Impact of firing temperature on fire-through metal contacts to P-doped (n) and B-doped (p) poly-Si

Abstract

Screen-printed contacts to doped polysilicon (poly-Si) layers with high-temperature fire-through pastes is one of the key enablers for widespread industrial acceptance of passivating contact solar cells. In recent years, rapid progress has been made in the development of pastes for contacting doped poly-Si layers. However, there are still gaps in understanding the behavior of fire-through contacts, especially regarding damage to poly-Si layers. In this work we present a comprehensive study of metal contacts formed with fire-through pastes to both phosphorus-doped ( n + ) poly-Si and boron-doped ( p + ) poly-Si layers. The contacts are fired at different temperatures and characterized by evaluating specific contact resistivity and recombination current density under the metal contacts. The metal crystallites formed at the interface between metal fingers and poly-Si is quantitatively analyzed to determine their area fraction. It is observed that the area fraction of the crystallites tends to saturate at higher temperatures for n + poly-Si, however such behavior is not observed for p + poly-Si within the range of firing temperatures relevant for industrial solar cells. Metal crystallites appear to agglomerate at higher temperatures for contacts to both n + and p + poly-Si layers which explains increased recombination due to aggressive etching of poly-Si. For Ag contacts to n + poly-Si best results are achieved when the metal crystallites covered 50% of the surface area at the optimum firing temperature. For Ag–Al contacts to p + poly-Si the crystallites covered 30% of the area, but resulted in higher recombination under the contacts. n-type bifacial solar cells are fabricated with n + poly-Si passivating contacts on the rear side and fired at different temperatures. The solar cell results followed the trends shown by the contacts and efficiency reached maximum for an optimized firing temperature. However, the change in efficiency was dominated by the front side contact characteristics. The results obtained on solar cells validate the observations about contact formation and metal recombination. Etch pits on p + poly-Si observed under SEM at different peak firing temperatures. Different splits represent the peak firing temperatures. A) 710 B) 730 C) 750 D) 770 E) 790. • In depth analysis of fired-through contacts formed to 150 nm doped poly-Si layers at different temperatures. • Contact resistivity improved with increasing temperature, attributed to increased number of metal crystallites. • Excellent contact resistivity ≈0.5 mΩ/cm 2 and ≈4 mΩ/cm 2 achieved for Ag and Ag–Al fired through contacts respectively. • The J 0,metal increased with increasing temperature due to increasingly aggressive etching of the poly-Si layer. • An ideal metal paste should result in optimum number of crystallites that do not extensively etch the poly-Si layer.