Al-Si contact formation involving back surface field and voids of PERC

Abstract

Local Al–Si contacts are a vital part of passivated emitter and rear cell (PERC) solar cells. They are generally fabricated using laser ablation to open the passivation dielectric layers, which is followed by Al paste printing and high-temperature alloying. However, the alloying process of Al–Si contacts and the formation mechanism of voids remain unclear in parts. In this study, based on the Al–Si binary phase diagram, we revealed the formation path associated with the back surface field (BSF) of the Al–Si alloying process and elucidated the formation mechanism of voids for dashed shape local contact openings. Also, a calculation model is proposed to determine and verify the Si concentration at the peak temperature. During peak-temperature firing, an equilibrium is established between the dissolution and diffusion of Si; subsequently, Si in the melt reaches its saturation concentration and laterally diffuses into the Al layer, thereby causing the net outflow of Si with the contact trench expansion and forming partial voids. During cooling, supersaturated Si precipitates to form BSF, while the degree of precipitated Si limits the thickness and forms the hypereutectic alloy. After the BSF formation, the melt was redistributed along the contact trench, resulting in partial voids turning into complete voids and fully filled contacts. This study also provides a new approach for regulating and optimizing Al–Si contacts. • The formation path of the back surface field of Al Si contact is revealed from the phase diagram. • Silicon saturated at peak temperature and BSF thickness is limited by the degree of silicon precipitation. • The contact of Hypereutectic component Al Si will affect the electrical properties. • The formation of voids is related to the lateral diffusion of silicon and the redistribution of melt. • The redistribution of melt transforms partial voids into complete voids.