Showing papers on "Polycrystalline silicon published in 2020"

High-Efficiency Silicon Heterojunction Solar Cells: Materials, Devices and Applications

Abstract: Photovoltaic (PV) technology offers an economic and sustainable solution to the challenge of increasing energy demand in times of global warming. The world PV market is currently dominated by the homo-junction crystalline silicon (c-Si) PV technology based on high temperature diffused p-n junctions, featuring a low power conversion efficiency (PCE). Recent years have seen the successful development of Si heterojunction technologies, boosting the PCE of c-Si solar cells over 26%. This article reviews the development status of high-efficiency c-Si heterojunction solar cells, from the materials to devices, mainly including hydrogenated amorphous silicon (a-Si:H) based silicon heterojunction technology, polycrystalline silicon (poly-Si) based carrier selective passivating contact technology, metal compounds and organic materials based dopant-free passivating contact technology. The application of silicon heterojunction solar cells for ultra-high efficiency perovskite/c-Si and III-V/c-Si tandem devices is also reviewed. In the last, the perspective, challenge and potential solutions of silicon heterojunction solar cells, as well as the tandem solar cells are discussed.

TFT Channel Materials for Display Applications: From Amorphous Silicon to Transition Metal Dichalcogenides

Abstract: As the need for super-high-resolution displays with various form factors has increased, it has become necessary to produce high-performance thin-film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a-Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light-emitting diode displays, and also to overcome the performance and reliability issues of a-Si:H, low-temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a-Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next-generation channel materials is discussed.

Photovoltaic efficiency enhancement of polycrystalline silicon solar cells by a highly stable luminescent film

Abstract: Si-based solar cells have dominated the entire photovoltaic market, but remain suffering from low power conversion efficiency (PCE), partly because of the poor utilization of ultraviolet (UV) light. Europium(III) (Eu3+) complexes with organic ligands are capable of converting UV light into strong visible light, which makes them ideal light converter to increase the efficiency of solar cells. However, the low stability of such complexes seriously hampers their practical applications. In this work, we report a highly stable and luminescent ethylene-vinyl acetate (EVA) copolymer film consisting of a Eu3+ complex as a down-shift material, Eu(ND)4CTAC (ND = 4-hydroxy-2-methyl-1,5-naphthyridine-3-carbonitrile, CTAC = hexadecyl trimethyl ammonium chloride), coating of which onto the surface of large area polycrystalline silicon solar cells (active area: 110 cm2) results in an increase of PCE from 15.06% to 15.57%. Remarkable stability of the luminescent film was also demonstrated under light-soaking test for 500 h, and no obvious luminescence degradation can be observed. The remarkable enhancement of the conversion efficiency by 0.51% absolute on such a large active area, together with the high stability of the luminescent film, demonstrates a prospect for the implementation of the films in photovoltaic industry.

Effect of Surface Texture on Pinhole Formation in SiOx-Based Passivated Contacts for High-Performance Silicon Solar Cells.

Abstract: High-efficiency silicon solar cells rely on some form of passivating contact structure to reduce recombination losses at the crystalline silicon surface and losses at the metal/Si contact interface. One such structure is polycrystalline silicon (poly-Si) on oxide, where heavily doped poly-Si is deposited on a SiOx layer grown directly on the crystalline silicon (c-Si) wafer. Depending on the thickness of the SiOx layer, the charge carriers can cross this layer by tunneling ( 2 nm). In this work, we study structures with tunneling- or pinhole-like SiOx contacts grown on pyramidally textured c-Si wafers and expose variations in the SiOx layer properties related to surface morphology using electron-beam-induced current (EBIC) imaging. Using EBIC, we identify and mark regions with potential pinholes in the SiOx layer. We further perform high-resolution transmission electron microscopy on the same areas, thus allowing us to directly correlate locally enhanced carrier collection with variations in the structure of the SiOx layer. Our results show that the pinholes in the SiOx layer preferentially form in different locations based on the annealing conditions used to form the device. With greater understanding of these processes and by controlling the surface texture geometry, there is potential to control the size and spatial distribution of oxide disruptions in silicon solar cells with poly-Si on oxide-type contacts; usually, this is a random phenomenon on polished or planar surfaces. Such control will enable us to consistently produce high-efficiency devices with low recombination currents and low junction resistances using this contact structure.

