Study on Dark Current of solar photovoltaic cells
Back
2022/11/05 13:53:17
In the absence of light, when the reverse bias voltage is applied to the PN junction (N area is positive, P area is negative), the reverse current will appear, which is the so-called dark current. For solar cells, dark current includes not only reverse saturation current, but also thin layer leakage current and bulk leakage current. Reverse saturation current refers to add a reverse bias voltage to PN junction, and the voltage of PN junction depletion layer width, the built-in electric field, the electron can new potential, P and N the number of majority carrier carrier (P area children for holes, N area children electronic) as it is difficult to cross the barrier, so the diffusion current is close to zero, but due to the electric field of the new, It makes it easier for the minority carriers in N region and P region to drift. In this case, the current in the PN junction is determined by the drift current from the dominant use. The direction of the drift current is opposite to the direction of the diffusion current, which is manifested in the external circuit that there is a reverse current flowing into the N region, which is formed by the drift movement of minority carriers. Since minority carriers are generated by intrinsic excitation, under a certain temperature, the number of minority carriers produced by ripe excitation is constant, and the current tends to be constant. The solar cell sheet can be divided into three layers: thin layer (N region), depletion layer (PN junction) and bulk area (P region). The process of recombination is always accompanied by the directional movement of carriers, and there will inevitably be small current, which will contribute to the value of the dark current obtained by the test. The part contributed by the thin layer is called the thin layer leakage current, and the part contributed by the body region is called the body leakage current.
2. Experiment and analysis
In the actual production of solar cells, the proportion of batteries with high dark current is high, which is the same as the debris rate, which is one of the two important factors affecting the qualified rate of finished batteries. The REASONS FOR THE EMERGENCE OF THESE batteries are various and extremely complex. After a long period of experiments, we analyzed the specific reasons for the emergence of high dark current and 5A batteries, and put forward the corresponding solutions in the production.
2.1 Reasons for internal quality of silicon wafers
The casting polysilicon has a simpler preparation process and lower cost than csilo monocrystalline silicon, but the ability to control impurities and defects is also weak. Oxygen, carbon, nitrogen, hydrogen and metal impurities in cast polysilicon, high density grain boundaries, dislocations and microdefects may cause the increase of the cell reverse current. Therefore, in the process of ingot casting, the impurity content of silicon material should be strictly controlled, the ingot casting process should be optimized, the impurity content and resistivity of silicon ingot and other parameters should be strictly controlled, and the appearance of microcrystalline defects should be strictly controlled. Figure 1 shows the damaged silicon wafer surface after cutting by wire saw. The surface is completely stripped from the silicon matrix, which greatly affects the surface properties of the silicon matrix.
2.2 Reasons for battery processing
2.2.1 fleece making process
: 1) wash embellish pollution in polysilicon acid system in the production of cashmere, and wash embellish within the system will generate insoluble salts, in the process of to embellish the wafer washing will contaminate the surface of the wafer and the p-n junction defects and, in turn, affect the coating, formed in the impurities without coating or coating defects, sintering pollution, especially the artificial pollution; It is very important to optimize the process of battery processing to prevent mechanical damage and reduce the dark current of batteries. The slurry will pass through the film at the impurity, causing the reverse dark current of the cell to increase.
2) Oil and water pollution of wind knife: the most important is the pollution of oil and water in compressed air. In the fleece-making equipment, there is a wind knife behind each process tank, which is used to remove the chemical liquid and water on the surface of the silicon wafer. The air used by the blade is compressed air, which contains a mixture of oil and water, which can stain the surface of the silicon wafer and form surface defects.
2.2.2 Diffusion process
PN junction uniformity is poor, high block resistance silicon wafer PN junction is relatively shallow, under the same sintering conditions, the shallow PN junction is easy to be penetrated by slurry, resulting in large leakage current. In THE production, THE size of block resistance should be strictly controlled, and the appropriate process should be adjusted to improve the uniformity of block resistance, so as to reduce the dark current of the battery.
2.2.3 Wet etching process
An important reason for the high dark current of the cell caused by wet etching is that the etching edge is large and the PN junction is destroyed, which leads to the leakage of electricity after coating printing and sintering. The edge of wet-etched silicon wafer is large for the following reasons:
1) The viscosity of the wet etching solution is insufficient, which is mainly manifested by the low concentration of H2S04 in the etching solution;
2) The chemical composition ratio is not suitable;
3) The etching groove flow is too large;
4) Exhaust air volume regulation is not suitable;
5) Slow transmission speed, etc.
Therefore, the proportion of high leakage batteries can be reduced to a certain extent by properly adjusting the wet etching process and minimizing the edge etching width under the condition of ensuring the etching depth.
2.2.4 Different plasma families have different components and performance, which have different effects on battery parameters
Different pastes have different penetration ability and high temperature performance to silicon nitride film, resulting in great differences in electrical parameters. In the type i sintering process, different sintering processes should be adopted according to the different properties of the slurry.
2.3 Causes of artificial pollution
The important thing is that the silicon wafer is contaminated by human hands, and the most important contaminant is sodium ions. The data in the following table are the comparison data of battery parameters when mechanical hand is used for loading and unloading in fleecing and diffusion processes, and manual chip collection and insert are used for fleecing and diffusion processes:
The above comparison data showed that the proportion of batteries using Irev2 > 5A battery parameters of willow arm insert was lower than 1.5%, which was significantly lower than that of manual insert.
2.4 Causes of mechanical damage in Cheng Shu
Due to the various processes in the battery production process will inevitably cause different degrees of mechanical damage to the silicon wafers, serious cases will cause dark cracks in the silicon wafers. If the silicon wafers have not broken after printing slurry, the slurry will enter the silicon wafers through dark cracks after sintering, resulting in large leakage of the battery. This kind of silicon wafer is difficult to find in the production process, but it can be observed under lR that there are micro cracks, which are shown as bright spots with high temperature. Therefore, the mechanical collision of silicon wafer should be reduced in the production process to prevent the appearance of dark cracks.
Conclusion 3.
In order to reduce the dark current of the cell and improve the quality of the cell, the internal quality of the silicon ingot should be improved first, and the content of internal impurities and defect state of the silicon ingot should be reduced. Secondly, reduce the production of all links of pollution, especially man-made pollution; It is very important to optimize the process of battery processing to prevent mechanical damage and reduce the dark current of batteries.
Previous
