Chemical surface passivation of low resistivity p-type Ge wafers for solar cell applications

The measurement of the bulk minority carrier lifetime of semiconductors requires efficient passivation of the recombination states at the surface. While numerous recipes have been published for Si-surface passivation, no adequate passivation methods are available for Ge. This paper presents a new and straightforward passivation method, based on a solution of iodine in polyvinyl acetate and acetone. The dependence of the carrier lifetime with time after passivation and with Ge resistivity has been investigated. It is found that the lifetime in this low resistivity material is strongly governed by Auger recombination.     

Defects generation by hydrogen passivation of polycrystalline silicon thin films

Hydrogen passivation technique is essential for improving the properties of polycrystalline silicon thin films. Elastic Recoil Detection Analysis (ERDA) indicated depth profiles of hydrogen concentration in poly-Si after hydrogen passivation. We have observed that plasma hydrogenation with duration up to 30 min effectively passivated the defects and improved photoluminescence intensity. Over 60 min of hydrogenation, PL intensity started to decrease. Raman scattering spectroscopy and X-ray rocking curve indicated that hydrogen created new defects and/or disorder with an increase in the hydrogen passivation time. Over 60 min, hydrogen started to form Si–H₂ bonding and hydrogen molecules (H₂), which lead to degradation of PL intensity. These peak positions were largely influenced by the grain size.These formations must be formed in quasi-stable sites, which are located at close to grain boundaries. Thus hydrogenation treatment may lead to defect passivation and new defects and/or disorder creation.     

Ambient stability of wet chemically passivated germanium wafer for crystalline solar cells

Surface passivation has been recognized as a crucial step in the evaluation of minority carrier lifetime of photovoltaic materials as well as in the fabrication of high efficient solar cells. Dilute acids of HF and HCl are employed for germanium (Ge) surface passivation. An effective lifetime of passivated Ge wafers has been evaluated by a microwave photoconductive decay (μ-PCD) measurement. Surface recombination velocities, S, of H- and Cl-terminated Ge surfaces are 23 and 37 cm/s, respectively. The stability of passivated Ge surfaces against exposure to air has also been examined. The HCl-passivated Ge surfaces are found to be more robust than HF-passivated surfaces.     

Hydrogenation of cyclohexene catalyzed by nanoporous SPE-PtNi/C membrane electrode

Implementing large-scale synthesis of nanoporous PtNi/C catalysts for hydrogenation processes of unsaturated organic compounds under atmospheric pressure at temperatures below 100 °C is an urgent task. In this paper, a nanoporous SPE-PtNi/C membrane electrode is prepared by ion beam sputtering, ultrasonic-assisted electrochemical dealloying, and hotpressing. Special attention is paid to the analysis of the electrochemical properties, phase structure, surface morphology, active constituent distribution, and catalytic efficiency for cyclohexene hydrogenation of the produced electrode in comparison with those of commercial Pt/C catalysts. The results show that the ultrasonic-assisted electrochemical etching temperature exerts the most significant effect on the catalytic activity of the catalyst under consideration. In particular, the treatment of PtNi/C in 0.7 mol/L HClO₄ for 1.5 h at the optimized temperature of 50 °C enables one to increase the catalytic activity by 25.20% at decreasing the Pt-loading content by 88.85% compared with commercial Pt/C. Furthermore, the nanoporous structure of the PtNi/C surface allows the binding energy of Pt 4f_7/2 to be reduced by 0.23 eV due to the Ni loss and the crystal plane shrinkage from NiPt, which enhances the compressive strain of the lattice structure of exposed Pt and increases the number of active sites. Finally, the use of a nanoporous SPE-PtNi/C membrane electrode could increase the yield of cyclohexene during the hydrogenation reaction by 179% at simultaneously improving the current efficiency by 69%.     

