Investigation On Metallization Of Crystalline Si Solar Cells

Screen-printing technology is the most commonly used in the photovoltaic industry due to its low cost, robustness, easy automation, and high throughput. The Series resistance limits the efficiency of screen-printed Si solar cells. As the raw material of screen-printing, silver paste directly affects the final Series resistance. This work was carried out on the glass frit in the paste. One point was to investigate the mechanism of the Ag-Si contact and discuss the possibility of the heavy metal oxide glass with Bi replace the glass contain Pb. Although the glass frit with Pb is most widely used in Ag thick-film paste, the environmental pollution and poisonous caused by Pb is very serious. It is necessary to reduce the use of glass which contains Pb.In this thesis, heavy metal oxide glass without Pb was prepared. The property of Bi2O3-ZnO-B2O3-SiO2system were examined by X-ray diffraction(XRD), Hemisphere point test and linear expansion test. The results are listed as follows:Bismuth borosilicate glass system has a wide formative region. The Tf and wettability of the glass can be improved by changing the ratio of silicon and boron or adding ZnO, Sb2O3. But, at the same time, this system become unstable, and it is easy to induce devitrification and parvafacies. During the firing process, there is no reaction between glass frit and silver particles. But silver will dissolve in the melted glass. The existence of melted glass frit is benefit for the firing of silver particles. The dissolve ability of silver in melted glass frit directly affects the amount of the recrystallization silver. Glass frit etch through SiNx film during the firing, this process is more likely a dissolve process. In order to achieve good electrical property of solar cells, it is important to achieve proper Tf temperature and recrystallization ability of glass frit.On the other hand, Forming gas annealing (FGA) is an effective process to repair low efficiency crystalline silicon (c-Si) solar cells with over-fired screen-printed paste electrodes. An experimental study was performed to investigate the effect and mechanism of FGA treatment on front silver electrodes of c-Si cells. To facilitate the FGA mechanistic study, special simulation samples were prepared to magnify the FGA effects on glass frit and over-fired electrodes. The micro-morphology (from cross-section X-SEM) and elemental composition (from EDX) data revealed few Ag crystallites in the paste/Si interface due to the thick glass layer from paste over-firing. The FGA treatment induced phase crystallization (from XRD) in the paste, increased the glass wettability on both Si and Ag substrates, and thus resulted in a thinner glass layer, which expedited the precipitation of more pyramidal Ag crystallites at the Ag/Si interface. The wetting angle data of glass samples measured before and after FGA confirmed the mechanism of FGA that the improvement of glass wettability benefits to reduce the glass layer thickness. As a result, more Ag crystallites diffused towards and precipitated at the Si interface, contributing to a lower contact resistance between the paste electrode and Si matrix, and improved electrical properties for over-fired c-Si cells.