Photoluminescence from photochemically etched silicon

The photoluminescence (PL) of photochemically etched silicon is studied. In the photochemical etching process, an n-type silicon wafer is immersed in an etchant solution of hydrofluoric acid (HF) and H₂O₂. A low-power visible laser (typically He–Ne) is used to illuminate the samples. The etching process occurs through the photogeneration of carriers. Although no electrodes are used in this etching method, the final samples show PL similar to electrochemically etched porous silicon. The samples were prepared using (1 0 0) n-type silicon with a resistivity of 1.0–5.5 Ω cm. An He–Ne laser with 20 mW of maximum power output was used and the spot radius (on the samples) was varied from 1 to 4 mm. A strong emission in the red-yellow optical region can be present in the final samples depending on the HF:H₂O₂ concentration ratio, etching time and laser intensity of the etching process. The PL spectra excited with the monochromated output of an Xe light source as excitation is studied. The peak wavelength of the PL intensity shifts to the blue region of the spectrum when increasing the laser intensity. Quantum confinement can explain this blue shifting if smaller silicon nanocrystallites are formed with higher laser intensity. The peak PL intensity also decreases when increasing the laser intensity, although a “threshold” condition must be reached to have measurable PL. Each sample also exhibits a shifting in peak PL wavelength when varying the PL excitation wavelength. The corresponding dependence and the variations of the PL intensity are studied. Other experimental conditions are discussed.     

Correlation between reflectivity and photoluminescent properties of porous silicon films

Porous silicon (PS) layers were formed on p-type, 〈1 0 0〉 oriented, 1-5 Ω cm resistivity Cz silicon wafers by electrochemical etching in an HF:C₂H₅OH (1:2 by volume) electrolyte at room temperature at a constant current density 20 mA/cm². The etching duration was varied to achieve PS layers of different morphologies and thicknesses. Both the photoluminescence (PL) and the total diffused reflectivity spectra of the PS layers were measured. It was found that for the PS layers grown for etching durations of less than 90 s the PL emission is insignificant and reflectivity is quite low. Such PS layers can be used as antireflection coatings (ARC) on solar cells. The PS layers formed for etching durations greater than 90 s show a significant PL emission in 500-800 nm range with peak lying in 630-660 nm wavelength range. When etching duration increases from 90 s to 8 min the PL intensity increases and the PL peak shows a blue shift. With further increase in etching duration the PL intensity decreases and PL peak shows a red shift. The reflectivity of the photoluminescent layers increases with etching duration showing a highest value for a sample grown for 8 min. Further increase in etching duration up to 20 min the reflectivity decreases and then increases. Striking observation is that both the PL emission intensity and reflectivity in the wavelength range of 550-800 nm are maximum for the PS layer grown for the etching duration of 8 min.     

Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies

The effect of freezing of a membrane-electrode assembly on its physical properties and performance was investigated. It was found that freeze-thaw cycles caused the electrode (i.e., catalyst layer) of a fully hydrated membrane-electrode assembly (MEA), either as a freestanding piece or as assembled in a cell, to crack. Accompanying the cracking was a reduction in the electrochemical active surface areas of the electrodes as measured by cyclic voltammetry, but the short-term performance of the fuel cell did not show much effect. When dry reactants were used to remove some water from a cell that had been previously tested at fully hydrated condition, freeze-thaw cycling did not cause apparent damage to the appearance of the electrodes. Also, for freestanding MEAs that were taken directly from the manufacturing line and only exposed to ambient temperature (e.g., 23 °C) and relative humidity (e.g., <50% RH), freezing did not cause apparent damage to the appearance of the electrodes.     

