Hydrogen Dilution Research Articles

Plasma-Enhanced Chemical Vapor Deposition of Silicon Films at Low Pressure in GEC Reference Cell

A self-consistent fluid simulation of inductively coupled plasma-enhanced chemical vapour deposition (ICP-CVD) using silane, argon, and hydrogen mixture is presented. The model solves the continuity equations for charged species, the drift-diffusion equations to describe their transport and the electron energy balance equation, coupled with Maxwell’s and Poisson’s equations, using COMSOL Multiphysics software. In order to obtain a better comprehending of the discharge parameters, a discharge in a Gaseous Electronics Conference cell (GEC) reactor is simulated. In this study, the GEC reactor with five-turn inductive coils is driven at a radio frequency of 13.56 MHz in order to sustain the plasma discharge at low temperature for a mixture of SiH4/Ar/H2, with an intial gas pressure fixed at 20 mTorr. The simulations yield the profile of plasma components such as electron density and temperature as well as the electrical potential in the centre of the discharge during silicon films deposition. The effects of external settings, such as chamber pressure, applied power and hydrogen dilution, on the growth rate of the silicon films deposited by ICP-CVD are then investigated. It is shown that the increase of the applied power from 1000 to 3000 W and the gas pressure from 10 to 40 mTorr results in a moderate increase in the deposition rate of the silicon films, whereas the increase in the dilution of the hydrogen in the mixture produces its decrease.

Role of hydrogen in modifying a-Si:H/c-Si interface passivation and band alignment for rear-emitter silicon heterojunction solar cells

Boosting the contact property of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) film is pivotal to achieving high-efficiency silicon heterojunction (SHJ) solar cells. Here, the microstructure of a-Si:H(i) film is modified with hydrogen dilution ratio using hot wire chemical vapor deposition (HWCVD) for the application into rear-emitter SHJ solar cells. A higher hydrogen content associated with high valence band offset was found to decrease the fill factor FF for low dilution, while high interface defect densities related to epitaxial growth are responsible for the deterioration of both FF and open-circuit voltage VOC for high dilution. In particular, the most compact film prepared at a moderate dilution exhibits the most compact structure with most hydrogen located as isolated hydrogen rather than clustered hydrogen. Finally, high efficiency of SHJ solar cells up to 22.5% was yielded using the optimized a-Si:H(i) layer thanks to a significant enhancement of FF, which is attributed to improved passivation quality and rational band alignment at the a-Si:H(i)/c-Si interface. This work clearly interpreted the correlation between SHJ device parameters and a-Si:H(i)/c-Si interface properties, which might guide the design of a-Si:H passivation layers in pursuit of high-efficiency SHJ solar cells.

Connected evaluation of polymer electrolyte membrane fuel cell with dehydrogenation reactor of liquid organic hydrogen carrier

Recently, hydrogen energy technologies attract attention as power systems. To develop hydrogen energy systems, hydrogen storage methods with high storage density and good safety are required. Liquid organic hydrogen carrier (LOHC) is one of the novel hydrogen storage technologies. LOHC has advantages of high storage density, good safety, and easy handling. In this study, a polymer electrolyte membrane fuel cell (PEMFC) stack is operated with hydrogen released from LOHC to evaluate the feasibility of the connected operation of the PEMFC stack and LOHC dehydrogenation reactor. Dibenzyltoluene (H0-DBT) is used as a LOHC material, and the dehydrogenation of perhydro dibenzyltoluene (H18-DBT) is conducted at 240–300 °C. Released hydrogen is purified by adsorbent of activated carbon to remove impurities. However, 100–1400 ppm of methane is observed after the purification, and the PEMFC stack power is reduced from 39.4 W to 39.0 W during the operation by hydrogen dilution and physical adsorption of methane. Then, to evaluate the irreversible damage, pure hydrogen was supplied to the PEMFC stack. The stack power is recovered to 39.4 W. It is concluded that the connected operation of the LOHC dehydrogenation reactor and PEMFC stack is feasible, and the activated carbon adsorbent can be a cost-effective purification method for LOHC.

