Effects of hydrogen plasma on passivation and generation of defects in multicrystalline silicon

Hydrogenation by plasma is a low cost and efficient method to improve the photovoltaic properties of multicrystalline silicon. The role of plasma parameters on the efficiency of hydrogenation was studied using secondary ion mass spectrometry (SIMS), hydrogen effusion, electrochemical impedance spectroscopy and electron beam induced current (EBIC). The experimental results showed a deuterium concentration of 10²⁰ atoms cm⁻³ could be reached in the sample after a 15-min treatment. Optimal treatment time depends on temperature and leads to maximum electrical conductivity and minority carrier diffusion length. The results confirm the reduction of defects densities and potential barriers associated with grain boundaries.     

Characterisation of direct epitaxial silicon thin film solar cells on a low-cost substrate

Direct epitaxial crystalline silicon thin film (CSiTF) solar cells on low-cost silicon sheets from powder (SSP) ribbons have been prepared using rapid thermal chemical vapour deposition (CVD) growth. The characterisation of CSiTF solar cells was investigated by electron and spectrally resolved light beam induced current (EBIC and SR-LBIC, respectively). All EBIC measurements were performed on both the front-side surface as well as on the cross-section of CSiTF solar cells. The electrical recombination was detected by EBIC and compared with their morphologies. The results of EBIC scan show that recombination centres are situated at grain boundaries (GBs); higher the density of grain, higher the recombination activities (higher contrast). Recombination of different intensity (strong and weak) takes place at vertical GBs. Compared with the high recombination at GBs, the contrast in intragrain is low. The dark contrast of the GBs and intragrain defects is clearly reduced near the surface due to the passivation by hydrogen, which indicates that the minority carrier diffusion length decreases gradually with the depth perpendicular to the surface. The diffusion length was determined by SR-LBIC. The results show that the diffusion length distribution is quite inhomogeneous over the whole cell area. A maximum L_eff of about 25 μm and mean values around 15 μm are calculated for the best solar cell.     

High accelerating voltage electron beam induced current (EBIC) of thick and thin solar silicon specimens

High and low accelerating voltage electron beam induced current, HAV- and LAV-EBIC, microscopy was used to investigate both thick and thin silicon solar cell specimens to determine a correlation between EBIC and solar cell performance. In thick specimens, we observed that, unliked LAV-EBIC (5–50 keV), HAV-EBIC (200 keV) correlates well with measured solar cell properties because HAV-EBIC, like light, probes a silicon sample to a depth of hundreds of microns.

Thinned specimens were investigated in a specially built transmission electron microscope (TEM) stage that permitted correlation between TEM defect microstructure and HAV-EBIC electrical activity. In these thin specimens, the calculated spreading of the 200 keV electron beam is approximately 0.1 μm. The experimental resolution, though, is an order of magnitude larger, because the low collection efficiency forces operation at very high incident beam currents, with a concurrent increase in spot size.     

Distinguishing morphological and electrical defects in polycrystalline silicon solar cells using scanning electron acoustic microscopy and electron beam induced current

Morphological and electrical defects in polycrystalline silicon solar cells are distinguished by scanning electron acoustic microscopy (SEAM) and electron beam induced current (EBIC) techniques, respectively. It was found that while some defects are both morphologically and electrically detectable, some are predominantly only either electrical or morphological in nature. Combining both SEAM and EBIC is therefore an ideal approach as the two techniques can provide complementary information on both the morphological and electrical manifestation of the defects.     

Optical characterization of polysilicon thin films for solar applications

We report on the results of the investigation of optical properties and structure of PECVD deposited thin films of hydrogenated polysilicon determined by UV–Vis and IR spectroscopy. The influence of the hydrogen dilution of silane plasma at the PEVCD deposition on the film properties was investigated. The refractive index, the optical band gap energy and the microstructure of hydrogen and oxygen were analysed. The changes are discussed and correlated with the structure, the changes of the surface morphology and the hydrogen to silicon bonding. The optical band gap becomes larger than that of the undiluted sample. The results show that at dilution between 20 and 30 the transition between amorphous and crystalline phase occurs and the sample becomes a mixture of amorphous, polycrystalline phase with nano-sized grains and voids with decreasing hydrogen concentration. The presence of interstitial oxygen and oxygen bonded in surface Si–OH groups was detected.     