Metal current collector-free freestanding silicon-carbon 1D nanocomposites for ultralight anodes in lithium ion batteries

Although current collectors take up more weight than active materials in most lithium ion battery cells, so far research has been focused mainly on improving gravimetric capacities of active materials. To address this issue of improving gravimetric capacities based on overall cell components, we develop freestanding nanocomposites made of carbon nanofibers (CNFs) and silicon nanowires (SiNWs) as metal current collector-free anode platforms. Intrinsically large capacities of SiNWs as active materials in conjunction with the light nature of freestanding CNF films allow the nanocomposites to achieve 3-5 times improved gravimetric capacities compared to what have been reported in the literature. Moreover, three-dimensional porous structures in the CNF films facilitate increased mass loadings of SiNWs when compared to flat substrates and result in good cycle lives over 40 cycles. This type of nanocomposite cell suggests that 3D porous platforms consisting of light nanomaterials can provide for higher gravimetric and areal capacities when compared to conventional battery cells based on flat, heavy metal substrates.     

Investigation of aluminosilicate as a solid oxide fuel cell refractory

Aluminosilicate represents a potential low cost alternative to alumina for solid oxide fuel cell (SOFC) refractory applications. The objectives of this investigation are to study: (1) changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) deposition of aluminosilicate vapors on yttria stabilized zirconia (YSZ) and nickel, and (3) effects of aluminosilicate vapors on SOFC electrochemical performance. Thermal treatment of aluminosilicate under high temperature SOFC conditions is shown to result in increased mullite concentrations at the surface due to diffusion of silicon from the bulk. Water vapor accelerates the rate of surface diffusion resulting in a more uniform distribution of silicon. The high temperature condensation of volatile gases released from aluminosilicate preferentially deposit on YSZ rather than nickel. Silicon vapor deposited on YSZ consists primarily of aluminum rich clusters enclosed in an amorphous siliceous layer. Increased concentrations of silicon are observed in enlarged grain boundaries indicating separation of YSZ grains by insulating glassy phase. The presence of aluminosilicate powder in the hot zone of a fuel line supplying humidified hydrogen to an SOFC anode impeded peak performance and accelerated degradation. Energy dispersive X-ray spectroscopy detected concentrations of silicon at the interface between the electrolyte and anode interlayer above impurity levels.     

Electrical and thermal performance of silicon concentrator solar cells immersed in dielectric liquids

Direct liquid-immersion cooling of concentrator solar cells was proposed as a solution for receiver thermal management of concentrating photovoltaic (CPV) and hybrid concentrating photovoltaic thermal (CPV-T) systems. De-ionized (DI) water, isopropyl alcohol (IPA), ethyl acetate, and dimethyl silicon oil were selected as potential immersion liquids based on optical transmittance measurement results. Improvements to the electrical performance of silicon CPV cells were observed under a range of concentrations in the candidate dielectric liquids, arising from improved light collection and reduced cell surface recombination losses from surface adsorption of polar molecules. Three-dimensional numerical simulations with the four candidate liquids as the working fluids, exploring the thermal performance of a silicon CPV cell array in a liquid immersion prototype receiver, have been performed. Simulation results show that the direct-immersion cooling approach can maintain low and uniform cell temperature in the designed liquid immersion receiver. The fluid inlet velocity and flow mode, along with the fluid thermal properties, all have a significant influence on the cell array temperature.     

Hydrogen production from macroalgae by simultaneous dark fermentation and microbial electrolysis cell with surface-modified stainless steel mesh cathode

An affordable cathode material for microbial electrolysis cells (MECs) was synthesized via surface-modification of stainless steel mesh (SSM) by anodization. The anodization parameters, such as wire mesh size, temperature, applied voltage, operating time, were optimized. The surface-modified SSM (smSSM) exhibited porous surface and higher specific surface area. The as synthesized smSSMs were utilized as freestanding cathodes in a conventional microbial electrolysis cell (MEC) and a simultaneous dark fermentation and MEC process (sDFMEC). The H₂ production in MEC and sDFMEC with smSSM as cathode was approximately 150% higher than that with SSM. The performance of smSSM was 67–75% of that of Pt/C. The sDFMEC with smSSM as cathode was stable for 12 cycles of fed-batch operation in 60 days. Overall, energy conversion from S. japonica by sDFMEC was as high as 23.4%.     