Hydrogen passivation of defects in EFG ribbon silicon

To enhance the bulk lifetime of multicrystalline silicon material, gettering of impurities and hydrogen passivation of defects are investigated. In edge-defined film-fed grown (EFG) ribbon silicon, an aluminium-enhanced hydrogenation of defects by silicon nitride has been reported. On thin wafers, the formation of a full area aluminium back surface field will lead to wafer bending due to different thermal expansion coefficients of aluminium and silicon. To circumvent this problem, remote plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) as passivation scheme for the front and rear surface is proposed. In this work, the bulk passivation by hydrogenation is investigated using two different hydrogen passivation techniques: (i) passivation in a remote hydrogen plasma and (ii) passivation due to a post-deposition anneal of remote PECVD-SiN_x in a lamp-heated conveyor belt furnace. Measurements of the bulk lifetime show that the lifetime improvement due to remote hydrogen plasma passivation degrades under illumination with white light. In contrast, the hydrogen passivation by a post-deposition SiN_x anneal is only effective if a phosphorous-doped emitter is present below the SiN_x layer during the hydrogenation. This lifetime improvement is stable under illumination.     

Passivation and etching of fine-grained polycrystalline silicon films by hydrogen treatment

Here we investigated the effects of hydrogen treatment on highly defected polycrystalline silicon solar cells in terms of defects passivation and surface etching. The poly-Si films were formed by high-temperature chemical vapour deposition. The hydrogen treatment was carried out through deposition of a-SiN_x:H layer followed by a thermal treatment or by direct hydrogen plasma. The deposition of silicon nitride layers on polysilicon cells led to a slight increase in the open-circuit voltage without damage to the surface. In contrast, after plasma hydrogenation, the results revealed an etching process of the emitter simultaneously with an important increase of the measured open-circuit voltage by a factor 2, reaching 420 mV.     

Mathematical modelling and simulation of nitrite hydrogenation in a membrane microreactor

Nitrite hydrogenation using heterogeneous catalysis is an important process for purification of wastewater or potable water. The main aim of this study is to explore a new mechanistic model and simulation for a heterogeneously catalysed reaction in a microporous catalytic layer. The system studied involves a liquid solution containing certain amount of nitrite, and a membrane reactor in which the nitrite penetrates into the catalytic layer to react with hydrogen. The developed model considers coupling between equations of momentum transfer in free and porous media and convection-diffusion of nitrite. It was found that there is great agreement between measured data and modelling values. Increasing velocity was the main reason for reduction of nitrite conversion and also there was a slight increase in nitrite conversion with increasing the thickness and porosity of catalytic layer. Furthermore, it was found that the diffusion mass transfer mechanism is favourable for nitrite hydrogenation while convective mass transfer of fluid flow has negative impact on nitrite hydrogenation.     

Efficiently catalytic transfer hydrogenation of aryl and heteroaryl halides by ultrafine palladium nanoparticles confined into UiO-66

The hydrodehalogenation of aryl and heteroaryl halides (AHHs) is very crucial for academic and industrial applications. Herein, ultrafine palladium nanoparticles (Pd NPs) with the size distribution about 1.77 ± 0.35 nm, were in-situ synthesized and confined into the metal-organic framework of UiO-66 (named as Pd@UiO-66) by impregnation reduction method without tedious post-reducing step. Pd@UiO-66 shows excellent activity with a high conversion (>90%) efficiency in the catalytic transfer hydrogenation (CTH) of AHHs under mild water systems utilizing ammonium formate as hydrogen donor. Furthermore, Pd@UiO-66 maintains highly excellent stability (conversion >95%) after 5 times reused cycles without losing catalytic activity and leaching Pd nanoparticles. This study supplies a new method for hybrid catalysts by immobilizing ultrafine Pd nanoparticles into crystalline MOFs, displaying efficient transform performance for halogen compounds by catalytic hydrogenation.     