Stochastic modelling of daily beam irradiation

A dynamical statistical analysis of the daily sum of the beam irradiation measured, on a horizontal surface, in Genoa, Italy, has been done using a 9-year time series, with two substantially different methods: the Markov chain model and the first order autoregressive model. In the first case, the data range has been divided into five different equiprobable classes or “states”. The sequential characteristics of the obtained discrete time series have been described by four “seasonal” 5 × 5 transition matrices between the states of the process. Yearly series of daily beam irradiation have been simulated by associating suitable values of irradiation to every state of the chain. In the second case, data have been first modified in order to obtain a standard Normal frequency distribution; an autoregressive process of order 1 has been fitted to the transformed series. The autoregressive parameter has been estimated keeping it time invariant. Synthetic sequences of daily solar irradiations have been generated with the fitted model. The reliability both of the Markov chain model and of AR(1) model has been verified by comparing the artificial series to the empirical one. The autoregressive model has shown an appreciable superiority in reproducing the stochastic law of the daily sums of beam irradiation with respect to the Markov chain model.     

Electrical properties of multicrystalline silicon produced by electromagnetic casting process: Degradation and improvement

The electrical properties of boron-doped multicrystalline silicon for photovoltaic applications, elaborated by the cold crucible pulling process, are studied by the photoconductivity decay method and the electron beam-induced current measurement technique. The bulk lifetime mapping of the minority carriers in the as-grown silicon wafers is drawn up using both the techniques. Moreover, the consequence of phosphorus doping on the recombination properties of extended defects are studied using the EBIC measurements. Two different treatments are investigated in order to improve the electrical properties of the as-grown silicon wafers: (a) thermal phosphorus diffusion, for which the gettering efficiency is determined by the different treatment parameters; (b) remote plasma hydrogen passivation which leads to increase of the minority carrier lifetime.     

Photoinduced hydrogen evolution by use of porphyrin, EDTA, viologens and hydrogenase in solutions and Langmuir–Blodgett films

Photoinduced hydrogen evolution was investigated by use of a zinc porphyrin, EDTA, viologens and hydrogenase (H₂ase) in the solutions and Langmuir–Blodgett (LB) films. An almost linear increase of hydrogen evolution rate was observed with the increase of H₂ase concentrations from 1 to 5 μg/ml. For the zinc porphyrin, EDTA and methyl viologen, when their concentrations increased to a given value, hydrogen evolution did not show obvious increase. Phospholipid-porphyrin mixed LB films were prepared and used as photosensitizer for the photoinduced hydrogen evolution. Spectroscopic studies of the deoxygenated solutions indicated a “new” absorption band (in the solutions) or sharp peaks (in the LB films) when the sample solutions were irradiated, which was ascribed to the formation of an excited complex of porphyrin–EDTA (or -EDTA breakdown products). This excited complex was unstable to air.     

Influence of the substrate geometrical parameters on microcrystalline silicon growth for thin-film solar cells

The effect of substrate morphology on the growth and electrical properties of single-junction microcrystalline silicon cells is investigated. A large variety of V-shaped and U-shaped substrates are characterized by scanning electron microscopy (SEM) and the growth of thin-film microcrystalline silicon (μc-Si:H) devices is observed by cross-sectional transmission electron microscopy (TEM). It is shown that enhanced electrical properties of solar cells are obtained when U-shaped substrates are used and the effect is universal, i.e. independent of the substrate or feature size. U-shaped substrates prevent the formation of two dimensional “cracks”, which are identified as zones of porous material, from propagating throughout the active part of the solar cell. A numerical growth simulation program reproduces satisfactorily these experimental observations. According to these simulations, shadowing effect due to surface morphology and low adatom surface diffusion length are responsible for the formation of cracks in μc-Si:H material.     