Impurities and defects in multicrystalline silicon for solar cells: low-temperature photoluminescence investigations

The low-temperature photoluminescence (PL) measurements (down to 4.2 K) were employed for the investigations of the defects and impurities in multicrystalline silicon (mc-Si) samples grown by block-casting method. The optical properties of as-grown, irradiated by gamma-rays, heat and hydrogen plasma treated samples were studied. It was found that carbon and oxygen as the residual impurity atoms are responsible for the formation of the zero-phonon PL lines with 0.9355 eV (T line) and 0.9652 eV (I line) after heat treatments at about 350–550°C. The appearance of PL lines with the energies of 0.9697 eV (A line) and 0.7894 eV (C line) after a gamma-rays irradiation can be attributed to the formation of carbon- and oxygen-related centers, respectively. The comparison of the PL properties of the mc-Si samples with the mono-crystalline one is performed. It is shown that the main peculiarities of the low-temperature PL spectra of mc-Si can be explained both by the influence of residual impurities and the residual strains in this material.     

Investigation of hydrogen storage behavior of silicon nanoparticles

Porous Silicon (PS) freestanding film is a derivative of single crystal Si wafer. PS films obtained on electrochemical etching of p-type silicon (Si) wafer were used to synthesize Si nanoparticles by ultrasonication. 12 μm thick and 29% porous freestanding PS films were sonicated for 4 h in 120 W ultrasonication bath at 42 kHz. HRTEM image shows Si nanoparticles in the range of 8–20 nm in size. In this paper we present results of hydrogen absorption experiments conducted on Si nanoparticles. Standard Seivert’s type apparatus was used to carry out hydrogen absorption pressure composition isotherm measurements in the pressure range of 1–10 bar and in the temperature range of 29 °C–150 °C. Theoretically SiH_x system has 3.44, 6.66 and 9.67 wt% of hydrogen for x = 1, 2, 3 respectively. Experimental results show maximum hydrogen uptake of 2.25 wt% at the temperature of 120 °C and at 9.76 bar pressure. Hydrogenation of Si nanoparticles exhibits frequency downshifts from 510.7 to 507.3 cm⁻¹ in Raman spectra. Raman peaks were de-convoluted in two bands to study effect of hydrogenation on FWHM, crystallanity and elastic strain of the nanoparticles. Bonding between Si, O and H atoms were investigated using Fourier transform infrared spectroscopy(FTIR) spectroscopy. UV–Vis spectra and Tauc plots were used to discuss the relation between hydrogenation and optical band gap of the Si nanoparticles. Optical band gap was found to increase from 1.6 to 2.25 eV on subjecting Si nanoparticles to hydrogenation.     

Optical response of grain boundaries in upgraded metallurgical-grade silicon for photovoltaics

Using upgraded metallurgical-grade silicon (UMG-Si) is a cost-effective and energy-efficient approach for the production of solar cells. Grain boundaries (GBs) play a major role in determining the device performance of multicrystalline Si (mc-Si) solar cells. In this study two UMG-Si wafers, one from the middle part of a brick and the other from the top part of the same brick, were investigated. An excellent correlation was found between the grain misorientation and the corresponding optical response of GBs as indicated by photoluminescence (PL) imaging, electron backscattered diffraction (EBSD), and cross-sectional transmission electron microscopy (TEM). In addition, the PL features at random GBs depend also on the impurity levels in the wafer. In particular the PL emission was greatly enhanced in the narrow regions close to the random GB in the top wafer, which is an interesting phenomenon that may have potential application in high efficiency light-emission diodes (LEDs) based on Si.     

Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching

High-efficiency silicon solar cells need a textured front surface to reduce reflectance and to improve light trapping. Texturing of monocrystalline silicon is usually done in alkaline solutions. These solutions are cheaper, but are pollutants of silicon technologies. In this paper, we investigate an alternative solution containing tetramethyl ammonium hydroxide ((CH₃)₄NOH, TMAH ). This study shows the influence of different parameters (concentration, agitation, duration and temperature), to obtain uniform and reliable pyramidal texturization on different silicon surfaces (as cut, etched and polished). Under optimized conditions, TMAH-textured surface led to an average weighted reflectance of 13%, without any antireflection coating independent of the initial silicon surface. Unlike potassium hydroxide (KOH) texturing solution, characterization of silicon oxide layer contamination after TMAH texturing process revealed no pollution, and passivation is less affected by TMAH than by KOH texturization.     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Mechanical Strain Dependence of Thermal Transport in Amorphous Silicon Thin Films

Recent computational studies predict mechanical strain–induced changes in thermal transport, which is yet to be validated by experimental data. In this article, we present experimental evidence of an increase in thermal conductivity of nominally 200-nm-thick freestanding amorphous silicon thin films under externally applied tensile loading. Using a combination of nanomechanical testing and infrared microscopy, we show that 2.5% tensile strain can increase thermal conductivity from 1 to 2.4 W/m-K. We propose that such an increase in thermal conductivity might be due to strain-induced changes in microstructure and/or carrier density. Microstructural and optical reflectivity characterization through Raman and infrared spectroscopy are presented to investigate this hypothesis.     

IMPORTANCE OF GRID REFINEMENT IN NUMERICAL MODELING OF CHEMICAL VAPOR DEPOSITION PROCESSES

The importance of grid resolution near the substrate surface for accurate prediction of the deposition rates in chemical vapor deposition modeling has been demonstrated. The exercise is conducted through numerical modeling of the chemical vapor deposition of silicon in an atmospheric-pressure, circular, impinging-jet reactor. Silicon is deposited from gaseous silane (SiH 4 ) supplied in a dilute condition premixed in a hydrogen carrier gas. The substrate temperature is kept fixed at 1,333 K. The model includes variable fluid properties and buoyancy forces in the hydrodynamic model. The Bousinesq approximation is not used because the temperature gradient is large. In addition to the hydrodynamic and thermal solution, both gas-phase reactions in the bulk gas and surface reactions on the susceptor are included in the model. The mesh-independent solution and the deposition rate of silicon on the wafer surface are presented. It is observed that a very fine mesh near the substrate surface, within the concentration boundary layer for the intermediate species such as silylene (SiH 2 ), is required to establish grid independency and accurate prediction of the deposition rate. For the specific deposition process modeled in this study, about 7 control volumes had to be placed within the SiH 2 concentration boundary layer at the substrate surface.     

Effect of surface condition on boiling heat transfer from silicon chip with submicron-scale roughness

Boiling heat transfer on treated silicon surfaces was studied. Experiments were conducted to investigate the effects of submicron-scale roughness on the boiling heat transfer at a subcooled condition in FC-72 at the ambient pressure. Two-type of treated silicon surfaces were prepared for boiling surfaces using anodisation with HF (hydrofluoric acid) based electrolyte and DMF (dimethylforamide) based one. The back side of the treated surface was glued to the back side of the other silicon chip on which thin film heaters and thin film temperature sensors were fabricated using conventional MUPs processes with doped polysilicon. The treated chips with submicron-scale roughness which provide many possible nucleation sites showed considerable enhancement in the nucleate boiling heat transfer coefficients compared to the untreated silicon surface. Further, the critical heat flux (CHF) of the treated surfaces increase linearly to the increase in the effective area for boiling.     

