Study On The Synthesis And Performances Of Silver Powder Used For Front Contacts Of Silicon Solar Cells

As the key material of making front-side Ag contacts of silicon solar cells, silver powders have a direct effect on photoelectric properties of solar cells, like conversion efficiency, series resistance, open circuit voltage, etc. In this paper, high-performance front contact silver powders were prepared by liquid-phase reduction method with silver nitrate as silver source and ascorbic acid as reductant. The silver particle’s morphology, size distribution and the microstructure of electrodes were analyzed by scanning electron microscope(SEM), laser particle size analyzer and other analysis methods. The effects of kind of dispersants and processing parameters on the performances of silver powders and corresponding solar cells were investigated.The influences of different dispersants, like PVP, arabic gum, citric acid, guar gum and gelatin on the performances of silver powders were studied. The results indicated spherical silver powders with high tap density(5.56g·cm-3), high dispersion and uniform particles size distribution were prepared with gelatin protection. Under the gelatin dispersive system, the effects of solution concentration, doses of dispersant, reaction PH and temperature on the performance of silver powder were studied. With the increasing of concentration of C6H8O6(5.1%-26.3%), the silver particle size increases from 1.49μm to 2.33μm firstly and then decreases to 2.09μm, the corresponding conversion efficiency increases from 15.62% to 18.18% and the sphericity is improved. With the increasing of gelatin/silver nitrate mass(0.6%-7%), the dispersion and the crystallinity are improved, and the silver particle size decreases from 3.44μm to 1.82μm. With the increasing of PH value(0.6-3), the dispersion is improved, the silver particle size increases from 1.65μm to 1.89μm firstly and then decrease to 1.41μm. And the conversion efficiency is relatively high(17.81%) when PH value is 1.2.Four silver powders with different particle size and morphology were prepared by changing processing parameters. The silver powders were used to make front silver contact to investigate the influences of silver powders on the screen printing, sintering process and photoelectric properties of front silver contact. The results indicated: the minor particle size(0.69μm) is difficult to prepare the high solid content of front silver paste; the silver paste prepared by the irregular silver powders cannot be used to print the uniform thin grating line due to the poor thixotropy. The microstructure of front silver paste was characterized by SEM. The smaller silver particles, the smaller the size of Ag crystallite is and the less number of Ag crystallite is at the Ag-Si interface, which resulted in the increase of the Ag-Si contact resistance. The oversize of silver particles leads to the uneven distribution of the Ag crystallite during the sintering process and the damage of emitter junction of solar cell because of the existence of the oversized Ag crystallite, which resulted in the poorer performance of solar cell. As the comparative number and size of the silver crystals at the Ag-Si interface, the conversion efficiency of solar cells fabricated using spherical silver powders with size of 1.47μm, high tap density and sintering activity is higher in four different silver powders.