Silicon Ribbon Research Articles

First solar cells on silicon wafers doped using sprayed boric acid

A new method for boron bulk doping of silicon ribbons is developed. The method is based on the spraying of the ribbons with a boric acid solution and is particularly suited for silicon ribbons that require a zone-melting recrystallization step. To analyse the quality of the material thus obtained, multicrystalline silicon samples doped with this doping process were used as substrate for solar cells and compared with solar cells made on commercial multicrystalline silicon wafers. The values obtained for the diffusion length and the IV curve parameters show that the method of doping with the boric acid solution is suitable to produce p-doped silicon ribbons for solar cell applications.

Solar cell fabrication using edge-defined film-fed growth (EFG) silicon wafers

The insufficient supply of polysilicon is limiting the growth of the segment of the photovoltaic industry using silicon materials. Because it is grown directly in the form of ribbon from a silicon melt, edge-defined film-fed growth (EFG) silicon ribbon is a promising alternative for cutting down wafer costs by reducing the polysilicon consumption and eliminating kerf loss. In this paper, we will discuss the various properties that can be achieved with for low cost and high-efficiency EFG silicon ribbon solar cell fabrication. Boron-doped p-type EFG ribbon silicon wafers with resistivities of 2–4 Ω cm and a size of 125 mm × 125 mm were used. The major fabrication steps we studied were mixed acid (HF, HNO 3, DI water) texturing, phosphorus diffusion with POCl 3, thermal oxide growth for surface passivation, laser process for edge isolation, and PECVD of SiN x :H for surface passivation and antireflection coating. By optimizing the processing steps, we achieved a conversion efficiency, open circuit voltage, short circuit current, and fill factor as high as 14.5%, 584 mV, 32.1 mA/cm 2, and 77%, respectively.

Stretchable Microfluidic Radiofrequency Antennas

www.MaterialsViews.com C O M Stretchable Microfl uidic Radiofrequency Antennas M U N I By Masahiro Kubo , Xiaofeng Li , Choongik Kim , Michinao Hashimoto , Benjamin J. Wiley , Donhee Ham , and George M. Whitesides * C A IO N This paper describes a new method for fabricating stretchable radiofrequency antennas. The antennas consist of liquid metal (eutectic gallium indium alloy, EGaIn [ 1 , 2 ] ) enclosed in elastomeric microfl uidic channels. In particular, a microfl uidic structure made of two types of elastomers (polydimethylsiloxane (PDMS) and Ecofl ex (type 0030, Reynolds Advanced Materials)) with different stiffness has been developed to improve the stretchability and mechanical stability of the antennas. These antennas can be stretched up to a strain [defi ned as the percentage change in length or ( l – l 0 )/ l 0 ] of 120 %. This high stretchability allows the resonance frequencies of the antennas to be mechanically tuned over a wide range of frequencies. The antennas can also be repeatedly stretched, while retaining a high effi ciency (> 95 %) in radiation. “Stretchability” in electronics has the potential to open new opportunities, particularly for large-area devices and systems, and in systems that require the device to conform to a nonplanar surface, or to bend and stretch while in use. [ 3–5 ] Compared to “fl exible” electronics built on nonstretchable polymer or paper substrates, [ 6 , 7 ] stretchable electronics can cover almost arbitrarily curved surfaces and movable parts. Mechanical compliance may increase the comfort of the user for wearable electronics or implantable medical devices, and simplify the integration for a range of applications. [ 3–5 , 8 ] New approaches to stretchable electronics are now being developed. In a recent advance, Rogers et al. [ 4 , 5 ] described stretchable integrated circuits with elongation of up to 100 % using wavy, thin silicon ribbons on pre-stretched elastic substrates. Antennas offer new, attractive applications for stretchable electronics; these applications might include reconfi gurable antennas, [ 9 ] antennas for limited and nonplanar spaces, [ 10 ] and wearable sensors. Two methods are commonly used to build antennas for commercial applications. The most common method uses sheet-metal processing; in this method, a metal sheet is punched, bent, and welded into the desired structure. A second method uses chemical etching and plating to make small patterns of metal. This method can make fl exible antennas by patterning metal on a fl exible substrate. Neither

Fabrication of microstructured silicon (µs-Si) from a bulk Si wafer and its use in the printing of high-performance thin-film transistors on plastic substrates

In this paper, we report a new fabrication route to generate microstructured, single-crystalline silicon (µs-Si) ribbons using (1 1 0) silicon. Two different methods were explored for producing these printable structures. This work also introduces a second-process innovation in the fabrication of microstructured semiconductor objects for printed large-area circuits, namely the direct integration of a high-quality, thermally grown silicon dioxide (SiO2) layer for use as a gate dielectric in top-gate metal-oxide-silicon field effect transistors. We also demonstrate and characterize a soft, conformable lamination process that considerably enhances the mechanical stability of devices printed on plastic, allowing bending radii as small as 0.8 cm. These structures enable a reduction of the bending strains localized at the device interface. These improvements were fully characterized by finite element simulations of the strain distribution present in a descriptive model of the multilayer laminated circuit.