电子束熔炼多晶硅温度场的数值模拟

采用有限元分析软件ANSYS对电子束熔炼多晶硅材料的工艺过程进行了数值模拟。在不同的电子束熔炼工艺参数条件下，进行了多晶硅温度场变化情况的数值模拟，并对其行了分析讨论。所得到的模拟结果较好地反应了电子柬熔炼多晶硅温度场变化的特点。     

Mean daily averages of beam radiation received by tilted surfaces as affected by the atmosphere

A method to compare the values of the monthly average beam radiation tilt factor, , obtained if atmospheric effects are accounted for or not, is presented. It is found that the difference between the two determinations may vary greatly with declination, latitude and ratio of daily averaged diffuse to global irradiation. In cases when this ratio is not obtained via direct measurement and its value is large enough, the accuracy of the “atmospheric” may be seriously affected. By comparison with measured radiation data from Montreal, Canada, the “atmospheric” as calculated by the proposed method appears very accurate for south vertical surfaces, and of acceptable accuracy for east or west vertical surfaces. In the latter case, a part of the discrepancy is attributed to various experimental errors, which are discussed.     

Complementary techniques for solid oxide electrolysis cell characterisation at the micro- and nano-scale

High-temperature steam electrolysis by solid oxide electrolysis cells (SOEC) is a method with great potential for transforming clean and renewable energy from non-fossil sources to synthetic fuels such as hydrogen, methane or dimethyl ether, which have been identified as promising alternative energy carriers. With the same technology, fuel gas can be used in a very efficient way to reconvert chemically stored energy into electrical energy, since SOECs also work in the reverse mode, operating as solid oxide fuel cells (SOFC). As solid oxide cells (SOC) perform at high-temperatures (700–900 °C), material degradation and evaporation can occur, e.g., from the cell-sealing material, leading to poisoning effects and aging mechanisms that decrease the cell efficiency and long-term durability. To investigate such cell degradation processes, thorough examination of SOCs often requires a chemical and structural characterisation at a microscopic and nanoscopic level. The combination of different microscopic techniques such as conventional scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and the focused ion beam (FIB) preparation technique for transmission electron microscopy (TEM) allows for post-mortem analysis at a multi-scale level. These complementary techniques can be used to characterise structural and chemical changes over a large and representative sample area (micro-scale) as well as at the nano-scale level for selected sample details. This article presents a methodical approach for the structural and chemical characterisation of changes in aged cathode-supported electrolysis cells produced at Risø DTU, Denmark. Additionally, we present results from the characterisation of impurities at the electrolyte/hydrogen interface caused by evaporation of sealing material.     

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10¹⁶/cm³ could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.     

Compositional changes in industrial hemp biomass (Cannabis sativa L.) induced by electron beam irradiation Pretreatment

The effects of electron beam irradiation on chemical decomposition of industrial hemp biomass were evaluated at doses of 150, 300, and 450 kGy. The quantity of decomposed components was indirectly estimated by measuring changes in alkaline extraction. The more severe degradation of structural components induced by higher irradiation doses resulted in larger amounts of alkaline extract. Carbohydrate compositional analysis using ¹H-NMR spectroscopy was applied to quantitatively investigate changes in the polysaccharides of the industrial hemp. The xylose peak intensity in the NMR spectra decreased with increasing electron irradiation dose, indicating that xylan was more sensitive to electron beam irradiation than cellulose.     

Strain gradient-induced diffusion of hydrogen in palladium and nickel membranes

The “uphill” diffusion of hydrogen during permeation through flat sheets of palladium and nickel has been studied by an electrochemical permeation method at 303 K. For both annealed and “as cold rolled” Pd samples, uphill diffusion effects on hydrogen absorption and desorption have been observed over a range of initial hydrogen contents from about H/Pd = 0.01, i.e. near or slightly less than the _max composition, up to H/Pd = 0.25–0.3. The occurrence of a non-Fickian component of permeation flux has been associated with temporary formation of lattice volume differences across the ( + β)/β and ( + β)/ interfaces during absorptions and desorptions, respectively. Influences of the magnitudes of galvanostatic hydrogen fluxes and of the membrane thickness on the uphill effects were examined. Analogous uphill effects were observed in similar studies with nickel membranes also in both annealed and “as cold rolled” states, which were much larger than those observed for palladium.     

