6.4 Smartwires cells
Photovoltaic technology is constantly evolving. Significant improvements have been made at the level of the photovoltaic cell, but there are limitations at the level of those connections into the photovoltaic panel. Swiss company Meyer Burger [1] has developed Smart Wire Connection Technology (SWCT), which does not reduce the performance of the entire photovoltaic panel compared to a single photovoltaic cell. It involves combining the lamination process and the interconnection of cells to form a photovoltaic panel in a single lamination step. During this process, material consumption and energy consumption are reduced and cell efficiency is at \( 25.4\% \).
Smartwire technology also involves the use of a grid of wires on the photovoltaic cell plane instead of the conventional solution of interconnecting busbars. Here, the number of electrical connections in a single cell reaches up to 2640, which provides resistance to mechanical stress and better performance in low-light conditions ( Fig. 1 ).
Using this process reduces electrical and optical losses in the photovoltaic panel, due to shorter charge paths to the electrodes and less shading of the cell by the electrodes than in standard busbar technologies.
SWCT technology transforms the appearance of the front surface of the photovoltaic cell and offers several additional benefits:
- connects multiple wires, reducing ohmic and optical losses by reducing the thickness of the busbars as the number of wires can be tailored to the design of a particular photovoltaic cell,
- the use of silver paste can be significantly reduced,
- the light absorption of the photovoltaic cell is improved by intelligent reflection of light through the wires,
- reducing the impact of photovoltaic cell micro-cracks by increasing the number of current collection paths,
- simplifies process steps, soldering and lamination processes are replaced by a single lamination process which are done together,
- stress in the photovoltaic cell is reduced because the temperature during the bonding process is uniform throughout the cell and is below \( 160_{}^{o}\textrm{C} \),
- the cost of making the photovoltaic panel is lower,
- the process is compatible with many types of materials such as Al, Cu, Ni, Ag and is therefore possible for new material combinations and joining new photovoltaic cell concepts such as back passivated cells, HJT, metal plating and IBC.
SWCT is an electrode-to-photovoltaic cell bonding technology based on wire bonding. It typically uses 15 to 38 wires on either side of the photovoltaic cell. The conductors are round copper-based wires coated with a low-temperature alloy, typically a layer 1-2 microns thick with \( 50\% \) indium alloy. The wires are embedded in a polymer film that is applied directly to the metallized cell.
The components arranged in this way are laminated ( Fig. 2 ). The busbars are associated with the photovoltaic cell metallization and provide electrical contact with most of the material (e.g., the number of busbars and their thickness can be customized for almost any cell metallization design and cell power). Busbars on the surface of the photovoltaic cell (both front and back) are not needed. This saves time and materials (the metallization process requires expensive material such as silver paste) and prevents shadowing. SWCT technology has an additional advantage, better passivation of the backside of the photovoltaic cell can be achieved with an aluminum screen printed on the backside of the surface field or with any backside passivation method (such as \( SiO_{2}, a-Si, AlO_{x} \) etc.).
Photovoltaic cells are fragile and therefore need to be protected to resist external conditions such as rain, hail, moisture, wind and snow. Protection is usually achieved by embedding the photovoltaic cell in glass and an encapsulation layer. Since the generated current must be transported from one cell to another, electrical losses occur. The most reliable, proven techniques used to date are ribbon soldering.
SWCT technology offers up to 38 coated copper wires to carry the charge generated by the photovoltaic cells. The finger length can be reduced from 39 mm to 4-8 millimeters, which in turn makes the power loss at the finger negligible. This reduction in finger length is achieved without changing the cross section of the transport material. The power loss associated with resistance drops and more energy can be extracted from each individual photovoltaic cell. The comparisons in the table below show that an SWCT with 30 wires of 0.2 mm diameter has the same optical shading as one with 3 busbars. The SWCT with 18 wires of 0.3 mm diameter has \( 85\% \) higher Cu cross section compared to 3 busbars, i.e., lower resistance. In addition, optical shadowing is reduced ( \( 2.6\% \) compared to \( 2.9\% \) for 3 busbars), and there are additional benefits from the reduced finger length to 8.2 mm. In summary, the data on the Fig. 3 below [1] show that SWCT has better performance than busbar technology.
In order to compare SWCT type technology with busbar technology, photovoltaic panels were prepared using the same type of cells. Two types of cells were used in the experiment, monocrystalline cells made by Hareon Sun (China) and cells made by HJT (Switzerland). The performance data of the c-Si monocrystalline cell were collected on Fig. 3. The panels thus fabricated were subjected to performance tests. Increasing the number of busbars to 5 raised the cell power to \( 102\% \), and in SWCT technology to \( 103\% \). If the microcracking resistance of the cells is added to this, the use of the SWCT type procedure is justified.
With SWCT technology, there is no drastic loss of power due to microcracks [2], because the generated charges can reach the collecting electrode through a different path.
The video "The Making of SmartWire Technology" shows the manufacturing technology of photovoltaic panels made with smart-wire technology.
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The video "The Technology Behind SolarTech Universal" shows the manufacturing technology of photovoltaic panels made with smart-wire technology.
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