Novel techniques to prevent edge isolation of silicon solar cells by avoiding leakage currents between the emitter and the aluminium rear contact

摘要

Whenever the base contacting is effected by overcompensating the rear side electrically active emitter, short-circuits occur between the emitter and the alloyed aluminium rear contact. Without additional costly processing steps for the avoidance of excessive leakage currents this transition is non-rectifying. The development of novel cell designs like POWER (POlycrystalline Wafer Engineering Result) or EWT (Emitter Wrap-Through) solar cells with interdigitated contact grids has made the problem more complicated, because standard edge isolation, where short circuits are limited to the cell edges, cannot be efficiently applied. The aim of our investigation was the formation of rectifying neighbouring p{sup}+n{sup}+-doped regions. Three methods have been investigated whereby two of them yielded a rectifying metal-emitter transition without the necessity of junction isolation. This was achieved by using an Al-P codiffusion process, i.e. the simultaneous formation of the emitter and the BSF (Back Surface Field) in one single high temperature step or by removing the excessive Al after alloying and subsequently depositing a new smaller base contact.