Use of carrier injection engineering to increase the light intensity of a polycrystalline silicon avalanche mode light-emitting device

Abstract: This paper demonstrates a polycrystalline silicon avalanche mode light-emitting device. The unique N+PN+PN+ cascade structure is designed to enhance light intensity via carrier injection engineering, in which the minority carriers are injected from the forward-biased junction to the light emission junction. Visible light can be observed at the reverse-biased PN junctions when the device operating voltage exceeds 20 V. In particular, the phonon-assisted indirect interband recombination of carriers with excess energy may be the main mechanism of photon emission. A specific junction model is proposed to explain that the light intensity peaks are generated primarily via carrier injection. Comparing the spectral measurements of a single polysilicon N+P junction device and the proposed cascade device shows that the strategy of improving the luminous intensity via carrier injection engineering is feasible and effective.

Influence of ultrathin gahnite anti-reflection coating on the power conversion efficiency of polycrystalline silicon solar cell

Abstract: Current research has concentrated on the development of ZnAl2O4 (gahnite) spinel nanostructure through anti-reflection coating (ARC) material for improved power conversion efficiency (PCE) of polycrystalline silicon solar cells. Radio frequency magnetron sputtering technique was adopted to deposit transparent polycrystalline gahnite nano-microfilms at room temperature. Material deposition was performed in a pure argon atmosphere on polycrystalline silicon solar cell substrates with a coating duration of 5–45 min. The influence of gahnite spinel nanostructure-integrated coating on the efficiency of silicon solar cell was explored by investigating physical, electrical, optical characteristics and temperature distribution profiles. The synthesized ARC material has gahnite spinel crystal structure composed of two-dimensional (2D) nanosheets. Atomic force microscopy study revealed that the thickness of synthesized gahnite 2D nanosheets was about 50 nm. The resistivity of gahnite coated with the time duration of 35 (T-IV) minutes on silicon solar cell was measured to be 1.93 × 10−3 Ω cm. The nano-microfilms showed a great optical transmittance (97%) in the wavelength range of 300–800 nm. The maximum PCE of 21.27% at open atmospheric condition and 23.83% at controlled atmospheric condition had been achieved for 35 (T-IV) minutes of gahnite nano-microfilm coating and it has been proved that gahnite nano-microfilms assists the absorption of more photons on a polycrystalline silicon solar cell substrate. The results acquired indicate that the gahnite nano-microfilm is an appropriate ARC material for polycrystalline silicon solar cells to enhance the PCE.

Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells

Abstract: A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.

Comparison of Monocrystalline and Polycrystalline Solar Modules

Abstract: As the typical representative of clean energy, solar energy generating systems has the characteristics of long development history, low manufacturing cost and high efficiency, and so on. Polycrystalline silicon modules and monocrystalline silicon modules have become the mainstream products in the photovoltaic market. Based on the comparisons of the microstructure, macrostructure and physicochemical properties, we can draw the following conclusions: monocrystalline silicon cells have the advantages of perfect lattice structure, high material purity, low grain boundary energy, weak internal resistance, and high efficiency, meanwhile, the monocrystalline modules show the good aesthetic value because of the color uniform and no spots. At present, the polycrystalline and monocrystalline modules are mainly used in the rooftop or ground photovoltaic systems, the monocrystalline module has the good power generation yield and save the cost of land or rooftop with the same installed capacity. The actual power generation yield of monocrystalline is higher than polycrystalline with the same amount of modules. If the PERC monocrystalline modules are used in solar energy project, more power generation yield will be generated than traditional modules.

Isolating p- and n-Doped Fingers With Intrinsic Poly-Si in Passivated Interdigitated Back Contact Silicon Solar Cells

Abstract: Polycrystalline silicon on silicon oxide (poly - Si/SiO x ) passivating contacts enable ultrahigh-efficiency interdigitated back contact silicon solar cells. To prevent shunt between n- and p-type-doped fingers, an insulating region is required between them. We evaluate the use of intrinsic poly - Si for this isolation region. Interdigitated fingers were formed by plasma deposition of doped hydrogenated amorphous silicon through mechanically aligned shadow masks on top of a full-area intrinsic hydrogenated amorphous silicon ( a -Si:H) layer. High-temperature annealing then crystallized the a- Si:H to poly - Si and drove in the dopants. Two mechanisms were identified which cause contamination of the intrinsic poly - Si gap during processing. During deposition of doped fingers, we show using secondary ion mass spectrometry and conductivity measurements that the intrinsic gap becomes contaminated by doped a- Si:H tails several nanometers thick to concentrations of ∼1020 cm−3. Another source of contamination occurs during high-temperature annealing, where dopants desorb from doped regions and readsorb onto intrinsic a- Si:H. Both pathways reduce the resistivity of the intrinsic gap from ∼105 to ∼10−1 Ω·cm. We show that plasma etching of the a- Si:H surface before crystallizing with a capping layer can eliminate the contamination of the intrinsic poly-Si, maintaining a resistivity of ∼105 Ω·cm. This demonstrates masked plasma deposition as a dopant patterning method for Si solar cells.

