Quasi-monocrystalline Silicon Research Articles

Role of Longitudinal Temperature Gradients in Eliminating Interleaving Inclusions in Casting of Monocrystalline Silicon Ingots

Infrared analysis reveals the presence of interwoven inclusions, primarily comprised of silicon nitride and silicon carbide, in the casting process of monocrystalline silicon ingots. This study investigates how the longitudinal temperature gradient affects the removal of inclusions during the casting of monocrystalline silicon ingots through simulations and comparative experiments. Two monocrystalline silicon ingots were cast, each using different longitudinal temperature gradients: one employing smaller gradients and the other conventional gradients. CGSim (Version Basic CGSim 23.1) simulation software was utilized to analyze the melt flow and temperature distribution during the growth process of quasi–monocrystalline silicon ingots. The findings indicate that smaller longitudinal temperature gradients lead to a more robust upward flow of molten silicon at the solid–liquid interface, effectively carrying impurities away from this interface and preventing their inclusion formation. Analysis of experimental photoluminescence and IR results reveals that although inclusions may not be observed, impurities persist but are gradually displaced to the top of the silicon melt through a stable growth process.

Quasi-monocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors

Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors are planned to have diameters of $\ensuremath{\approx}50$ cm and a mass of $\ensuremath{\approx}200$ kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than $\ensuremath{\approx}50$ cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors.

Kinetics of the Light and Elevated Temperature Induced Degradation and Regeneration of Quasi-Monocrystalline Silicon Solar Cells

We investigate the degradation and regeneration behavior of quasi-monocrystalline silicon passivated emitter and rear cells under illumination at elevated temperatures. The decrease and increase of the solar cell efficiencies over time is accelerated under increased temperature or illumination intensity. We examine the defect activation kinetics and determine rate constants both for the degradation and regeneration. We apply temperatures in the range of 37-140 °C and illumination intensities in the range of 0.1-1.4 suns. These conditions typically occur when operating solar modules in the field. The rate constants are strongly increased with increasing temperature and increasing illumination intensity. We perform multiple regressions fits of the degradation and regeneration data with different approaches for the illumination intensity dependence. A linear illumination intensity dependence on the rates of degradation and regeneration is found. Activation energies for the degradation and regeneration of (0.89 ± 0.04) eV and (1.07 ± 0.07) eV, respectively, are extracted that allow for identification of the defect activation and deactivation mechanisms.

Performance of thin silicon solar cells with a quasi-monocrystalline porous silicon layer on the rear side

The present study employs porous silicon (PS) or quasi-monocrystalline porous silicon (QMPS) as a reflector material on the rear side. It presents an analytical model that simulates the performance of n+–p–p+ thin silicon solar cell with a QMPS layer on the rear side. The development of the model involves the formulation of a complete set of equations for the photocurrent density that is then solved analytically in the base region, including the photocurrent generated under the effect of the light reflected by QMPS layer. This also takes the contribution of the back p+-region (back surface field) to the generated photocurrent into consideration. The enhancements brought by the thin film QMPS with regard to photovoltaic (PV) parameters are then investigated and compared to those brought by the conventional silicon solar cell. Moreover, the effect of the QMPS layer on the current–voltage characteristics J–V and the internal quantum efficiency (IQE) of thin silicon solar cells are simulated by means of AFORS-HET software. These simulations show that the improvement of the PV parameters is due to an increase in the transport parameters of minority carriers in the p-region.

Dislocation formation in seed crystals induced by feedstock indentation during growth of quasimono crystalline silicon ingots

In this work the dislocation formation in the seed crystal induced by feedstock indentation during the growth of quasimono (QM) silicon ingots for photovoltaic application was investigated. It could be shown by special laboratory indentation experiments that the formed dislocations propagate up to several millimeters deep into the volume of the seed crystal in dependence on the applied pressure of the feedstock particles on the surface of the seed crystal. Further, it was demonstrated that these dislocations if they were not back-melted during the seeding process grow further into the silicon ingot and drastically reduce its material quality. An estimation of the apparent pressure values in a G5 industrial crucible/feedstock setup reveals that the indentation phenomenon is a critical issue for the industrial production of QM silicon ingots. Therefore, some approaches to avoid/reduce the indentation events were tested with the result, that the most promising solution should be the usage of suitable feedstock particles as coverage of the seed.

Numerical simulation of stresses and dislocations in quasi-mono silicon

The Alexander–Haasen model is applied for the analysis of dislocation dynamics in quasi-mono crystalline silicon. Model constants are re-calibrated using stress–strain measurements on small silicon samples under uniaxial compression. It is observed that the activation energy may decrease at low temperatures and the hardening parameter generally increases due to the presence of grown-in dislocation clusters. The calibrated model is applied to an idealized cooling process which allows for a discussion of the basic physical mechanisms leading to residual stresses in quasi-mono ingots. Residual stresses can be reduced by minimizing thermal stresses during the elastic–plastic transition, which was observed approximately between 1100°C and 750°C in the present case.

Laser Enhanced Hydrogen Passivation of Silicon Wafers

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

Dislocation formation in seeds for quasi-monocrystalline silicon for solar cells

An investigation of two industrially cast quasi-monocrystalline silicon blocks revealed a high dislocation density originating at intersections between the seed crystals. This may be ascribed to three different generation mechanisms. Firstly, a dislocation cell structure was observed in the seed crystals, probably as an effect of poor surface preparation of the seeds. Furthermore, clusters of dislocations form around contact points in the interface between two neighbouring seeds. At contact points, the two monocrystalline silicon seeds plastically deform and sinter together. Dislocation rosettes form as a result of an indentation mechanism at high temperatures. A third mechanism acts at the bottom surface, where dislocation clusters also form by indentation of contact points between the seed and the crucible. Since dislocations forming in the seeds will continue into the growing ingot, it is crucial to depress the dislocation formation in the seeds.

