spectrum solar cells, full-color emission, and widely tunable lasing. In this talk, we will review the most recent progress in the growth and applications of semiconductor nanowires and nanosheets in producing extremely widely tunable lasing from a given substrate, or from a given nanostructure. Both material growth and device characterization will be discussed.

MBE growth of entire-indium-composition-tunable In&amp;lt;italic&amp;gt;&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;&amp;lt;/italic&amp;gt;Ga&amp;lt;sub&amp;gt;1-&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;N alloy

As a direct band-gap semiconductor, ternary nitride alloy In x Ga1- x N has a tunable energy band-gap from 3.43 eV (for GaN) to 0.64 eV (for InN) with increasing In content, which covers the spectrum from 0.36 to 1.9 μm. In x Ga1- x N alloy has high electron drift velocity and absorption coefficient, which shows great potential application in devices such as full spectrum solar cells and light-emitting devices. Due to lack of proper substrate, In x Ga1- x N is usually grown on sapphire or GaN template. In this article, we review the growth and properties of InN and InGaN with tuable In composition grown on sapphire substrate and GaN/sapphire template by MBE. A boundary-temperature-controlled method is proposed, which successfully lead to a high-electron-mobility InN layers on sapphire substrate by MBE. The temperature-controlled epitaxy method used to grow In x Ga1- x N layers with tunable In composition on GaN/sapphire template.

Properties of GaN-Related Epitaxial Thin Films Grown on Sapphire Substrates as Transparent Conducting Electrodes

Thin films of gallium nitride (GaN) and related nitride materials were prepared, and their properties as transparent conducting electrodes were investigated. GaN thin films were directly grown on sapphire single crystal substrates by the molecular beam epitaxy. Heavy doping of germanium was employed to reduce resistivity of the films, with sufficient reduction found to be possible while maintaining their epitaxial growth state. Optical transmission spectra of the films in the short wavelength region were slightly deteriorated by the heavy doping; however, this was successfully improved by growing GaN films under metal-rich conditions to increase the electron mobility and suppress unwanted increase of the carrier densities. In addition, the optical transmission spectra in the short wavelength region was improved also by alloying GaN with aluminum nitride, though the resistivities of these films were relatively higher than those of the unmodified GaN films. The prepared nitride thin films exhibited sufficiently suitable properties as transparent conducting electrodes for use in applications such as full-spectrum nitride-based solar cells.

Preparation of Narrow Band-Gap Cu2Sn(S,Se)3 and Fabrication of Film by Non-Vacuum Process

We successfully prepared a Cu2Sn(S1-xSex)3 (CTSSe) solid solution with 0≤x≤1.0. CTSSe solid solution powders were synthesized by mixing the elemental powders and post-annealing at 600 °C. The crystal structure of Cu2SnS3 (CTS) was characterized by Rietveld refinement of the powder X-ray diffraction data and determined to be a monoclinic crystal system. The band gaps of CTSSe solid solution were determined by the diffuse reflectance spectra of the powder samples and the transmittance spectrum of the film fabricated by a non-vacuum thin-film fabrication process called printing and high-pressure sintering (PHS). The band gap (Eg) of CTS is 0.87 eV, which is in good agreement with the recently reported value of monoclinic CTS film. The band gap of the Cu2Sn(S1-xSex)3 solid solution linearly decreases from 0.87 eV (x = 0.0) to 0.67 eV (x = 0.6) with increasing Se content. The CTSSe solid solution has potential as a narrow band-gap absorber material for thin-film full spectrum solar cells.

ChemInform Abstract: Composition and Bandgap‐Graded Semiconductor Alloy Nanowires

AbstractReview: 95 refs.

F404 薄膜フルスペクトル太陽電池開発における材料設計(F4.物質・材料と計算力学～無機・有機・ナノ材料の新展開～,フォーラム)

F404 薄膜フルスペクトル太陽電池開発における材料設計(F4.物質・材料と計算力学～無機・有機・ナノ材料の新展開～,フォーラム)

Composition and Bandgap‐Graded Semiconductor Alloy Nanowires

Semiconductor alloy nanowires with spatially graded compositions (and bandgaps) provide a new material platform for many new multifunctional optoelectronic devices, such as broadly tunable lasers, multispectral photodetectors, broad-band light emitting diodes (LEDs) and high-efficiency solar cells. In this review, we will summarize the recent progress on composition graded semiconductor alloy nanowires with bandgaps graded in a wide range. Depending on different growth methods and material systems, two typical nanowire composition grading approaches will be presented in detail, including composition graded alloy nanowires along a single substrate and those along single nanowires. Furthermore, selected examples of applications of these composition graded semiconductor nanowires will be presented and discussed, including tunable nanolasers, multi-terminal on-nanowire photodetectors, full-spectrum solar cells, and white-light LEDs. Finally, we will make some concluding remarks with future perspectives including opportunities and challenges in this research area.