Optimizing polymer electrolyte membrane thickness to maximize fuel cell vehicle range

Automotive hydrogen polymer electrolyte membrane (PEM) fuel cell systems require periodic purges to remove nitrogen and water from the anode. Purging increases system performance by limiting anode hydrogen dilution, but reduces hydrogen utilization. State of the art fuel cell membrane electrode assemblies utilize thin ionomer membranes in an effort to increase performance and reduce cost. Thinner membranes also increase the required anode purge rates due to the increased transport of inert gases. A model was developed to examine the relationship between membrane thickness and vehicle range which takes into account anode purge rate. The model includes changes in efficiency and hydrogen utilization as a function of PEM thickness for a variety of operating conditions. The model predicts that an optimal membrane thickness which maximizes vehicle range exists, but this thickness is highly dependent on other system conditions. The results of this study can be extended to help optimize stack development and balance of plant design.

Electron energy loss spectroscopy and energy filtered transmission electron microscopy of hydrogenated nano-crystalline Si thin films deposited via hydrogen profiling during hot-wire CVD for application in photovoltaics

Energy filtered transmission electron microscope imaging gives direct evidence of the nano-void network within hydrogenated nano-crystalline silicon thin films, deposited by reducing the hydrogen dilution as a function of deposition time (so called hydrogen profiling) during hot-wire chemical vapor deposition. Electron thickness maps show a t/λ value of approximately 0.38 in the void-rich areas compared to 0.47 in the continuous film, where t is the depth through which the electron beam is scattered and λ the electron mean free path in the Si thin film. Three-window elemental mapping shows no oxygen diffusion throughout the film architecture, whereas spectral imaging in combination with scanning transmission electron microscopy-coupled energy loss near edge fine-structure analyses along the growth direction of the film, across the tip and along the grain boundaries of the conically shaped crystals reveal changes in the Si-L3,2 and L1 energy-loss edges. In particular, the amorphous tissue between cone tips in the incubation layer typically shows a poorly defined L2 ionization edge, which progressively becomes more pronounced in the crystalline areas. Subtle variations in the L1 lineshape along the length of the film, characteristic of hydrogen passivation, confirm the profiling during synthesis.

Large‐scale data principal component analysis of phase transition from a‐Si:H to nc‐Si:H using PECVD optical emission spectra

Phase transformation of hydrogenated nanocrystalline silicon (nc-Si:H) films was essential in the solar cells industry. Therefore, the plasma chemistry monitoring of in-situ diagnostics optical emission spectroscopy (OES) of plasma-enhanced chemical vapour deposition (PECVD) is deemed crucial for investigating the phase transformation. The proposed PC2-OES algorithm can be used to monitor the PECVD process health condition of the crystallisation rate at different hydrogen dilution ratios. Principal component analysis (PCA) was performed to distinguish the crucial role of nc-Si:H emission characteristics of plasma chemistry and the resultant crystallisation rate. Measurement results revealed that OES spectra characterisation confirmed that the crystallisation rate index (Hα*/SiH*) was highly correlated to hydrogen dilution ratio (R) and phase transformation of nc-Si:H film. The proposed PCA-based evaluation method will provide valuable information to reflect consistently the crystallisation rate of deposited films with different hydrogen dilution ratio and possibly other processing parameters of mass flow rate and applied power density.