Characterization and Analysis of Local Series Resistance on Quasi-Monocrystalline Solar Cells

This work evaluates the application of Quasi-Monocrystalline silicon (QM-Si) wafers on the conventional structure of silicon solar cells textured by an alkaline solution to generate random pyramids. Although most areas of QM-Si wafers are <100> orientated by using a seeded-grown and directional-solidification process, there are still some none-<100> orientated areas randomly distributed on the wafers. These areas stay more planar, featuring worse reflectance and light trapping. Furthermore, series resistance (Rs) mapping on the final cells shows that these areas also have a higher local Rs. Our experiments and analysis show that the higher local Rs is mainly due to the poor Ag/Si contacts according to the results of our TLM measurement. Such higher local Rs values have led to only 76.3% fill factor and 17.1% efficiency performance of 500 cells on average. Further detailed investigations show that the local Rs, or local contact resistance, depends greatly on the surface roughness for screen printed cells, which implies that to take advantage of better quality of the QM-Si wafers, silver paste needs to be improved to form good contacts on both textured and planar silicon surface, when these wafers are textured with a random-pyramid alkaline process.

Reorganized Porous Silicon Bragg Reflectors for Thin-Film Silicon Solar Cells

Stacks of porous silicon layers have been successfully applied to maximize internal reflection at the interface between a silicon substrate and an epitaxially grown layer. The stack is consist of alternating porous layers of high and low porosity, defined by the quarter-wavelength rule. During the hydrogen bake prior to epitaxial growth of the epitaxial layer, the porous silicon stack crystallizes in the form of thin quasi-monocrystalline silicon layers incorporating large voids. Experimental data of the measured external reflectance have been linked to the internal reflectance. An optical-path-length enhancement factor of seven was calculated in the wavelength range of 900-1200 nm. Application on thin-film epitaxial solar cells showed a 12% increase in short-circuit current and efficiency

Progress in thin film free-standing monocrystalline silicon solar cells

We present an improved process for free-standing monocrystalline silicon (FMS) thin film solar cells developed at IMEC. We demonstrate that a thick annealed porous layer, or the quasi-monocrystalline silicon (QMS) layer, incorporated into a two-side contacted thin film solar cell structure does not produce any considerable series resistance. The best cell so far exhibits a conversion efficiency of 12.6% with a fill factor of 75.7% for an active layer thickness of 20 μm. This cell process includes emitter formation by phosphorous diffusion, silicon nitride deposition for antireflection coating and front surface passivation, and 50-μm-thick annealed porous layer remained on the rear. Despite the expected small reduction in solar cell's efficiency, maintaining the porous layer in the device structure renders the processing and handling of the thin FMS film much easier, thus leading to a better yield and up-scalability. PC1D simulations show that a thick QMS layer can lessen the short circuit current density in some degree, depending on the active layer quality, thickness, optical confinement, etc., while the other cell parameters remain substantially unvaried; a 10% to 15% higher process yield is expected to be sufficient to reach a break-even between the two processes.

Structural, optical and electrical properties of quasi-monocrystalline silicon thin films obtained by rapid thermal annealing of porous silicon layers

Quasi-mono-crystalline silicon (QMS) layers have a top surface like crystalline silicon with small voids in the body. Such layers are reported to have a higher absorption coefficient than crystalline silicon at the interesting range of the solar spectrum for photovoltaic application. In this work we present a study of the structural, optical and electrical properties of quasimonocrystalline silicon thin films. Quasimonocrystalline silicon thin films were obtained from porous silicon, which has been annealed at a temperature ranging from 950 to 1050 °C under H 2 atmosphere for different annealing durations. The porous layers were prepared by conventional electrochemical anodization using a double tank cell and a HF / Ethanol electrolyte. Porous silicon is formed on highly doped p +-type silicon substrates that enable us to prevent back contacts for the anodization. Atomic Force Microscope (AFM) was used to study the morphological quality of the prepared layers. Optical properties were extracted from transmission and reflectivity spectra. Dark I– V characteristics were used to determine the electrical conductivity of quasimonocrystalline silicon thin films. Results show an important improvement of the absorption coefficient of the material and electrical conductivity reaches a value of twenty orders higher than that of starting mesoporous silicon.

Structure and Properties of Quasi-Monocrystalline Silicon Thin-Films

ABSTRACTThis contribution describes the preparation of a single crystalline Si thin-film separable from a reusable Si wafer. The method relies on: i) etching of a porous silicon layer ii) high-temperature annealing and iii) transfer of the recrystallized film to a foreign substrate. As a result of the process we obtain 1 to 30 μm thick monocrystalline Si films that contain voids with a size of several 100 nm. Due to its “swiss-cheese-like” structure the material is termed as “quasi-monocrystalline Si”. Sub micrometer thin layers are almost compact, while in several micron thick films voids cause scattering of incident light. This effect increases the effective absorption coefficient by light trapping and seems promising for the application of our quasi-monocrystalline films in thin film solar cells. Quasi-monocrystalline p-type silicon reaches a hole mobility of 78 cm2/ Vs measured by room-temperature Hall-effect. High carrier mobility and adjustable optical characteristics make these films suitable for display and photovoltaic applications. Quasi-monocrystalline films are processed using conventional high-temperature Si processing; finished devices can be transferred to a foreign substrate such as glass, while the starting wafer can be reused several times.