Epitaxial Silicon Junctions for Solar Cell Bow Reduction
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Solution Overview
Problem
The conventional aluminum back surface field (BSF) fabrication process in single crystal silicon solar cells induces wafer bow, leading to breakage during processing, especially with thinner wafers, and requires costly diffusion and screen printing steps, which are inefficient and yield-reducing.
Innovation Solution
The method involves epitaxially depositing a highly doped p++ silicon BSF and n++ front surface field, eliminating the need for aluminum screen printing and diffusion, allowing for thinner solar cells with reduced manufacturing costs and process complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If aluminum screen printing is used to create back surface field, then electrical conductivity is improved, but wafer bow increases causing breakage
Solution Approach 1:
The patent changes the material parameter from aluminum paste to epitaxial silicon layer, fundamentally altering the physical and chemical properties of the back surface field. This epitaxial silicon layer provides electrical conductivity through controlled doping while maintaining mechanical compatibility with the silicon wafer, eliminating the bowing issue caused by aluminum paste shrinkage during firing.
Solution Approach 2:
The patent replaces the mechanical screen printing process with an epitaxial growth process. Instead of applying aluminum paste mechanically and firing it (which causes shrinkage and bowing), the back surface field is grown epitaxially layer-by-layer in a controlled environment, eliminating the mechanical stress that causes wafer bow and breakage.
2Quantity of substance
If wafer thickness is reduced to lower cost, then material cost decreases, but wafer bow becomes a yield killer
Solution Approach 1:
The patent changes the back surface field formation method from aluminum screen printing to epitaxial silicon growth, which fundamentally alters how thin wafers are handled. The epitaxial process allows for precise control of thin wafer properties and creates a mechanically compatible structure that prevents bowing even in wafers as thin as 150 microns or less, thereby maintaining high manufacturing yield.
3Reliability
If conventional diffusion process is used for front junction, then doping is achieved, but process complexity and cost increase
Solution Approach 1:
The patent merges the back surface field formation and front junction creation into a single epitaxial growth process. Both the p-type back surface field and the n-type front junction are formed sequentially in the same epitaxial reactor without requiring separate diffusion steps, post-diffusion cleans, or multiple screen printing operations, thereby reducing process complexity while maintaining junction quality.
Solution Approach 2:
The epitaxial growth process serves multiple functions simultaneously: it forms the back surface field, creates the front junction, and deposits anti-reflective coating materials in sequence within a single reactor system. This multi-functional approach eliminates the need for separate specialized processes for each function, reducing overall process complexity.
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 prevents wafer bow, enables the production of thinner solar cells, reduces manufacturing costs, and improves efficiency by avoiding the conventional Al BSF-induced bow and diffusion steps, resulting in higher open circuit voltage and lower production costs.
Implementation Method 1
depositing an epitaxial film of highly doped p-type silicon on a porous silicon layer on a silicon wafer
Implementation Method 2
a porous silicon layer on a silicon wafer
Data Source
AI summary
Fabrication of a single crystal silicon solar cell with an insitu epitaxially deposited very highly doped p-type silicon back surface field obviates the need for the conventional aluminum screen printing step, thus enabling a thinner silicon solar cell because of no aluminum induced bow in the cell. Furthermore, fabrication of a single crystal silicon solar cell with insitu epitaxial p-n junction formation and very highly doped n-type silicon front surface field completely avoids the conventional dopant diffusion step and one screen printing step, thus enabling a cheaper manufacturing process.