Suppression of Parasitic Epitaxy Growth and Realization of High-Quality Wafer Surface Passivation of Silicon Heterojunction Solar Cells

Intrinsic hydrogenated amorphous silicon (i-a-Si:H) films lead to excellent surface passivation of crystalline silicon wafers. However, a-Si:H deposition on a crystalline silicon wafer often results in undesired epitaxy growth, which deteriorates the passivation property, In this paper, we studied the influence of varying plasma enhanced chemical vapor deposition (PECVD) parameters, such as the product of the gas pressure (P) and the electrode distance (Di) and the hydrogen dilution ratio (P = SiH4/(H2 + SiH4) × 100), on the passivation quality and the properties of silicon heterojunction (SHJ) solar cells. Measurements showed that proper combinations of high P × Di and large P values can yield high minority carrier lifetimes (MCLTs) of passivated silicon wafers. Also, the tendency of MCLTs measured from passivated wafers is the same as that for open circuit voltages (Voc) of fabricated SHJ solar cells. A high Voc is obtained from SHJ solar cells when unwanted epitaxial growth is minimized at the wafer surface.

Study of Si and Ge Atoms Termination Using H-Dilution in SiGe:H Alloys Deposited by Radio Frequency (13.56 MHz) Plasma Discharge at Low Temperature.

In this work, we present the study of the atomic composition in amorphous SiXGeY:HZ films deposited by radio frequency (RF—13.56 MHz) plasma discharge at low deposition temperature. A study and control of Si and Ge atoms termination using H-dilution in SiGe:H alloys deposited by RF plasma discharge was conducted and we made a comparison with low-frequency plasma discharge studies. Solid contents of the main elements and contaminants were determined by SIMS technique. It was found that for low dilution rates from RH = 9 to 30, the germanium content in the solid phase strongly depends on the hydrogen dilution and varies from Y = 0.49 to 0.68. On the other hand, with a higher presence of hydrogen in the mixture, the germanium content does not change and remains close to the value of Y = 0.69. The coefficient of Ge preferential incorporation depended on RH and varied from PGe = 0.8 to 4.3. Also, the termination of Si and Ge atoms with hydrogen was studied using FTIR spectroscopy. Preferential termination of Si atoms was observed in the films deposited with low RH < 20, while preferential termination of Ge atoms was found in the films deposited with high RH > 40. In the range of 20 < RH < 40, hydrogen created chemical bonds with both Si and Ge atoms without preference.

Effect of Alloy Composition on the Optoelectronic Properties of Hydrogenated Microcrystalline Silicon-Germanium Films Deposited with Various Hydrogen Dilution

Hydrogenated microcrystalline silicon-germanium (μc-Si1-xGex: H) films were fabricated by plasma-enhanced chemical vapor deposition. The influence of hydrogen dilution on the optoelectronic and crystal structural properties of μc-Si1-xGex: H films were investigated upon alloy composition. Under the relatively low hydrogen dilution condition, the Si atoms of films are easier to crystallize than the Ge atoms, and the dark and photo-conductivity decreased with the Ge concentration because of the reduced crystalline volume fraction of the Si atoms (X Si-Si ) and the increased Ge dangling bond density. However, under higher hydrogen dilution condition, crystallization of the Si atoms decreased while crystallization of the Ge atoms increased with Ge incorporation, and the optoelectronic properties were strongly influenced by the defect density and crystallization of Ge atoms in films.

Very low-temperature growth of silicon thin films using chlorinated precursors and optical properties

In this work, the effect of the very low substrate temperature and hydrogen dilution on chemical, structural, and optical properties of polymorphous silicon thin films (pm-Si:H) using dichlorosilane as a silicon gas precursor in the plasma enhanced chemical vapor deposition (PECVD) was analyzed. The films were synthesized at a lower deposition temperature in the range from 60 to 150 °C and two H2 dilutions of 60 and 100 sccm. Hydrogen incorporation in silicon thin films has been studied by Elastic Recoil Detection Analysis (ERDA) and Fourier Transform Infrared Spectroscopy (FTIR). FTIR was also used to verify the chemical stability of the material as a function of oxidation state and hydrogen effusion. The ERDA analysis evidenced that the hydrogen content typically does not exceed 30 at. %, and the lowest value obtained is around 10 at. %. With Raman spectroscopy, the crystalline fraction was obtained in the range of 5–20%, and the average size of the embedded nanocrystals was found to be between 2 and 3 nm, which was cross-checked by High-Resolution Transmission Electron Microscopy (HRTEM). Finally, by UV–Vis spectroscopy, the effective optical band gap of this material has been calculated, and the value was found to be around 1.6 eV (absorber layer) for the samples with 100 sccm of H2 dilution and around 2.2 eV (window layer) for the samples with 60 sccm of H2 dilution. Overall the increase in the substrate temperature resulted a better ordering in the amorphous matrix, whereas, with the increase in the hydrogen dilution an improvement in the structure factor was observed. Suitable properties of the deposited material in the present work could be useful for the development of a thin silicon layer for different silicon solar cell technologies.

Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition

We investigated the effect of hydrogen dilution on the Si cluster volume fraction of hydrogenated amorphous films by varying the hydrogen dilution ratio at 0.5 Torr and compared it to that obtained at pure silane discharge at 0.3, 0.4, and 0.5 Torr. The correlation between the plasma emission characteristic, deposition rate, and cluster volume fraction in the hydrogen dilution plasma was described. The cluster volume fractions of films under hydrogen dilution conditions were similar to those of the pure silane but showed a higher deposition rate. The results suggest that under hydrogen dilution conditions, it is possible to maintain a higher deposition rate with a lower cluster incorporation rate.

Effects of Hot Wire Temperature on Properties of GeSi:H Films with High Hydrogen Dilution by Hot-Wire Chemical Vapor Deposition

GeSi:H films are prepared by hot-wire chemical vapor deposition (CVD) with high hydrogen dilution, DH=98%. Effects of hot wire temperature (Tw) on deposition rate, structural properties and bandgap of GeSi:H films are studied with surface profilemeter, Raman spectroscopy, Fourier transformed infrared spectroscopy, and UV-VIS-NIR spectrophotometer. It is found that the deposition rate (Rd) goes up with increasing of Tw, but increasing rate of Rd declines when Tw⩾1 550 °C. High Tw is beneficial to the formation of Ge-Si, but it has little effect on relative contents of the hydrogen bonds (Ge-H, Si-H, etc.) in the films. In the Tw range of 1 400–1 850 °C, the maximum bandgap of the GeSi:H films is 1.39 eV at Tw =1 450 °C and the band gap decreases with Tw increasing when Tw⩾1 450 °C.

Analysis of Defects and Surface Roughness on the Hydrogenated Amorphous Silicon (a-Si:H) Intrinsic Thin Film for Solar Cells

Effect of defect - through observation of energy absorption Urbach, on deposition rate, energy band gap, and surface roughness of intrinsic thin film are investigated using Radio Frequency Plasma Enhance Chemical Vapor Deposition (RF-PECVD). Films are grown on ITO (Indium Tin Oxide) glass substrate. Analysis of energy band gap is conducted to determine changes in the structure of a thin film of a-Si:H. Energy band gap is important to determine the portion of the spectrum of sunlight that is absorbed solar cells. From the characterization using UV-Vis spectrometer and the Tauc’s plot method, the width of the resulting energy band gap is greater if the hydrogen dilution is increased. It can be shown that the increase of the hydrogen dilution, will increase the energy band gap, and the surface roughness of thin layers. Instead, the improvement of the hydrogen dilution decrease the rate of deposition and Urbach energy. It is estimated that with greater hydrogen dilution, an intrinsic thin film of a-Si:H is more conductive for more reduction in residual of band tail defects or dangling bond defects.