Suppression of Edge Effect Induced by Positive Gate Bias Stress in Low-Temperature Polycrystalline Silicon TFTs With Channel Width Extension Over Source/Drain Regions

Abstract: This study demonstrated that the edge effect induced by positive gate bias stress (PBS) was effectively eliminated by applying channel width extensions over source/drain regions in low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs). After PBS, a stress-induced hump current in subthreshold region is observed and is caused by the charge trapping into parasitic transistors along the channel width edges. This phenomenon is attributed to carrier injection occurring along the width edges since a high electric field is located at that region, where lots of defects are generated due to the definition of the active area during the etching process. In this study, we show that the degradation in electrical characteristics during the gate bias operation resulted from the edge effect can be eliminated when the TFT devices utilize the channel width extension in the active layer. Electrical measurements, numeral calculations, and a TCAD electrical simulation confirm that this geometry, which includes a channel extension along the width direction, can dramatically reduce the carrier injection at the edges even though high electric field is mainly located at the width edges during gate bias operation.

Influence of the Carbon Concentration on ( p ) Poly-SiC x Layer Properties With Focus on Parasitic Absorption in Front Side Poly-SiC x /SiO x Passivating Contacts of Solar Cells

Abstract: Passivating contacts based on polycrystalline silicon (poly-Si) on an interfacial oxide are limited by parasitic absorption, which may be reduced by incorporation of foreign elements in the poly-Si layer. In this study, the influence of carbon incorporation in the concentration range of 6.9–21.5 at% on boron-doped polycrystalline silicon carbide (poly-SiC x ) layer properties is investigated and interpreted in the context of an application as full-area passivating contact on the front side of a solar cell. For constant annealing parameters, higher carbon concentrations reduce the crystallinity of the layers. A high crystallinity in turn is confirmed to be a key parameter for the application in a solar cell as it ensures both low resistivity as well as low parasitic absorption. Low recombination current densities in the range of 7.2–12.2 fA/cm2 are determined for all layers on interfacial oxides on planar surfaces, whereas the differences are rather related to variations in the boron concentration than to the carbon concentration or the deposition parameters. A reduction of the ( p ) poly-SiC x layer thickness down to 10 nm would yield a parasitic absorption current density of 1.13 ± 0.13 mA/cm2. Using this value and the lowest measured recombination current density, a simple model predicts a theoretical solar cell efficiency limit of 26.7 ± 0.2%.

Enhancement of Mechanical Bending Stress Endurance Using an Organic Trench Structure in Foldable Polycrystalline Silicon TFTs

Abstract: This letter proposes a new organic trench structure which can enhance the endurance of polycrystalline silicon thin film transistors (TFTs) under mechanical bending stress. It compares conventional structure TFTs to devices with an organic trench to examine reliability after undergoing 100,000 iterations of channel width-axis (100,000’WC) compression at R = 2mm. Due to the low Young’s modulus of organic materials, stress while undergoing bending strain is desorbed to the buffer layer, which can lower the overall stress in TFTs. Furthermore, an optimal location and ideal length for such an organic trench is evaluated by mechanical bending simulation.

Resilience of Fluorinated Indium-Gallium-Zinc Oxide Thin-Film Transistor Against Hydrogen-Induced Degradation

Abstract: Thin-film transistors (TFTs) based on amorphous indium-gallium-zinc oxide (IGZO) with or without plasma fluorination treatment were fabricated and the sensitivity of their characteristics to hydrogen exposure was compared. Consistent with the lower hydrogen content revealed using secondary ion-mass spectrometry, TFTs built with fluorinated IGZO were shown to exhibit improved intrinsic resilience against hydrogen-induced degradation. Further enhanced by the incorporation of aluminum oxide as a hydrogen diffusion-barrier, such resilience is beneficial to the integration of fluorinated IGZO TFTs with hydrogen-containing devices, such as photodiodes based on amorphous hydrogenated silicon and TFTs based on low-temperature polycrystalline silicon.

Large-Scale and Localized Laser Crystallization of Optically Thick Amorphous Silicon Films by Near-IR Femtosecond Pulses.

