Electroplated Metal Grid for Solar Cell Series Resistance Reduction
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Solution Overview
Problem
Conventional solar cell manufacturing methods, such as screen-printing silver paste, result in high series resistance, increased material costs, and limitations in line width and height, which hinder the achievement of high energy-conversion efficiency due to suboptimal adhesion and stress issues with metal grids.
Innovation Solution
A method involving a metal stack with a metal-adhesive layer formed using physical vapor deposition, including Ti or Ta, and a conducting layer of Cu or Ag, where a soldering layer of Sn or Ag is added on top, and the front-side electrode grid is formed through electroplating with a patterned masking layer and subsequent etching to achieve lower resistance and improved adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If screen-printing Ag paste is used to form the electrode grid, then the manufacturing process is simple and widely adopted, but the series resistance is high and material cost increases
Solution Approach 1:
A metal-adhesive layer (Ti or Ta, 5-50 nm thick) is introduced as an intermediary between the Si emitter and the Ag electrode grid. This adhesive layer enables direct electroplating of Ag onto the Si surface, replacing the conventional screen-printing paste method. The adhesive layer provides excellent adhesion and enables low-resistance electrical contact, resolving the high series resistance issue while maintaining manufacturing feasibility through electroplating processes.
Solution Approach 2:
The conventional mechanical screen-printing process is replaced with an electrochemical electroplating process. Instead of mechanically depositing Ag paste and firing it at high temperatures, the invention uses electroplating to deposit Ag directly onto the metal-adhesive layer, achieving lower resistance and better adhesion without the limitations of mechanical printing methods.
2Ease of manufacture
If screen-printing Ag paste is used, then the process is conventional and established, but the line width cannot be reduced further and line height is limited
Solution Approach 1:
The mechanical screen-printing process is replaced with electroplating, which offers superior control over line dimensions. Electroplating allows for precise control of line width (reducing to 50-100 microns) and line height (increasing to 25-50 microns) through control of plating time, current density, and electrolyte composition, overcoming the dimensional limitations of screen-printing.
Solution Approach 2:
The invention changes the fundamental deposition parameters from mechanical paste printing to electrochemical plating. By controlling electroplating parameters (current density, plating time, electrolyte concentration, temperature), precise control over line width and height is achieved, enabling narrower lines and greater height control compared to conventional printing methods.
3Temperature
If Ag paste is fired at high temperature, then good contact between Ag and Si is achieved, but the resistivity remains high compared to bulk silver
Solution Approach 1:
The high-temperature firing process is replaced with electroplating. Instead of heating the Ag paste to 700-800°C to achieve contact, the invention uses electroplating to deposit Ag directly onto the metal-adhesive layer, achieving excellent electrical contact at lower temperatures. The electroplated Ag forms a metallurgical bond with the adhesive layer, resulting in resistivity much closer to bulk silver (2-3 × 10^-6 ohm-cm) compared to fired paste (5-8 × 10^-6 ohm-cm).
4Length of stationary object
If multiple printing is used to increase line height, then the height increases, but the line width also increases which is undesirable
Solution Approach 1:
Multiple sequential printing operations are replaced with a single electroplating process. Electroplating can achieve the desired line height (25-50 microns) in one continuous deposition step without affecting line width. The electroplating process deposits material uniformly vertically, maintaining precise lateral dimensions defined by the masking layer while achieving the required height for effective current collection.
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 reduces the series resistance of the metal grid, enhances adhesion, and allows for narrower line widths and increased line heights, thereby improving the energy-conversion efficiency of solar cells while minimizing material costs.
Implementation Method 1
a metal-adhesive layer formed using physical vapor deposition, including Ti or Ta
Implementation Method 2
the front-side electrode grid is formed through electroplating with a patterned masking layer and subsequent etching
Data Source
AI summary
One embodiment of the present invention provides a method for fabricating solar cells. During operation, an anti-reflection layer is deposited on top of a semiconductor structure to form a photovoltaic structure, and a front-side electrode grid comprising a metal stack is formed on top of the photovoltaic structure. The metal stack comprises a metal-adhesive layer comprising Ti or Ta, and a conducting layer comprising Cu or Ag situated above the metal-adhesive layer.