Hydrogen Defect Passivation of Silicon Transistor on Plastic for High Performance Flexible Device Application

The electrical characteristics of a transistor on a transferred silicon ribbon are demonstrated. The process temperature is limited to 200°C for potential use on plastic sheets. Additional hydrogen annealing reduces the threshold voltage and improves the transistor properties. A high mobility of around 160 cm 2 /V s, with a high on/off ratio and an off current of as low as < 10 11 A, is achieved. The flexibility of the device is evaluated after applying stress in the bended condition. The device shows very little change in properties with a bending radius <4 mm. Overall, good electrical and mechanical properties are demonstrated for future use on flexible device applications.

SEM Characterization of Silicon Layers Grown on Carbon Foil

Characterization of defect structure in silicon ribbon grown on carbon foil has been carried out. The structure of grown Si layers and a dislocation density in these layers have been studied using selective chemical etching and the Electron Backscattering Diffraction. It is observed that the layers consist of rather large grains, the majority of which is elongated along the growth direction with a similar surface orientation and with a misorientation angle between neighboring grains of 60º. This means that such grains are separated by the (111) twin boundaries. The dislocation density in different grains is found to vary from 102 to 107cm-2. The energy dispersive X-Ray microanalysis has shown that some twin boundaries are enriched with carbon.

Hydrogenation in Crystalline Silicon Materials for Photovoltaic Application

In this contribution an overview of hydrogenation issues for (multi-)crystalline silicon material is given. Crystalline silicon material for photovoltaic application contains more defects than material used for other semiconductor device fabrication. Therefore passivation of bulk defects has to be performed to reach higher efficiencies and exploit the cost reduction potential of these materials. Especially minority charge carrier lifetimes of ribbon silicon can be drastically improved by hydrogenation in combination with a gettering step. Apart from bulk passivation atomic hydrogen plays an important role in surface passivation via dielectric layers. Performance of single dielectric layers or stack systems can be increased after a hydrogenation step. It is believed that hydrogen can passivate defects at the silicon/dielectric interface allowing for lower surface recombination velocities. In industrial application hydrogenation is performed via deposition of a hydrogen-rich PECVD SiNx layer followed by a belt furnace annealing step. Surface passivation for characterization of charge carrier bulk lifetime is often performed with the same technique, omitting the annealing step to avoid in-diffusion of hydrogen. It is shown that for some crystalline silicon materials even the PECVD SiNx deposition alone (without annealing step) can cause significant bulk defect passivation, which in this case causes an unwanted change of bulk lifetime.

Dislocation Engineering in Multicrystalline Silicon

Dislocations are known to be among the most deleterious performance-limiting defects in multicrystalline silicon (mc-Si) based solar cells. In this work, we propose a method to remove dislocations based on a high temperature treatment. Dislocation density reductions of &gt;95% are achieved in commercial ribbon silicon with a double-sided silicon nitride coating via high temperature annealing under ambient conditions. The dislocation density reduction follows temperature-dependent and time-dependent models developed by Kuhlmann et al. for the annealing of dislocations in face-centered cubic metals. It is believed that higher annealing temperatures (&gt;1170°C) allow dislocation movement unconstrained by crystallographic glide planes, leading to pairwise dislocation annihilation within minutes.

Modeling a linear electric molten zone in a silicon ribbon

AbstractThe linear electric molten zone produced by passing an electric current in a silicon ribbon may be used to create a floating molten zone specially suited for a recrystallization process. A one‐dimensional heat transfer numerical model of a linear electric molten zone is developed. Simulation results show the electric current concentration mechanism, and describe the effect of parameters such as the auxiliary radiative input power, or the ribbon thermal conditioning and geometry on the temperature profile and the molten zone width. It is shown that producing a linear molten zone electrically, is about half as energy intensive as if this were done solely by radiative heating. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The silicon on dust substrate path to make solar cells directly from a gaseous feedstock

