Ribbon Growth On Substrate Research Articles

Stability and thermoelectric performance of doped higher manganese silicide materials solidified by RGS (ribbon growth on substrate) synthesis

Abstract Large scale deployment of thermoelectric devices requires that the thermoelectric materials have stable electrical, thermal and mechanical properties under the conditions of operation. In this study we examine the high temperature stability of higher manganese silicide (HMS) materials prepared by the RGS (ribbon growth on substrate) technique. In particular we characterize the effect of element substitution on the structural and electrical changes occurring at the hot side of temperatures of thermoelectric devices relevant to this material (600 °C). Only by using suitable substitution (4% vanadium at the Mn site) can we obtain temperature-independent structural parameters in the range 20 °C–600 °C, a condition that results in stable electrical properties. Additionally, we show that 4% vanadium substitution at the Mn site offers the best thermoelectric figure of merit among the different compositions reported here with ZTmax = 0.52, a value comparable to the state of the art for HMS materials. Our analysis suggests that ionized impurity scattering is responsible for the better performance of this material.

Modeling electromagnetically driven free-surface flows motivated by the Ribbon Growth on Substrate (RGS) process

The Ribbon Growth on Substrate (RGS) technology is a crystallization technique that allows direct casting of silicon wafers and sheets of advanced metal-silicide compounds. With the potential of reaching high crystallization rates, it promises a very efficient approach for future photo-voltaic silicon wafer production compared to well-established processes in industry. However, a number of remaining problems, like process stability and controllability, need to be addressed for the RGS technology to eventually become a competitor in the near future. In this regard, it is very desirable to gain detailed insights into the characteristic process dynamics. To comply with this demand, we have developed a new numerical tool based on OpenFOAM (foam-extend), capable of simulating the free-surface dynamics of the melt flow under the influence of an applied alternating magnetic field. Our corresponding model thereby resolves the interaction of hydrodynamic and magnetodynamic effects in three-dimensional space. Although we currently focus on the RGS process, the modeling itself has been formulated in a more general form, which may be used for the investigation of similar problems, too. Here we provide a brief overview of these developments.

Free-surface dynamics in the Ribbon Growth on Substrate (RGS) process

With the Ribbon Growth on Substrate (RGS) technology, a new crystallization technique is available that allows controlled high crystallization rate production of silicon wafers and advanced metal-silicide alloys. Compared to other casting methods, such as e.g. directional solidification, the RGS pr ocess allows better crystallization control, high volume manufacturing and high material yield due to its continuous, substrate-driven design. Insights from modelling the characteristic melt flow in the casting frame are very desirable. To address this demand, we are developing a new numerical tool based on OpenFOAM [1] which can be utilized to simulate the free-surface dynamics of the melt flow under the influence of alternating electromagnetic fields. The underlying multi-physical model involves three-dimensional hydrodynamic and magnetodynamic effects and their interaction.

Novel RGS materials with high fill factors and no material-induced shunts with record solar cell efficiencies exceeding 16%

Kerf losses due to ingot and wafer sawing can be avoided by solidifying the silicon wafers directly from the melt by the Ribbon Growth on Substrate (RGS) process, thus significantly reducing the wafer cost. However, up to now solar cells made from standard RGS material suffered from shunting problems due to current collecting structures. This resulted in lower fill factor values and hence in lower efficiencies compared to solar cells made from block-cast multicrystalline silicon (mc-Si) materials. In this contribution two novel RGS materials are presented and investigated. Solar cells processed from these new materials have fill factor values above 78%, comparable to those of mc-Si. The increased fill factor values can be explained by the absence of current collecting structures as concluded from a comparative analysis of spatially resolved Light Beam Induced Current (LBIC) measurements and Electroluminescence (EL) images, and from infrared transmission microscopy investigations. Additionally the improved material quality resulted in open-circuit voltage VOC values up to 608mV. This enhanced material quality, in combination with increased fill factor values, resulted in record efficiencies above 16% (certified by Fraunhofer ISE CalLab). This represents a significant improvement compared to the former efficiency record of 14.4% for standard RGS material.

