Analysis of laser processing application of the ho

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Application analysis of laser processing of thin film solar cells

ultrashort pulse lasers are used in the production and processing of high-precision structures of many materials, because ultrashort laser technology is neither thermal ablation nor "cold" ablation. However, these ultrashort pulse laser systems require more investment and operating capital than traditional industrial laser sources. In the material processing market, picosecond laser light source has received more attention, because the traditional laser light source has been unable to meet the requirements of ablation quality

today's picosecond laser systems have good performance and can operate normally under manufacturing conditions. Another important factor for their successful application in industry is that they meet the requirements of processing and manufacturing cycle. The current repetition rate of picosecond laser source is 500KHz, which can meet the needs of industrial processing speed. One of the ultrafast laser sources that meet the requirements of the manufacturing industry is the picoregen industrial laser, which is provided by high Q laser production company. The output power is 30 W, the pulse width is 10 PS, and the repetition rate is 500 kHz

an interesting application field of this laser is thin film processing. In this application, the selective ablation process combines the characteristics of high precision and high speed, which makes the ultrafast laser have unparalleled advantages. In addition, it is particularly noteworthy that the current processing of thin-film solar cells is still based on nanosecond laser ablation and mechanical marking. These processing processes are feasible in theory, but in the process of large-scale production, it is still necessary to improve the repetition frequency and production cost

historically, silicon wafer production technology has been used in the production of solar cells. This technology uses low-cost float glass as the base material instead of expensive silicon wafer. The low-cost substrate combines the film coating as the action layer, which greatly reduces the production cost. Thin film solar modules can be produced more economically and effectively by using embedded means. What's more, some super hard materials reduce the required materials, so they can compete effectively with traditional energy sources

the film must have a "row" structure to achieve electrical isolation, and the substrate and other layers must not be damaged when processing this structure. At present, the processing and rectification of "row" structure are realized by nanosecond laser ablation or mechanical line cutting. Using ultrafast laser to process molybdenum (MO), copper indium selenide (CIS), zinc oxide (ZnO) and other transparent oxides, the research shows that ultrashort laser pulse reduces the thermal effect and has better repeatability, but the processing speed in industrial application still needs to be improved

recently, copper indium selenide (CIS) thin-film solar cells have attracted much attention because of their high efficiency compared with other thin-film solar cells. Copper indium selenide thin film solar cells have three functional layers (as shown in Figure 1)

first, a molybdenum layer with a thickness of 500 nm is sprayed onto a glass substrate several millimeters thick, and then the so-called P1 process is carried out by nanosecond laser ablation. Subsequently, a cis layer with a thickness of 1 m to 3 m was deposited on the molybdenum layer structure, which formed an absorption layer for light and formed a p-type semiconductor. In P2 process, technicians use mechanical marking to process CIS layer to realize electrical isolation. Subsequently, a thin layer of transparent conductive ZnO with a thickness of 1 m to 2 m was deposited on the surface of the CIS layer, forming the n-type part of the solar cell, which is the path to the industrialization of R & D achievements. In order to improve the performance of the battery, a buffer layer (CDs, 100 nm) can be added between CIS and ZnO. Molybdenum layer is usually called "backward contact", and ZnO layer is usually called "forward contact". In the last P3 process, the technician processed the ZnO and CIS layers on the molybdenum layer at the same time by means of mechanical marking

etching process P1

in flow process, nanosecond laser is usually used for P1 processing (as shown in Figure 2). This technology may make the edge of the material too high, resulting in partial diversion or short circuit to the upper layer, and the height of the edge is almost equal to the thickness of the molybdenum layer. Moreover, there are micro cracks and local material spalling in the range of 5 to 10 nm at the edge of the groove, which further shortens the life of the solar cell. In addition, where the laser pulses coincide, the base glass may be melted. These conditions hinder the normal production and processing process. In the glass melting area caused by laser coincidence, the barrier layer may be destroyed. All these damages can be traced back to the thermal effect caused by the nanosecond ablation process

Figure 3 shows the shunting effect obtained by picosecond laser on the molybdenum layer. From this figure, we can see that the image has some wavy shapes, and the upper and lower ends of the edge are slightly asymmetric in height. Fortunately, the edges formed here are not as obvious as those formed by nanosecond laser ablation

etching process P2

the processing process of P2 is to use mechanical means to etch lines on the CIS layer, which is on the relatively hard molybdenum layer. The drawback of this technology is that the linewidth may fluctuate between 60 and 120 M. Figure 4 shows the P2 structure obtained by picosecond laser ablation. Using picosecond laser, technicians can remove the CIS layer with pertinence and high quality, and the groove width is only 22 M. Using picoregen industrial laser, the processing speed can reach m/s. The figure shows that using picosecond laser technology, technicians can selectively remove without causing problems such as microcracks or material spalling caused by thermal effects

etching process P3

at present, the processing of zno/cis double layers on molybdenum layer is similar to the processing process of P2 structure. The width of the scribed line is greater than 70 m, accompanied by irregular material peeling, while the debris and residual materials of ZnO and CIS layers are also scattered in the scribed line. The marking speed is only a few cm/s

by using picosecond laser, technicians can selectively process zno/cis double layers above molybdenum layer without any damage. Figure 5 shows the effect diagram of laser processing zno/cis double layers. The stepped profile on the side of the groove clearly reflects the advantages of selective processing. In addition, the line width is only 18 m, which shows that the processing process is very reliable and reduces material loss

these results show that 1064 nm picosecond laser can be applied as a general technology. 3 If the user finds that the pendulum and other institutions are inflexible in the process of using, it is used for CIS thin film solar and forms a long-term friendly cooperative relationship with domestic well-known universities, enterprises and quality inspection centers to process all processes of battery (P1-P3). The latest picosecond laser has a high repetition rate, which can ensure the processing speed of industrial level (several m/s). In addition, the processed reticles have the characteristics of high quality and high stability, which further shows that this technology can reduce processing losses and prolong the life of solar cell modules

authors: Dr. Sandra zoppel and Dr. Heinz Huber are from high Q laser production in Hohenems, Austria

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