A highly adaptable Ni catalyst for Liquid Organic Hydrogen Carriers hydrogenation

Reducing the cost of hydrogenation/dehydrogenation catalysts and improving the catalytic activity are essential steps to promote the commercial application of Liquid Organic Hydrogen Carriers (LOHCs) technology. We reported a series of highly adaptable 70 wt% Ni supported catalysts prepared by a facile co-precipitation method. The as-prepared catalysts were used in the hydrogenation of several promising LOHCs candidates, including benzene, N-propylcarbazole, N-ethylcarbazole and dibenzyltoluene. By adjusting the ratio of Al and Si, the Ni₇₀/AlSiO-1/1 catalyst with Al and Si in a molar ratio of 1:1 presents highest catalytic activity for hydrogenation of the above LOHCs, indicating the catalyst is highly adaptable for different LOHCs. The characterization results proved that the presence of SiO₂ could significantly weaken the interaction between metal and carrier and decrease the formation of NiAl₂O₄ species, which is beneficial to the reducibility of Ni. The introduced Al₂O₃ can inhibit the agglomeration of Ni and increase the dispersion of the metal. Besides, the Ni₇₀/AlSiO-1/1 catalyst was used to hydrogenate N-propylcarbazole by 5 cycles. In the fifth cycle, the hydrogen uptake reached the theoretical hydrogenation storage within 1.5 h, which suggested the excellent stability of the catalyst. Because of its low cost, high efficiency, high adaptation and highly stable, the self-made Ni catalyst has potential prospect in large-scale LOHCs application.     

Selection of competitive adsorption additives to relieve product inhibition of maleic acid hydrogenation in proton exchange membrane flow cell reactor: A molecular dynamics simulation

Adsorption of products on carbon carriers and the sluggish mass transport of products in the catalytic layer lead to product inhibition in proton exchange membrane flow cell reactor (PEMFCR), which seriously decreases hydrogenation performance. Here, via molecular dynamics simulations, an alleviative mechanism of competitive adsorption additives on product inhibition is proposed by choosing biomass derivatives maleic acid (MA) as hydrogenation model compound and ethanol (EtOH) as an additive model. The enhanced solvation and competitive adsorption synergistically promote the outflow of product succinic acid (SA) from the catalytic layer. Strong hydrophobic-hydrophobic interactions between EtOH and SA promote solvation and solubility of SA in the solvent water, and meanwhile the competitive adsorption steric hindrance of EtOH on adsorption interfaces weakens van der Waals interactions between SA and carbon carriers. Accordingly, the main structural characteristics that influence competitive adsorption of additives, such as, amphiphilic structure, polarity, molecular weight, and size of hydrophobic moiety, are exacted for selection of favorable additives. Among the studied water miscible alcohol and ketone additives, isopropanol (iPrOH) is proved to be favorable, which is further verified by the highest increase (64.3%) of hydrogenation conversion of MA in experiments. This work could provide theoretical guidance for selecting high-performance additives, intensifying heterogeneous hydrogenation in PEMFCR.     

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN_x film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1–10 μs to over 20 μs. Our results indicate that the optimum anneal temperature for SiN_x-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. We propose a three-step physical model, based on our results, in which defect passivation is governed by the release of hydrogen from the SiN_x film due to annealing, the generation of vacancies during Al–Si alloying, and the retention of hydrogen at defect sites due to rapid cooling. Controlled rapid cooling was implemented after the hydrogenation anneal to improve the retention of hydrogen at defect sites by incorporating an RTP contact firing scheme. RTP contact firing improved the performance of ribbon solar cells by 1.3–1.5% absolute when compared to slow, belt furnace contact firing. This enhancement was due to improved back surface recombination velocity, fill factor, and bulk lifetime. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%.     