Efficiencies above 16% on Novel Quasi-mono RGS Material

To date, solar cells made from kerf-less and cost-effective Ribbon Growth on Substrate (RGS) Si material suffered heavily from shunting problems. In combination with the low material quality, this led to decreased fill factor values and thus to significantly lower efficiency values compared to block-cast multicrystalline (mc) Si. In this study, a novel RGS material seeded on silicon substrate is investigated. Solar cells made from this new ‘quasi-mono’ material using a lab-type process show VOC values >600 mV and average fill factor values >78%, similar to standard mc-Si. This leads to cell efficiencies >16% (certified by ISE CalLab), which is an enormous progress compared to the former efficiency record of 14.4%. The obtained higher fill factor values are most probably due to the absence of current collecting structures, in contrast to standard RGS material, as shown by infrared microscopy investigation. This was also found comparing Internal Quantum Efficiency topograms and Electroluminescence maps. For this new material, minority carrier lifetime is in the range of 18 μs (after P-gettering and hydrogenation) and hence approximately two times higher than for standard RGS, which explains the higher achieved VOC. Furthermore, the bulk quality of this novel RGS material is more homogeneous and its grain orientation is predominantly (100), allowing the use of an alkaline texture.

Cost Efficient Manufacturing of Silicide Thermoelectric Materials and Modules using RGS Technique

Thermoelectric (TE) power generation presents a promising and attractive way to convert high temperature waste heat into electricity. However in many situations, such as in industrial waste heat recovery, the market is looking for affordable solutions, while the search for efficient and stable materials is still ongoing. The relatively high TE system costs, are partially due to material costs such as for Te in BiTe, or PbTe, but also due to the relatively small scale leg- and module manufacturing process. Especially in the leg manufacturing, there are often a number of steps involved such as ingot crystallization, ball milling, hot pressing or plasma sintering and leg sawing. With the availability of the ribbon-growth-on-substrate (RGS) technology, it is possible to cast semiconductor material in a net-shape form directly from the melt, thus reducing the number of manufacturing steps, material losses and production costs. In combination with a strip leg based module concept, the potential for a major cost reduction step is within reach. Besides affordability, other challenges are to demonstrate a material that is stable at elevated temperatures, and comparable or better than material produced by other semiconductor manufacturing processes.In this paper, the progress in manufacturing higher manganese silicon material by the RGS process is shown, demonstrating ZT values above 0.5. It could be shown that the phase composition can be controlled by the RGS process and that phase geometry is tunable by changing crystallization velocity. This phase structuring ability is seen as an opportunity to enhance phonon scattering by phase boundaries, and hence further improve ZT values. Controlling the Si phase in HMS was found to be key in the RGS material development process. Especially the addition of chromium doping not only changed carrier concentration and conductivity, but also resulted in a different phase composition of the cast material.In addition to the material development the strip material based module concept is introduced and first demonstrators devices are shown.

Computational Study of Contact Solidification for Silicon Film Growth in the Ribbon Growth on Substrate System

Ribbon growth on substrate (RGS) has emerged as a new method for growing silicon films at low cost for photovoltaic applications by contact solidification. Thermal conditions play an important role in determining the thickness and quality of the as-grown films. In this study, we have developed a mathematical model for heat transfer, fluid flow, and solidification in the RGS process. In particular, a semi-analytical approach is used in this model to predict solidification with a sharp solid–liquid interface without using a moving grid system. A more realistic analytical relationship that considers the varying rate of heat removal at the interface has been developed to evaluate the effective heat transfer rate, solidification rate, and solidification front. These models were used to predict the flow patterns in the crucible, the temperature distributions in the system, the velocity fields in the crucible, the solidification rates, and the film thicknesses. The effects of important operational parameters, such as pulling speed, preheat temperature, and thermal properties of the substrate material, have been examined. In addition, an order of magnitude analysis has been performed to understand heat transfer in the growing film and substrate. This analysis leads to a simplified mathematical model for heat transfer and solidification, which can be resolved analytically to derive theoretical solutions for the effective heat transfer coefficient, the rate of solidification, and the film thickness. The results show that the solidification rate varies largely on the substrate. The non-uniformity can be mitigated by altering the temperature distribution in the silicon melt through manipulating heat generation in the top heater. The rates of solidification and film thickness are very sensitive to both the thermal conductivity and preheat temperature of the substrate. Increasing pulling velocity will increase the rate of solidification at the leading edge but reduce the film thickness. The numerical model and the theoretical solution provide an important tool for thermal design and optimization of the RGS system.

