Silicon Ribbon Research Articles

Physical mechanisms of in situ surface gettering of metals in ribbon silicon for solar cells

The authors have employed synchrotron-based x-ray absorption near-edge spectroscopy and x-ray fluorescence microscopy to identify the chemical state and distribution of metals gettered to the surface of String Ribbon silicon grown with CO-ambient gas. Copper and nickel precipitates were observed in their equilibrium silicide phases, indicating a dominant relaxation gettering mechanism. In addition, microwave photoconductive decay measurements show a decrease in bulk iron concentration and a tenfold increase in minority carrier lifetime in CO-ambient grown material. Implications of the observed gettering mechanism on ribbon-type solar cells are discussed.

3D coupled electromagnetic and thermal modelling of EFG silicon tube growth

The edge-defined film-fed growth (EFG) process is mainly used to grow silicon ribbons and hollow cylinders as well as hollow polygons of various geometries. The development of the EFG process is primarily focused on technology improvements and cost reduction of wafers for photovoltaic applications. In case of the growth of hollow polygons, three-dimensional (3D) numerical analysis is extensively used because the system of polygonal geometry cannot be adequately described in axisymmetric model. Electromagnetic simulation predicts a 3D distribution of electromagnetic energy in all the system. The calculated three-dimensional temperature fields allow the analysis of the temperature profiles along and across the growing silicon tubes and the investigation of thermally induced stress and strain in different cases. The non-linear model consisting on electromagnetic, thermal and structural simulations has been adjusted and successfully validated by experimental tests with industrial installations.

Heterogeneous Three-Dimensional Electronics by Use of Printed Semiconductor Nanomaterials

We developed a simple approach to combine broad classes of dissimilar materials into heterogeneously integrated electronic systems with two- or three-dimensional layouts. The process begins with the synthesis of different semiconductor nanomaterials, such as single-walled carbon nanotubes and single-crystal micro- and nanoscale wires and ribbons of gallium nitride, silicon, and gallium arsenide on separate substrates. Repeated application of an additive, transfer printing process that uses soft stamps with these substrates as donors, followed by device and interconnect formation, yields high-performance heterogeneously integrated electronics that incorporate any combination of semiconductor nanomaterials on rigid or flexible device substrates. This versatile methodology can produce a wide range of unusual electronic systems that would be impossible to achieve with other techniques.

Fabrication and analysis of high-efficiency String Ribbon Si solar cells

This paper reports on the solar cell efficiencies of 18.3% and 16.8% (both 4cm2) achieved on String Ribbon Si material using photolithography-defined and screen-printed front grid contacts, respectively, reinforcing the potential of String Ribbon Si for high-performance industrial cells. Rapid co-firing of screen-printed Al on the rear and low-frequency plasma-enhanced chemical vapor deposited single-layer SiNx antireflection coating on the front in a rapid thermal processing unit was combined to enhance the as-grown minority carrier lifetime of 2–3μs to 91μs in processed String Ribbon Si cells. The performance of the best screen- printed String Ribbon Si cell (16.8%) was comparable to the 17.0% efficient planar float-zone (FZ) Si cell, which was processed simultaneously in the same run. However, the average performance of String Ribbon Si cells was 0.9%–1.4% lower in absolute efficiency relative to the FZ Si cells. Detailed characterization and analysis revealed that this is due to the presence of distributed electrically active defects, which could not be fully passivated, in spite of very effective defect hydrogenation during the cell processing.

High-speed mechanically flexible single-crystal silicon thin-film transistors on plastic substrates

This letter describes the fabrication and properties of bendable single-crystal-silicon thin film transistors formed on plastic substrates. These devices use ultrathin single-crystal silicon ribbons for the semiconductor, with optimized device layouts and low-temperature gate dielectrics. The level of performance that can be achieved approaches that of traditional silicon transistors on rigid bulk wafers: effective mobilities>500cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V/spl middot/s, ON/OFF ratios >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , and response frequencies > 500 MHz at channel lengths of 2 μm. This type of device might provide a promising route to flexible digital circuits for classes of applications whose performance requirements cannot be satisfied with organic semiconductors, amorphous silicon, or other related approaches.