Improving the performance of textured silicon solar cells through the field-effect passivation of aluminum oxide layers and up-conversion via multiple coatings with Er/Yb-doped phosphors

This paper examines the influence of field-effect passivation (from a coating of aluminum oxide) in conjunction with up-conversion (from multiple coatings containing Er/Yb-doped phosphors) on the performance of silicon solar cells. Note that the phosphors were applied to the rear surface of the cells. The surface morphology of the coatings was characterized by scanning electron microscopy and the chemical composition of Er/Yb-doped phosphors coating was examined using energy-dispersive X-ray spectroscopy. The fluorescence emissions of the coatings were examined using photoluminescence and optical image measurements. We examined the influence of field-effect passivation on dark current-voltage as well as photo-current density and external quantum efficiency (EQE). Improvements in photovoltaic performance after applying coatings containing Er/Yb-doped phosphors were estimated in terms of EQE and conversion efficiency. The field-effect passivation of Al₂O₃ and up-conversion provided by Er/Yb-doped phosphors resulted in EQE enhancements over a wavelength range of 600 to 1050 nm. Field-effect passivation was shown to enhance the conversion efficiency by 1.77% (from 16.91% to 17.21%), up-conversion enhanced conversion efficiency by 2.9% (from 17.21% to 17.71%), and a combination of field-effect passivation and up-conversion enhanced conversion efficiency by 4.73% (from 16.91% to 17.71%).     

Impact of thermal processes on multi-crystalline silicon

Fabrication of modern multi-crystalline silicon solar cells involves multiple processes that are thermally intensive. These include emitter diffusion, thermal oxidation and firing of the metal contacts. This paper illustrates the variation and potential effects upon recombination in the wafers due to these thermal processes. The use of light emitter diffusions more compatible with selective emitter designs had a more detrimental effect on the bulk lifetime of the silicon than that of heavier diffusions compatible with a homogenous emitter design and screen-printed contacts. This was primarily due to a reduced effectiveness of gettering for the light emitter. This reduction in lifetime could be mitigated through the use of a dedicated gettering process applied before emitter diffusion. Thermal oxidations could greatly improve surface passivation in the intragrain regions, with the higher temperatures yielding the highest quality surface passivation. However, the higher temperatures also led to an increase in bulk recombination either due to a reduced effectiveness of gettering, or due to the presence of a thicker oxide layer, which may interrupt hydrogen passivation. The effects of fast firing were separated into thermal effects and hydrogenation effects. While hydrogen can passivate defects hence improving the performance, thermal effects during fast firing can dissolve precipitating impurities such as iron or de-getter impurities hence lower the performance, leading to a poisoning of the intra-grain regions.     

Boosting the efficiency of solar cells fabricated on electromagnetic cold crucible cast multicrystalline silicon by means of hydrogen passivation

In this work the improvement of the quality of the electromagnetic cold crucible cast multicrystalline silicon (EMC) material produced by Sumitomo Sitix Co. (SCC, previously Osaka Titanium Co.) by hydrogen plasma is investigated with the final goal of realizing solar cells with the maximum posible efficiency. Two different hydrogen passivation techniques are implemented: hydrogen passivation by means of rf (radio frequency) plasma treatment and hydrogen passivation using microwave induced remote plasma treatment. By combining the oxide surface passivation and hydrogen passivation by remote plasma from the front side and by rf plasma from the back side, a significant improvement in short-circuit current, in open-circuit voltage, and in fill factor is obtained. A maximum efficiency of 16% on 2 × 2 cm² cells and of 14.5% on 10 × 10 cm² cells is achieved. This 16% efficiency is the highest ever reported on EMC multicrystalline silicon.     