Crystal defects and their impact on ribbon growth on substrate (RGS) silicon solar cells

Ribbon Growth on Substrate (RGS) silicon wafers are casted directly from the silicon melt onto reusable substrates. Material losses by wafer sawing are omitted and high production speeds can be achieved. However, multicrystalline RGS silicon as it is produced today incorporates high densities of crystal defects and impurities limiting the efficiency of the corresponding solar cells. The local impact of crystal defects on material quality is estimated via models developed by Donolato and Micard et al.. By theoretically negating the impact of grain boundaries and dislocations, charge carrier diffusion lengths are still limited to values <100µm. In addition to crystal defects which are common in other multicrystalline silicon materials, we found current collecting structures within grain boundaries. These structures can be associated with carbon and oxygen precipitation and are the cause for shunting phenomena. We conclude that high impurity concentrations are the dominant factor for limiting the performance of RGS silicon solar cells.

SEM Investigation of the electrical properties of silicon ribbons

The structure and electrical properties of silicon ribbons grown on a substrate by the Ribbon Growth on Substrate (RGS) method method for solar cell applications have been investigated in secondary electron and electron beam induced current modes of scanning electron microscopy. The growth method and growth conditions have provided the formation of the coarse-grained structure of silicon, in which the majority of grains are separated by twin boundaries and the dislocation density does not exceed 106 cm−2. According to the electron beam induced current investigations, the recombination contrast from twin boundaries is extremely low at 300 K, only a small amount of twin boundaries show an increase in the contrast upon cooling, and the contrast from dislocations is almost absent in the temperature range from 100 to 300 K.

Computational simulations of Ribbon-Growth on substrate for photovoltaic silicon wafer

Computational simulations of horizontal ribbon growth on substrate (RGS), used for production of silicon wafers for photovoltaic applications, have been made based on FLUENT-based solutions of the fundamental governing equations of mass, momentum and energy. A conservation equation for the liquid volume fraction, along with a solidification model, is used in addition to find the phase distributions. Validations of both the melt flow and solidification components of the computational model are made by comparing with available data on Czochralski bulk process and vertical ribbon growth process, with good agreements for these components. This provides the basis for validity of the method for silicon melt flow and solidification processes, including the RGS. The pull speed and the heat extraction rates are varied to find the optimum production conditions during RGS. The pull speed can be directly input in the current model, and shows the effects of decreased residence time at high pull speeds. At intermediate heat extraction rates, the solidification dynamics can lead to disruptions in the melt flow on the substrate, leading to inhomogeneous solidification conditions. A test matrix involving the pull speed and the heat extraction rate shows that a pull speed of less than 0.1 m/s and heat extraction rate of greater than 100 W/cm 2 are the necessary conditions for achieving complete and stable solidification over a length scale of 0.8 m in the current configuration. These numbers translate to 2 kJ/m 2 as the minimum necessary enthalpy flux during stable RGS.

Observation of transition metals at shunt locations in multicrystalline silicon solar cells

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescence microscopy, transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cell materials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Current collecting channels in RGS silicon solar cells—are they useful?

Ribbon growth on substrate (RGS) silicon could be the crystalline silicon material for PV of the future. The extremely fast production technique avoiding any material losses due to sawing drastically reduces the wafering costs. On the other hand, one has to deal with more crystal defects (grain boundaries, dislocations, impurities), which especially limit the diffusion length and normally result in small short-circuit current densities J sc. The charge carrier collection probability can be increased by a macroscopic V-texture of the surface, but even more effective would be a 3-dimensional emitter structure within the whole bulk cell volume. This was observed in some RGS solar cells showing minority carrier lifetimes of only around 0.4 μs after cell processing, but J sc of above 34 mA/cm 2. In these cells, the whole bulk volume collects current despite the small diffusion lengths. This behaviour was investigated using spatially resolved lifetime and internal quantum efficiency mappings, capacitance measurements and a special EBIC technique, where the electron beam hits the backside of the wedge-shaped solar cell. From our results, we conclude that the collecting structures may be caused by inversion in combination with a high O content. Cells with large areas of collecting channels exhibit lower fill factors, but nearly no loss in open-circuit voltage as compared to the standard RGS cells. For both types of cells, confirmed record efficiencies of 12.5% have been obtained.

