| Abstract:With non-renewable resources such as oil, natural gas and coal in the decades to be constantly consumed, the contradiction betewwn supply and demand is becoming sharper. Solar energy as a kind of pollution-free renewable energy with rich source has the world pro-gaze and crystalline silicon solar cell grows the fastest. Front silver paste for solar cell is the key materials of crystalline silicon solar cell, usually printed on the silicon substrate which is coated antireflection film. The electrode will be created after drying and high-temperature sintering. Front silver paste contains silver powder, glass frits, organic vehicle and doping agent. The preparation technology of front silver paste is more advanced in some foreign countries. However, the performance of domestic front silver paste is unsatisfying and unstable because of late start and abroad technical security, where silver powder is the bottleneck of localization of front silver paste. So the research and development of high-grade ultrafine silver powder is badly in need to realize localization of front silver paste. This paper mainly researches on the preparation and the characterization of the ultrafine silver powder for front silver paste for Solar cell, which is prepared with chemical reduction method. And we have prepared front silver paste with home-made ultrafine silver powder. With the help of scanning electron microscope, laser scattering particle analyzer, PF-100B vibration solid density spectrometer and D/Max2500XRD, we observed and analysed the morphologies, mean grain sizes, tap density, crystallinity of silver powder and microstructure of thick film. The conductivity of front silver paste and the photoelectric property of solar cell are measured by four-probe meter and solar cell sorting machine. The results are as follows:1. Utrafine silver powders were prepared by chemical reduction method with ascorbic acid as the reducing agent, methylcellulose as the dispersant. The result shows that the reaction temperature has a great effect on the morphology of the ultrafine silver powders. Spherical-like silver powders can be prepared when the temperature is25℃and30℃; Dendritic silver powders can be prepared when the temperature is40℃, 50℃and60℃. The dosage of dispersant has a great effect on the dispersibility of ultrafine silver powder. Silver powder has better dispersibility as the increasing of dosage of dispersant. The pH value has a great effect on the mean size of silver powder. Silver powder has smaller size as the increasing of pH value. When pH value is2and10, the particle size of silver powder is2.26μm and0.05μm respectively. And finally we got the optimization process of the experiments. We can obtain spherical silver powders with high crystallinity, fine dispersion and irregular surface, whose tap density is4.1g·mL-1and mean particle size is2.21μm, in the condition that the reaction temperature is25℃, pH value is2, the mass ratio of methylcellulose and ascorbic acid is0.02.2. Compared dispersion effect between Carboxymethocel and methylcellulose,the results show that carboxymethocel has better dispersion effect than methylcellulose. We can obtain spherical silver powders with fine dispersion and smooth surface, whose tap density is4.6g·mL-1and mean particle size is3.95μm, when the reaction temperature is30℃, pH value is2, the mass ratio of carboxymethocel and ascorbic acid is0.015.3. Utrafine silver powders prepared with a mixture dispersant consists of Twain and Arabic gum as has fine dispersion, small partical size and high tap density, which can better meet the technical requirements of the fine grid. We can obtain polyhedron silver powders, whose tap density is5.4g·ml-1and mean particle size is1.5μm, in the condition that the reaction temperature is30℃, pH value is2, the mass ratio of Twain, Arabic gum and ascorbic acid is0.1:0.06:3. Besides, with the treatment of heating and stirring, the dispersion and tap density of silver powder can be greatly improved.4. Dispersibility of silver powders has a great effect on the printing performance of front silver paste for solar cell. Silver paste prepared by silver powders with fine dispersibility and small partical size has better printing performance. Silver thick film prepared by spherical silver powder with smooth surface has denser structure, lower resistivity and stronger adhesive force. The resistivity is0.15mΩ·cm and the adhesive force is4.2N. In additon, the density of the sintering thick films increase as the increasing of peak sintering temperature. When the peak sintering temperature is870℃, solar cell has the highest IPCE of17.84%。... |
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