The nature of mobile hydrogen in a-Si:H—Electrochemical studies

Electrochemical studies of both the a-Si:H based metal/oxide/semiconductor structures and a-Si:H modified Pt electrode as a part of two-electrode cell compartment with electrolyte were performed to obtain information on the nature of mobile hydrogen in a-Si:H. We assume, that the complex formed by a hydrogen molecule and a silicon dangling bond (the origin of D_h states) provides for mobile hydrogen. Information on the process of electron selfexchange (hopping) in a-Si:H driven by diffusion in the presence of a concentration gradient of redox centers is obtained. The capability of the electrochemical methods for studying the role of hydrogen in degradation of a-Si:H caused by bias annealing and/or illumination is demonstrated. Also, it was shown that because of the unique electrocatalytic features of the a-Si:H modified Pt electrode, the latter can act as a “programmable” and selective sensor of heavy metal ions and/or metal complexes.     

Sustainable energy planning based on a stand-alone hybrid renewableenergy/hydrogen power system: Application in Karpathos island, Greece

This study presents the sustainable planning of a renewables-based energy system, which aims to fulfil the electric needs of the island by replacing the existing diesel generators with new wind farms, photovoltaic installations and hydrogen production systems. Electric system design and least cost planning analysis were concluded using historic data from both demand and supply sides. An optimal “sustainable island” scheme should ensure 100% use of renewable energy resources for power generation, while hydrogen production is ideal for covering storage and transportation needs. Due to its morphology and scale, Karpathos applies perfectly for wind and solar energy systems, due to increased solar resource (about 1790 kWh/m².year of global irradiation) and high wind potential (average of 9 m/s in specific locations). Therefore, this case study examines an increase in RES penetration up to 20% in the electric energy mixture, a hydrogen production plan just for the needs of transport and a more aggressive, 100% renewables scheme that ensures a self-fulfilling energy system based on indigenous renewable resources.     

Precursor route poly(thienylene vinylene) for organic solar cells: Photophysics and photovoltaic performance

Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65 eV (750 nm), which corresponds to the absorption spectrum of the polymer.     

Key factors affecting microbial anode potential in a microbial electrolysis cell for H2 production

In order to optimize operations of microbial electrolysis cell (MEC) for hydrogen production, microbial anode potential (MAP) was analyzed as a function of factors in biofilm anode system, including pH, substrate and applied voltage. The results in “H” shape reactor showed that MAP reflected the information when any factor became limiting for hydrogen production. Commonly, hydrogen generation started around anode potential of −250 mV to −300 mV. While, higher current density and higher hydrogen rate were obtained when MAP went down to −400 mV or even lower in this study. Biofilm anode could work normally between pH 6.5 and 7.0, while the lowest anode potential appeared around 6.8–7.0. However, when pH was lower 6.0 or substrate concentration was less than 50 mg L⁻¹ in anode chamber, MAP went up to −300 mV or above, leading to hydrogen reduction. Applied voltage did not affect MAP much during the process of hydrogen production. Anode potential analysis also showed that planktonic bacteria in suspended solution presented positive effects on biofilm anode system and they contributed to enhance electron transfer by reducing internal resistance and lowering minimum voltage needed for hydrogen production to some extent.     

Investigation of single-walled carbon nanotubes-titanium metal composite as a possible hydrogen storage medium

The present experimental work deals with the investigation of hydrogen uptake study of single-walled carbon nanotubes (SWCNTs-Ti)-titanium metal composite. The mixture containing SWCNTs and Ti powder is made into tablet by cold pressing. The composite has been prepared and hydrogenated by evaporating the tablet in hydrogen ambient on glass substrates using electron beam (EB) evaporation technique. Efficient hydrogen uptake of 4.74 wt.% is achieved with the composite and the adsorbed hydrogen posses the average hydrogen binding energy of 0.4 eV. The obtained hydrogen uptake is due to the cumulative adsorption of hydrogen by CNTs and Ti nanostructured materials. The physical properties are characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction study (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Raman analysis. Hydrogenation and dehydrogenation behavior of the composite are studied using CHN-elemental analysis and thermo gravimetric/thermal desorption spectroscopy (TG/TDS) studies, respectively. The stored hydrogen is found to be 100% reversible in the temperature range of 160–310 °C.