Simulation of the crystallisation of silicon ribbons on substrate

Today's photovoltaic market is divided into multicrystalline silicon wafers from ingot casting and monocrystalline wafers from Czochralski crystals. In both cases large crystals have to be cut into thin wafer geometry. As an alternative approach the EFG process and the string ribbon process are in industrial use for the production of silicon ribbons directly out of the melt. One future alternative for a high production process is the Ribbon Growth on Substrate (RGS) process which is now in the final development stage for industrial production of silicon ribbons. In this paper we will report simulation results for the crystallisation of silicon melt in contact with a substrate. This is the basis not only for the RGS process but also for thin film processes like laser remelting of amorphous Si. The results show the general solidification behaviour at the tip region of a growing Si sheet. Different crystal growth modes are predicted resulting in different grain structures of the silicon sheet as observed experimentally.

EBIC Investigation of a 3-Dimensional Network of Inversion Channels in Solar Cells on Silicon Ribbons

Ribbon growth on substrate (RGS) is a novel technique to produce thin sheets of silicon for solar cells with high throughput directly from the melt without a slicing step. By this kind of process the amount of impurities incorporated and the temperature gradients during the solidification are remarkable higher then e.g. in a block casting process. Solar cells produced on RGS material can reach sufficient efficiencies above 12 %, but show unusual properties. The short circuit current may reach exceptionally high values beyond 30 mA/cm2, which are usually only observed in monocrystalline silicon solar cells. The fill factor and the open circuit voltage, however, are lower than expected. The dark current-voltage (I-V) characteristic of such solar cells shows ideality factors (giving the steepness of the forward IV-characteristic) well above two, in disagreement with the theory for a simple emitter-base diode. These unusual properties are caused by the existence of a 3-dimensional network of inversion channels formed by densely packed precipitates around dislocations. These channels are able to collect minority carriers in the bulk of the solar cells and lead them to the pn junction. Thus, in spite of the low diffusion length, the collecting volume is drastically enhanced. It is possible to visualize and investigate the inversion channels by EBIC (electron beam induced current) measurements using special sample geometries. Under forward bias these channels may act as preferred injection sites. A physical model for the inversion channels is proposed explaining for the first time ideality factors above two over a large bias range.

Formation and annihilation of oxygen donors in multicrystalline silicon for solar cells

The efficiencies of solar cells based on multicrystalline silicon (mc-Si) have reached 17% even employing high-throughput crystallization steps and industrial-relevant solar cell processes. The efficiency of multicrystalline solar cells is governed by crystal defects, impurities and the interaction of both. The number of crystal defects, such as dislocations and grain boundaries, crucially depends on the crystallization conditions, while, with regard to impurities, electrically active transition metals, such as iron, are well-known to seriously reduce the minority carrier lifetime. A similarly important role, however, is played by oxygen. Various oxygen or oxygen-containing defect centers showing strong recombination activity may form in monocrystalline silicon as well as in mc-Si. In mc-Si blocks the formation of so-called thermal donors and nitrogen-oxygen complexes can take place during the relatively slow cooling of the ingots. Thermal donors and nitrogen-oxygen complexes lead to reduced lifetimes especially in the edge regions of the ingot. Whereas this lifetime reduction is hardly efficiency-relevant as long as annealing steps above 600°C for several minutes are implemented in solar cell processing, another species of oxygen donor, the new donor, forms in the temperature range between 600 and 900°C that is frequently used for solar cell fabrication. For silicon with a high oxygen content such as the Bayer RGS (ribbon growth on substrate) material, the new donors seem to be the most efficiency-relevant defects which can only be prevented using well-adjusted temperature profiles during crystallization and solar cell processing. Whereas monocrystalline silicon can benefit from high oxygen content through internal gettering steps in microelectronic device processing, a substantial improvement of mc-Si for solar cells is achievable by lowering the oxygen content. Oxygen contents considerably below those of monocrystalline silicon are therefore state of the art for modern high-throughput production material fabricated by the block-casting technology.

99/02666 Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11 %

99/02666 Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11 %

99/02669 Solar hydrogen production by using ferrites

For the first time efficiencies above 11% for solar cells (4 cm2) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO2 double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm−2 and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.

Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11%

For the first time efficiencies above 11% for solar cells (4 cm 2) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO 2 double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm −2 and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.

Solar cells with 11% efficiency on ribbon‐growth‐on‐substrate (RGS) silicon

Solar cells with 11% efficiency on ribbon‐growth‐on‐substrate (RGS) silicon

Solar cells with 11% efficiency on ribbon-growth-on-substrate (RGS) silicon

Solar cells with 11% efficiency on ribbon-growth-on-substrate (RGS) silicon