Preparation Of Ultra-fine Silver For Solar Cells And The Effect Of The Property Of Thick Film

Abstract:Solar power is a kind of safe, stable as well as immeasurable energy. It can be used to produce electricity according to photoelectric effect and the solar cell is the device to realize the process, in which the crystalline silicon solar cell grows fast and have the mose mature technology. Silver paste is the most important component in solar cell manufacture. The silver paste is made with silver powder, glass frits, organic vehicle as well as the doping agent, in which the silver powder occupies over85%weight. Thus the silver powder has an important effect on the electronic, mechanical and printing properties. The production of high quality silver paste in our native land is hindered by the property of native silver powder, whose dispersibility and tap density fall far behind the silver powder made abroad, while producers in other countries adopt technical security to protect its intellectual property. In the research we use different dispersant to prepare the ultra-fine silver powder measured less than1μm by liquid phase reaction.We also studied the influence of size, dispersibility and the mixture of silver powder on the densification of the front electrode in solar cells. The SEM, XRD, tap density spectrometer were used to analyze the pattern, size, dispersibility, tap density, purity and crystallinity of silver powders and the densification of electrodes.The solar cell sorting machine was used to measure the photoelectric transformation efficiency of the solar cells. The results are as follows:1.Dodecyltrimethylammonium Bromide (CTAB) was used as protective agent to prepare highly dispersive ultra-fine spherical silver powder for solar cells by reducing AgNO3solution with ascorbic acid. The experiment shows that the size of silver powder gets smaller as the concentration of Ag+decreases. Silver powder of0.58μm can be prepared when the concentration of Ag+reaches0.1mol/L. As the amount of CTAB increasing, the size of the powder decreases and the dispersibility gets better and the become worse. When m(CTAB)/m(AgNO3)=0.6, we can obtain silver powder of0.68μm with good dispersibility. The size decreases as the pH of the reaction increases, while the dispersibility become worse. The silver powder good dispersibility can be prepared when the pH is6-8. Adding appropriate amount of glycerol can change the medium of the reaction, which does good to the reduce of powder size. Silver powder with average particle size of0.75μm, tap density of4.0g/mL can be obtained by process optimization when the c(Ag+) is0.3mol/L, m(CTAB)/m(AgNO3)=0.6, pH is6.2.Triethanolamine(TEA) was used as protective agent to prepare highly dispersive ultra-fine spherical silver powder for solar cells by reducing AgNO3solution with ascorbic acid. The result shows that the reaction contains two step, TEA and Ag+form the AgOH colloid, AgOH is reduced by ascorbic acid to form Ag. In alkaline condition, the size fo silver powder decreases as the pH goes up as well as the amount of TEA rises, while in acidic condition TEA will lose its function. With process optimization, silver powder with average particle size of0.8μm can be obtained by adding TEA to the AgNO3solution when the pH is8, the concentration of TEA is4g/L.The dispersibility of the powder is not improved obviously, while the organic remains less.3. The research studied the influence of size, dispersibility and the mixture of silver powder on the densification of the front electrode in solar cells. Results show that when the dispersibility of silver powder are similar, the silver powder with smaller size can obtain more compact electrode. In the experiment, comparing with the2.5μm silver powder, the pore space ratio of electrodes prepared with the powder below1.0μm can have a decrease of18%, and the conversion efficiency has an increase of1.2%. The pore space ratio of silver powder which has high dispersibility is28%lower than the electrodes made by low dispersive silver powder, the conversion efficiency has a rise of3.6%, when the two kind of powder are of the same size.4.Mixed powder was prepared by mix the silver powder of micron grade and sub-micron grade in certain proportion. Result shows that the thinner powder can filling in the void between the stacking coarse powder. The thick electrode film of high density can be formed more easily when the stacking density of silver powder increases. Therefore, in a certain range, the pore space ratio decreases and the conversion efficiency increases as the thinner powder’s amount rises. When the thinner powder is excessive, the thinner powder can have negative effect on the densification of solar cell electrodes. In the experiment, the electrodes of15.55%void ratio and17.83%conversion efficiency can be achieved when the mass ratio of micron silver powder S1and sub-micron silver powder S2reaches7:3.