Semiconductor Alloy Nanowires and Nanobelts With Tunable Optical Properties

Recent advancements in the study of alloy semiconductor nanowires (NWs) and nanobelts are reviewed with a special perspective on their applications in optoelectronics with widely tunable bandgaps. Special emphasis is on the composition-graded alloy NWs. An extremely wide range of alloy compositions (thus bandgaps) can be achieved on a single substrate in a single growth run, creating an unprecedented materials capability that is not possible with planar epitaxial growth. Applications of such unique materials in widely tunable lasers and full-spectrum solar cells are discussed.

Present Status and Future Prospects of Silicon Thin-Film Solar Cells

In this report, an overview of the recent status of photovoltaic (PV) power generation is first presented from the viewpoint of reducing CO2 emission. Next, the Japanese roadmap for the research and development (R&D) of PV power generation and the progress in the development of various solar cells are explained. In addition, the present status and future prospects of amorphous silicon (a-Si) thin-film solar cells, which are expected to enter the stage of full-scale practical application in the near future, are described. For a-Si single-junction solar cells, the conversion efficiency of their large-area modules has now reached 6–8%, and their practical application to megawatt solar systems has started. Meanwhile, the focus of R&D has been shifting to a-Si and microcrystalline silicon (µc-Si) tandem solar cells. Thus far, a-Si/µc-Si tandem solar cell modules with conversion efficiency exceeding 13% have been reported. In addition, triple-junction solar cells, whose target year for practical application is 2025 or later, are introduced, as well as innovative thin-film full-spectrum solar cells, whose target year of realization is 2050.

Spatial Composition Grading of Quaternary ZnCdSSe Alloy Nanowires with Tunable Light Emission between 350 and 710 nm on a Single Substrate

We demonstrated a general methodology of growing spatially composition-controlled alloys by combining spatial source reagent gradient with a temperature gradient. Using this dual gradient method, we achieved for the first time a continuous spatial composition grading of single-crystal quaternary Zn(x)Cd(1-x)S(y)Se(1-y) alloy nanowires over the complete band gap range along the length of a substrate. The band gap grading spans between 3.55 eV (ZnS) and 1.75 eV (CdSe) on a single substrate, with the corresponding light emission over the entire visible spectrum. We also showed that the dual gradient method can be extended to achieve alloy composition control in two spatial dimensions. The unique material platform achieved will open a wide range of applications from color engineered display and lighting, full spectrum solar cells, multispectral detectors, or spectrometer on-a-chip to superbroadly tunable nanolasers. The growth methodology can be extended more generally to other alloy systems.

Quaternary Alloy Semiconductor Nanobelts with Bandgap Spanning the Entire Visible Spectrum

We used an improved cothermal evaporation route for the first time to achieve quaternary semiconductor nanostructured alloys, using an example of Zn(x)Cd(1-x)S(y)Se(1-y) nanobelts. The PL (bandgap) of these as-grown nanostructured alloys can be continuously tunable across the entire visible spectrum through experimentally controlling their compositions. Such widely controlled alloy nanostructures via composition/light emission provide a new material platform for applications in wavelength-tunable lasers, multicolor detectors, full-spectrum solar cells, LEDs, and color displays.

Characteristics of High In-Content InGaN Alloys Grown by MOCVD

InN and In0.46Ga0.54N films are grown on sapphire with aGaN buffer by metalorganic chemical vapour deposition (MOCVD). Bothhigh resolution x-ray diffraction and high resolution transmissionelectron microscopy results reveal that these films have a hexagonalstructure of single crystal. The thin InN film has a high mobility of475 cm2V−1s−1 and that ofIn0.46Ga0.54N is 163 cm2V−1s−1.Room-temperature photoluminescence measurement of the InN film shows apeak at 0.72 eV, confirming that a high quality InN film is fabricatedfor applications to full spectrum solar cells.