FRONT SIDE METALLIZATION OF P- AND N-TYPE SI SOLAR CELLS: MICROSTRUCTURE OF THE GLASS LAYER

摘要

High-efficiency cells (～18.0% for p-type and ～20% for n-type cells) show similar microstructure and chemical composition of the glass layer and a similar temperature dependence of the series resistance. Details of the glass layer with respect to its quantitative chemical composition, microstructure, and electrical conductivity have not been reported in the literature. However, we consider it as an important microstructural feature affecting the physical properties of the cell: (i) the glass layer reflects the chemical composition of the applied paste, (ii) it contributes to the current path. Therefore, a detailed analysis of glass layers is provided here by applying a combined SEM and TEM-EDX spectroscopy. The chemical composition of the investigated glass layers was (SiO_x)Pb, with Pb >10 at.%. Other elements like Ag, Zn, Al, and Ti were detected in the glass layer as minor constitutes. In high-efficiency n- and p-type cells, Al and Zn mole fractions in the of range 1-2 at.% were detected. The chemical composition of the glass layer serves as a chemical bar code of the applied paste used for the screen printing process and affects the contact resistance at the Si/metallization interface. Cells processed with an Al-free paste and a Zn-rich paste were investigated and yielded Al-free and Zn-rich glass layers, respectively. The role of additives in the glass layer, like Al and Zn, were found to be important for the reduction of the contact resistance. Glass layers of high-efficiency cells contain a large density of precipitates and Ag colloids and are considered as dirty semiconductors rather than insulators. The percolation model is, therefore, appropriate to describe the current path in such layers.