Study of electron tunneling mechanism by co-firing process for high efficiency crystalline Si solar cells

It is well known that performance of industrial crystalline silicon solar cell greatly depends on the final step of co-firing condition. In order to achieve higher efficiency solar cell, control of co-firing condition by a fast firing process is very critical. Using Dual-Beam-Focused Ion-Beam (DBFIB) and Transmission Electron Microscopy (TEM), the microstructure of the interface between the front-side silver paste contact and emitter under different co-firing conditions are investigated by increasing co-firing temperatures and belt speeds. It is found that there is a thin glassy-phase layer just above Si surface plays an important role in contact properties. The location where silver to Si surface direct contact is observed through SEM, and there is actually a very thin glass layer in between Ag/Si contact and Ag embryos. In addition, high-density of silver embryos above on Si surface is found for sample with an optimized co-fired process. The thin glass layer between Ag-embryos and Ag-bulk is an effective barrier for electron tunneling through. All these phenomena presented in this study suggest that Ag-bulk/thin-glass-layer/Si contact be the most decisive path which can be optimized through co-firing process. It is found that the optimal co-firing process can be improved greatly and provided a good silver paste contact. An average conversion efficiency of 18.40 of a sample size of 100 pieces of 6” p-type mono crystalline Si 2 bus-bars solar cell without any extra process can be finally achieved.