Crucible Removable Barrier Reduces Silicon Contamination
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Solution Overview
Problem
The contamination of silicon ingots during directional solidification due to impurities from crucible materials and coatings degrades the electrical properties and efficiency of photovoltaic cells, particularly affecting the 'red-zone' of the ingot, which is unusable and increases production costs.
Innovation Solution
A crucible with a removable diffusion barrier made from high-purity materials, such as silicon carbide or silicon nitride, is used between the molten silicon and the crucible bottom to concentrate and retain impurities, reducing contamination and enabling the reuse of seeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a crucible made from silica with 99.6% purity is used for directional solidification, then the crucible can withstand the melting temperature of silicon and facilitate ingot production, but the impurities (0.4% by mass) from the crucible material contaminate the silicon ingot, degrading electrical properties and creating a unusable 'red-zone'
Solution Approach 1:
A removable barrier element made of high-purity material (such as high-purity silica, graphite, or silicon carbide) is introduced as an intermediary between the crucible bottom and the molten silicon. This barrier element prevents direct contact between the silicon and the crucible walls, thereby blocking the diffusion path of impurities from the crucible to the silicon ingot while still allowing thermal energy to be transmitted for solidification.
Solution Approach 2:
The crucible bottom is segmented into two distinct parts: the permanent crucible structure and the removable barrier element. This segmentation allows the barrier element to be separated from the crucible, enabling it to be replaced after use. The barrier element can be removed from the crucible bottom, facilitating both the prevention of contamination during use and the recovery of high-purity silicon without crucible contamination in subsequent operations.
2Ease of operation
If an anti-adherent coating based on silicon nitride is deposited on the crucible interior walls to facilitate demolding, then the silicon ingot can be easily removed, but the coating itself becomes a source of contamination, introducing additional impurities such as Fe, Al, Cu, Co, Mn, and Cr into the silicon
Solution Approach 1:
The removable barrier element serves as an intermediary that eliminates the need for an anti-adherent coating on the crucible walls. By placing the barrier element between the crucible and the silicon, demolding can be achieved without requiring a coating that would otherwise contaminate the silicon. The barrier element itself can be designed with appropriate surface properties to facilitate easy removal of the solidified ingot.
Solution Approach 2:
The function of the anti-adherent coating is extracted and transferred to the removable barrier element. Instead of having the coating permanently attached to the crucible where it contacts the silicon, the coating function is moved to a separate, removable component that can be discarded or cleaned, preventing contamination of the silicon ingot.
3Manufacturing precision
If monocrystalline seeds are placed at the bottom of the crucible for epitaxial growth of quasi-mono-crystalline silicon, then high-quality silicon ingots can be produced, but the seeds become contaminated by diffusion of metallic impurities and oxygen from the crucible/coating assembly during the first use, preventing their reuse and increasing production costs
Solution Approach 1:
The removable barrier element is positioned directly beneath the monocrystalline seeds, acting as a protective intermediary that prevents diffusion of metallic impurities and oxygen from the crucible assembly to the seeds during the epitaxial growth process. This protection allows the seeds to maintain their high crystalline quality and be reused for multiple ingot productions.
Solution Approach 2:
The barrier element is placed in position before the seeds are introduced to the crucible, providing beforehand protection against contamination. This prior cushioning ensures that when the seeds are placed on top of the barrier element, they are immediately protected from exposure to impurities in the crucible environment during the solidification process.
4Device complexity
If the crucible bottom is in direct contact with molten silicon during directional solidification, then the solidification process can be simplified, but the resulting silicon ingot has a thick 'red-zone' (2-3 cm for multicrystalline, 3-5 cm for mono-like) with degraded electrical properties that represents 30% of the crystallized charge and cannot be used for wafer manufacture
Solution Approach 1:
The removable barrier element is introduced as a temporary intermediary during the solidification process to prevent impurity diffusion into the silicon. After solidification is complete, the barrier element is removed, allowing for easy demolding of the ingot. This approach maintains process simplicity while dramatically reducing the red-zone thickness from 2-5 cm to less than 1 cm, thereby recovering 30% or more of the previously unusable silicon material.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Significantly reduces the volume of the 'red-zone' in silicon ingots, increasing the yield of purified material and allowing for the reuse of seeds, thereby enhancing the efficiency and cost-effectiveness of photovoltaic cell production.
Implementation Method 1
a removable barrier element (5), arranged directly or indirectly on the bottom of the mold, to be inserted between the molten silicon (10) and the bottom of the mold (4)
Implementation Method 2
subjecting said silicon in the molten state to conditions conducive to its solidification in said cavity
Implementation Method 3
manufacture of multi-crystalline or quasi-mono-crystalline silicon ingots advantageously equipped with a lower part of the ingot also called 'red-zone'
Implementation Method 4
Multi-crystalline silicon ingots are produced there by melting a silicon charge followed by directional solidification
Data Source
Figure 1~4
Figure 3~5
AI summary
The present invention relates to a crucible for directional solidification of a silicon ingot, said crucible comprising a mould intended to receive the molten silicon, and a removable element forming a barrier to the diffusion of metallic impurities and arranged on the base of the inner face of said mould, the removable element being totally or partially formed of a barrier layer formed by at least one material selected from the group comprising silica SiO2, silicon nitride Si3N4, silicon carbide SiC, silicon carbonitride, silicon oxynitride, graphite and mixtures thereof, and having a purity of at least 99.95 % by mass, the barrier layer having a thickness conducive to the retention of impurities capable of diffusing out of the base during the directional solidification and cooling of the silicon.