Reduction of plasma-induced damage by electron beam excited plasma CVD

The electron beam excited plasma (EBEP-) CVD has succeeded in making nano-crystaline films. On the other hand, the existence of the plasma-induced damage by EBEP-CVD has been confirmed using the hydrogen plasma by measuring the photoluminescence (PL). After plasma exposure, broad band peak appears in the region of 1.0–0.78 eV (1.2–1.6 μm), and intensity of bound exciton peak with energy of 1.093 eV, which is measured and the non-irradiated silicon has been decreased. The same experiment was also performed with RF plasma and the peak appeared not only for EBEP but also for conventional RF plasma. The damage peak tends to disappear over 420°C of the substrate temperature. The damage recovery analysis has been done in relation to the annealing temperature of the substrate after the plasma exposure. Exciton peak has been increased by increasing the temperature especially at 350°C. Furthermore, plasma-induced peak intensity has been decreased at temperature higher than 500°C. Similar peak has been observed in the samples irradiated with high-energy protons. Therefore, positive ions in the plasma are thought to be the source of the damage of the silicon. The origin of the plasma-induced defect in Si is also considered. According to these results, the electric potential of substrate was controlled in order to avoid collision with positive ions in the plasma. When it was set to zero, plasma-induced peaks did not appear.     

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.     

Silicon sheet from silane: first results

A process to obtain self-supported thin silicon films is being developed. Films are grown by optical chemical vapour deposition (CVD) (using halogen lamps as heating system) from silane, at low temperature and relatively high growth rates, on silicon substrates with a sacrificial layer of porous Si (PS), which allows film detachment. The PS layer was formed by anodisation of the Si-substrate surface in a HF:ethanol solution. Film deposition was carried out in an optical cold wall horizontal CVD reactor, operating at atmospheric pressure, and specially designed for this study. Deposition rates and film morphology were studied as a function of substrate nominal temperature, gas concentration and flux. In the final chosen conditions, deposition occurs at a nominal temperature of 840°C, with a silicon growth rate between 2 and 4 μm/min, which is relatively high for the low temperature used. A good usage of silane gas was already achieved, with 80–85% of the silicon in the silane gas being deposited on a 40×40 mm² substrate, with very low deposition rate on the reactor wall. Films of thicknesses from 10 to 150 μm were deposited. The films were found to be continuous with surfaces coated with whiskers. Film detachment from multicrystalline substrates has proved unsuccessful so far, while it readily occurs when monocrystalline substrates are used. The reason for this is macropore collapse and film rupture, usually occurring in the smaller grain regions of the multicrystalline substrates.     

晶体硅太阳电池选择性扩散的研究

该文主要介绍一种制造太阳电池的选择性扩散新工艺,采用印刷电极的方法在硅片上的电极位置印刷高 磷浆料,然后在整个硅片表面喷涂一层浓度较低的磷源,放入高温炉中扩散后形成电极下具有较高的表面掺杂浓 度(-1020／cm3),电极间具有相对较低的表面掺杂浓度(-1019／cm3)。这样在电极下及具附近将构成一个浓度差结。 这种发射极结构有利于减小发射区复合电流、提高短波响应、作好欧姆接触和提高太阳电池的开路电压等优点。     

Enhancement of the optical absorption of thin-film of amorphorized silicon for photovoltaic energy conversion

In this study we report for the first time a method for simple, precise and single-step generation of thin film of amorphous silicon (a-Si) on silicon substrate induced by laser pulses with the frequency of megahertz under ambient conditions for solar cell fabrication. Also, the effect of laser parameters such as pulse width is investigated by developing an analytical model for the calculation of the non-dimensional surface temperature with various pulse widths; it was found from experimental and analytical results that for a constant power and repetition rate, an increase in the pulse duration corresponds to a significant increase in the surface temperature which results in an increase in the amount of amorphorized material as well as improvement of light absorption. A Scanning Electron Microscope (SEM), scanning near-field optical microscope (SNOM), a Micro-Raman, Energy Dispersive X-ray (EDX) spectroscopy and an optical spectrometer were used to analyze the optical and material properties of the amorphorized thin layer on Si-substrate.