Catalytic mechanism study on the gasification of depolymerizing slag in supercritical water for hydrogen production

Supercritical water gasification technology can realize efficient conversion of biomass, coal and other organics into hydrogen rich gas. But the efficiency of non-catalytic gasification at relative low temperature is not high. Besides, as for catalytic gasification, catalysis mechanism is complex. Thus how to improve efficiency and master the catalysis mechanism is a challenging issue. In this thesis, supercritical water gasification of depolymerizing slag experiments with the catalysis of different kinds of catalysts are conducted and the catalysis mechanism is analyzed. The results indicate that catalyst mechanism of K₂CO₃ is that it can promote the swelling and hydrolysis of lignocellulose and increase the amounts of phenolic intermediates. Ru/Al₂O₃ presents some different catalytic properties. It facilitates hydrogenation reaction of hydrolysis products, ring-opening reaction and the cleavage of carbon-carbon bonds then enhances gasification degree and increases gasification efficiency. Moreover, the binary catalyst displays a good synergic effect and the catalytic activity is higher than that of any single catalyst since these two catalysts promote various gasification stages. The gasification efficiency and hydrogen yield increase 13.22 mmol g^?1 and 66.46% respectively with the synergic catalyst of K₂CO₃ and Ru/Al₂O₃.     

Passivation and gettering of defective crystalline silicon solar cell

Different polycrystalline silicon and single-crystalline silicon with dislocations were used for passivation and gettering processes. These materials have defects and more impurity in the crystal. The dominant increase of electronic performance was found for wafers with more defects by using a different casting method. The wafers of single crystalline silicon with dislocations also have higher increase of efficiency of cell in comparison with that wafer without dislocations during oxide passivation processes used. POCl₃ was used for gettering processes. Single-crystal wafer with or without dislocations was used for comparison of gettering.     

Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.     

A study on the catalytic hydrogenation of N-ethylcarbazole on the mesoporous Pd/MoO3 catalyst

Mesoporous MoO₃ shows an apparent activity in the catalytic hydrogenation of N-ethylcarbazole (NEC), where a significant amount of tetrahydro-N-ethylcarbazole (4H-NEC) and perhydro-N-ethylcarbazole (PNEC) are detected with the hydrogen uptake of 0.97 wt% after 6 h when the temperature rises to 220 °C. 0.5 wt% Pd/MoO₃ catalyst shows a superior catalytic efficiency than the traditional precious metal catalysts 0.5 wt% Ru/Al₂O₃ and 0.5 wt% Pd/Al₂O₃, especially in the conversion of Octahydro-N-ethylcarbazole (8H-NEC) to PNEC. The hydrogenation mechanism of MoO₃ is completely different from the traditional precious metal catalysts. With the presence of a small amount of Pd, the breaking of HH bond is greatly accelerated, result in the promotion of hydrogen spillover rate and the increase of the concentration of hydrogen molybdenum bronze H_xMoO₃, which improves the catalytic efficiency of the MoO₃ catalyst. Rise the temperature also helps increasing the concentration of H in H_xMoO₃.     

Effect of hydrogen passivation on electrical transport in polycrystalline silicon

Hydrogen passivation of grain boundaries in polycrystalline silicon is shown to improve the efficiencies of devices fabricated from these materials. We have carried out hydrogen plasma passivation of polycrystalline silicon in RF discharge and have studied the variation in room temperature resistivity of the polysilicon wafer after H plasma treatment. In order to study intergrain changes in the polysilicon wafer after optimum H plasma treatment, we have carried out J(V,T) measurement in the temperature range 130 to 330 K. It is shown that after optimum hydrogenation, the temperature range over which thermionic emission occurs extrapolates to a much lower temperature value which is due to reduction of the grain boundary potential barrier after H passivation.     

生物油催化加氢提质的研究进展

生物油是生物质经快速热解技术制得的一种液体燃料。生物油含氧量较高,必须经过提质才能转化为高品位燃料。在生物油提质技术中,催化加氢由于能显著降低生物油含氧量,提高能量密度而受到广泛关注。文中首先介绍了催化加氢的基本原理,并从生物油中不同的加氢对象着手,根据工艺的温度、压力、时间、催化剂和反应器等条件,综述了国内外相关研究工作。最后针对目前生物油催化加氢技术存在的问题,提出其改进设想和前景展望。