Localized Back Surface Field Solar Cell Rear Passivation
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Solution Overview
Problem
Conventional solar cells face efficiency losses due to aluminum back surface fields (BSF) causing warping, suboptimal reflection, and shunting issues, which are costly and not compatible with high-throughput manufacturing.
Innovation Solution
A method involving a boron-containing doping paste to form a highly doped p-type region around rear metal electrodes, reducing minority charge carrier recombination and using a selective emitter with a silicon-containing nanoparticle ink to prevent shunting, along with a rear floating junction for improved passivation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If aluminum back surface field (BSF) is used, then rear surface passivation is improved, but solar cell warping occurs and manufacturing yield decreases
Solution Approach 1:
The patent changes the material parameter from aluminum to silver for the BSF layer, and adjusts the doping concentration parameter to form a localized p+ region. This parameter change eliminates the warping issue while maintaining effective rear surface passivation, as silver does not cause the same thermal expansion and mechanical stress problems as aluminum during the firing process.
Solution Approach 2:
The patent implements a localized BSF structure where the silver-containing paste is applied only in specific regions around the rear metal contacts rather than as a uniform aluminum layer across the entire rear surface. This local quality approach creates heavily doped p+ regions precisely where needed for carrier collection and passivation, while avoiding the global warping effects of conventional aluminum BSF.
2Ease of manufacture
If aluminum BSF is used, then rear contact formation is simplified, but long wavelength light absorption is reduced due to suboptimal reflection
Solution Approach 1:
The patent changes the optical parameter of the BSF material from aluminum to silver, which has superior reflectivity in the long wavelength region. This parameter change enhances the reflection of long wavelength light back into the silicon substrate, increasing the optical path length and improving light absorption efficiency without complicating the manufacturing process.
3Reliability
If aluminum BSF is used, then rear metal contact is established, but shunting issues occur reducing cell efficiency
Solution Approach 1:
The patent creates a localized p+ doped region around the rear metal contacts where silver-containing paste is applied. This local quality approach ensures that the highly doped region forms precisely at the contact interface, providing low-resistance electrical contact while the surrounding passivation layers maintain carrier separation. This prevents shunting paths that would otherwise allow carriers to recombine at the metal-silicon interface.
Solution Approach 2:
The patent uses a composite paste formulation containing silver particles, glass frit, and organic vehicle. The silver provides conductive pathways and BSF functionality, the glass frit facilitates sintering and adhesion, and the organic vehicle enables screen printing deposition. This composite material approach achieves both reliable electrical contact and effective passivation while avoiding the shunting problems of pure aluminum BSF.
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 enhances solar cell efficiency by reducing recombination, increasing long wavelength light absorption, and preventing shunting, while being cost-effective and compatible with high-throughput manufacturing.
Implementation Method 1
heating the silicon substrate in a diffusion ambient to a second temperature and for a second time period in order to diffuse the p-type dopant into the rear surface to form a rear p+ diffusion area on the rear surface
Implementation Method 2
heating the silicon substrate in a baking ambient to a first temperature and for a first time period in order to remove residual solvent from the deposited ink
Implementation Method 3
exposing the silicon substrate to a phosphorous deposition ambient at a third temperature for a third time period, wherein a front PSG layer and a front phosphorous diffusion layer are formed on the front surface
Data Source
AI summary
A solar cell, comprising: a doped silicon substrate, the silicon substrate comprising a front surface and a rear surface; a front phosphorous diffusion layer formed on the front surface; a front anti-reflective layer formed on the front phosphorous diffusion layer; a front metal electrode on the front surface in ohmic contact with the front phosphorous diffusion layer through the front anti-reflective layer; a rear passivation layer formed on the rear surface; a rear metal electrode in a pattern on the rear surface passing through the rear passivation layer; and a rear p+ diffusion area on the rear surface between the rear passivation layer and a boron-doped region of the silicon substrate, the rear p+ diffusion area surrounding the rear metal electrode.