In this paper, we present a silicon on dust substrate (SDS) process, a new method for the growth of silicon ribbons. As a demonstration of the concept, we also present results on solar cells made of these new silicon ribbons. SDS ribbons were obtained directly from a gaseous feedstock by a fast CVD step using silane. The resulting self-supported intrinsic ribbons were microcrystalline and porous. To make these ribbon films suitable for photovoltaic applications, a novel recrystallization with an in situ doping step was developed. To this purpose, the ribbons were sprayed with boric acid and then recrystallized by float zone melting. Simple solar cells were prepared by employing: aluminium back contacts, Ti/Pd/Ag front grid contacts, with no anti-reflective coating, doping optimization, passivation or gettering. The 1-sun I–V characteristics of the cells were: Voc ∼ 530 mV and Jsc ∼ 24 mA cm−2. The minority carrier diffusion length obtained from a spectral response at long wavelengths gave values of Ln ∼ 70 µm.

Iron point defect reduction in multicrystalline silicon solar cells

In this work, we propose and demonstrate an annealing procedure designed to improve the performance of iron-contaminated silicon solar cells. Specifically, we put forward the idea that cells contaminated with iron should be annealed at appropriate times and temperatures to allow for the transformation from supersaturated point defects to distributed iron silicide precipitates. We examine the optimal transformation rate for string ribbon multicrystalline silicon and demonstrate that a 30min annealing can improve the efficiency of cells manufactured from low-purity feedstock.

Emissions from Photovoltaic Life Cycles

Photovoltaic (PV) technologies have shown remarkable progress recently in terms of annual production capacity and life cycle environmental performances, which necessitate timely updates of environmental indicators. Based on PV production data of 2004-2006, this study presents the life-cycle greenhouse gas emissions, criteria pollutant emissions, and heavy metal emissions from four types of major commercial PV systems: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride. Life-cycle emissions were determined by employing average electricity mixtures in Europe and the United States during the materials and module production for each PV system. Among the current vintage of PV technologies, thin-film cadmium telluride (CdTe) PV emits the least amount of harmful air emissions as it requires the least amount of energy during the module production. However, the differences in the emissions between different PV technologies are very small in comparison to the emissions from conventional energy technologies that PV could displace. As a part of prospective analysis, the effect of PV breeder was investigated. Overall, all PV technologies generate far less life-cycle air emissions per GWh than conventional fossil-fuel-based electricity generation technologies. At least 89% of air emissions associated with electricity generation could be prevented if electricity from photovoltaics displaces electricity from the grid.

Carrier lifetime limitation defects in polycrystalline silicon ribbons grown on substrate (RGS)

Carrier lifetime limitation defects in polycrystalline silicon ribbons have been examined in samples with high oxygen and carbon content. Infrared spectroscopy showed that essentially all supersaturated oxygen impurities precipitated within 1 h annealing at over 800 °C. Preferential defect etching revealed that a much higher density of oxygen precipitates were generated in dislocation-free grains than in those highly dislocated (10 5–10 7 cm −2) ones. Correlated with electron-beam-induced current imaging, we found that oxygen precipitates are the dominant carrier recombination defects in dislocation-free grains, while dislocations are the lifetime killer for highly dislocated grains. It is suggested that eliminating dislocations alone will not improve the carrier lifetime, considering that a higher density of oxygen precipitates formed in the absence of dislocation-related heterogeneous nucleation sites will significantly degrade the carrier lifetime.

Deformation and Fracture Behavior of Rapidly Solidified and Annealed Iron-Silicon Alloys

In this study, the mechanical properties, deformation behavior, and fracture modes of iron-silicon melt-spun ribbons are related to changes in silicon composition from 4.5 to 6.5 wt pct and the influence of ordering phase transformations. The as-solidified melt-spun ribbons, which exhibit plasticity even for the Fe-6.5 wt pct Si composition, provide the opportunity to characterize dislocation glide. Tensile deformation with a plastic strain of ∼2 pct produced planar slip of pure edge and pure screw mainly on {112} slip planes although slip on {011} and {123} planes was also observed. The extended dislocations became qualitatively more planar as silicon concentration increased owing to reduced cross-slip. For the Fe-6.5 wt pct Si material, pairs of dislocations with the same Burgers vector were observed. As ribbon thickness increased, the material’s ductility decreased. Thinner ribbons provide a reduced mean free path of gliding dislocations and fewer impediments to glide before they reach the ribbon surface, which removes strain hardening effects. In the as-solidified state, the B2 and DO3 order has been suppressed. Heat treating the 6.5 wt pct silicon ribbons induces the ordering phase transformations and reduces the ductility. The most embrittled condition occurs for the coexistence of B2 and D03 ordered domains after annealing at 400 °C to 500 °C.