Mechanically flexible thin-film transistors that use ultrathin ribbons of silicon derived from bulk wafers

This letter introduces a type of thin-film transistor that uses aligned arrays of thin (submicron) ribbons of single-crystal silicon created by lithographic patterning and anisotropic etching of bulk silicon (111) wafers. Devices that incorporate such ribbons printed onto thin plastic substrates show good electrical properties and mechanical flexibility. Effective device mobilities, as evaluated in the linear regime, were as high as 360cm2V−1s−1, and on/off ratios were &amp;gt;103. These results may represent important steps toward a low-cost approach to large-area, high-performance, mechanically flexible electronic systems for structural health monitors, sensors, displays, and other applications.

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

Abstract We have fabricated 4 cm2 solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (Voc) of 620 mV, a short circuit current density (Jsc) of 36.8 mA/cm2 and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a Voc of 620 mV, a Jsc of 37.5 mA/cm2 and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiNx and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (∼1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.

Epitaxial thin-film Si solar cells

Most types of thin-film solar cells imply a radical departure from the dominant bulk crystalline Si technology. This is not the case for epitaxial thin-film solar cells. In this technology, a high quality Si layer is deposited epitaxially on a low-cost Si substrate (e.g. cast Upgraded Metallurgical Grade silicon or high-throughput Si ribbons) and processed into a solar cell. This technology combines the well-known advantages of crystalline Si (high efficiency potential, stability and reliability) with a substantial cost reduction potential. Moreover, the similarity to the traditional Si technology lowers the threshold for adoption by the PV industry. This paper gives an overview of the field of epitaxial thin-film solar cells, covering substrates, deposition techniques and solar cell processing. Achievements reported in the literature are summarized and recent results are presented. Special attention is given to the crucial issue of achieving high currents by increasing absorbance in the active layer. This can be achieved by increased absorption through material engineering and through implementation of advanced light confinement schemes.

Comparison of String and Ribbon Samplers in Orchard Spray Applications

The main objective of this study was to establish the relationship between spray deposition on ribbon and string samplers in field applications. The experiment involved five commonly used citrus air-carrier sprayers, which included both conventional and tower configurations, equipped with axial- or cross-flow fans and hydraulic nozzles or rotary atomizers. A sampling structure, consisting of vertical and horizontal lines of polyester string and cotton ribbon, was constructed in a citrus grove. Spray mixtures, containing a fluorescent tracer, were applied at different volume rates and ground speeds (nine treatments) on a 30 m course between two tree rows. Shortly after spraying each replication, the lines were collected in pieces. In the laboratory, the amount of deposit was determined by fluorometry. Weather data recorded during the applications were used as co-variables in the analysis of variance. Each treatment was replicated 3 to 5 times. The results showed a significant difference between capture efficiencies of the two target types. In all sample locations, the amount of spray deposit on the string was higher than that captured on cotton ribbon. Overall, the mean and standard deviation of the string/ribbon deposit ratios were 2.3 and 0.7, respectively.

Large silicon crystal hollow-tube growth by the edge-defined film-fed growth (EFG) method

Commercialization of the edge-defined film-fed growth (EFG) process for growth of silicon ribbon crystals in the form of hollow tubes has directed the development of EFG furnaces toward increasing tube diameters in order to maximize productivity in manufacture of wafers for solar cells. The current state-of-the-art EFG production is based on thin-walled octagon tubes with 12.5 cm faces, where tube diameter is of the order of 38 cm. We describe here the challenges faced in bringing EFG technology into large-scale manufacturing, and ongoing development of furnace designs for growth of tubes for larger wafer production using hexagons with 15 cm face widths, and wall thicknesses in the range 250–300 μm.

