Research on sintering matching of crystalline silicon solar cells

　　The purpose of sintering is to sinter the electrodes printed on the silicon wafer at high temperatures to form good ohmic contact with the silicon wafer, thereby better collecting the current generated by the photovoltaic effect on the surface of the cell. The conventional production process of solar cells is: surface pre-cleaning → chemical etching to produce texture → diffusion bonding → plasma etching to remove edges → dephosphorization of silicate glass → anti-reflective coating → screen printing → sintering → performance testing, among which Sintering is one of the core links in battery cell production.

　　The quality of sintering directly affects several important parameters of the battery sheet: series resistance, parallel resistance and fill factor, thus determining the quality of the battery sheet. Due to the differences in equipment and process conditions used by many manufacturers, as well as the continuous updating of printing pastes, each manufacturer needs to continuously adjust and modify in practice according to its own actual situation to achieve better process matching. This paper theoretically analyzes the judgment of series resistance, parallel resistance and filling factor on the sintering effect, and uses experiments to verify these judgment indicators, and then obtains the relationship between each parameter and sintering temperature. This has certain guiding significance for the sintering matching adjustment in the industrial production process.

　　1 Sintering principle

　　The principle of sintering is that when the metal material and crystalline silicon are heated to the eutectic temperature, the crystalline silicon atoms will diffuse into the metal in a certain proportion, and the number of diffused crystalline silicon atoms depends on the temperature of the metal and the volume of the metal. At this time, the state in which the crystalline silicon atoms are integrated into the metal is called an alloy system. After that, the temperature is lowered, and the silicon atoms integrated into the metal will crystallize out in a solid state again, thus forming an epitaxial layer between the metal and the crystalline silicon. If this epitaxial layer contains the same type of impurities as the original crystal, the metal forms an ohmic contact with the crystalline silicon.

　　Each batch of silicon wafers has its optimal sintering point due to its different impurity doping ratio and depth. When the temperature is too high or too low, our ideal sintering effect cannot be achieved. When under-burning, the ohmic contact is not completely formed, the series resistance will be too large, and the filling factor will be low; when over-burning, the diffused phosphorus on the surface of the silicon wafer is driven deep into the silicon wafer at high temperature, and the phosphorus in the silver paste If sufficient phosphorus source replenishment cannot be formed, the impurity concentration on the silicon wafer surface will decrease, the contact resistance will increase, and the silver-silicon alloy will also consume too much silver. At this time, the silver-silicon alloy layer is equivalent to an isolation layer, preventing The output of carriers will also increase the contact resistance and reduce the fill factor.

　　Therefore, we can judge the quality of sintering by examining the contact resistance and p-n junction characteristics. Combined with the investigation of parallel resistance and reverse current, we can roughly judge the adjustment direction of sintering. Over-burning will cause the electrode to burn through, and more impurities will be dispersed near the p-n junction, increasing the probability of local leakage. At this time, the The characteristic is that the parallel resistance is too small and the reverse current is too large. At the same time, when the temperature is too high, the probability of surface recombination increases, and the short-circuit current will also decrease accordingly. Therefore, the judgment of sintering matching is mainly a comprehensive judgment of the contact resistance, parallel resistance, fill factor, reverse current and short-circuit current in the series resistance.

　　2 experiments

　　2.1 Sample

　　The experiment selected a p-type single crystal silicon wafer with an area of 156mm×156mm and a crystal orientation of <100> as the experimental sample. Its conductivity was 0.5Ω·cm～2.0Ω·cm, and the thickness of the original silicon wafer was about 200μm. The experimental samples were divided into 11 groups, with 20 samples in each group.

　　2.2 Experimental steps

　　The experimental steps are as follows: cleaning and texturing → diffusion and bonding → etching → dephosphorization of silica glass → anti-reflective coating → printing → high-temperature sintering → test statistical average data → adjust the sintering process → analyze the conclusion.

　　(1) Cleaning and texturing, using low-concentration alkali solution to etch and prepare the textured surface to reduce the reflectivity of the silicon wafer.

　　(2) Diffusion junction, use liquid POCl3 as the phosphorus source to diffuse at high temperature to form a p-n junction, use Semilab's sheet resistance tester SHR-1000 to test the sheet resistance, and control the sheet resistance range from 53Ω/□ to 57Ω/□.

　　(3) Use CF4 and O2 plasma to etch the edge of the silicon wafer.

　　(4) Use HF acid of appropriate concentration to remove the attached phosphosilicate glass.

　　(5) Use conventional processes to deposit a layer of SiNx:H anti-reflective film on the surface of the silicon wafer. Use semilab's LE-100PV ellipsometer to measure the thickness and refractive index of the film. The film thickness is 75nm and the refractive index is 2.01. .