Abstract: Amorphous silicon (α-Si) film present an inexpensive and promising material for optoelectronic and nanophotonic applications. Its basic optical and optoelectronic properties are known to be improved via phase transition from amorphous to polycrystalline phase. Infrared femtosecond laser radiation can be considered to be a promising nondestructive and facile way to drive uniform in-depth and lateral crystallization of α-Si films that are typically opaque in UV-visible spectral range. However, so far only a few studies reported on use of near-IR radiation for laser-induced crystallization of α-Si providing less information regarding optical properties of the resultant polycrystalline Si films demonstrating rather high surface roughness. The present work demonstrates efficient and gentle single-pass crystallization of α-Si films induced by their direct irradiation with near-IR femtosecond laser pulses coming at sub-MHz repetition rate. Comprehensive analysis of morphology and composition of laser-annealed films by atomic-force microscopy, optical, micro-Raman and energy-dispersive X-ray spectroscopy, as well as numerical modeling of optical spectra, confirmed efficient crystallization of α-Si and high-quality of the obtained films. Moreover, we highlight localized laser-induced crystallization of α-Si as a promising way for optical information encryption, anti-counterfeiting and fabrication of micro-optical elements.

Experimental investigation on the machining characteristics of fixed-free abrasive combined wire sawing PV polycrystalline silicon solar cell

Abstract: At present, the fixed abrasive wire sawing (FAWS) technology is gradually used in the photovoltaic industry to cut polycrystalline silicon slices. However, there are obvious directional wire marks, parallel grooves, and amorphous silicon layer on the surface of the slices formed by the FAWS, which leads to a high optic reflectivity of the textured surface obtained after the mature acid etching texturization technology. So the slices cannot meet the requirements of the photovoltaic cell. In the paper, a novel fixed-free abrasive combined wire sawing (FFACWS) technology for cutting PV polycrystalline silicon is presented to solve this problem, by adding loose SiC abrasives to cooling lubricant during the fixed abrasive wire sawing. A single-factor and orthogonal experimental study on sawing characteristics was carried out. The effect of size and mass fraction of SiC abrasives in the slurry, workpiece feed speed and wire speed on the surface morphology, roughness, and kerf loss were studied. The results show that within the range of the processing parameters in the paper studied, the obvious wire marks, parallel grooves, and ductile layers on the surface of the slices can be removed by the FFACWS. The surface roughness of the slices along the wire movement direction and the workpiece feed direction increases with the increase of the mass fraction of SiC abrasives in the slurry and workpiece feed speed and it decreases with the increase of wire speed. But the effect of the size of SiC abrasives is related to the matching of the protruding height of the fixed abrasives on the wire surface along the workpiece feed direction. In the wire movement direction, it increases with the size of SiC abrasives. The kerf loss increases with the increase of size and mass fraction of SiC abrasives in the slurry and the wire speed but has little effect with the change of workpiece feed speed.

Nonlinear properties of laser-processed polycrystalline silicon waveguides for integrated photonics.

Abstract: We report nonlinear optical characterization of cm-long polycrystalline silicon (poly-Si) waveguides at telecom wavelengths. Laser post-processing of lithographically-patterned amorphous silicon deposited on silica-on-silicon substrates provides low-loss poly-Si waveguides with surface-tension-shaped boundaries. Achieving optical losses as low as 4 dB cm-1 enabled us to demonstrate effects of self-phase modulation (SPM) and two-photon absorption (TPA). Analysis of the spectral broadening and nonlinear losses with numerical modeling reveals the best fit values of the Kerr coefficient n2=4.5×10-18 m W-1 and TPA coefficient βTPA=9.0×10-12 m2 W-1, which are within the range reported for crystalline silicon. On-chip low-loss poly-Si paves the way for flexible integration of nonlinear components in multi-layered photonic systems.

Latent ion tracks in amorphous and radiation amorphized silicon nitride

Abstract: The microstructure of 265, 402 and 710 MeV Bi ion induced latent tracks in amorphous thin films of silicon nitride is studied using HRSTEM combined with EELS techniques. The results are compared with latent ion tracks in 220 MeV Xe and 710 MeV Bi irradiated polycrystalline silicon nitride, which was amorphized as a result of irradiation with swift heavy ions only. The results suggest that the track sizes are weakly dependent on defect structure of Si3N4. 710 MeV Bi ions induce tracks with radii that are similar in both amorphous and radiation-amorphized Si3N4, 1.5 ± 0.3 nm and 1.4 ± 0.1 nm, respectively. The situation for Xe is not as clear cut most likely due to the much higher irradiation fluence as compared to Bi.