Rear Passivated Solar Cell Wet Etch Opening
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Solution Overview
Problem
Conventional methods for contacting rear-passivated solar cells, such as laser ablation, result in damage to the silicon substrate, are time-consuming, and lack the precision and control needed for high efficiency, particularly in creating intricate shapes and controlling the depth of the opened surface.
Innovation Solution
A wet etch process using screen printable etch resists is employed to selectively remove the passivation layer and optionally deepen the opening, allowing for higher resolution and throughput, with the etch resist composition designed for high precision and resistance to etching, enabling the use of aluminum paste without firing through the passivation layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If laser ablation is used to open the rear surface, then the passivation layer can be removed, but thermal damage to the silicon substrate occurs and cell efficiency decreases
Solution Approach 1:
The patent replaces the mechanical/thermal laser ablation process with a chemical wet etching process. The etch resist pattern is applied to the rear surface, and chemical etchants selectively remove the passivation layer and silicon material without generating thermal damage, thus achieving precise openings while preserving substrate integrity
Solution Approach 2:
The patent introduces an etch resist layer as an intermediary material that enables selective etching. This resist layer is applied in a desired pattern, protects areas where passivation should remain, and allows controlled removal of passivation and silicon in exposed areas through wet etching, preventing unwanted thermal damage
2Manufacturing precision
If laser ablation is used to remove material, then the passivation layer can be opened, but the process is time-consuming due to rastering over entire surface
Solution Approach 1:
The patent applies the etch resist pattern before the etching process, defining the exact areas where material removal is desired. This preliminary patterning step allows subsequent batch wet etching of multiple wafers simultaneously, avoiding the sequential rastering motion of lasers and dramatically increasing throughput while maintaining precise depth control through etch time and chemistry parameters
3Manufacturing precision
If laser ablation is used, then openings can be created, but intricate shapes and high resolution detail cannot be achieved
Solution Approach 1:
The patent replaces the mechanical rastering limitation of lasers with chemical etching that follows the etch resist pattern. This allows intricate shapes and high-resolution details to be achieved through precise resist application and photolithography techniques, while the chemical etching process naturally conforms to the pattern boundaries, enabling complex geometries that would be difficult with laser ablation
4Length of stationary object
If laser ablation removes more material to increase depth, then opening depth increases, but thermal damage to substrate increases and efficiency decreases
Solution Approach 1:
The patent changes the etching parameters (etchant composition, concentration, temperature, etch time) to control the depth and profile of openings. Wet etching provides self-limiting behavior and anisotropic etching capabilities that allow precise depth control without the thermal runaway effect of laser ablation, enabling deeper openings when needed while maintaining substrate integrity through controlled chemical reactions
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
The wet etch process results in higher solar cell efficiencies by minimizing substrate damage, achieving thicker and more uniform back-surface fields, and improving the conformity of these fields to the perimeter, thereby enhancing carrier flow and reducing shunting currents.
Implementation Method 1
The passivation layer can then be removed using a wet etch
Implementation Method 2
screen printable etch resists are described... with the etch resist composition designed for high precision and resistance to etching
Data Source
Figure 1
Figure 2
Figure 2A
AI summary
Compositions used in, and methods for, fabricating a rear-passivated silicon solar cell are described. A novel method of opening the back surface for contacting the silicon with a conventional aluminum paste is described. Various novel screen printable etch resists are described. First, such an etch resist can be printed on the rear of a rear-passivated solar cell wafer. The passivation layer can then be removed using a wet etch, and next the etch resist layer can be removed. Further, an additional wet etch step may optionally be used to deepen the opening into the silicon and enhance the BSF (back-surface field). Aluminum paste can then be printed over the entire backside of the now etched cell. The entire cell is then fired. In exemplary embodiments of the present invention, the paste can be chosen so that it will not fire through the passivation layer.