　　(6) Printing and sintering: Use a Baccini screen printer to print electrodes on silicon wafers, and use a Despatch sintering furnace in groups of 20 wafers to repeatedly adjust the sintering furnace according to the sintering results. The main method here is to use the control variable method. First, control the temperature of zones 1 to 8 of the sintering furnace to remain unchanged, adjust zone 9 of the sintering furnace to obtain a relatively good sintering point, and then control the temperature of zones 1 to 7 and 9 to remain unchanged. 8 temperature zones for optimal matching temperature. Zones 1 to 7 were not adjusted during the entire experiment and were common steep slope sintering curves. The temperature settings are shown in Table 1.

　　(7) Keep the temperature in zone 8 unchanged and set it to 820°C. The results obtained by adjusting the temperature in zone 9 are shown in Table 2.

　　(8) Keep the temperature in zone 9 unchanged and set it to 925°C. Adjust the temperature in zone 8 to obtain the results in Table 3.

　　3Data and analysis

　　The experimental data results were obtained from the pss10 solar photovoltaic simulator test, using the average data of 20 pieces as a reference to avoid data distortion caused by the fluctuation of a single piece. Comparing the sintering data in Table 2 and Table 3, we can easily find that Uoc, Isc, Rs, Rsh, FF and eta have changed at different sintering temperatures. In particular, the changes in Rs, Rsh, FF and eta are obvious.

　　During the experiment, we controlled the remaining eight areas and adjusted them in the direction of over-burning and under-burning respectively, in order to find the most suitable sintering point. In order to facilitate data analysis, we adjusted the sintering data to the process of gradually increasing temperature.

　　Analyzing the sintering data of the nine regions, as shown in Figure 1, we found that as the temperature continues to increase, the back field of the battery is more fully sintered, the series resistance of the battery continues to improve, the fill factor increases, and the efficiency increases. Continuing to increase the temperature, in addition to a large decrease in the series resistance and an increase in the fill factor, other parameters become worse. If the temperature continues to increase, all parameters of the battery become worse. The series resistance improved at the fifth set of temperatures. It is likely that the higher temperature caused the front silver electrode to burn through, causing the series resistance to drop significantly. The fill factor can be viewed as a function of the series resistance, as shown in the formula.

　　In the formula: FF is the fill factor; Uoc is the open circuit voltage; Rs is the series resistance.

　　According to the formula, when the series resistance drops significantly, FF has an improvement process. Analyzing the above sintering process, when the temperature in zone 8 remains unchanged, the sintering process temperature setting of the fourth set of data is the most ideal. Compared with the original sintering process, the efficiency increased by 0.28%.

　　According to the formula, when the series resistance drops significantly, FF has an improvement process. Analyzing the above sintering process, when the temperature in zone 8 remains unchanged, the sintering process temperature setting of the fourth set of data is the most ideal. Compared with the original sintering process, the efficiency increased by 0.28%.

　　According to the formula, when the series resistance drops significantly, FF has an improvement process. Analyzing the above sintering process, when the temperature in zone 8 remains unchanged, the sintering process temperature setting of the fourth set of data is the most ideal. Compared with the original sintering process, the efficiency increased by 0.28%. According to the formula, when the series resistance drops significantly, FF has an improvement process. Analyzing the above sintering process, when the temperature in zone 8 remains unchanged, the sintering process temperature setting of the fourth set of data is the most ideal. Compared with the original sintering process, the efficiency increased by 0.28%.

　　After repeated debugging, the optimal sintering temperature is shown in Table 4.

　　At this sintering temperature, we conducted multiple tests and obtained good repeatability. At this temperature, the efficiency improved by 0.51% compared to the initial sintering process. This shows that the above analysis is correct.

　　4 Conclusion

　　By controlling variables and adjusting the temperatures in zones 8 and 9 respectively, a relatively good sintering process was obtained. In the process of arriving at this process, we combined theory and analysis of experimental results to draw the following conclusions :

　　(1) The series resistance of the battery is closely related to the matching of the sintering process. Generally, with a good sintering process, the series resistance will be reduced accordingly;

　　(2) FF is closely related to the series resistance. Generally, as the series resistance decreases, FF will increase accordingly;

　　(3) Generally, there is a phenomenon of high series resistance in both overheating and underheating. At this time, the judgment should be made based on the parallel resistance and backflow. Generally, in overheating, there is a phenomenon of reduced parallel resistance and high backflow;

　　(4) The improvement of contact resistance can well predict the improvement of sintering effect. Without damaging the p-n junction, the smaller the contact resistance, the better the sintering effect;

　　(5) In the process of adjusting the cell sheet from under-fired to the optimal sintering point, the series resistance continues to decrease. Just after passing the optimal point, the series resistance will still decrease. At this time, it needs to be judged based on efficiency and backflow;

　　(6) It is really impossible to judge the sintering point at this time. You can use the control variable method to change a temperature zone in one direction and observe the sintering effect to determine the direction of temperature adjustment;

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