Lasers are an important tool for the production of thin-film solar modules, especially high-performance ultrashort pulse lasers, which provide ultrashort pulses with a duration of only a few picoseconds, which not only helps manufacturers increase production, but also optimizes the process . At present, in the discussion on solving future energy problems, photovoltaic energy plays an important role as a renewable energy source. Technological advancement is a crucial prerequisite for achieving energy parity consumption, such as reducing the cost of photovoltaic power generation to near the cost of traditional energy through technological advances.
At present, crystalline silicon solar cells are the leading products in the photovoltaic market, with conversion efficiencies of up to 20%. In the manufacturing process of crystalline silicon solar cells, lasers are mainly used for wafer cutting and edge insulation. In the laser edge insulation process, a laser-assisted doping process is used to prevent power loss caused by a short circuit between the front and back sides of the battery. More and more lasers are being used in laser assisted doping processes to improve carrier mobility, especially for electrode contact fingers. In the past few years, thin-film solar cells have made great progress, and industry experts hope that they will occupy about 20% of the market share in the photovoltaic market in the future.
The film used in thin-film solar cells is only a few microns thick, so it can save a lot of material in production. Lasers play a decisive role in the manufacture of thin film solar cells. Throughout the manufacturing process, the laser structuring and connecting the cells into modules and etching the modules accordingly ensures the required insulation properties.
Mature laser engraving process
In the production process of amorphous silicon or cadmium telluride (CdTe) thin film solar cell modules, a conductive film and a photovoltaic film are deposited on a large-area glass substrate. After each layer of film is deposited, the film is etched by a laser and the cells are automatically connected in series. In this way, the current of the battery and the module can be set according to the battery width. Accurate, selective, non-contact laser processing that can be reliably integrated into the production line of thin film solar cell modules. The so-called engraved line (see Figure 2) is a coherent process of single laser pulse etching. The spot size of the pulse after focusing is 30~80μm. Therefore, in the P1 layer engraving, the pulse width should be tens of nanometers. Pulsed light of seconds (10 to 80 ns) etches the glass substrate.
Transparent conductive oxides (TCOs such as ZnO and SnO2) are typically processed using near infrared lasers and relatively high pulse repetition frequencies. The pulse repetition frequency usually required exceeds 100 kHz. A high pulse repetition rate ensures thorough cleaning at the cut.
Depending on the material's absorption coefficient for the laser, it is necessary to select the appropriate laser wavelength for the particular process. The green laser's damage threshold for silicon is much lower than its destruction threshold for TCO, so the green laser can safely penetrate the TCO film layer and scribe the absorption layer. The scribe line mechanism of the P2 layer and the P3 layer is the same as that of the P1 layer. The process parameters of the P2 layer and the P3 layer with respect to the P1 layer have been listed above.
The characteristics of the single-pulse reticle mechanism itself impose certain limitations on the pulse repetition frequency. In order to prevent the semiconductor layer from falling off, the typical pulse repetition frequency required during processing is 35 to 45 kHz. A commonly used etch threshold is about 2 J/cm 2 , that is, it is capable of focusing 25 μJ of laser energy onto an area having a diameter of 40 μm, and the average power is very low. Since the average power of the green laser is on the order of several watts, it is possible to split the beam and perform multi-beam parallel processing, thereby further improving the work efficiency.
For the scribe line applications of the P1, P2 and P3 layers, the compact and compact diode-pumped lasers for micromachining applications with output wavelengths of 1064 nm and 532 nm are undoubtedly an ideal choice. The laser provides extremely high pulse stability. These lasers have a pulse duration of 8 to 40 ns and a pulse repetition rate of 1 to 100 kHz.
Clear protection
In order to prevent the solar cell module from being corroded or short-circuited, it is necessary to leave a margin of about 1 cm at its edge for the encapsulation of the entire battery module. Sandblasting is currently used to remove this edge. Although the investment cost of the sandblasting method is low, this process brings costs in terms of wear, sand removal, and dust pollution. The production of thin film solar cell modules requires clean, cost-effective solutions, and laser processing solutions are undoubtedly the best choice. Excellent processing quality can be achieved by increasing the average power of the laser. Laser processing can achieve a removal speed of approximately 50 cm 2 / s, and even a standard size solar cell module can be processed within 30 s.
In fact, all edge film layers can be removed with the same pulse, and the increase in removal rate is closely related to the average power of the laser. Lasers with high average power and high pulse energy can remove specific areas at once. The most suitable for this processing application is a laser system with fiber optic transmission that outputs a square or rectangular spot. The energy distribution of the laser after transmission through the fiber is more uniform, resulting in a high degree of consistency in the removal effect. By using the parallel combination of the spots, the processing efficiency can be increased by more than 50% compared with the conventional fiber, and the pulse repetition frequency is reduced while ensuring the processing safety. In addition, it can be combined with a scanning galvanometer to reduce non-production cycles during processing. Of course, the laser should also provide a corresponding time-sharing output option to reduce non-production time. In addition, several different workstations can be used to share the same laser processing solution, so that the loading and unloading time of the product does not affect the laser production efficiency.
Future laser technology
The use of special materials in the manufacture of CI(G)S solar cell modules poses a huge challenge to laser processing technology. If the applicable substrate material is glass, the molybdenum material is deposited onto the glass. However, since molybdenum has high melting point, good thermal conductivity, and high heat capacity, cracks and shedding may occur during heating. Since these disadvantages are unavoidable when processing with a nanosecond laser, the use of the laser is inseparable from the quality of the processing obtained. Similarly, the material of the absorbing layer is also quite sensitive to heat. Selenium (Se) has a lower melting point than metal materials such as copper (Cu), indium (In), and gallium (Ga), and it can be obtained at low temperatures. The place of bonding is separated. As a result, the semiconductor without the selenium layer becomes an alloy layer, causing the heat generated by the long pulse laser to short the edge.
Picosecond lasers will provide an ideal solution to these problems. Ultra-short pulse laser removal of the film material does not result in severe edge heat affected zones. High-performance picosecond lasers with wavelengths of 1030 nm, 515 nm, and 343 nm can be applied to the structuring of CI(G)S thin film solar cell modules. Ultrashort pulse lasers will replace the mechanical scribing process to further improve processing quality and processing efficiency.
Prospects for laser applications In the future, laser technology is expected to gain more application space in the photovoltaic manufacturing process, such as selective ablation of passivation layers of crystalline silicon solar cells, ultra-short pulses with high beam quality and high pulse energy lasers are particularly suitable for this type of application. application. Currently, only disc laser technology is available on the market to meet this standard. The output power of the disc laser is adjustable to achieve higher throughput, and the excellent beam quality of the ultra-short pulses of the output can significantly improve the conversion efficiency of the solar cell.
Laser technology has won a place in solar cell production, and its selective, non-contact processing has surpassed other processes. As the cost pressures on solar cell production increase, high-power, high-performance lasers will be widely adopted in large-scale production. Moreover, new laser technology with ultra-short pulses will also bring new production processes. In the future, the advancement and wide adoption of laser technology will inevitably increase the cost per watt of solar cell production.
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