Effect of Material Inhomogeneity on the Open-Circuit Voltage of String Ribbon Si Solar Cells

The effect of material inhomogeneity on open-circuit voltage (V/sub OC/) of string ribbon Si solar cells is investigated by a combination of experimental results and a simple analytical model. Light beam-induced current (LBIC) measurements showed that a cell with no detectable defective region gave an efficiency of 15.9% with a V/sub OC/ of 616 mV. However, a neighboring cell with highly defective regions covering 38% of its area, as determined by LBIC measurements, gave an efficiency of 14.1% with V/sub OC/ of 578 mV. Another cell with 19% highly defective regions gave an efficiency of 15.0% with V/sub OC/ of 592 mV. A simple and approximate analytical model was developed to quantify the loss in V/sub OC/ on the basis of recombination intensity and the area fraction of defective regions. This model showed that the majority of the loss in V/sub OC/ is associated with the most defective region, even if its area fraction is relatively small.

Fabrication of silicon submicrometer ribbons by glancing angle deposition

Ribbon structures on the submicrometer scale are of interest for the development of nanodevices in various fields. We fabricate periodic arrays of silicon ribbons using electron beam lithography and thin film deposition at highly oblique incident angles. A periodicity of 1 µm and a line width less than 100 nm is used for the lithographically prepatterned substrate seed layers to ensure that the planar fill factor was less than the equilibrium volume fill factor of the thin film. Individual ribbons exhibit a width of approximately 1.8 µm, controlled by the length of deposition, and a thickness of approximately 100 nm. The ribbons fabricated for this experiment have a length of 4 mm, and exhibit an amorphous structure with scattered crystallites throughout the matrix.

Influence of low-energy ion-beam treatment by hydrogen on electrical activity of grain boundaries in polycrystalline silicon

The results of investigation into the effect of hydrogen plasma treatment and annealing on the electrical activity of grain boundaries (GBs) in silicon ribbons manufactured with the edge-defined film-fed growth (EFG) technique are presented. It is shown that hydrogenation with small doses of introduced hydrogen leads to decrease of the GB electrical activity, whereas further growth of hydrogenation time gives rise either to saturation or increase of electrical activity for annealed and non-annealed ribbons, respectively. The latter is explained in terms of a model taking into account the concurrence of GB defects passivation and neutralization of boron ions in the vicinity of GB during hydrogenation.

Electronic and structural properties of grain boundaries in electron-irradiated edge-defined film-fed growth silicon

Edge-defined film-fed growth (EFG) is an economical method of producing multicrystalline silicon ribbon for solar cells. Such silicon is heavily doped with carbon. After electron irradiation, the dominant defect found in this material is the G-centre, which is associated with the CsCi defect. In this paper, the techniques of scanning cathodoluminescence and electron backscattered diffraction pattern analysis are used to correlate the luminescence from the G-centre with the grain boundary structure in electron-irradiated EFG silicon. A localized enhancement of G-centre luminescence is found near twin boundaries at temperatures above 20 K, whereas no such enhancement is found near low-angle grain boundaries at temperatures up to 80 K or at twin boundaries below 20 K. This behaviour may be caused by thermal ionization of excitons from traps at the twin boundaries, and their subsequent capture at G-centres.

Impact of oxygen on carbon precipitation in polycrystalline ribbon silicon

This article reports experimental evidence for the effect of oxygen on carbon precipitation in polycrystalline ribbon silicon. Four sets of wafers subject to various heat treatments have been examined by infrared spectroscopy. It is found that carbon precipitation in an oxygen-containing wafer consists of two distinct steps, namely, an initial rapid oxygen–carbon coprecipitation in the very first hour annealing, followed by slow precipitation during subsequent prolonged annealing. A high oxygen content enhances carbon precipitation throughout the two steps. It is shown that the formation of interstitial carbon in the presence of excess silicon self-interstitials generated during oxygen precipitation plays an important role in increasing the carbon precipitation rate in the first hour annealing. Because of the absence of interstitial injection during the following slow precipitation process, the enhancement effect of oxygen can only arise from an increase in precipitation sites. It is proposed that the oxygen–carbon coprecipitates formed in the very first hour annealing provide sites for continuous carbon precipitation. This explains why carbon impurities precipitate faster in a high oxygen-containing wafer, even after removal of all the interstitial oxygen from the silicon matrix.