High-Performance Amorphous Silicon Thin Film Solar Cells Prepared at 100 °C: Toward Flexible Building-Integrated Photovoltaics

For low-cost and lightweight polymer/plastic substrates in flexible building-integrated photovoltaic (BIPV) modules, low-temperature processing is essential. Amorphous silicon (a-Si:H) requires processing at a temperature of 200–250 °C by plasma-enhanced chemical vapor deposition to obtain satisfactory optoelectronic properties, which limits such substrates in terms of thermal budget. This study is focused on the fabrication of p–i–n-type a-Si:H solar cells at relatively low temperatures (100 °C). Intrinsic a-Si:H films with large optical gaps (1.83 eV) were prepared at 100 °C using a high hydrogen dilution ratio. In addition, p-type amorphous silicon oxide and n-type microcrystalline silicon oxide films with large optical gaps and suitable conductivities were prepared at 100 °C using a gas mixture containing the dopant B2H6 or PH3 and CO2. Finally, an a-Si:H p–i–n cell was fabricated at 100 °C; it exhibited an excellent power conversion efficiency of 9.0%, which was higher than those reported for a-Si:H thin film photovoltaics prepared at 100 °C. We believe that this study will open promising routes for the development of high-performance and flexible BIPVs.

Effect of Hydrogen Addition on the Sooting Tendency of 1,3-Butadiene Premixed Flames

Mole fractions and formation-depletion pathways of the first aromatic ring and its precursors, acetylene and propargyl radical as well as those of naphthalene are numerically investigated during the co-combustion of 1,3-butadiene/H2 mixtures in low pressure premixed laminar flames conditions. The molar fraction of H2 in the mixture is varied from 0 to 60% by adding 10% of hydrogen to the neat butadiene flame and by keeping the inert mole fraction (argon) and the equivalence ratio constants. The effect of hydrogen is investigated by using a modified Chemkin II version and by differentiating its chemical effect from its thermal and dilution effects. The obtained results indicate that the butadiene/H2 oxidation process is controlled by the synergism between hydrogen dilution and chemical effects. Regardless of the hydrogen amount added to the fuel mixture, two routes are evolved in the benzene formation process, the direct C4 + C2 route and the indirect C3 route, evolving fulvene as an intermediate species. Besides, the collected data reveal that hydrogen doping induces a noticeable decrease in the concentrations of all the studied species inferring that the hydrogen blended fuels are less prolific in soot formation than the neat 1,3-butadiene fuel. The observed decrease is dependent on the hydrogen initial concentration in the fuel mixture as well as on the nature of its effect.

Structural analysis of hydrogenated nano-crystalline silicon suboxide (nc-SiOx:H, x < 1) thin films with SAXS for a potential application as phase change material

Hydrogenated nano-crystalline silicon suboxide (nc-SiOx:H; x < 1) is a two-phase material in which nanometer-sized silicon (nc-Si) region enclosed by hydrogenated amorphous silicon suboxide (a-SiOx:H) matrix. nc-SiOx:H thin films are produced by using highly hydrogen-diluted (dH ≥ 90%) plasma of silane and carbon-dioxide mixture, in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system. The high concentration of the H-atoms in the plasma assist the silicon atoms on the growing film surface to crystallize. Small Angle X-Ray Scattering (SAXS) method was used to determine the nano-scale structures of nc-SiOx:H thin films, for varying hydrogen dilution ratios. The size, shape and the distribution of the nano-formations are found to depend strongly on the preparation conditions. From FTIR absorption and Raman scattering experiments, atomic oxygen and hydrogen contents and also the crystal ratio of the samples are evaluated. As results of nano-structural analyses, the morphologies, sizes and distributions of the nano-aggregations are found to be closely related to the crystallization of the films. From the SAXS results of the samples prepared at the hydrogen dilution ratio of 90%, the nano-aggregations are found to form in uniformly distributed core-shells. However, as the hydrogen dilution is increased to higher ratios to 95% and further to 99%, the nano-scaled fractals are deduced to form. The oxygen and hydrogen contents in nc-SiOx:H are strictly related to the crystal ratio of the samples. The crystal ratio is determined by the hydrogen dilution ratio during the deposition. The two-phase structure of nc-SiOx:H samples and the dependence on the hydrogen dilution ratio may suggest these materials are candidates for phase change materials. Depending on the deposition parameters, smaller nc-Si structures may be prepared which may have different (lower) electron densities required to melt the amorphous region and make the phase change possible due to explosive crystallization.

