Mono-crystalline Silicon Seed Layer Orientation for Directional Solidification
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Solution Overview
Problem
Current directional solidification processes for producing crystalline silicon for photovoltaic cells result in multi-crystalline regions encroaching into the ingot, leading to reduced electrical efficiency and inhomogeneous wafer quality, making it difficult to achieve both high-quality and cost-effective mono-crystalline silicon production.
Innovation Solution
A method involving a crucible with a seed layer comprising peripheral seed tiles arranged with specific crystallographic orientations, where the first {110} plane is parallel to the crucible wall and the second {110} plane is normal to the direction of solidification, inhibiting multi-crystalline silicon encroachment and promoting a higher proportion of mono-crystalline silicon growth, along with optimized cutting and etching processes to enhance wafer quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional directional solidification processes are used, then production cost is reduced, but the silicon structure becomes multi-crystalline with grain boundaries and dislocations, leading to reduced electrical performance
Solution Approach 1:
A mono-crystalline seed layer is placed at the bottom of the crucible before melting the silicon feedstock. This preliminary action establishes a controlled crystalline structure that guides the solidification process, enabling mono-crystalline growth from the seed layer while using the cost-effective directional solidification method rather than the expensive Czochralski process
Solution Approach 2:
The crystallographic orientation parameters of the seed layer are specifically controlled to have a <100> orientation normal to the crucible base. This parameter change in crystallographic orientation enables the seed layer to effectively guide mono-crystalline growth and inhibit multi-crystalline encroachment from the crucible walls
2Reliability
If mono-crystalline silicon is produced by the Czochralski process, then electrical efficiency is improved, but production volume is limited and cost increases
Solution Approach 1:
A mono-crystalline seed layer is placed at the bottom of the crucible before melting the silicon feedstock. This preliminary action establishes a controlled crystalline structure that guides the solidification process, enabling mono-crystalline growth from the seed layer while using the cost-effective directional solidification method rather than the expensive Czochralski process
Solution Approach 2:
The seed layer serves multiple functions: it initiates mono-crystalline growth, establishes crystallographic orientation, and inhibits multi-crystalline encroachment from the crucible walls. This multi-functionality allows a single process to achieve both high electrical efficiency and large production volume
3Manufacturing precision
If multi-crystalline silicon is produced by directional solidification, then production cost is reduced, but wafer quality becomes inhomogeneous due to multi-crystalline regions encroaching into the ingot
Solution Approach 1:
The seed layer is positioned specifically at the bottom center of the crucible with a defined area that is optimized to control crystal growth locally. This local control of crystallographic orientation propagates through the solidification process, ensuring uniform mono-crystalline quality throughout the entire ingot while maintaining cost-effective production
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
This approach significantly increases the proportion of mono-crystalline silicon in the ingot, reducing defects and impurities, resulting in improved electrical performance and homogeneous wafer quality, suitable for efficient photovoltaic cell production.
Implementation Method 1
the molten silicon is then gradually solidified in a directional process which allows the crystalline structure to form in a solid silicon ingot
Implementation Method 2
a crystalline silicon seed layer is provided in a crucible... directionally solidifying the molten silicon to form a silicon ingot
Data Source
Figure 1~2C
Figure 3
Figure 4A~4B
AI summary
A crystalline silicon ingot is produced using a directional solidification process. In particular, a crucible is loaded with silicon feedstock above a seed layer of uniform crystalline orientation. The silicon feedstock and an upper part of the seed layer are melted forming molten material in the crucible. This molten material is then solidified, during which process a crystalline structure based on that of the seed layer is formed in a silicon ingot. The seed layer is arranged such that a {110} crystallographic plane is normal to the direction of solidification and also so that a peripheral surface of the seed layer predominantly also lies in a {110}crystallographic plane. It is found that this arrangement offers a substantial improvement in the proportion of mono-crystalline silicon formed in the ingot as compared to alternative crystallographic orientations.