Study on epitaxial silicon thin film solar cells on low cost silicon ribbon substrates

Heavily B-doped silicon ribbons were prepared by modified silicon sheets from powder (SSP) technique using 9 N purity electronic-grade silicon powder. Utilizing the SSP ribbons as substrates, crystalline silicon thin film (CSiTF) solar cells have been fabricated by direct epitaxy method. As-prepared CSiTF solar cells were characterized by illuminated and dark I– V characteristic measurement, Suns- V oc measurement, quantum efficiency (QE) and reflectance measurement. The results show that the CSiTF solar cells have poor performance with typical efficiency η only about 4.25%, and nearly all the characteristic parameters are far away from the ideal values. By detailed analysis and discussion on the measured results, it was found that grain boundary (GB) recombination in the active layers and electrical leakage at the edge of the PN junctions play a major role in deteriorating the performance of the CSiTF solar cells, which lead to higher dark saturation current I s and lower parallel resistance R sh, respectively. By improving the preparation process, such as optimization of the epitaxy process, use of remote plasma hydrogen passivation (RPHP), optimization of the metallization and use of double layer anti-reflection (DLAR) coatings, etc., all the characteristic parameters of the CSiTF solar cells have been greatly improved, and the best efficiency value increased to 8.02%.

Measurement of residual stress in multicrystalline silicon ribbons by a self-calibrating infrared photoelastic method

This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on the infrared photoelastic technique. This self-calibrating method allows the in situ determination of the photoelastic coefficients and can thus be used for any crystal orientation. The method was validated by the experimental determination of the photoelastic coefficient of monocrystalline (100) silicon wafers and by comparison with strain measurements using asymmetrical x-ray diffraction. The distribution of residual stress in multicrystalline silicon ribbons was also measured. The results showed strong evidence for tensile stress in the central region and compressive stress near the edges of the ribbons. Both the measured residual stress and the photoelastic coefficient distributions are correlated to grain boundaries.

Measurement of residual stress in EFG ribbons using a phase-shifting IR photoelastic method

Abstract This paper reports on the measurement of residual stress in EFG silicon ribbons for solar cell applications using the phase-shifting infrared (IR) photoelastic method. The samples analysed were wafers cut from EFG octagons with 100 mm face width and from EFG 125 mm face-width octagon under development. Experimental results show that the distribution of residual stress in both types of samples is similar, within measurement uncertainties. The average residual stress in the samples is about 8 MPa. Maximum stresses of around 30 MPa are associated with twin and grain boundaries. Significant variations of stress along the growth direction, possibly related to buckling, were also measured.

Spatially resolved investigations of lifetime enhancement in vertically grown, multicrystalline silicon ribbons

Abstract The influence of gettering or defect passivation steps on recombination activity in the vertically grown, multicrystalline ribbon materials edge-defined film-fed growth and string ribbon silicon has been investigated with the help of photoconductance decay. In contrast to well-known results of integral measurements, spatially resolved lifetime mappings have been obtained by applying microwave detection technique. This aspect of spatial resolution has been found to be indispensable for investigating the impact of different processing steps on material quality in an accurate way. Apart from strong variations in as-grown lifetimes that have been found throughout vertically grown silicon wafers, this is due to areas of comparable starting lifetimes which have been revealed to react very differently to applied processing steps. After processing, some of them reach minority charge carrier lifetimes of more than 300 μ s whereas others just show values of a few microseconds. As a consequence, the results of integral measurements strongly depend on the nature of areas incorporated in the specific sample. An impression of the corresponding uncertainties inherent to integral measurements has been obtained by statistical evaluation of spatially resolved lifetime data.

Processing step-related upgrading of silicon-based solar cells detected by photoluminescence spectroscopy

The aim of this work was to study the effect of solar cell processing steps on the electrical quality of the material, using photoluminescence spectroscopy as a non-contact technique. It was confirmed that consecutive steps in solar cell processing not only lead to electronic quality upgrading in Ribbon and multicrystalline silicon wafers, but also to the introduction of recombining defects. Furthermore, the importance and usefulness of photoluminescence technique on the overall improvement of the substrate material is discussed.