Complementary Logic Gates and Ring Oscillators on Plastic Substrates by Use of Printed Ribbons of Single-Crystalline Silicon

CMOS inverters and three-stage ring oscillators were formed on flexible plastic substrates by transfer printing of p-type and n-type single crystalline ribbons of silicon. The gain and the sum of high and low noise margins of the inverters were as high as ~150 and 4.5 V at supply voltages of 5 V, respectively. The frequencies of the ring oscillators reached 2.6 MHz at supply voltages of 10 V. These results, as obtained with devices that have relatively large critical dimensions (i.e., channel lengths in the several micrometer range), taken together with good mechanical bendability, suggest promise for the use of this type of technology for flexible electronic systems.

Finite deformation mechanics in buckled thin films on compliant supports

We present detailed experimental and theoretical studies of the mechanics of thin buckled films on compliant substrates. In particular, accurate measurements of the wavelengths and amplitudes in structures that consist of thin, single-crystal ribbons of silicon covalently bonded to elastomeric substrates of poly(dimethylsiloxane) reveal responses that include wavelengths that change in an approximately linear fashion with strain in the substrate, for all values of strain above the critical strain for buckling. Theoretical reexamination of this system yields analytical models that can explain these and other experimental observations at a quantitative level. We show that the resulting mechanics has many features in common with that of a simple accordion bellows. These results have relevance to the many emerging applications of controlled buckling structures in stretchable electronics, microelectromechanical systems, thin-film metrology, optical devices, and others.

Sprayed boric acid as a dopant source for silicon ribbons

This paper presents a new method for bulk boron doping of silicon ribbons for solar cells. The method is based on the spraying of the ribbon with a solution of boric acid followed by zone melting recrystallization in an argon atmosphere. Dopant incorporation is evaluated by measuring the spreading resistance of the silicon ribbon. A numerical model for the incorporation of boron into silicon was developed, considering two competing mechanisms: boron diffusion during the heating step and evaporated boron transport in the gas phase. The dependence of the incorporation of boron on experimental parameters such as the direction of recrystallization and the flux of argon in and out of the furnace was measured, yielding results that are compatible with the numerical model. It is also shown that the mean incorporation of boron into the samples depends linearly on the initial concentration of boric acid.

Printable Single‐Crystal Silicon Micro/Nanoscale Ribbons, Platelets and Bars Generated from Bulk Wafers

AbstractThis article demonstrates a method for fabricating high quality single‐crystal silicon ribbons, platelets and bars with dimensions between ∼ 100 nm and ∼ 5 cm from bulk (111) wafers by using phase shift and amplitude photolithographic methods in conjunction with anisotropic chemical etching procedures. This “top‐down” approach affords excellent control over the thicknesses, lengths, and widths of these structures and yields almost defect‐free, monodisperse elements with well defined doping levels, surface morphologies and crystalline orientations. Dry transfer printing these elements from the source wafers to target substrates by use of soft, elastomeric stamps enables high yield integration onto wafers, glass plates, plastic sheets, rubber slabs or other surfaces. As one application example, bottom gate thin‐film transistors that use aligned arrays of ribbons as the channel material exhibit good electrical properties, with mobilites as high as ∼ 200 cm2 V–1 s–1 and on/off ratios &gt; 104.

Three dimensional coupled simulation of the induction heated EFG crystallization process

The edge-defined film-fed growth (EFG) process is used to grow silicon ribbons and hollow cylinders as well as hollow polygons of various geometries. In the case of the hollow polygons growth, three-dimensional (3D) numerical analysis is extensively used because the system of polygonal geometry cannot be adequately described in axisymmetric statement. The paper presents a modelling concept consisting of electromagnetic, thermal and structural simulations. Non-linear effects like temperature dependent material properties and heat exchange conditions are taken into account by numerical coupling of different physics. An extended parametric study includes a sensitivity analysis of one industrial polygonal EFG system to all significant parameters like induction coil currents and their phase relations, position of the coils and geometry of the growth system.

Bendable integrated circuits on plastic substrates by use of printed ribbons of single-crystalline silicon

This letter presents studies of several simple integrated circuits—n-channel metal-oxide semiconductor inverters, five-stage ring oscillators, and differential amplifiers—formed on thin, bendable plastic substrates with printed ribbons of ultrathin single-crystalline silicon as the semiconductor. The inverters exhibit gains as high as 2.5, the ring oscillators operate with oscillation frequencies between 8 and 9MHz at low supply voltages (∼4V), and the differential amplifiers show good performance and voltage gains of 1.3 for 500mV input signals. The responses of these systems to bending-induced strains show that relatively moderate changes of individual transistors can be significant for the operation of circuits that incorporate many transistors.