Contact formation of front side metallization in p-type, single crystalline Si solar cells: Microstructure, temperature dependent series resistance and percolation model

Screen printed front side contacts were investigated in single-crystalline (planar and textured) Si solar cells with n-type emitters, yielding maximum efficiencies of 18.0%. The crystallographic orientation of the Si surface and the paste strongly affect the contact formation as well as the contact resistance of the cells. For textured cells a continuous glass layer together with the formation of Ag colloids yielded a small contact resistance. Planar < 111 > oriented Si yielded specifically lower contact resistance (< 5 m Omega cm(2)) as compared to planar < 100 > orientation (> 10-40 m Omega cm(2)) for different pastes. Pyramidal Ag crystals are formed only on < 100 > oriented Si, whereas lens shaped Ag crystals are grown on < 111 > surfaces. From this it was concluded that the shape of the Ag nanocrystals determines the contact resistance, pyramidal Ag crystals formed on < 100 > planar surfaces yielded cells with large contact resistance and are, therefore, not considered to be necessary for a low contact resistance. Temperature dependent series resistance measurements yielded metallic behavior for cells with the lowest contact resistance bound to a certain paste. For other pastes and processing conditions a semiconducting behavior of the series resistance was found. However, cells with significant density of colloids in the glass layer yielded a small series and contact resistance. By considering the above arguments, a percolation model has been introduced in which metallic Ag colloids generate current filaments across the glass layer. This reduces the resistivity of the glass layer and thereby introduces a percolative nature of the current via Ag nanocolloids. The percolation limit for the 2d case was calculated for periodically arranged colloids with equal size and yields a minimum volume fraction of 15% for the Ag colloids in the glass layer. (C) 2015 Elsevier B.V. All rights reserved.