During the preparation of chalcogenide thin film cells, four main laser processing processes are involved, namely three selective film layer etching, i.e., scribing processes (called P1, P2 and P3), and one full film layer removal, i.e., laser edge clearing process (called P4). The whole cell is divided into n sub-cells in series structure, aiming to reduce the impact on the module caused by the performance difference in different areas due to the inhomogeneity of the chalcogenide material, and at the same time, it can increase the output voltage of the module, reduce the output current of the module, and reduce the series resistance between sub-cells and the thermal loss of the external circuit resistance.
Laser P1 scribing: laser etching of the bottom TCO film layer to form mutually independent TCO substrates;
Laser P2 scribing: laser etching of other film layers above TCO to provide transmission channels for the positive and negative electrodes of two neighboring sub-cells;
Laser P3 scribing: after depositing the back electrode, laser etching other film layers above TCO, separating the sub-cells from each other;
Laser P4 edge clearing process: the laser removes the deposited film from the edge of the cell to prevent leakage and ensure the reliability of the battery package.
Figure 1. The distance between the P1 and P3 lines is called the "dead zone", the dead zone does not contribute to the power generation of the cell, the role of the laser is to minimize the width of the dead zone in order to increase the effective area.
The necessity of laser scribing for the calomel battery:
Mechanical scribing, as one of the traditional cell interconnections, is widely used in the P2P3 process of CIGS cells, despite its high efficiency. However, there are obvious limitations, which include the disadvantages that mechanical scribing is contact type, the mechanical needle tip is subject to continuous wear and tear, has a short life span and needs to be replaced periodically, serious chipping of edge debris, insufficient depth control accuracy, poor line consistency, and large dead zones. Therefore, this scribing method is completely unsuitable for the total thickness of less than 1 μm of the membrane layer of the chalcogenide thin film battery.
In contrast, laser scribing has the advantages of being non-contact, easy to maintain, less thermal influence on the edge, easy to control the scribing depth, good line consistency, and smaller dead zones, which enables a more accurate, cleaner, and more efficient scribing process, as well as precise control of the removal of the film layer, and a clean and smooth bottom and edges of the groove.
Figure 2. Comparison of mechanical scribing (left) and laser scribing (right).
For the choice of pulse length, nanosecond laser processing thermal effect is obvious, there are shortcomings such as rough edges, surface debris, and slow processing speed; while the use of picosecond laser as a representative of the ultrashort pulse can show the advantages of high peak power, small thermal effect, smooth edges, precision and so on. In the process of laser etching, picosecond laser can be focused to the ultra-fine space area, quickly vaporize the material evaporation, avoid the material of the laser's linear absorption, energy transfer, conversion and the existence of heat and thermal diffusion, can do almost no thermal impact, to achieve the laser "cold" processing.
Figure 3. Picosecond laser processing effect
With regard to the choice of laser wavelength, different materials have different absorption rates for different wavelengths of laser light, and the higher the absorption rate, the smaller the thermal effect produced.
Figure 4. This is the spectral absorbance diagram of FTO materials, usually using a short wavelength of ultraviolet light when the absorption rate is very high, more appropriate. However, because the 355nm green laser is generally 1064nm infrared laser through the frequency doubling crystal frequency doubling, in the conversion process will produce about 40 ~ 50% power loss, therefore, from the point of view of cost-effective considerations, the P1 scribing process will choose to use the 1064nm infrared laser, and the FTO has a certain degree of absorption of infrared light. P2 layer should be used in the case of the FTO not to damage the 532nm laser, the FTO is not damaged. The P2 layer should be scribed with 532nm green light without damaging the FTO, while the P3 process can choose the appropriate wavelength of light source according to the absorbance and thickness of the layer, and the 532nm wavelength can be used to remove the layer.
As for the choice of scribing direction, there are generally two ways: direct scribing on the film surface and scribing through the glass surface. The former refers to the laser beam focused on the film layer to be removed above, the material absorbs the laser energy gasification to form an engraved line, this method has a large thermal effect, easy to form a "crater", and requires precise control of the process. Scribing through glass is to focus the laser beam on the interface between the glass and the film layer to be removed, the interface absorbs the energy and vaporizes the film layer, which rapidly expands in volume to form an "explosive shockwave", thus forming the scribe line. This process has a smaller heat-affected area and is less likely to form a "crater", but requires stronger laser energy and a higher process window.
Laser scribing in the thin film module to ensure that the whole cell is divided into more than n sub-cells in series under the premise of the dead zone should be compressed as much as possible, and the laser in the role of the film layer, may produce obvious heat-affected zone, craters, the bottom of the debris burrs, film delamination and other process adverse effects.
Figure 5. Possible adverse process effects of laser processing of battery film layers include obvious heat-affected zones (top left), craters (top right), bottom slag burrs (bottom left), and film delamination (bottom right).
Therefore, the dead zone control and a series of factors affecting the processing results, in addition to the above mentioned laser pulse width (ns / ps / fs); laser wavelength (355 / 532 / 1064 nm); processing mode (film surface processing / glass surface processing), but also covers the corresponding laser parameters, including the choice of power, beam quality, focused spot size and overlap rate, and the subsequent process results. In addition to the processing method (film side/glass side), it also covers the corresponding laser parameters including power, beam quality, focused spot size and overlap rate, as well as the subsequent process effects such as scribing width, heat effects, craters, line scribing consistency of P123, parallelism, etc. Additional considerations include beam shaping and dust handling during processing.