Correlation of Impedance Matching and Optical Emission Spectroscopy during Plasma-Enhanced Chemical Vapor Deposition of Nanocrystalline Silicon Thin Films

In this paper, the correlation of impedance matching and optical emission spectroscopy during plasma-enhanced chemical vapor deposition (PECVD) was systematically investigated in SiH4 plasma diluted by various hydrogen dilution ratios. At the onset of nanocrystallinity in SiH4− depleted plasma condition, the SiH+ radical reached a threshold value as the dominant radical, such that a-Si to nc-Si transition was obtained. Furthermore, the experimental data of impedance analysis showed that matching behavior can be greatly influenced by variable plasma parameters due to the change of various hydrogen dilution ratios, which is consistent with the recorded optical emission spectra (OES) of Hα* radicals. Quadruple mass spectrometry (QMS) and transmission electron microscopy (TEM) were employed as associated diagnostic and characterization tools to confirm the phase transformation and existence of silicon nanocrystals.

Preparation and Crystallization Characteristics of Hydrogenated Nanocrystalline Silicon Thin Films by Plasma-Enhanced Chemical Vapor Deposition

Both intrinsic and doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films were prepared by plasma-enhanced chemical vapor deposition (PECVD) with various process parameters, such as a hydrogen dilution ratio, power, substrate temperature, and doping ratios of phosphorus or boron. The crystallization characteristics of nc-Si:H thin films grown with various process parameters were carefully and systematically investigated by Raman spectroscopy. Generally speaking, the results show that the higher the hydrogen dilution ratio and power or the lower the doping ratio, the higher the average grain size and the crystalline volume fraction of both thin films prepared and investigated here. In addition, a p-i-n type nc-Si:H thin film solar cell, which has an open circuit voltage of 660 mV and a short circuit current intensity of 13.06 mA/cm2, was directly prepared on a flat transparent conductive glass substrate.

Experimental study on explosion characteristics of hydrogen–propane mixtures

The explosion characteristics of a hydrogen–propane mixture under different initial temperatures and at different hydrogen addition, equivalence, and dilution ratios were studied using a 20-L apparatus. First, the effects of hydrogen addition and equivalence ratio were studied. The results showed that the maximum explosion pressure and the maximum rate of pressure rise first increased, then decreased from lean to an equivalence ratio of 1.6, and reached the maximum value at the equivalence ratio of 1.2. The maximum explosion pressure and the maximum rate of pressure rise increased with the increasing hydrogen proportion of the mixtures, and the time to the maximum explosion pressure decreased. Furthermore, the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, was explored under nitrogen or carbon dioxide dilution. Carbon dioxide exhibited a stronger dilution effect than nitrogen. For the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, the maximum explosion pressure saliently decreased with the increasing initial temperature in the range of 25 °C-120 °C. The variation trend of experimental results was consistent with theoretical predictions.

Pyrolysis of Natural Gas: Effects of Process Variables and Reactor Materials on the Product Gas Composition

AbstractHigh‐temperature pyrolysis of natural gas is the basis of the standard method for the manufacture of acetylene. The study of methane pyrolysis was designed to find optimum process conditions that would produce high yields of acetylene with minimal carbon formation. High temperatures and short residence times enhanced the selectivity for acetylene, while hydrogen dilution was found to suppress the generation of carbonous products. Carbon formation on reactor surfaces over time may be mainly responsible for the misalignment of predicted and measured product gas compositions, as the mechanisms reported do not consider the surface chemistry. In essence, the pyrolysis system favors the highest possible temperature and shortest possible residence time, suggesting that the selection of reactor materials is the key for pyrolysis process optimization. The operating temperature is likely dictated by the physical properties of the reactor materials rather than the selection of optimal pyrolysis conditions.